VEETEBEATE EMBEYOLOGY
MAESHALL
BY THE SAME AUTHOR.
Fourth Edition, Revised and Illustrated. Crown 8vo. is.
THE FROG : an Introduction to Anatomy, Histology, and Embryology.
DR. MILNES MARSHALL AND C. HERBERT HURST.
A JUNIOR COURSE OF PRACTICAL ZOOLOGY.
By A. Milnes Marshall, M.D., D.Sc, M.A., F.R.S., Professor in the Victoria University ; Beyer Professor of Zoology in Owens College ; late Fellow of St. John's College, Cambridge. Assisted by C. Herbert Hurst, Ph.D., Lecturer in the Victoria University ; Demonstrator and Assistant-Lecturer in Zoology, Owens College, Manchester. Third Edition, Revised. With additional Illustra- tions. Crown 8vo. 10s. 6d.
• This book cannot fail to be of great value to those who are studying zoology in their laboratory work ; and to sueh we have great pleasure in strongly recommending it.' — Loxdox Medical Record.
'We have, in all, a most successful
and important book. . . . The illustra- tions are excellent, reflecting the great- est credit upon all concerned. . . . The book is highly welcome and most admir- able. It is provided with an exceedingly good index, and presented in a form de- manding our sincere thanks.' — Natuke.
London : SMITH, ELDEE, & CO., 15 Waterloo Place.
VERTEBRATE EMBRYOLOGY
A TEXT-BOOK FOE STUDENTS AND PRACTITIONERS
BY /
A. MILNES MARSHALL, M.D., D.Sc, M.A., F.R.S.
PROFESSOR IN THE VICTORIA UNIVERSITY; BEYER PROFESSOR
OF ZOOLOGY IN OWENS COLLEGE ; LATE FELLOW OF
ST JOHN'S COLLEGE, CAMBRIDGE
LONDON SMITH, ELDER, & CO., 15 WATERLOO PLACE
1893
[All right* rrserrrd]
OL frf
Mil
PBEFACE
Gkeat attention has of recent years been given to the study of Embryology, and yet it is curiously difficult to find straight- forward accounts of the development even of the commonest animals. The special memoirs and monographs are usually limited to particular phases in the life-history of the forms with which they are concerned ; while the text-books of embryology aim rather at explaining the general progress of development within the several groups than at supplying complete descrip- tions of individual examples.
Up to the present time there has been no reasonably com- plete account of the development of the common frog, or of the rabbit, in our own or in any other language; while in works professing to deal with human embryology it is more common than not to find that the descriptions, and the figures given in illustration of them, are really taken, not from human embryos at all, but from rabbits, pigs, chicken, or even dogfish.
This latter practice is a most unfortunate one, and has been the cause of much confusion. The student is led to suppose that our knowledge is more complete than is really the case, while at the same time he finds the greatest difficulty in obtain- ing definite information on any particular point in which he is interested. Moreover, the implication that the details of develop- ment are identical in members of the same or of allied groups is directly opposed to the results of recent investigations, which are showing more and more clearly that marked differences,
VI PKEFACE.
both in the earlier and later stages of development, may occur between allied genera and species, or even amongst individual members of the same species.
The present book is an attempt to fill the gap, thus indicated, so far as the elements of Vertebrate Embryology are concerned. In it a few selected types are alone dealt with, and to each of these a separate chapter is devoted.
In the choice of types I have been mainly guided by the following considerations. Amphioxus is taken first, partly on account of its great morphological importance, and partly because of the extreme simplicity of its earlier developmental history, and of the clue which this affords to the more compli- cated conditions obtaining in the higher vertebrates. The next three chapters deal with the frog, the chick, and the rabbit respectively ; these have been selected as good representatives of the classes to which they belong, and as being the most easily obtained and the most suitable forms for laboratory purposes. The final chapter, and the longest in the book, is devoted to the development of the Human Embryo ; this has been included on account of its great intrinsic interest, and of the difficulty the student experiences, owing to the scattered and comparatively inaccessible nature of the original memoirs, in obtaining a reliable account of the present state of our knowledge. I have taken much pains to make this chapter as complete as our knowledge will allow, and venture to hope that it will be found useful not only by students of science and of medicine, but also by those engaged in medical practice.
I have not attempted to write a series of complete mono- graphs ; my purpose has been to give consecutive and straight- forward accounts which shall contain, in a form convenient for reference, the main facts known to us concerning the development of the animals I have selected as types. Many points of detail have been purposely omitted, as have also some of the more recent statements which appear to me to require confirmation. Science is better served by clearly stating in what points our knowledge is defective than by ignoring or evading difficulties ;
PREFACE. Vil
and I have purposely emphasised the more important of these gaps in the hope of drawing to them the attention of those who may have opportunity of filling up the deficiencies. The bibliographical lists at the ends of the several chapters have been deliberately curtailed, and include only those books and papers which appear to me of real importance : my object in this, as indeed in all respects, has been to produce a book which shall be useful rather than encyclopaedic.
I have, in the text, made no attempt to assign the several statements to their original authors : to do so would have bur- dened the book unduly. It will be well, however, to give here the main sources from which the facts are gathered, in order that I should not receive credit which is really due to others.
In the chapter on Amphioxus I have had to rely entirely on the work of other observers. The descriptions of the earlier stages are from the well-known accounts by Kowalevsky and by Hatschek : for the later stages I have depended mainly on the recent researches of Professor Lankester and Mr. Willey.
Except as regards the processes of maturation and fertilisa- tion of the egg, which are described from Oskar Schultze's papers, the chapter on the Frog is based almost entirely on my own observations, supplemented by the work of some of my pupils.
The development of the Chick has been described more often than that of any other animal. I have, however, worked over the greater part of the ground again, with special reference to this book. I have derived much assistance from the researches of Duval, especially in regard to the earlier stages of develop- ment.
I have not myself studied the processes of segmentation, and formation of the blastodermic vesicle in the Kabbit ; but I have had the advantage of examining a very excellent series of pre- parations by my friend Mr. Assheton. In my descriptions of these earlier stages I have relied mainly on the accounts of E. van Beneden, and of Kolliker. The later stages, from the first appearance of the embryo onwards, I have studied in con-
via PREFACE,
siderable detail, and 1 he descriptions are mainly from my own observations. In the study of the placenta, especially in its earlier stages, I have been greatly helped by Duval's careful investigations.
In the chapter dealing with the Human Embryo I have been compelled to obtain my material almost entirely from the obser- vations of others ; and notably from the splendid and long- continued work of Professor His, to whom it is due that our knowledge of human embryology is in so many respects more precise than that of any other mammal. It is a source of great regret to me that my friend Professor Minot's important treatise on Human Embryology only came into my hands while the last sheets of my own book were passing through the press, and that I have been unable to avail myself of the rich store of facts, and of the numerous suggestive explanations which his work con- tains.
A large proportion of the figures are new, and have been made expressly for this book from my own drawings. In the new figures, as well as in a large number of those which I have copied from the works of others, I have adopted, so far as practicable, a uniform mode of treatment and of lettering, which will, I hope, facilitate comparison of the figures of the several types with one another. I am under great obligations to the publishers of the works from which figures have been borrowed, for permission to reproduce these; and more especially to Messrs. Vogel, of Leipzig, for their ready consent to supply electrotypes, and to allow a large number of figures to be copied from Professor His' great monograph on the development of the Human Embryo ; a courteous liberality that is not always to be met with in this country.
I wish also to record my indebtedness to my friends Dr. Robinson and Mr. Assheton for many valuable suggestions and criticisms in the course of the work, and for much kind assist- ance in the correction of the proofs. To Mr. P. Hundley and Mr. G. Pearson, by whom the drawings on the wood were made, and the blocks engraved, my thanks are due for the great care
PBEFACE. > ix
they have bestowed on what, in my judgment, is one of the most important parts of the book.
I shall be very grateful for corrections or suggestions from those who use the book. I would further venture to make an earnest appeal for assistance to those who have opportunity of obtaining human embryos, and who do not require them for their own purposes. Our knowledge of the early stages of development of the human embryo is still very imperfect, and it is of the utmost importance that any opportunities that may occur of extending it should not be lost. Embryos of any age, but more particularly those of the first month or six weeks, would be of the greatest service to myself : they should be put into strong spirit as quickly as possible, a little cotton-wool being placed in the bottle to support the embryos, and to pre- vent them from shaking about during transit ; and any acts, such as the date of the last occurring menstruation, which would aid in determining the age, should be carefully recorded.
A. M. M.
Owens College: March 1893.
CONTENTS
CHAPTER I
INTRODUCTION
TAGE
General account of the development of animals — Structure of the egg — Maturation or ripening of the egg — Fertilisation of the egg— The early stages of development of the embryo— Theory of fertilisation — Segmentation of the egg — The germinal layers — The general history of development — The recapitulation theory — The origin of sex — Bibliography 1
CHAPTER II
AMPHIOXUS
Structure of the adult Amphioxus — Morphological importance of Am- phioxus — General account of the development of Amphioxus — The early embryonic development — The condition at the time of hatching — The later embryonic development — The condition at the close of the embryonic period — The larval period — The adolescent period — Bibliography 37
CHAPTER III
THE FROG
General account of the development of the frog — The egg — The early stages of development — The nervous system — The sense organs — The alimentary canal — The gill-clefts and the gills — The heart and blood- vessels— The urinary and reproductive organs — The skeleton and teeth — Bibliography 90
CHAPTER IV
THE CHICK
General account of the development of the chick — The egg — The early stages of development — The nervous system — The sense organs — The alimentary canal — The heart and blood-vessels — The urinary organs — The body cavity and the muscular system — The skeleton — The feathers— Bibliography 219
Xll CONTENTS.
CHAPTER V
THE RABBIT
I'AGK
Preliminary account of the development of the rabbit— The egg— Tin: early stages of development— General history of the embryo — The nervous system — The sense organs — The digestive system — The heart and blood-vessels — The urinary organs — The ccelom — The muscular system — The skeleton— The skin — The placenta — Bibliography. , .'541
CHAPTER VI
THE HUMAN EMBRYO
Preliminary account of the development of the human embryo — The human ovum — General history of the human embryo — The nervous system — The sense organs — The digestive system — The heart and blood-vessels — The urinary organs— The reproductive organs— The foetal membranes and the placenta — Bibliography . . . .448
INDEX (52 i
LIST OP ILLUSTRATIONS
INTRODUCTORY CHAPTER
FIG. TAGK
1. Stages in the maturation of the egg of the frog. (After 0. Schultze) 9
2. Segmentation of the egg of Amphioxus. (After Hatschek) . . 18
3. Segmentation of the frog's egg 20
4. The hen's egg, freshly laid 21
5. Segmentation of the germinal disc of the hen's egg. (After Coste
and Duval) . 21
6. Later stage in the segmentation of the germinal disc of the hen's
egg. (After Coste. and Duval) 21
7. Vertical section of the germinal disc of the hen's egg at the close of
segmentation. (After Duval) 21
8. Vertical section of early larval stage of Amphioxus. (After Hats-
chek) 23
9. Horizontal section of early larval stage of Amphioxus. (After Hats-
chek) 23
10. Transverse section through the head of a chick embryo at the end of
the first day of incubation .24
AMPHIOXUS
11. Young specimen of Amphioxus, viewed as a transparent object.
(From Marshall and Hurst) 38
12. Transverse section through the anterior part of the pharynx of an
adult Amphioxus. (From Marshall and Hurst) .... 39
13. Transverse section through the posterior part of the pharynx of an
adult female Amphioxus. (From Marshall and Hurst) . . 42
14. Segmentation of the egg of Amphioxus. (After Hatschek) . . 50
15. Formation of the gastrula of Amphioxus. (After Hatschek) . . 53
16. Later stage in the formation of the gastrula of Amphioxus. (After
Hatschek) 53
17. Further stage in the formation of the gastrula of Amphioxus. (After
Hatschek) 54
18. Completion of the gastrula of Amphioxus. (After Hatschek) . . 54
19. The gastrula of Amphioxus, bisected vertically. (After Hatschek) . 5f>
20. The gastrula of Amphioxus, bisected horizontally. (After Hatschek) 50 21-24. Transverse sections across the bodies of Amphioxus embryos,
showing the mode of formation of the nervous system and of
the mesoblastic somites. (After Hatschek) .... 58
xiv Mst ok ILLUSTRATIONS.
WVi, ''AI'K
25. Amphioxus embryo at the time of hatching; bisected vertically.
(AfterHatschek) B9
26. Amphiozof embryo at the time of hatching; bisected horizontally.
(AfterHatschek) 59
27. Amphioxus embryo shortly after hatching ; seen in optical section
from the right side. (AfterHatschek) 02
28 and 29. Transverse sections of Amphioxus embryos shortly after hatching ; showing stages in the formation of the notochord and mesoblastic somites. (After Hatschek) 64
30. Amphioxus embryo with nine pairs of mesoblastic somites, seen in
optical section from the right side. (After Hatschek) . . 66
31. Amphioxus embryo with nine pairs of mesoblastic somites, seen in
horizontal section. (After Hatschek) 66
32. Transverse section through the middle of an Amphioxus embryo with
nine pairs of mesoblastic somites. (After Hatschek) . . 67
33. Amphioxus embryo with fourteen pairs of somites ; seen in optical
section from the right side. (After Hatschek) .... 69
34. Amphioxus larva at the commencement of the second or ' larval '
period of development. (After Hatschek) . . . . .74
35. Young Amphioxus during the ' adolescent ' period. (After Kowa-
levsky) 74
36. The anterior end of an Amphioxus larva with four primary gill-slits,
seen from the left side. (After Lankester and Willey) . . 76
37. The anterior end of an Amphioxus larva with fourteen primary gill-
slits, seen from the right side. (After Willey) .... 76
38. The anterior end of an Amphioxus larva with thirteen primary and
eight secondary gill-slits, seen from the right side. (After Willey) 77
39. The anterior end of an Amphioxus larva with twelve primary gill-
slits, of which the first and twelfth are disappearing, and eight secondary gill-slits ; seen from the ventral surface. (After Willey) * 78
40. Diagrammatic transverse section across an Amphioxus larva with
eleven or twelve primary gill-slits, but no secondary ones. (Slightly modified from Lankester and Willey) .... 82
41. Diagrammatic transverse section through an advanced Amphioxus
larva with fully formed atrial cavity. (Slightly modified from Lankester and Willey, and from Boveri) 83
42. Diagrammatic transverse section across the intestinal region of an
Amphioxus larva with five primary gill- slits. (After Hatschek) 85
43. Diagrammatic transverse section across a young Amphioxus imme-
diately after the completion of the larval period. (After Hatschek) 86
THE FKOG
44. Various stages in the development of the frog. (From Brehm's
'Thierleben') 93
45. Stages in the maturation of the egg of the frog. (After 0. Schultze) 9« 46-48. Segmentation of the frog's egg 101
i
LIST OF ILLUSTRATIONS. XV
HO. PAGB
49. The blastula stage in the development of the frog's egg . , . 103
50. The frog's egg at the close of segmentation 103
51. Median sagittal section of a frog embryo, showing the spreading of
the epiblast, and the commencing formation of the mesenteron. 104
52. Sagittal section of a frog embryo during the formation of the
mesenteron 106
r>:\. Horizontal section of a frog embryo during the formation of the
mesenteron 107
54. Sagittal section of a frog embryo just before the disappearance of
the segmentation cavity 108
55. Sagittal section of a frog embryo after the disappearance of the
segmentation cavity and completion of the mesenteron . . 109
56. A transverse section through the middle of a frog embryo at about
the stage represented in Fig. 55 110
57. A frog embryo at the time of appearance of the neural folds : seen
from the dorsal surface 113
58. Stages in the early development of the frog embryo, seen obliquely
from the hinder end. (From a series of wax models by Dr. F. Ziegler of Freiburg i/B) 114
59. Transverse section through a frog embryo, showing the neural folds
shortly before they meet each other to complete the neural tube 115
60. Sagittal section of a frog embryo, shortly before closure of the
blastopore 116
Gl. Sagittal section of a frog embryo, shortly after closure of the blasto- pore and formation of the anus 117
62. The brain of the adult frog : dorsal surface 119
63. The brain of the adult frog : ventral surface 119
64. Sagittal section of the head end of a tadpole, just before the opening
of the mouth 120
65. Sagittal section through the head and body of a tadpole of 12 mm.
length, at the time of appearance of the hind limbs . . .121
66. Diagrammatic horizontal section of a 12 mm. tadpole, at the time of
appearance of the hind limbs 135
67. Transverse section through the head of a tadpole of 6| mm. length,
about the time of hatching . .187
68. Transverse section across the posterior part of the head of an adult
frog, showing the position and relations of the auditory organs, Eustachian tube, and hyoid apparatus 144
69. Sagittal section through a tadpole at the time of hatching . . 146
70. Transverse section across the middle of the length of a frog embryo
3| mm. in length •. 147
71. Horizontal section of the head and body of a 12 mm. tadpole . . 155
72. Side view of a tadpole at the time of hatching 157
3. Ventral view of a tadpole at the time of hatching . . . 157
4. Horizontal section of a tadpole at the time of hatching . . .1 58
5. Transverse section through the head of a 12 mm. tadpole . . .162
76. Diagrammatic figure of a 12 nun. tadpole, about the- time of appear-
ance of the hind limbs 166
77. Diagrammatic figure of the head and anterior part of the body of a
THE CHICK
17:5 17s
187
189 190
xvi LIST OF ILLUSTRATIONS.
no. . ''"''■
7 mm. tadpole shortly after hatching; showing the branchial
blood-vessels from the ventral surface 170
78. Diagrammatic figure of the same embryo as in fig. 77, seen from the
right side 17°
79. Diagrammatic) transverse section across the bead of a 7 mm. tadpole 171
80. Diagrammatic figure of the head of a 12 mm. tadpole from the right
side, showing the heart and branchial blood-vessels .
81. Diagrammatic figure of the arterial system of an adult frog
82. Transverse section through the body of a tadpole at the time of
hatching, showing the nephrostomes of the head-kidney .
83. Diagrammatic figure of a 12 mm. tadpole dissected from the ventral
surface, to show the heart and branchial vessels, and the head kidneys and commencing Wolffian bodies ....
84. Transverse section across a 12 mm. tadpole, passing through the
middle of the head-kidney
85. A 40 mm. tadpole dissected from the ventral surface, to show the
heart, the branchial vessels, and the urinary and reproductive organs
86. A tailed frog, during the metamorphosis, dissected from the ventral
surface to show the urinary and reproductive organs . . .193
87. Transverse section through the hinder part of the body of a tailed
frog during the metamorphosis 194
88. Transverse section through the anterior part of the body of a tailed
frog during the metamorphosis 195
89. Sagittal section of a tailed frog during the metamorphosis . . 200
90. The skull of a 12 mm. tadpole, seen from the right side . . . 202
91. The skull of a 12 mm. tadpole, from the dorsal surface . . . 202
92. The skull of a 12 mm. tadpole, from the ventral surface . . . 202
93. The skull of a tailed frog, towards the close of the metamorphosis.
seen from the right side 208
94. The skull of an adult frog, seen from the right side .... 209
95. The skull of an adult frog, seen from the ventral surface . . . 210
96. The skeleton of the frog, seen from the dorsal surface . . . 214
193
221
97. The hen's egg at the time of laying
98. The yolk of a hen's egg at the thirty-sixth hour from the commence
ment of incubation
99. The yolk of a hen's egg at the end of the third day of incubation
100. The hen's egg at the end of the fifth day of incubation .
101. The hen's egg at the end of the ninth day of incubation .
102. An early stage in the segmentation of the germinal disc of the hen's
egg. (After Coste and Duval)
103. A later stage in the segmentation of the germinal disc of the hen's
egg. (After Coste and Duval) 233
104. Section through the germinal disc and adjacent parts of the yolk of
a hen's egg about the middle of its stay in the uterus. (After Duval) 231
105. Vertical section of the blastoderm and adjacent part of a hen's egg
towards the close of segmentation. (After Duval) . . . 234
223 224
225
227
233
LIST OF ILLUSTRATIONS. XVll
FIG. PAGE
106. Vertical section of the blastoderm and adjacent part of the yolk of
a hen's egg at the time of laying, but before the commencement
of incubation. (After Duval) 235
107. A diagrammatic figure of the blastoderm of a hen's egg about the
twentieth hour of incubation. (In part after Duval) . . 238
108. Transverse section across the blastoderm of a hen's egg about the
twentieth hour of incubation 239
109. A diagrammatic figure of the blastoderm of a hen's egg about the
twenty-fourth hour of incubation. (In part after Duval) . . 241
110. A chick embryo at the twenty-fourth hour of incubation ; seen from
the dorsal surface 248
111. A chick embryo at the thirty-sixth hour of incubation; seen from
the dorsal surface 251
112. A median longitudinal, or sagittal, section of a chick embryo at the
thirty-sixth hour of incubation .251
113. A chick embryo at the end of the third day of incubation . . 253
114. A median longitudinal, or sagittal, section through a chick embryo
at the end of the third day of incubation ..... 255
115. A chick embryo at the end of the fifth day of incubation . . 257 lit). A median longitudinal, or sagittal, section of the head and anterior
part of the neck of a chick embryo at the end of the eighth day
of incubation 258
117. Transverse section across the body of a chick embryo at the twenty-
fourth hour of incubation 2G1
118. Transverse section across the head of a chick embryo at the twenty-
fourth hour of incubation 263
119. Transverse section across the head of a chick embryo at the forty-
third hour of incubation 264
120. Transverse section across the head of a chick embryo at the forty-
third hour of incubation, the section passing through the com- mencing auditory pits and the heart 265
121. Transverse section across the head of a chick embryo at the forty-
eighth hour of incubation 276
122. Transverse section across the fore-brain and eye of a chick embryo
at the sixtieth hour of incubation 277
123. A median longitudinal, or sagittal, section through a chick embryo
at the end of the fifth day of incubation 282
1 24. A section through the head of a chick embryo at the end of the third
day of incubation 284
125. The head of an embryo chick at the end of the fifth day of incuba-
tion 287
126. The head of an embryo chick at the end of the seventh day of in-
cubation 288
127. The anterior end of a chick embryo at the thirty-sixth hour of incu-
bation 299
128. A diagrammatic figure showing the arrangement of the blood-
vessels in a chick embryo at the end of the fifth day of incu- bation 304
129. A transverse section across the body of a chick embryo at the forty-
eighth hour of incubation 317
a
XVlli LIST OF ILLUSTRATIONS.
net. pAai
130. The left half of the skeleton of the common fowl. (From Marshal]
and Hurst) 326
131. The skull of a chick embryo at the end of the eighth day of incu-
bation 829
132. The skull of the fowl, from the right side. (From Marshall and
Hurst) 831
THE RABBIT
133. Section through part of the ovary of an adult rabbit . . . 347
134. A fully formed ovum of a rabbit, shortly before its discharge from
the ovary. (After Bischoff) 350
135. A rabbit's ovum, from the upper end of the oviduct, after extrusion
of the two polar bodies. (After Bischoff) 350
136. A rabbit's ovum, about twenty-two hours after copulation, showing
division of the ovum into two cells. (After Bischoff) . . 353
137. A rabbit's ovum about the middle of the third day, showing the
morula stage, shortly before the completion of segmentation. (After Bischoff) 353
138. A rabbit's ovum seventy hours after copulation, showing the condi-
tion at the close of segmentation. (After Van Beneden) . . 354
139. A rabbit's ovum seventy-five hours after copulation, showing the
first stage in the formation of the blastodermic vesicle. (After Van Beneden) 354
140. Section of the blastodermic vesicle of a rabbit at the end of the
fourth day. (After Van Beneden) 355
141. A vertical section across the embryonal area of the blastodermic
vesicle of a rabbit at the end of the fifth day. (After Kolliker) 359
142. A transverse section across the hinder part of the embryonal area of
a rabbit embryo at the end of the seventh day. (After Kolliker) 359
143. The blastodermic vesicle of a rabbit at the end of the seventh day.
(Modified from Kolliker) 360
144. The embryonal area of a rabbit at the middle of the eighth day.
(Modified from Kolliker) 360
145. A rabbit embryo and blastodermic vesicle at the end of the ninth day 363
146. A median longitudinal, or sagittal, section through a rabbit embryo
and blastodermic vesicle at the end of the ninth day. (In part after Van Beneden and Julin) 364
147. A rabbit embryo and blastodermic vesicle at the end of the tenth
day. (In part after Van Beneden and Julin) .... 365
148. A rabbit embryo and foetal appendages at the end of the twelfth
day. (In part after Van Beneden and Julin) .... 366
149. A rabbit embryo of the twentieth day 367
150. A median longitudinal, or sagittal, section through a rabbit embryo
at the end of the twelfth day 373
151. A median longitudinal, or sagittal, section through the head of a
rabbit embryo of the eighteenth day 375
152. The brain of an adult rabbit, dissected from above. (From Marshall
and Hurst) 377
153. A median longitudinal, or sagittal, section of the brain of an adult
rabbit. (From Marshall and Hurst) 378
LIST OF ILLUSTRATIONS. xix
FIG. TAGB
154. The brain of an adult rabbit from the ventral surface. (From
Marshall and Hurst) 383
155. A transverse section across the head of a rabbit embryo of the
fourteenth day 388
156. A transverse section across the head of a rabbit embryo of the
twenty -first day 391
157. A diagrammatic section across the head of an adult rabbit, to show
the relations of the internal ear, tympanic cavity and membrane,
and the auditory ossicles. (From Marshall and Hurst) . . 394
158. A transverse section across the head of a rabbit embryo at the end
of the eleventh day, the section passing through the medulla oblongata, the ears, and the pharynx 395
159. A transverse section across the head of a rabbit embr}ro of the
fifteenth day, passing through the medulla oblongata, the ears,
and the pharynx 396
160. A diagrammatic view of an adult male rabbit from the left side.
(From Marshall and Hurst) 401
161. A rabbit embryo at the end of the twelfth day, seen from the right
side 402
162. The skull of the rabbit from the right side. (From Marshall and
Hurst) 405
163. A transverse section across the thorax of a rabbit embryo of the
sixteenth day 409
164. A transverse section across the body of a rabbit embryo of the
early part of the tenth day, showing the supposed epiblastic origin of the Wolffian duct. (After Hensen) .... 422
165. A transverse section across the body of a rabbit embryo at the end
of the eleventh day 423
166. A transverse section across the hinder part of the body of a rabbit
embryo of the fourteenth day 424
167. Selected vertebrae from the rabbit. (From Marshall and Hurst) . 431
168. A transverse section across the uterus, with the contained blasto-
dermic vesicle, of a rabbit at the end of the seventh day. (In part after Duval) 436
169. A transverse section across the uterus and the contained blasto-
dermic vesicle of a rabbit at the end of the ninth day. (In part after Duval) 439
170. A transverse section across the uterus and the contained embryo
of a rabbit at the end of the nineteenth day .... 443
THE HUMAN EMBRYO
171. Part of a vertical section of the ovary of a new-born infant. (From
Strieker's ' Histology') 451
172. Front view of Reichert's ovum. (From Kolliker, after Rei chert) . 472
173. Side view of Reichert's ovum. (From Kolliker, after Reichert) . 472
174. Diagrammatic section of Reichert's ovum. (From His) . . . 473
175. A longitudinal section of the uterus, with an ovum in situ, esti-
mated as about the thirteenth day. (After Kollmann) . . 474
176. Outline figure of a human embryo lettered by Professor His, E, and
estimated as about the thirteenth day. (From His) . . . 477
X* LIST OF ILLUSTRATIONS.
no, TAGK
177. Outline figure of a human embryo described by Allen Thomson, and
estimated as about the thirteenth day. (From His) . . . 477
178. Outline figure of a human embryo, lettered by Professor His, S R,
and estimated as of the thirteenth day. (From His) . . 477
179. Human embryo lettered by Professor His, S R, and estimated as of
the thirteenth day. (After His) 478
180. Human embryo of about the thirteenth day, from the left side.
(After V. Spee) 479
181. The same embryo as in Fig. 180, from the dorsal surface. (After
V. Spee) 479
182. Transverse section across the head end of the human embryo shown
in Figs. 180 and 181. (After V. Spee) 480
183. Transverse section across the middle of the body of the human
embryo shown in Figs. 180 and 181. (After V. Spee) . . 480
184. Transverse section across the hinder end of the human embryo
shown in Figs. 180 and 181. (After V. Spee) . . . .480
185. Human embryo of about the fourteenth day, from the right side.
(After Kollmann) 481
186-188. Diagrammatic longitudinal sections through human embryos, representing hypothetical stages intermediate between Reichert's ovum and His' embryos, E or S R. (From His) .... 484
189. Outline figure of a human embryo lettered by Professor His, Lg, and
estimated as fifteen days old. (From His) .... 487
190. Outline figure of a human embryo lettered by Professor His, Sch,
and estimated as fifteen days old. (From His) .... 487
191. Outline figure of a human embryo lettered by Professor His, M, and
estimated as eighteen days old. (From His) .... 487
192. Outline figure of a human embryo figured by Allen Thomson, and
probably about eighteen days old. (From His) . . . 487
193. Outline figure of a human embryo lettered by Professor His, B B,
and estimated as about eighteen days old. (From His) . . 487
194. Outline figure of a human embryo lettered by Professor His, Kin,
and estimated as about twenty days old. (From His) . . 487
195. Outline figure of a human embryo lettered by Professor His, Lr,
and estimated as twenty or twenty-one days old. (From His) . 487
196. Human embryo at the commencement of the third week. (From
His, after Coste) 488
197. Human embryo lettered by Professor His, Lg, and estimated as
fifteen days old. (After His) 489
198. Human embryo lettered by Professor His, Lr, and estimated as
twenty or twenty-one days old. (After His) .... 491
199. Outline figure of a human embryo figured and described by Coste,
and estimated as about twenty-three days old. (From His) . 493
200. Outline figure of a human embryo lettered by Professor His, a, and
estimated as about twenty-three days old. (From His) . . 493
201. Outline figure of a human embryo figured and described by Allen
Thomson, and estimated as about twenty-three days old. (From His) 493
202. Outline figure of a human embryo lettered by Professor His, B, and
estimated as twenty-seven days old. (From His) . 493
LIST OF ILLUSTRATIONS. . XXI
TIG. PAQE
203. Outline figure of a human embryo lettered by Professor His, A, and
estimated as twenty-seven days old. (From His) . . . 493
204. Human embryo lettered by Professor His, A, and estimated as
twenty-seven days old. (After His) 494
205. Outline figure of a human embryo lettered by Professor His, Rg, and
estimated as thirty-two or thirty-three days old. (From His) . 497
206. The under surface of the head of a human embryo lettered by Pro-
fessor His, Hn, and estimated as about twenty-nine days old. (After His) 498
207. The under surface of the head of a human embryo lettered by Pro-
fessor His, C.IL, and estimated as about thirty-four days old. (After His) 498
208. The left ear of a human embryo lettered by Professor His, Br. 2, and
estimated as thirty-five days old. (From His) .... 500
209. The left ear of a human embryo lettered by Professor His, Dr, and
estimated as thirty-eight days old. (From His) .... 500
210. A pregnant uterus of about the fortieth day. (From Kolliker, after
Coste) 501
211. Outline figure of a human embryo about the middle of the sixth
week. (From His) 503
212. Outline figure of a human embryo at the end of the second month.
(From His) 504
213. Head of a human embryo at the end of the seventh week. (After
His) 505
214. Head of a human embryo at the end of the second month. (After
His) 505
215. The head and fore part of the body of a human embryo lettered by
Professor His, Lr, and estimated as twenty or twenty-one days old. (After His).. 511
216. Human embryo lettered by Professor His, Pr, and estimated as
twenty-eight days old. The brain is exposed from the left side, and the body of the embryo has been dissected to show the heart and aortic arches and the alimentary canal. (After His) 512
217. The brain of a human embryo lettered by Professor His, ZW, and
estimated as about the middle of the eighth week. (After His) 513
218. A human foetus three months old, dissected from the dorsal surface
to expose the brain and spinal cord. (From Kolliker) . . 514
219. The brain of a human foetus three months old, from the right side.
(From Kolliker) 515
220. The brain of a human foetus three months old, dissected from the
dorsal surface. (From Kolliker) 515
221. The brain of a human foetus three months old, from the ventral
surface. (From Kolliker) 515
222. The brain and spinal cord of a human foetus four months old, from
the dorsal surface. (From Kolliker) 515
223. The brain of a human foetus sjx months old, from the right side.
(From Kolliker) 515
224. The brain of a human foetus of the fifth month, bisected, and seen
from the inner surface. (From Kolliker) 518
225. A transverse section through a portion of the wall of the spinal
xxii LIST OF ILLUSTKATIONS.
J'lO. J'AGB
cord of a human embryo at the beginning of the fourth week. (After His) 522
226. A diagrammatic transverse section across the spinal cord of a human
embryo of the fourth week. (After His) 525
227. A diagrammatic figure of a human embryo lettered by Professor
His, Ko, and estimated as thirty-one days old. The figure shows the brain and spinal cord, and the cranial and spinal nerves. (After His) 529
228. Transverse section across the medulla oblongata of a human embryo
lettered by Professor His, Ko, and estimated as thirty-one days old. The section passes through one of the roots of the hypo- glossal nerve, and through both the motor and sensory roots of the pneumogastric nerve. (After His) 532
229. The left auditory vesicle of a human embryo four weeks old. (After
W. His, jun.) £42
230. The left auditory vesicle of a human embryo five weeks old. (After
W. His, jun.) 542
231. The left auditory vesicle, or internal ear, of a human embryo of the
eighth week. (After W. His, jun.) 543
232. Human embryo lettered by Professor His, Lg, and estimated as
fifteen days old. The brain and heart are exposed from the right side ; the alimentary canal and the yolk-stalk are repre- sented in median sagittal section. (After His) .... 545
233. Outline figure of the alimentary canal of a human embryo lettered
by Professor His, Pr, and estimated as twenty-eight days old. (From His) 546
234. Outline figure of the alimentary canal of a human embryo lettered
by Professor His, Sch, and estimated as thirty-five days old. (From His) 547
235. Outline figure of the alimentary canal of a human embryo estimated
as thirty- two days old. (From His) 548
236. Outline figure of the alimentary canal of a human embryo estimated
as thirty-five days old. (From His) 548
237. The floor cf the pharynx of a human embryo fifteen days old, seen
from above. (After His) 550
238. The floor of the pharynx of a human embryo twenty-three days old,
seen from above. (After His) 551
239. The floor of the pharynx of a human embryo twenty-eight days
old, seen from above. (After His) 552
240. The head and neck of a human embryo thirty-two days old, seen
from the ventral surface. (After His) 553
241. The roof of the mouth of a human embryo about two and a half
months old, showing the formation of the palate. (After His) . 554
242. The tongue and the floor of the mouth of a human embryo at the
end of the second month. (After His) 556
243. Human embryo lettered by Professor His, 131, and estimated as
twenty -three days old. The brain and spinal cord are exposed from the right side: and the body is dissected to show the heart, the blood-vessels, and the alimentary canal. (After His) 567
LIST OF ILLUSTRATIONS. XXlii
FIO. PAGE
244. The dorsal half of the heart of a human embryo twenty-eight days
old, seen from within. (After His) 569
245. The aortic arches of a human embryo thirty-two days old, from the
left side. (After His) 576
246. The aortic arches of a human embryo thirty-five days old, from the
left side. (After His) 577
247. The liver, and the veins in connection with it, of a human embryo
twenty-four or twenty-five days old, seen from the ventral sur- face. (After His) 580
248. Transverse section across the body of a human embryo estimated
as fourteen days old. (After Kollmann) 589
24D. The adult ovary, parovarium and Fallopian tube. (From Quain's
* Anatomy,' after Kobelt) 595
250. The external genitalia of a human embryo of about the ninth week.
(From Kolliker, after Ecker) 596
251 . The external genitalia of a human embryo of about the tenth week.
(From Kolliker, after Ecker) 596
252. The external genitalia of a male human embryo towards the end of
the third month. (From Kolliker, after Ecker) .... 597
253. The external genitalia of a female human embryo towards the end
of the third month. (From Kolliker, after Ecker) . . . 597
254. A diagrammatic section of the pregnant human uterus at the
seventh or eighth week. (From Quain's 'Anatomy,' after Allen Thomson) 600
255. A pregnant human uterus of about the twenty-fifth day. (From
Quain's ' Anatomy,' after Coste) 608
FERTILISATION OF THE EGG. 17
The connection between the formation of polar bodies and the process of fertilisation still remains to be explained. Such cases as those of the gipsy moth and the drone bee indicate that this connection is to be regarded rather as a normal than as a necessary one. Rapid cell division is an exhausting process, and Maupas has shown that in the Ciliate Infusoria the act of fission, which is the most frequent mode of reproduction, although it commences and at first proceeds with great rapidity, after a certain number of generations becomes less rapid, then irregular, and finally ceases altogether. To set it going again, a process of rejuvenescence or constitutional invigoration is ne- cessary ; this is effected by conjugation, during which an inter- change of nuclear matter is effected between the two individuals concerned in the act.
It seems very possible that the repeated cell division, which takes place in the formation of polar bodies, has a similar ex- hausting effect on the nucleus of the ovum, rendering a process of rejuvenescence desirable, and in most cases absolutely necessary, before any further division can take place ; this rejuvenescence being effected by conjugation, or fusion, of the nuclei of the spermatozoon and of the ovum.
This view, as Hartog points out, is in accordance with Balfour's theory in so far as it regards the formation of polar bodies as a process the object of which is to prevent parthenogenesis ; but differs from this theory in regarding the polar bodies, not as male elements extruded from an originally hermaphrodite egg, but as cells, the rapid formation of which has reduced the part of the nucleus still remaining in the egg, i.e. the [female pronucleus, to a condition of exhaustion which renders the stimulus of ferti- lisation necessary, or at least highly advantageous, if further cell-division is to take place.
Segmentation of the Egg.
The actual details of segmentation vary considerably in different cases, the differences depending chiefly on the relative amount of food-yolk present, and on its distribution within the egg.
The simplest form of segmentation is presented by alecithal eggs, such as those of Amphioxus. It is characterised by the almost geometrical regularity with which the successive divisions
18
INTRODUCTION,
\^-3
Fig. 2. — Segmentation of the egg of Amphioxus. x 220. (After Hatschek.)
I, the egg before the commencement of development : only one polar body, PB, has been seen, but from analogy with other animals it is probable that there are really two present. II, the ovum in the act of dividing, by a vertical cleft, into two equal blasto- meres. Ill, stage with four equal blastomeres. IV, stage with eight blastomeres ; an upper tier of four slightly smaller ones, and a lower tier of four slightly larger ones. V, stage with sixteen blastomeres, in two tiers, each of eight. VI, stage with thirty- two blastomeres, in four tiers, each of eight : the embryo is represented bisected, to show the segmentation cavity or blastoccel, B. VII, later stage : the blastomeres have in- creased in number by further division. VIII, blastula stage : bisected to show the blastoccel, B.
SEGMENTATION OF THE EGG. 19
occur, and by the fact that the cells, or blastomeres, into which the egg is divided are approximately equal to one another in size. The first cleft, Fig. 2, II, is a vertical one, and divides the egg into two perfectly similar halves. The second cleft is also vertical, but at right angles to the first one : on its completion the egg is divided into four cells or blastomeres of equal size, Fig. 2, in. The third cleft, Fig. 2, iv, is a horizontal one, and divides each of the four blastomeres of the previous stage into two, of which the lower one is slightly the larger. Two vertical clefts next appear simultaneously, at angles of 45° with the two first clefts : by these the number of the blastomeres is again doubled, giving sixteen in all, Fig. 2, v. Two new horizontal clefts double the number of blastomeres once more ; the stage, with thirty-two blastomeres, being shown in Fig. 2, VI. From this time segmentation continues rapidly, but with less regu- larity : later stages are shown in Fig. 2, vn and vm.
Segmentation is said to be complete, or holoblastic, when, as in Amphioxus, the whole egg is divided up at once into blasto- meres : it is further distinguished as equal when, as again in Amphioxus, the several blastomeres are from the first approxi- mately equal in size.
In the frog's egg, Fig. 3, segmentation is holoblastic, but unequal. The first two clefts, which, as in Amphioxus, are vertical, divide the egg equally and symmetrically ; but the third, or hori- zontal cleft, Fig. 3, in, is much nearer the upper than the lower pole, and throughout the later stages of segmentation, Fig. 3, IV and V, there is marked inequality in size between the blasto- meres of the upper and lower halves of the egg. Unequal seg- mentation is due to food yolk, which, in a telolecithal egg like the frog's, is specially accumulated in the lower pole, and retards the developmental processes in this as compared with the upper half of the egg.
An exaggeration of this condition is seen in the hen's egg, in which food-yolk is present in such quantity as to absolutely stop the processes of development in all parts of the egg, except in a small circular patch on the surface, corresponding to the upper pole of the egg of Amphioxus or the frog. To this circular patch, or germinal disc, Fig. 4, ba, segmentation is restricted. Figs. 5 and 6 represent surface views of the germinal disc during the process of segmentation, and show the
c 2
20
INTRODUCTION.
irregular manner in which the several clefts appear ; while Fig. 7 represents a vertical section of the germinal disc, with the
Fig. 3. — Segmentation of the Frog's Egg. The second figure is a surface view, the remaining four figures represent the egg in section, x 20.
I, the ovum just before the completion of the first cleft, by which it is divided into two equal blastomeres. II, stage with eight blastomeres : an upper tier of four small ones, and a lower tier of four much larger ones. Ill, the same stage, with eigbt blas- tomeres, in section. IV, V, later stages, showing further increase in the number of the blastomeres, with great inequality in their size. B, segmentation cavity or blastocoel. U , nucleus.
SEGMENTATION OF THE EGG.
21
Fig. 4.— The Hen's Egg, freshly laid. x |. BA, germinal disc. SH, egg shell. SM, shell membrane. SV, air chamber. WA, white or albumen. WC, chalaza. Y yolk. Z, vitelline membrane.
1\
Vf
mc
/ X
Fig. 5.
Fig. 6.
Figs. 5, 6. — Stages in the segmentation of the germinal disc of the Hen's Egg. x 10. (After Coste and Duval.)
B ZL
Fig. 7. — -Vertical section of the germinal disc of the Hen's Egg at the close of segmentation, x 25. (After Duval.) B, segmentation cavity or blastoccel. E, upper layer of blastomeres, or epiblast. TsT', nucleus of incompletely formed blastomere. VXj, vacuole in yolk. Y, yolk ZL, lower layer of blastomeres.
V
22 INTRODUCTION.
adjacent parts of the yolk, at the close of segmentation. Seg- mentation, when confined to part of the egg, is spoken of as meroblastic ; and when, as in the hen's egg, it is limited to a circular patch on the surface of the egg it is further distin- guished as discoidal.
Another type of meroblastic segmentation is presented by the centrolecithal eggs of Arthropods. Here, there is no localised germinal disc, but the whole surface of the egg consists of a layer of protoplasm free from yolk-granules, in which segmenta- tion occurs almost simultaneously at all parts ; such a mode of segmentation may be distinguished as superficial.
The principal types of segmentation, described above, may be tabulated as follows : —
I. Holoblastic or complete segmentation.
A. Equal : as in the alecithal egg of Amphioxus.
B. Unequal : as in the telolecithal egg of the frog. II. Meroblastic or partial segmentation.
c. Discoidal : as in the telolecithal egg of the chick.
D. Superficial : as in the centrolecithal eggs of Arthropods.
The Germinal Layers.
At the close of segmentation the whole of the egg, or, in cases of meroblastic segmentation, a part only of it, is divided up into cells or blastomeres. These blastomeres very early become arranged in two layers ; an outer layer, the epiblast, which covers the surface of the embryo ; and an inner layer, the hypoblast, which lines a cavity within its interior. Epiblast and hypoblast form the two primary germinal layers of the embryo : the epi- blast becomes ultimately the epidermis or outer layer of the skin ; while the hypoblast becomes the epithelial lining of the alimentary canal ; the cavity surrounded by the hypoblast, spoken of as the archenteron, forming the first commencement of the digestive tract. Figs. 8 and 9 represent early larvse of Amphioxus which have reached the stage described.
The details of development of epiblast and hypoblast, and more especially the mode of appearance of the archenteric cavity, are subject to great modifications in different groups of animals, but the essential relations are in all cases as described above.
Between epiblast and hypoblast a third layer of cells, the mesoblast, appears at a later stage, usually derived, directly or
THE GEKMIXAL LAYERS.
23
indirectly, from the hypoblast. Though appearing after the other two germinal layers, the mesoblast grows very rapidly, and in the higher animals forms a larger part of the embryo than the other two layers together.
The two primary germinal layers, epiblast and hypoblast, occur, and with essentially similar relations, in all groups of Metazoa, from sponges up to mammals. The middle germinal layer, or mesoblast, presents far greater variations, and it is by
Fig. 0.
Figs. 8, 9. — Vertical and horizontal sections of early larval stages of Amphioxvts. x 220. (After Hatschek.)
CE, commencing mesoblastic outgrowth. E, epiblast. Gr, archenteron. H, hypo- blast. N"F, neural fold. NT, neurenteric canal, leading from neural tube to archenteron. PC, polar mesoblast cell.
no means clear that all the structures spoken of as mesoblastic in the different groups of animals have any real community of origin or relations. In Sponges and Ccelenterates a mesoblastic layer cannot be said to exist, but in all other groups of Metazoa it is present.
The three germinal layers together make up the whole of the embryo, and from them all parts of the adult animal are derived : the principal organs and parts to which the layers give origin respectively are as follows.
The epiblast, or outer layer, gives rise to the epidermis, cover- ing the body generally ; and to the various organs derived from the epidermis. Of these, the more important are : — the nervous system, both central and peripheral ; the olfactory and auditory epithelium, the retina and lens of the eye, and the other organs of sensation ; the epithelial lining of the mouth and anus ; the pineal and pituitary bodies ; the enamel of the teeth ; the hairs,
24
INTRODUCTION.
nails, claws, and other epidermal modifications; and the epi- thelial lining of the mammary, sweat, and other glands formed from the skin.
The hypoblast, or inner layer, gives rise to the epithelium lining the alimentary canal and its various diverticula ; inclu- ding the glands of the oesophagus, stomach, and intestine, the lungs, the bladder, the bile ducts, gall bladder, and pancreatic- ducts, the hepatic cells of the liver, and the secreting cells of the pancreas. The notochord also is formed from hypoblast.
Fig. 10. — Transverse section through the head of a Chick Embryo at the end of the first day of incubation, showing the relations of the three germinal layers, x 100.
B, cavity of the brain : the origin of the walls of the brain from the epiblast is well seen. CH, notochord, arising from the hypoblast. E, epiblast. H, hypoblast. M"A, root of one of the cranial nerves. TP, cavity of the alimentary canal, in the pharyngeal region. R,T, blood-vessel. The whole of" the part of the figure covered by the lighter shading is mesoblast.
From the mesoblast, or middle layer, are derived all struc- tures lying between the epiblast and hypoblast ; i.e. the con- nective tissue, muscles, skeleton (except the notochord), blood- vessels, and lymphatics * and also the peritoneum, and the urinary and reproductive organs.
The General History of Development : the Recapitulation Theory.
It is a familiar fact that animals in the earlier stages of their existence differ greatly in form, in structure, and in habits from the adult condition.
In some cases, as, for example, in Amphioxus, the whole history of development is a steady upward progress towards the adult condition, the several organs and parts gradually approxi-
THE RECAPITULATION THEORY. 25
mating towards the fully formed state, and each stage bringing the animal, not merely as a whole, but as regards each of its organs and parts, one step nearer to the perfect form.
In the great majority of animals, however, the course of development is not so straightforward. Even in Amphioxus there are features in the early embryonic stages, such as the communication between the neural tube and the digestive cavity, which completely disappear during development, and which have no relation to the adult condition of the animal.
In the higher Vertebrates, far more striking instances occur. In the embryo of a chick or of a mammal the structure and relations of the heart and blood-vessels are for a time those of a fish ; and for the attainment of the adult condition it is neces- sary, not merely that new structures should appear and new relations be acquired, but that parts once present should actually become obliterated. The frog, again, commences its free exists ence as a tadpole, which is really a fish, not merely as regards its breathing organs, but in all details of its organisation ; and the change from the tadpole to the frog involves great modifica= tion in the shape, size, and relations of almost all its organs, with complete obliteration of parts such as gills and tail, which were essential to the tadpole but are absent from the frog.
It is to cases such as the frog, or as the butterfly, in which the transition from larva to adult is even more extensive and more abrupt, that the term metamorphosis is applied ; cases in which the animal, instead of developing straight towards the adult condition, in place of aiming straight at its goal, deviates from the direct path, spends time and energy in developing and elaborating organs which, though in perfect keeping with its actual mode of existence, yet have no relation to the state it is ultimately to reach, and must indeed be got rid of before that final condition can be attained.
Cases of this kind forcibly illustrate the necessity for some explanation of the facts of development. Much attention has been given to the subject, especially of recent years, and it is now possible to frame a consistent theory which will explain the general history of development in all groups of animals, and which will also be in harmony with the accepted views con- cerning the mutual relations of these groups.
The doctrine of descent, or of evolution, teaches us that as
26 INTRODUCTION.
individual animals arise, not spontaneously, but by direct descent from pre-existing animals, so also is it with species, with families, and with larger groups of animals, and so also has it been for all time ; that as the animals of succeeding generations are related together, so also are those of successive geologic periods; that all animals, living or that ever have lived, are united together by blood relationship of varying nearness or remoteness ; and that every animal now in existence has a pedigree stretching back, not merely for ten or a hundred gene- rations, but through all geologic time since life first commenced on the earth.
The study of development has in its turn revealed to us that each animal bears the mark of its ancestry, and is compelled to discover its parentage in its own development ; that the phases through which an animal passes in its progress from the egg to the adult are no accidental freaks, no mere matters of develop- mental convenience, but represent more or less closely, in more or less modified manner, the successive ancestral stages through which the present condition has been acquired.
Evolution tells us that each animal has had a pedigree in the past. Embryology reveals to us this ancestry, because every animal in its own development repeats its history, climbs up its own genealogical tree.
This Recapitulation Theory, as it is termed, was obscurely hinted at by the elder Agassiz, and suggested more directly in the writings of Von Baer ; but it was first clearly enunciated by Fritz Miiller in 1863, and has since that date formed the foun- dation on which the explanation of the facts of embryology is based.
The fact that a frog commences its free existence as a tad- pole, i.e. to all intents and purposes as a fish, is a very extra- ordinary one, but it becomes at once intelligible if we interpret it as meaning that frogs are descended from fish, and that every frog is constrained to repeat or recapitulate its pedigree in the course of its own individual development.
Similarly, the long-tailed condition of the young crab at the time of leaving the egg is to be viewed as an indication of the descent of the short-tailed or brachyurous crustaceans from macrurous ancestors; and the presence of gill clefts in the young stages of chicks or rabbits, which when adult are totally
THE RECAPITULATION THEORY. 27
devoid of them, or of teeth in the embryo of the whalebone whale, are in like manner to be regarded as reminiscences of former ancestral conditions, and as indicating that the ancestors of chicks and rabbits breathed by gills, and that the toothless whalebone whales are descended from toothed progenitors.
It is on this fact of Recapitulation that the great value of embryology depends. The study of development acquires a new and striking interest when it is realised that through it we are enabled to obtain knowledge, in many cases unattainable by any other means, of the real or blood relationships between animals and groups of animals.
It is with animals as with men, the only natural classification is a genealogical or phylogenetic one, and the possibility of framing such a classification of animals depends very largely on the success with which we are able to reconstruct their pedigrees from a study of the stages through which they pass in their actual development or ontogeny.
Recapitulation must apply, not merely to the development of an animal as a whole, but to that of each one of its organs and parts : the formation of the ear, for example, as a pit of the skin, must be interpreted as meaning that the ear, like the other organs of sensation,' was in its earliest commencement merely a spe- cialised patch of skin.
The theory must also apply to the earliest stages of develop- ment equally with the later ones ; and the fact that all Metazoa commence their existence as eggs — perhaps the, most sd^ciking of all ftmhryologifial fants — receives an entirely new significance when we interpret it as a reminiscence of a unicellular ancestry for all Metazoa, and as an indication that all the multicellular animals, or Metazoa, are descended from unicellular Protozoa.
From this point of view the earliest developmental stages of Metazoa deserve special attention, as possibly indicating the actual lines of descent of Metazoa from Protozoa. Segmentation is simply cell-division ; and the main difference between cell division in Protozoa and segmentation of the egg of a Metazoon is that, in the former case, the products of division separate from each other as independent unicellular animals, while in the latter they remain in close contact and become constituent units of one multicellular animal. The several stages of segmentation, Fig. 2, ii to vii, may be compared with colonies of Protozoa ;
28 INTRODUCTION.
while the blastula stage, Fig. 2, VIII, reached at the close of segmentation, bears a striking resemblance to such adult forme as Volvox or Pandorina.
There is, however, another side of the question which must not be overlooked. Although it is undoubtedly true that deve- lopment is to be regarded as a recapitulation of ancestral phases, and that the embryonic history of an animal presents to us a record of the race history, yet it is also an undoubted fact, recognised by all writers on embryology, that the record so obtained is neither a complete nor a straightforward one.
It is indeed a history, but a history of which entire chapters are lost, while in those that remain many pages are misplaced, and others are so blurred as to be illegible ; words, sentences, or entire paragraphs are omitted, and, worse still, alterations or spurious additions of later date have been freely introduced, and at times so cunningly as to defy detection.
Very slight consideration will show that development cannot in all cases be strictly a recapitulation of ancestral stages. It is well known that closely allied_.&nimals_max differ markedly in their modes of development, which could not be the case if both recapitulated correctly. The common frog, for example, is at first a tadpole breathing by gills, a stage which is entirely omitted by the little West Indian fr_o_g, Hy lodes. A crayfish, a lobster, and a prawn are allied animals, yet they leave the egg in totally different forms. Some developmental stages, as the pupa condition of insects, or the stage in the development of a tadpole in which the oesophagus is imperforate, cannot possibly be ancestral. Or again, a chick embryo, of say the third day, Fig. 113, is clearly not an animal capable of independent existence, and cannot therefore correctly represent any ancestral condition ; an objection which applies to the earlier developmental histories of many, perhaps of most, animals.
HaBckel long ago urged the necessity of distinguishing, in actual development, between those characters which are really historical and inherited, and those which are acquired or spurious additions to the record. The former he terms palin- genetic or ancestral characters, the latter cenogenetic or acquired. The distinction is certainly a true one, but an exceedingly difficult one to draw in practice. The causes which prevent development from being a strict recapitulation of ancestral
THE EE CAPITULATION THEORY. 29
history, the modes in which these came about, and the influence which they respectively exert, are problems which are as yet only partially solved.
Of these disturbing causes, the most potent and the most widely spread arises from the necessity of supplying the embryo with nutriment. This acts in two ways.
If the amount of nutritive matter within the egg be small, then, as we have already seen, the young animal must hatch early and in a very imperfectly developed condition. In such cases, as in Amphioxus or the frog, there is of necessity a long period of larval life, during which natural selection may act so as to introduce modifications of the ancestral history, spurious additions to the text. Of such ' larval organs,' the long spines that form conspicuous features in the young, free swimming larvae of sea urchins, or of crabs, are good examples.
If, on the other hand, the egg contain within itself a con- siderable quantity of nutrient matter, then the period of hatch- ing can be postponed until this nutrient matter has been used up. The consequence is that the embryo hatches at a much later stage of its development, and, if the amount of food material is sufficient, may even, as in the case of the chick, leave the egg in the form of the parent. In such cases the earlier developmental phases are often greatly condensed and abbre- viated ; and as the embryo does not lead a free existence, and has no need to exert itself to obtain food, it commonly happens that these stages are passed through in a very modified form, the embryo being, as in a three-day chick, in a condition in which it is clearly incapable of independent existence.
The effect of a greater or less amount of foocUyolk on the recapitulation of ancestral characters has been summed up by Balfour thus : ' There is a greater chance, of the ancestral history being lost in forms which develop in the egg, and of its being masked in those which are hatched as larvse.'
There are a number of other causes, besides food-yolk, which tend to modify the ancestral history as preserved in individual development. The following list gives a brief summary of the more important of these.
30 INTRODUCTION.
Causes tending to falsify the ancestral history ; or to prevent ontogeny from being a true record of phylogeny.
1. The general tendency to condensation of the ancestral history. Except perhaps in the lowest groups of Metazoa, such as sponges, no animal can possibly repeat, in its own develop- ment, all the ancestral stages in the history of the race. There is a tendency in all animals towards striking a direct path from the egg to the adult : a tendency best marked in the higher, the more complicated members of a group, i.e. those which have the longest and most tortuous pedigrees.
2. The tendency to the omission of ancestral stages. This has been already noticed as one of the commonest effects of abundance of food-yolk. The omission of the gill-breathing stage in Hylodes and in all Amniote Vertebrates is a typical example.
3. The tendency to distortion, either in time or space. All erubryologists have noticed the tendency to anticipation, or pre- cocious development, of characters which really belong to a later stage in the pedigree. Many early larvae show it markedly, the explanation in this case being that it is essential for them to possess at the time of hatching all the organs necessary for independent existence.
Anachronisms, or actual reversals of the historical order of development of organs or parts, occur frequently. Thus the joint surfaces of bones acquire their characteristic curvatures before movement of one part on another is effected, and even before the joint cavities are formed.
4. The tendency to the accentuation or undue prolongation of certain stages. This is best seen in cases of abrupt metamor- phosis, as of the caterpillar to the butterfly ; or of the pelagic pluteus larva to the sea urchin, slowly crawling on the sea- bottom ; or of the herbivorous aquatic tadpole to the terrestrial and carnivorous frog. In such cases there is usually a great differ- ence between larva and adult in external form and appearance, in manner of life, and very usually in mode of nutrition ; and a gradual transition is inadmissible, because in the intermediate stages the animal would be adapted neither to the larval nor to the adult conditions ; a gradual conversion of the biting mouth parts of the caterpillar to the sucking proboscis of the moth would
THE RECAPITULATION THEORY. 31
inevitably lead to starvation. The difficulty is evaded by retaining the external form and habits of one particular stage for an unduly long period, so that the relation of the animal to its surrounding environment remains unaltered, while, internally, preparations for the later changes are in progress.
5. The tendency to the acquisition of new characters. This has been dealt with already ; it arises from the fact that the larval forms of animals, like the adults, are exposed to the action of natural selection, and so are liable to acquire characters that, do not belong to the ancestral history.
Before leaving the subject it is worth while inquiring whether any explanation can be found of recapitulation. A complete answer can certainly not be given at present, but a partial one may, perhaps, be found.
Darwin himself suggested that the clue might be found in the consideration that at whatever age a variation first appears in the parent, it tends to reappear at a corresponding age in the offspring ; but this must be regarded rather as a statement of the fundamental fact of embryology than as an explanation of it.
It is probably safe to assume that animals would not recapitulate unless they were compelled to do so : that there must be some constraining influence at work, forcing them to repeat more or less closely the ancestral stages. It is impossible, for instance, to conceive what advantage it can be to a chick or a rabbit embryo to develop gill clefts which are never used, and which disappear at a slightly later stage ; or how it can benefit a whale, that in its embryonic condition it should possess teeth which never cut the gum, and which are lost before birth.
Moreover, the whole history of development in different animals or groups of animals offers to us, as we have seen, a series of ingenious, determined, varied, but more or less unsuccessful efforts to escape from the necessity of recapi- tulating, and to substitute for the ancestral process a more direct method.
A further consideration of importance is that recapitulation is not seen in all forms of development, but only in development from the egg. In the several forms of asexual development, of which budding is the most frequent and most familiar, there is no repetition of ancestral phases ; neither is there in cases of
32 INTRODUCTION.
regeneration of lost parts, such as the tentacle of a snail, the arm of a starfish, or the tail of a lizard ; in such regeneration it is not a larval tentacle, or arm, or tail that is produced, but an adult one.
The most striking point about the development of the higher animals is that they all alike commence as eggs. Looking more closely at the egg, and the conditions of its development, two facts impress us as of special importance : first, the egg is a single cell, and therefore represents morphologically the Proto- zoan, or earliest, ancestral stage ; secondly, the egg, before it can develop, must, in the great majority of case3, be fertilised by a spermatozoon, just as the stimulus of fertilisation by the pollen grain is necessary before the ovum of a plant will commence to develop into the plant-embryo.
The advantage of cross-fertilisation in increasing the vigour of the offspring is well known, and in plants devices of the the most varied and even extraordinary kind are adopted to ensure that such cross-fertilisation occurs. The essence of the act of cross-fertilisation consists in combination of the nuclei of two cells, male and female, derived from different individuals. The nature of the process is of such a kind that two individual cells are alone concerned in it ; and it may reasonably be argued that the reason why animals commence their existence as eggs, i.e. as single cells, is because it is in this way alone that the advantage of cross- fertilisation can be secured, an advantage admittedly of the greatest importance, and to secure which natural selection would operate powerfully.
The occurrence of parthenogenesis in certain groups, either occasionally or normally, is not so serious an objection to this view as it appears at first. There are strong reasons for holding that parthenogenetic development is a modified form, derived from the sexual method. Moreover, it is the very essence of the view advanced above, that it does not state that cross-fertilisation is essential to individual development, but merely that it is in the highest degree advantageous to the species ; and hence room is left for the occurrence, exceptionally, of parthenogenetic development.
It may be objected that this is laying too much stress on sexual reproduction, and on the advantage of cross-fertilisation ; but it must not be forgotten that sexual reproduction is the
THE RECAPITULATION THEORY. 33
characteristic and essential mode of multiplication among Metazoa ; that it occurs in all Metazoa ; and that when asexual reproduction, as by budding, &c, occurs, this merely alternates with the sexual process, which sooner or later becomes necessary.
If the fundamental importance of sexual reproduction to the welfare of the species be granted, and if it be further admitted that Metazoa are descended from Protozoa, then we see that there is a most powerful influence constraining every animal to commence its life history in the unicellular condition, the only condition in which the advantage of cross-fertilisation can be obtained ; i.e. constraining every animal to begin its develop- ment at its earliest ancestral stage, at the very bottom of its genealogical tree.
On this view the actual development of any animal is strictly limited at both ends ; it must commence as an egg, and it must end in the likeness of the parent. The problem of recapitulation becomes thereby greatly narrowed ; all that remains being to explain why the intermediate stages in the actual development should repeat, more or less closely, the inter- mediate stages of the ancestral history. Although narrowed in this way, the problem still remains one of extreme difficulty, and no final solution can yet be given of it.
It is a consequence of the Theory of Natural Selection that identity of structure involves community of descent; a given result can only be arrived at through a given sequence of events. A negro and a white man have had common ancestors in the past ; and it is through the long-continued action of selection and environment that the two types have gradually been evolved. You cannot turn a white man into a negro merely by sending him to live in Africa : to create a negro the whole ancestral history would have to be repeated, and it may be that it is for the same reason that the embryo must repeat, or recapitulate, its ancestral history in order to reach the adult goal.
Kleinenberg, in his { Theory of the Development of Organs by Substitution,' has suggested that each historic stage in the evolution of an organ is necessary as a stimulus to the develop- ment of the next succeeding stage, and that the reason for the extraordinary persistence, in embryonic life, of organs which are rudimentary and functionless in the adult, may be that the
84 INTRODUCTION.
presence of such organs in the embryo is indispensable as a stimulus to the development of the permanent structures of the adult. Should this theory prove to be well founded, it will afford a ready and welcome explanation of many perplexing facts in the development of animals.
The Origin of Sex.
The simplest mode of reproduction is a mere act of fission or cell division, as seen in an Amoeba or in an ordinary epithelial cell. Such a form of reproduction is characteristic of the sinvpler Protozoa, and of the component tissues of Metazoa. It may concern one individual alone, or may be preceded by the con- jugation or fusion of two or more originally separate individuals or cells.
The higher Protozoa, or Infusoria, show considerable advance on this simple method. In Paramecium, or Stylonychia, reproduc- tion is effected, as before, by fission, i.e. by division of the single animal into two separate animals ; and under favourable circum- stances this process may be repeated again and again with great rapidity. Sooner or later the rate slackens, and ultimately the process stops altogether ; and it does not recommence until conjugation, usually temporary, has occurred between two indi- viduals, which on the completion of the process begin to divide actively once more. Maupas' researches have shown that this conjugation is absolutely necessary, and that it must not take place between two closely allied individuals, but between ones of different broods.
In Vorticella there is further complication, for the conjuga- ting individuals are in this case unlike ; one being an ordinary large, stalked Vorticella ; the other a small free-swimming indi- vidual, of which a number, usually eight, are formed by simul- taneous division of a large Vorticella. The conjugation is in this case a permanent one, the small Vorticella fusing completely with the large one; and the whole process corresponds singularly closely with the sexual reproduction of Metazoa, the small free-swimming Vorticella playing the part of the spermatozoon, while the large fixed one behaves as the ovum. This may be taken as the first definite establishment amongst animals of sexual differentiation, and the two Vorticella? may not inappropriately be spoken of as male and female respectively.
THE ORIGIN OF SEX. 35
In the colonial Protozoa, such as Volvox, which take the form of hollow balls of cells, certain of the cells become large and stationary, forming the female cells or ova; these are fertilised by small active male cells, derived from the same or from other colonies ; and then, by division of the fertilised ova, new balls or colonies are formed.
This process is essentially the same as the sexual reproduc- tion of Metazoa, and there can be little doubt that the process has been inherited by the Metazoa from their Protozoan ancestors.
The reason for the occurrence of sexual reproduction in all Metazoa is probably to be found, as suggested above, in the consideration that it is through sexual reproduction alone that the full advantage of cross-fertilisation can be obtained. This view, that sexual reproduction is to be regarded as highly advantageous rather than as absolutely essential to the species, is of great importance, as it leaves room for, and renders intelligible, the occurrence of other and asexual modes of reproduction such as are seen in so many groups of Invertebrates. It also affords a clue to the extraordinary condition of things described in certain of the pelagic Timicates. Salensky has shown that in Salpa, and to a less marked degree in Pyrosoma, certain of the follicle-cells surrounding the ovum pass into its interior, and take an active part in the formation of the embryo ; so that, although the egg is fertilised in the ordinary manner, the blasto- meres resulting from its segmentation only give rise to certain of the component cells of the embryo, and not, as is usually the case, to all of them. This mode of development may be regarded as a combination of the ordinary sexual process with an asexual process similar to that by which the gemmules of sponges or the statoblasts of Polyzoa are formed.
List of the more important Books and Memoirs hearing on the Subjects of Chajoter I.
Balfour, F. M. : ' Treatise on Comparative Embryology.' Vol. i. chaps, i. ii. iii. ;
vol. ii. chap. xiii. 1880-81. Beneden, E. v. : ' Recherches sur la maturation de l'ceuf et la f econdation.'
Archives de Biologie, iv. 1884. Beneden, E. v., et Neyt, A. : ' Nouvelles recherches sur la fecondation et
la division mitotique chez l'Ascaride megalocephale.' Bulletin de
l'Academie Royale des Sciences de Belgique, 3e ser., tome xiv. 1887.
8G CNTBODUCTION.
Blochmann, F. : ' Uebcr die Riohtungskorpei bei [nsekteneiern.' Morpho-
logischefl Jalirbuch, Bd. xii. 1887, und Bd. xv. 1889. Boveri, T. : ' Zellenstudien,' Heft i. ii. iii. Jenaische Zeitschrif t fiir Natur-
wissenscbaft, 1887, 1880, 1890. Biitschli, 0. : ' Gedanken iiber die morphologische Bedeutung der sogenannten
Richtungskorperchen.' Biologisches Centralblatt, iv. 1884. Carnoy, J. B. : ' Les globes polaires de l'Ascaris.' La Cellule, tcme ii. iii. 1887. Geddes and Thomson : ' The Evolution of Sex.' 1889. Hartog, M. M. : 'Some Problems of Reproduction.' Quarterly Journal of
Microscopical Science, vol. xxxiii. 1891. Hertwig, 0. : ' Lehrbuch der Entwicklungsgeschichte des Menschen und der
Wirbelthiere.' Dritte Auflage. 1890.
' Vergleich der Ei- und Samenbildung bei Nematoden.' Archiv fiir
mikroskopische Anatomie, Bd. xxxvi. 1890. Kleinenberg, N. : ' Die Entstehung des Annelids aus der Larve von Lopado-
rhynchus.' Zeitschrif t fiir wissenschaftliche Zoologie, Bd. xliv. 1886. Marshall, A. Milnes : ' Address to the Biological Section of the British Asso- ciation.' British Association Report, 1890 ; and Nature, vol. xlii. 1890. Maupas, E. : ' Recherches experimentales sur la multiplication des Infusoires
cilies.' Archives de Zoologie Experimentale, deuxieme serie, tome vi.
1888.
' Le rajeunissement karyogamique chez les Cilies.' Archives de-
Zoologie Experimentale, deuxieme serie, tome vii. 1889. Minot, C. S. : ' Theorie der Gonoblasten.' Biologisches Centralblatt, Bd. ii.
1882. Nussbaum, M. : ' Ueber die Veranderungen der Geschlechtsproducte bis zur
Eifurchung.' Archiv fiir mikroskopische Anatomie, Bd. xxiii. 1884. • Bildung und Anzahl der Richtungskorper bei Cirripedien.' Zoo-
logischer Anzeiger, xii. 1889. Salensky, W. : ' Beitrage zur Embryonal-Entwioklung der Pyrosomen.' Zoo-
logische Jahrbiicher ; Abtheilung fiir Anatomie und Ontogenie, Bd.
iv. u. v. 1890-91. Schultze, O. : ' Untersuchungen iiber die Reifung und Befruchtung des Amphi-
bieneies.' Zeitschrif t fiir wissenschaftliche Zoologie, Bd. xlv. 1887. AValdeyer, W. : ' Karyokinesis and its relation to the process of Fertilisation,'
(translation). Quarterly Journal of Microscopical Science, vol. xxx.
1889. Weismann, A. : Essays upon Heredity and kindred Biological Problems
(translations). 1889 and 1892. Weismann, A., und Ischikawa, C. : ' Ueber die Bildung der Richtungskorper bei
thierischen Eiern.' Berichte der naturforschenden Gesellschaft zu
Freiburg i. Br. Bd. iii. 1887. Zacharias, O. : ' Neue Untersuchungen iiber die Copulation der Geschlechts- producte und den Befruchtungsvorgang bei Ascaris megalocephala/
Archiv fiir mikroskopische Anatomie, Bd. xxx. 1887.
37
Chapter II. THE DEVELOPMENT OF AMPHIOXUS.
I. GENERAL ACCOUNT. 1. Structure of Amphioxus.
Amphioxus is a small, semi-transparent, fish-like animal, about a couple of inches in length, found in shallow parts of the Mediterranean and other seas. It is of sluggish habits, and usually remains buried in the sand, either completely or with the anterior end alone protruding ; but if disturbed it swims actively, by rapid lateral movements of the body.
In the general plan of its organisation Amphioxus agrees with the more familiar members of the group of Vertebrates, but in a large number of important respects it is far simpler than any of these.
The external appearance of Amphioxus is shown in Fig. 11. The body is elongated, laterally compressed, and pointed at both ends. There is no distinct head, and no trace of limbs.
A low dorsal fin runs along the middorsal line from end to end of the animal, becoming more prominent at the hinder end as the upper lobe of the caudal fin. The ventral surface bears a median fin along its posterior third, but in front of this is flattened, so that the body is triangular in section. The sides of this flattened ventral surface are bordered by the lateral fins or metapleural folds. {Of. Figs. 11, 12, 13.)
The skeleton is in an extremely simple condition. Neither cartilage nor bone is present, and the principal skeletal structure is an elongated elastic rod, the notochord (Fig. 11, k), which extends the entire length of the animal, lying dorsal to the ali- mentary canal and between this and the spmal cord. The noto- chord is surrounded by a thick sheath of dense connective tissue (Fig. 12, d), which is prolonged dorsalwards to form a tubular investment around the spinal cord. From these sheaths to the
38
A.Mi'iimxrs.
notocliord and spinal cord, connective tissue partitions or septa arise, which, running outwards to the skin, divide the great lateral muscles of the body into muscle-segments or myotomes
(Fig. 11, K, and Fig. 12, x.) The attachments of these septa to the skin are indi- cated by a series of >- shaped markings, very clearly seen on the sides of the animal along its whole length (Fig. 11).
The only other skeletal structures of importance are a series of elastic chiti- nous rods, supporting the side walls of the pharynx ; and an oval hoop, sur- rounding the mouth.
The great lateral mus- cles, noticed above, are the most important part of the muscular system. They form the side walls of the body along its whole length (of. Figs. 12 and 13), and are divided, as already described, into muscle seg- ments or myotomes by the connective tissue septa. The muscle fibres of each myotome run longitudi- nally, i.e. parallel to the axis of the body, the fibres taking origin from the connective tissue septa. The myotomes have been found to be sixty-one on each side of the body in a considerable number of specimens, and it seems probable that this number is constant. The myotomes of the two sides
STRUCTURE OF THE ADULT ANIMAL.
39
of the body are not arranged in pairs, but alternate with one another along the whole length of the animal ; and this lateral asymmetry, one of the most marked features of the adult Amphioxus, affects the nerves, blood-vessels, and other structures as well. The ventral surface of the body in the anterior two-
Fig. 12.— Amphioxus lanceolatus. Transverse section through the anterior part of the pharynx of an adult specimen. The boundary of the atrial cavity is indicated by a thick black line. The section is taken at about the level of the reference line R in Fig. 11. (From Marshall and Hurst.)
A, skeleton of dorsal fin. B, spinal cord. C, notochord. D, connective-tissue sheath surrounding notochord. E, cavity of pharynx. F, epibranchial groove of pharynx. G, endostyle, which in this anterior part is flattened out or even convex. H, atrial cavity, j, transverse muscles in floor of atrial cavity. M, dorsal ccelomio canal. P, metapleural canal. B-, left dorsal aorta. S, cardiac aorta. X, myotome. Y, suspensory fold of pharynx, separating the dorsal coclomic canal from the atrial cavity. Z, gill-arch or branchial bar ; the white triangular spot represents the cut surface of the skeletal rod of the arch.
thirds of the animal is covered by a thin sheet of muscle (Fig. 12, j), the fibres of which run transversely from side to side.
The alimentary canal is a nearly straight tube, the anterior part of which is modified for respiration, as in fish.
The buccal orifice (Fig. 11) is a large oval opening, on the ventral surface of the anterior end of the body; it is fringed on each side by a series of ciliated tentacles, but there are no
■10 AMPHIOXUS.
jaws. The buccal orifice opens into a buccal cavity (Fig. 11, a), which is bounded laterally by the buccal hood, and posteriorly by a muscular diaphragm, the velum ; a small perforation in the velum, a little way below its middle, is the true mouth and leads into the pharynx.
The pharynx (Fig. 11, c) is a wide sac, forming about half the length of the alimentary canal, and attached along its mid- dorsal line to the under surface of the sheath of the notochord, (Fig. 12). The sides of the pharynx are perforated by a large number of slit-like apertures, the gill- slits, which run obliquely downwards and backwards, and of which in the adult animal there may be one hundred or more on each side. The parts of the pharyngeal wall left between successive slits are narrow bars, the gill-arches, each of which is strengthened by an axial rod of a chitinous substance. These arches are of two kinds, arranged alternately ; the axial rods of the second, fourth, &c, or primary arches, being forked at their ventral ends, while the rods of the alternate, or secondary arches, are unsplit. Each double gill-slit is originally a single one, but becomes divided in the course of development (vide p. 78), by the downgrowth of the unsplit bar, or tongue-bar as it is termed, from its dorsal end. The successive gill-arches are connected by horizontal bars, of which there are usually three or more crossing each slit, so that the pharynx has the character of an open meshwork.
Along the mid-dorsal line of the pharynx is a deep epibran- chial groove (Fig. 12, f), lined by a single layer of long columnar ciliated cells. A band of similar cells, the endostyle (Fig. 12, g), runs along the mid-ventral wall of the pharynx; it is folded longitudinally in its hinder part to form a groove (Fig. 13, g).
The intestine (Fig. 11) commences at the hinder end of the pharynx, close to the dorsal surface ; and runs straight back to the anus, which is on the ventral surface, some little distance from the hinder end of the body, and slightly to the left of the median plane. The intestine is extremely narrow at its com- mencement ; further back it dilates to form an expanded part or stomach, from which a large pouch-like outgrowth, the liver (Fig. 11, d), extends forwards some distance along the right side of the pharynx, ending blindly in front.
During life a stream of water passes through the mouth into the pharynx, and then out through the gill-slits in the sides
STRUCTURE OF THE ADULT ANIMAL. 41
of the pharynx, the stream being kept up by the action of columnar flagellate cells which clothe the gill-arches, and the water serving to aerate the blood in the vessels of the arches as it swills over them.
The water that has passed through the gill-slits escapes into a large space, the atrial or epipleural cavity (Fig. 12, H) : this lies between the pharynx and the body wall, and into it the pharynx hangs freely, slung up to the body walls by suspensory folds (Fig. 12, y). The atrial cavity extends back some distance behind the pharynx, and along it the water passes, escaping finally by the atrial pore (Fig. 11, i), an aperture on the ventral surface of the body, bordered by prominent lips, and about one- third the length of the animal from its hinder end. The atrial cavity of Amphioxus is a very characteristic feature in its ana- tomy, and is apparently unrepresented in the higher Verte- brates.
The ccelom or body cavity is quite distinct from the atrial cavity, though its boundaries are not easy to follow. In the posterior part of the body, behind the atrial pore, the ccelom is a cavity of some width, surrounding the intestine and separating this from the body wall ; in front of the atrial pore it becomes greatly reduced owing to the increased size of the atrial cavity ; it is, however, readily recognisable as a narrow space im- mediately surrounding the intestine and the liver. Further forwards, in the region of the pharynx, the ccelom becomes much subdivided, and more difficult to trace; its principal divisions are a pair of dorsal ccelomic canals (Figs. 12 and 13, m), lying at the sides of the dorsal part of the pharynx, between the body walls and the suspensory folds of the pharynx. From the dorsal ccelomic canals a series of tubular diverticula extend down the outer sides of the primary gill-arches, as far as their ventral ends. A series of spaces surrounding the reproductive organs (Fig. 13, ov) are also parts of the ccelom.
The large spaces, P, in the metapleural folds do not belong to the ccelom, but are apparently lymphatic in nature.
In the circulatory system the more important features are the following. There is no heart, but the general course of the circulation is the same as in a fish. A median longitudinal vessel, the cardiac aorta or endostylar artery (Figs. 12 and 13, s), receives venous blood from the body at its hinder end, and carries it
42
AMNI10XCS.
forwards along the floor of the pharynx: from the cardiac aorta the blood passes along a series of vessels in the gill-arches, becom - ing aerated on the way, to the dorsal aortse, a pair of longitudinal vessels (Figs. 12 and 13, it), lying just beneath the notochord :
K —
Fig. 13. — Amphioxus lanceolatus. Transverse section through the hinder part of the pharynx of an adult female, passing- through the liver and the ovaries. The boundary of the atrial cavity is indicated b}7 a thick black line. The section is taken at about the level of the reference line K in Fig. 11. (From Marshall and Hurst.)
A, skeleton of dorsal fin. B, spinal cord. C, notochord. D, connective-tissue sheath of notochord. E, cavity of pharynx. F, epibranehial groove. Gr, endostyle. H, atrial cavity. L, liver. M, dorsal coelomic canal. M", branchial ccelomic canal. O, ccelomic space surrounding liver. OV, ovary. P, metapleural canal. B, left dorsal aorta. S, cardiac aorta. T, hepatic veins.
these unite behind the pharynx to form a single dorsal aorta, from which branches supply the various parts of the body.
The nervous system consists of a tube of nervous matter, the spinal cord, which lies immediately above the notochord, and extends almost the entire length of the body. It tapers slightly at its anterior end, and more markedly behind. The
STRUCTURE OF THE ADULT ANIMAL. 43
central canal of this tube is very small along the whole length of the cord, except at the extreme anterior end, where it expands to form a thin- walled chamber, or ventricle. This dilatation of the central canal constitutes the only indication, if indeed it can be regarded as such, of anything corresponding to the brain of higher Vertebrates. The nerves arise either by single roots from the dorsal surface of the cord, or by multiple roots from its ventral surface : the two sets of nerves, which are quite independent of each other, appear to correspond with the dorsal and ventral roots of the spinal nerves of other Vertebrates, although the dorsal roots have no ganglia and are both sensory and motor in function. Excepting the anterior three or four of each side, the nerves arise, not in pairs, but alternately from the right and left sides of the cord.
The sense organs are in a very simple condition, and can only doubtfully be compared with those of higher Vertebrates. From the anterior end of the ventricle of the central nervous system, a hollow outgrowth arises, which is in close relation with a ciliated pit on the dorsal surface and left side of the anterior end of the animal. This pit is commonly regarded as an olfac- tory organ.
The ' eye ' is a rounded pigment-spot in the anterior wall of the ventricle ; i.e. at the anterior end of the central nervous system (Fig. 11, m). There is no trace of an ear.
The sexes are distinct, but the male and female are similar, except as regards the microscopic structure of the reproductive organs. There are no genital ducts.
In the female, the ovaries (Fig. 13, ov) are a series of saccular organs, arranged in a row along the inner surface of the body wall, on each side of the pharynx, in the segments from the tenth to the thirty-sixth. They lie in cavities, which are specialised portions of the ccelom, and the true relations of which , will be described when the development of the reproductive organs is considered.
The ova, when ripe, are discharged into the atrial cavity by dehiscence of the proper wall of the ovary and of the atrial membrane. The discharged ova, together with the ovaries, form a bulky mass, which causes great distension of the atrial cavity, and distortion, through pressure, of the pharynx and other organs.
44 AMPHlOXTJS.
The ova, which measure 0405 mm. in diameter, appear to escape from the atrial cavity, as a rule, through the atrial pore ; but in some cases they have been seen to pass through the gill- slits into the pharynx, and to make their exit through the mouth.
In the male, the testes are similar in form and position to the ovaries of the female ; and the spermatozoa when ripe are discharged, like the ova, into the atrial cavity, from which they escape by the atrial pore.
2. Morphological Importance of Amphioxus.
It will be seen from the preceding account of its anatomy that Amphioxus, while clearly and undoubtedly a Vertebrate, yet differs from all ordinary Vertebrates, whether fish, amphibians, reptiles, birds, or mammals, in a number of points which are of great importance and affect almost every part of its body.
A closer examination shows that these points of difference between Amphioxus and the higher Vertebrates may be grouped under two chief heads.
(1) The atrial cavity, the large number of the gill-slits, the regular alternation of gill-arches of two kinds, the azygos character of the sense organs, the extension of the notochord to the extreme anterior end of the animal, and the curious lateral asymmetry shown by the myotomes, nerves, and other organs, are examples of a group of characters in which Amphioxus differs from the higher Vertebrates, not only in their adult con- dition, but at all stages of their existence.
(2) There is another and even more striking series of cha- racters, in which Amphioxus differs from the adult forms of the higher Vertebrates, but resembles these in their early develop- mental stages. Thus in all the higher Vertebrates there is a stage in development when the notochord is the only skeletal structure present, neither cartilage nor bone having yet appeared ; a stage in which the limbs are absent ; and a stage in which the muscles of the body have the simple and definite segmental arrangement seen in Amphioxus throughout life. In all higher Vertebrates the heart is at first straight, like the cardiac aorta of Amphioxus ; the liver arises as one or more outgrowths of the intestine ; and the dorsal and ventral roots of the spinal nerves are at first independent of each other. In these and in
MORPHOLOGICAL IMPORTANCE. ' 45
many other points, Amphioxns remains throughout life in a condition characteristic of the early developmental phases of the higher Vertebrates. Amphioxus halts permanently at a stage through which all the higher Vertebrates pass during their development.
The Recapitulation Theory explains this as indicating that in these respects Amphioxus represents, more or less exactly, a phase through which the higher Vertebrates have passed in the history of their evolution ; that, as regards the organs in ques- tion, Amphioxus may be viewed as figuring, with more or less exactness, an ancestral form from which the higher types of Vertebrates are descended.
From this standpoint Amphioxus is an animal of very special importance to morphologists ; and the development of Amphioxus acquires peculiar interest from the consideration that, if the adult animal is far more primitive than any other existing Vertebrate, then the earlier stages in its life history may reasonably be expected, in accordance with the law of Recapi- tulation, to yield valuable evidence as to the relations of Verte- brates with the simpler groups of Metazoa.
The above considerations do not imply that Amphioxus itself stands in the direct line of ancestry of any of the higher Vertebrates, but that it is a surviving representative of a type of animals which preceded the higher Vertebrates in point of time, and from which type, though not necessarily from Amphi- oxus itself, the higher Vertebrates have arisen.
Amphioxus shows us that, in attempting to reconstruct the characters of the ancestors of Vertebrates, we are almost certainly justified in omitting such features as paired limbs, a cartila- ginous or bony skeleton, jawTs, a twisted or chambered heart, a highly specialised brain, and paired sense organs ; characters which Amphioxus shows us are not necessary to an adult Vertebrate, and in the absence of which the embryos of higher Vertebrates agree with Amphioxus.
A different explanation of the peculiarities of Amphioxus has been offered by many zoologists, who consider that the simplicity that characterises so many of its organs, as the brain, heart, liver, &c, is not primitive, but due to degeneration ; that the immediate ancestors of Amphioxus were, in fact, animals higher in the zoological scale than itself. No distinct evidence of such
46 AMPHIOXUS.
degeneration has, however, been brought forward ; and the theory of degeneration would leave altogether unexplained what is after all the most important fact, namely, the resemblance so often referred to above, and seen not in one organ only, but in almost every part of its structure, between the adult Amphioxus and the embryonic stages of development of the higher Vertebrates.
3. General Account of the Development of Amphioxus.
The development of Amphioxus has, as yet, been studied by a very limited number of investigators ; and many points, especially in the later stages, are still only imperfectly understood.
Our actual knowledge is due in the first instance to Kowa- levsky, who published in 1867 an account of observations made by him at Naples in 1864. His descriptions, though brief, are exceedingly precise and well illustrated, and deal with both the earlier and later stages of development ; they were supplemented by further papers in 1870 and 1876.
In 1881, Hatschek published a detailed and admirably illustrated account of the earlier stages of development, from the laying of the eggs up to the formation of the mouth and first gill-cleft. His observations were made near Messina, the specimens being obtained from a small salt lake, communicating with the sea by a narrow channel two or three hundred yards in length.
The later stages of development, and more especially the mode of formation of the gill-clefts, the endostyle, and the atrial cavity were described very fully by Mr. Willey and Professor Lankester in 1890 and 1891, from observations on specimens obtained by Mr. Willey from the same locality as Hatschek. Quite recently, 1892, Boveri has described the mode of formation of the reproductive organs.
The spawning period, in the Mediterranean, begins early in spring, towards the end of March, and continues throughout the summer, up to September ; June being apparently the month of greatest activity. The eggs are laid about sunset, usually between 7 and 8 p.m. ; they are very small, 0*105 mm. in diameter, and consequently contain but little food-yolk. They are fertilised at once, by spermatozoa shed over them by the male, and begin to develop almost directly. The early stages are passed through with great rapidity, and early on the following morning, about
GENERAL ACCOUNT OF DEVELOPMENT. 47
eight hours after the eggs were laid, the little embryos work their way out of the egg-membranes and swim freely. Their condition at hatching is shown in Figs. 25 and 26, p. 59.
After hatching, the embryos continue to develop rapidly, and in about thirty-six hours from the time of spawning they reach the stage shown in Fig. 34, p. 74. The mouth is not formed until the end of this period, and development up to this stage is apparently effected at the expense of the small amount of food-yolk contained in the egg.
After the formation of the mouth, the embryo continues its pelagic life, but from this time develops slowly, increasing in length, and gradually acquiring the shape and characters of the adult. During this period the anterior part of the body presents an extraordinary asymmetry, by which the mode of formation of the gill-clefts, which appear in order from before backwards, is profoundly modified. The mouth is a large oval opening (Fig. 3G) placed, not on the ventral surface, but on the left side of the pharynx. The gill-slits of the two sides appear, not simulta- neously, but successively ; those of the left side, which may be termed primary slits, being formed before those of the right side, or secondary slits. The primary slits, of which there are as a rule fourteen, are not at first on the left side, but in the mid- ventral wall of the pharynx, and shift upwards so as to actually lie for a time on the right side of the pharynx. The secondary slits, usually eight in number, appear along the right side of the pharynx dorsal to the primary slits (Fig. 38) ; while between the two series of gill-slits, primary and secondary, the endostyle is formed at the anterior end of the pharynx.
During the later stages of pelagic life, the total duration of which is about three months, this curious asymmetry is gradually rectified. The mouth assumes its median position, the primary gill-slits shift across the median line, and take up their permanent position on the left side of the pharynx ; the endostyle shifts from the right side to the mid- ventral wall ; and, by disappearance of some of the primary gill-slits, the number of primary and of secondary gill-slits becomes equalised, eight being present on each side of the pharynx.
At the close of the pelagic period, which may be called the critical stage, the young Amphioxus, now about 3 -5 mm. in length, adopts the mode of life of the adult, burrowing in the sand, and
48 AMPHIOXUS.
gradually, by increase in the number of the gill-slits and in other ways, acquires the structure and size of the fully formed animal.
The whole developmental history of Amphioxus may, in accordance with the above account, be conveniently divided into periods, which will be dealt with in succession in the remainder of this chapter.
I. The Embryonic Period : including the stages from the com- mencement of development to the formation of the mouth. This lasts about thirty-six hours, and is characterised by the extreme rapidity with which the stages, especially the earlier ones, are passed through ; and by the fact that throughout the period the embryo is dependent for nutrition on the yolk granules contained within the egg- The actual rate of development varies to a certain extent with the temperature. The times here given are those recorded by Willey during the summer months ; in spring, Hatschek found the rate of development to be slower. The period may be subdivided into two parts.
1. Before hatching : from the commencement of develop- ment up to the hatching of the embryo ; a period of about eight hours.
2. After hatching : from the hatching of the embryo to the formation of the mouth ; a period of about twenty-eight hours, during which the embryo leads a pelagic life.
II. The Larval Period : from the formation of the mouth to the critical stage. This lasts about three months, and during it the larva is pelagic. Development takes place slowly ; and the most notable events are the formation of the gill-slits and the atrial cavity ; and the curious series of changes by which the symmetrical condition of the larva is re-established.
III. The Adolescent Period : during which the young Am- phioxus, having adopted the mode of life of the adult, gradually acquires its full structure, by increase in the number of the gill- slits, ripening of the reproductive organs, &c.
I. THE EMBRYONIC PERIOD.
From the laying of the eggs to the formation of the mouth. Duration of the period, from thirty-two to thirty-six hours.
Part I. From the laying of the eggs to the hatching of the embryo : a period of about eight hours.
EMBRYONIC PERIOD. 49
1. The Egg.
The ripe egg of Amphioxus is a spherical mass of proto- plasm, 0*105 mm. in diameter on the average, and inclosed in an elastic vitelline membrane. The protoplasm is studded with numerous yolk granules, which are sufficiently opaque to hide the nucleus. At one pole, which will be spoken of as the upper pole, there is a slightly flattened patch of protoplasm compara- tively free from yolk granules ; and on the top of this patch is a sharply defined polar body (Fig. 14, I, PB). A second polar body has not been seen.
The vitelline membrane, prior to fertilisation, adheres closely to the egg.
2. Fertilisation.
The male Amphioxus, as described above (p. 46), sheds spermatozoa over the eggs as these are laid by the female ; and they may be seen adhering in numbers to the vitelline mem- branes. The details of fertilisation have not been studied, but shortly after the spermatozoa gain access to the egg the vitelline membrane, which previously invested the egg closely, swells up rapidly by imbibition of water, and becomes separated from the egg by a considerable space; the egg ultimately lying in the centre of a capsule three or four times its own diameter. The purpose of this swelling up of the vitelline membrane, and its separation from the egg, is probably to prevent the entrance ot other spermatozoa after the egg has been fertilised.
3. Segmentation.
The process of segmentation commences at dusk, usually about 8 p.m., and is completed in about three hours.
The first cleft appears about an hour after the eggs are laid and fertilised. It commences as a depression at the upper pole of the egg, close to the polar body, extends rapidly across the upper pole, and then spreads quickly round the egg as a groove (Fig. 14, Ji). The groove deepens rapidly, being always more prominent at the upper than the lower pole ; and in about five minutes from its first appearance the cleft is completed, the egg being divided by it into two halves or blastomeres of equal size, and similar in all respects save for the presence of the polar body on the apex of one of them.
E
AMPHIOXUS.
Fig. 14. — Segmentation of the egg of Amphioxus. x 220. (After Hatschek.)
I, the egg before the commencement of development. PB, polar body. II, the egg in the act of dividing, by a vertical cleft, into two equal blastomeres ; about one hour after fertilisation. Ill, stage with four equal blastomeres ; about two hours after fertilisation. IV, stage with eight blastomeres : an upper tier of four slightly smaller ones, and a lower tier of four slightly larger ones. V, stage with sixteen blastomeres. in two tiers, each of eight. VI, stage with thirty two-blastomeres, in four tiers, each of eight ; about three hours after fertilisation : the embryo is represented bisected ver- tically to show the segmentation cavity or blastoccel, B. VII, later stage : the blasto- meres have increased in number by further division. VIII, blastula stage: bisected to show the blastoccel, B ; about four hours after fertilisation.
SEGMENTATION OF THE EGG. 51
A pause of about an hour now ensues, and then the second cleft is formed. This also is vertical, but in a plane at right angles to the first ; it bisects each of the two first blastomeres, and so gives rise to four equal and similar blastomeres (Fig. 14, in) ; these are ovoid in shape, with their apposed surfaces slightly flattened by mutual pressure.
The third cleft, which appears about a quarter of an hour later, is a horizontal one, dividing each of the four blastomeres of the previous stage into two (Fig. 14, iv). The cleft is a little above the equator, so that the four blastomeres of the upper tier are a little smaller than those of the lower tier. The blastomeres are in contact with one another laterally, but do not quite meet along the axis of the embryo. Hence the embryo is at this stage in the form of a ring, or short tube, with a central cavity, the segmentation cavity or blastoccel, which at present is open at both the upper and lower poles.
About a quarter of an hour later, the number of the blasto- meres is again doubled by two new vertical clefts, which appear simultaneously, in planes at right angles to each other, and at angles of 45° with the first two clefts. The embryo now consists (Fig. 14, v) of sixteen blastomeres, arranged in an upper tier of eight rather smaller ones, and a lower tier of eight rather larger ones.
A little later, about three hours from the time of fertilisa- tion, two more horizontal clefts appear simultaneously, dividing* each of the tiers into two, and again doubling the number of the blastomeres. The embryo now (Fig. 14, vi) consists of four tiers, each of eight cells ; the cells of the lowest tier, as shown in the figure, being decidedly larger than those of the other tiers. The blastoccel (Fig. 14, vi, b) still opens to the exterior at both poles, although the apertures are considerably narrowed by approxi- mation of the cells of the upper and lower tiers respectively.
In the next stage (Fig. 14, vii)the lowest tier of blastomeres of the preceding stage has divided horizontally, giving five tiers in all ; and each of the blastomeres of the four upper tiers has divided vertically into two. The embryo now, as shown in the figure, consists of five tiers of blastomeres, the four upper of which consist each of sixteen blastomeres, while the lowest tier consists of eight much larger blastomeres. The larva is nearly spherical in shape, and by approximation of the blastomeres of
E 2
52 AMPHIOXUS.
the top and bottom tiers the blastococl is now completely closed.
From this time the blastomeres continue to increase in number by division, but in less regular fashion than before, so that the arrangement in tiers soon becomes lost : the blastomeres at the lower pole, however, remain throughout of larger size than those in other parts of the embryo. The polar body is often visible, resting on the upper pole of the egg, but it has sometimes disappeared by this stage. The blastomeres, which have hitherto been of somewhat irregular shape, rounded at their outer and inner ends, and flattened through mutual pres- sure at their sides, now begin to assume more definite form ; and from this stage, which marks the close of segmentation, they may be more appropriately spoken of as cells.
4. The Blastula.
The embryo has now reached the stage to which the name blastula is given ; a stage which occurs at corresponding periods in the development of a number of the lower animals, and which is therefore of interest as possibly representing a very early ancestral form of animal life. Pandorina and Vol vox are exam- ples of organisms in which the blastula stage forms the adult condition.
The blastula (Fig. 14, vm) is a spherical or ovoid embryo, consisting of a single layer of cells, inclosing a central segmen- tation cavity or blastocoel, filled with fluid. In the blastula of Amphioxus the cells are not all of equal size, those of the lower half, and especially those at the lower pole, being distinctly larger than those of the upper half; the greater size and more opaque appearance of these lower cells are due to the greater quantity of yolk granules which they contain. At first, the cells of the blastula, though flattened laterally where they press against one another, remain rounded at their ends, both inner and outer. These ends, however, soon become flattened ; and the cells, in which the nuclei are now clearly visible, acquire the characters of columnar epithelial cells. These changes appear first at the upper pole of the embryo, and gradually extend to the lower pole. The blastula stage (Fig. 14, vm) is reached by the Amphioxus embryo at about the end of the fourth hour from the time of fertilisation of the eggs.
THE OASTEULA STAGE.
53
5. The Gastrula.
On the completion of the blastula, as described above, the multiplication of the cells ceases for a time, and the embryo undergoes a great change in shape, whereby it becomes converted into the form to which the name gastrula is given. This change is brought about as follows.
The lower surface of the blastula, consisting of the larger cells, becomes flattened (Fig. 15, h), and then invaginated within the upper surface (Fig. 16). The embryo thus becomes cup- shaped, its walls consisting of two layers : an outer layer, E, formed from the original upper part of the blastula ; and an inner layer, H, consisting of the invaginated cells, which originally formed the lower pole of the blastula.
Fig. 15.
Fig. Hi.
Figs. 15 and 16. — Formation of the gastrula of Amphioxus. The embryos are bisected vertically, one half alone being represented. x 220. (After Hatschek.)
Fig. 15. — Flattening of the lower pole of the blastula prior to invagination. Fig. 16. — Commencing invagination of the lower pole to form the gastrula.
B. blastocoel or segmentation cavity. E, epiblast. G, archenteron or gastroccel. H, hypoblast .
As the invagination proceeds, the blastocoel becomes gradually diminished in size, and is ultimately completely obliterated, the inner and outer layers of the gastrula coming in contact with each other (Fig. 17, H, e).
The two layers of cells of which the wall of the gastrula con- sists are the two primary germinal layers. The outer layer is spoken of as the epiblast, e, and the cells of which it consists are called epiblast cells : the inner layer is the hypoblast, H, and its cells, which originally were those forming the lower half of the blastula, are called hypoblast cells.
The cavity of the cup, formed by invagination of the hypo-
54 A \I I'JIIOXUS.
blast, is called the archenteron or gastrocoel, G : it gives rise to the greater part of the alimentary canal of the larva and adult. The mouth of the cup is called the blastopore; it is at first (Fig. 17) a very large aperture, but in the later stages becomes greatly reduced in size (Figs. 18 and 19).
Like the blastula, the gastrula is a very widely spread em- bryonic form, occurring not only in Vertebrates, but in a simple or modified condition in certain members of each of the great groups of Invertebrates as well. It has therefore, like the blastula, claims to be regarded as an ancestral form ; claims which are greatly strengthened by the fact that some of the simpler sponges, and some of the Ccelenterates, such as Hydra, are closely com- parable even in their adult condition to gastrulae.
Fig. 17. Fig. 18.
Figs. 17 and 18. — Completion of the gastrula of Amphioxus. The embryos are bisected vertically, and one half only of each is represented, x 220. (After Hatschek.)
Pig. 17. — Completion of the process of invagination, and consequent obliteration of the blastoccel. Pig. 18. — Narrowing of the blastopore, through growth backwards of its dorsal lip. E, epiblast. G-, archenteron or gastrocoel. H, hypoblast. PC, polar meso- blast cell.
The mechanical causes that lead to invagination, i.e. that actually occasion the change from the blastula to the gastrula condition, are not easy to determine. The epiblast cells appear to take no part in the process, and to undergo no appreciable change or alteration during it ; the active cells in the change are the hypoblast cells. By comparison of Figs. 15, 16, and 17, it will be seen that during invagination there is an increase in the number of the hypoblast cells ; and there is also, which is not so clearly brought out in the figures, an increase in the actual size of the individual cells. This increase in size is perhaps due to the cells absorbing the fluid of the blastoccel ; and this absorption
THE GASTRULA STAGE. 55
of fluid may perhaps be one of the factors that determine or aid the process of invagination. It seems more probable, however, that invagination is due rather to inequality in the rates of growth of the cells at different parts, than to direct pressure from any cause on the surface of the embryo.
The later stages in the development of the gastrula show some features of importance. At its earliest formation, as shown in Fig. 16, the axis of the gastrula coincides with that of the blastula; and the blastopore or gastrula mouth is circular in outline. Later on, as shown by the careful observations of Hatschek, owing to unequal rates of growth in different directions, the blastopore becomes oval instead of circular in outline, and the shape of the embryo changes (Fig. 18) in such way that the axis of the gastrula no longer coincides with the original axis of the blastula, but forms a considerable angle with this.
At the stage shown in Fig. 18 there may be seen at the lower lip of the blastopore, and placed one on each side of the median plane, a pair of cells, PC, which differ from the other hypoblast cells in their larger size and more rounded form, and in having very large nuclei. These two cells give rise ■at a later stage to important portions of the middle germinal layer or mesoblast : they may be named the polar mesoblast cells.
The further stages in the completion of the gastrula will be understood from a comparison of Figs. 18, 19, and 20. The embryo elongates, becoming ovoid or egg-shaped : at the same time the blastopore becomes still further reduced in size ; the narrowing being effected, according to Hatschek, entirely by growth backwards of its anterior lip, the posterior lip, indicated by the pair of polar mesoblast cells, remaining quiescent through- out the process.
In the fully formed gastrula (Figs. 19 and 20), the ends and surfaces of the larva may be clearly recognised. The polar mesoblast cells, P C, mark the posterior end of the embryo ; the blastopore, B p, now reduced to a small circular aperture, lies at the hinder end of the embryo, and slightly on the dorsal surface- The anterior end of the embryo is rounded and imperforate. The dorsal surface is flattened, and is further indicated by the blastopore ; while the ventral surface is strongly convex.
If Hatschek is right in stating that the narrowing of the
56
AMPHIOXUS.
blastopore is effected entirely by growth backwards of Lte anterior lip, then it is evident from a comparison of Figs. 17, 18, and 19 that the blastopore originally occupies almost the whole of what will afterwards be the dorsal surface of the larva; while the outer or convex surface of the young gastrin1 a (Fig. 17) corresponds to the ventral surface, and perhaps also to the anterior end of the larva. If these determinations are correct, Figs. 15, 16, and 17 show that the lower pole of the blastula corresponds to the dorsal surface of the larva, and the upper pole to its ventral surface.
Before leaving the gastrula the cells of the two layers, epi- blast and hypoblast, should be noticed more fully. The epiblast
Fig. 19.
Fig. 20.
Figs. 19 and 20. — The fully formed gastrula of Amphioxus. x 220. (After Hatschek.)
Fig. 19.— The gastrula bisected vertically : the left half is represented, as seen from the inner surface.
Fig. 20.-The gastrula bisected horizontally : the ventral half is represented, as seen from above.
BP, blastopore. Gr, archenteron. PC, polar mesoblast cell.
(Figs. 19 and 20) is a single layer of very short columnar or almost cubical cells ; at about the stage represented by Fig. 18 these cells develop on their outer surfaces rlagella or lash- like processes, one from each cell, by which the embryo is caused to rotate within the vitelline membrane. These flagella persist during the greater part of the pelagic existence of the embryo, but are not represented in the figures given here.
The hypoblast is a single layer of elongated columnar cells, with nuclei near their inner ends. At the lip of the blasto- pore the epiblast and hypoblast cells are necessarily continuous with one another ; in the mid-ventral line the two polar meso- blast cells render the transition an abrupt one ; but all round
THE GASTKULA STAGE. 57
the rest of the lip, and especially at its dorsal or anterior border, the two layers pass gradually into each other. In the figures this transition has, for diagrammatic purposes, been represented as an abrupt one.
The fully formed gastrula stage, seen in Figs. 19 and 20, is reached, in the summer, in from seven to eight hours from the time of fertilisation of the eggs. In the spring, according to Hatschek's observations, the time taken to reach the same stage is about fourteen hours. A comparison of Figs. 14 and 19 will show that the gastrula, though of different shape, is approxi- mately the same size as the egg itself.
6. Development of the Embryo from the Completion of the Gas- trula to the Time of Hatching.
The completion of the gastrula stage is followed by a short but well-marked and important period during which the rudi- ments of the nervous system, of the body cavity, and of the notochord are established, and at the close of which the embryo works its way out of the egg membrane, swims to the surface of the water by means of the flagella of the epiblast cells, and becomes a free living pelagic animal.
During this period it increases slightly in length but dimin- ishes in breadth, so that at the time of hatching (Fig. 26) it is about twice as long as it is wide. Its bulk remains practically the same as before, for the mouth is not yet formed, and the embryo consequently cannot obtain food from without, but is still dependent for nourishment on the yolk granules contained in the cells, more especially in the hypoblast cells.
The nervous system is formed in the following manner. At the time of completion of the gastrula the epiblast is slightly flattened along the dorsal surface, as shown in Fig. 19, and still better in the transverse section, Fig. 21.
This flattened band of epiblast now becomes slightly de- pressed, and at the same time becomes marked off along its sides from the lateral epiblast (Fig. 22, NP). The lines of demarcation are at first indicated by slight modifications in the shape and arrangement of the cells, but soon become more pro- nounced, the edges of the lateral plates of epiblast overlapping the central depressed plate (Fig. 23), and ultimately meeting
58
AMPHI0XU8,
each other in the median plane so as to completely cover over the central plate (Fig. 24).
The central plate of epiblast, which thus becomes roofed over, is spoken of as the neural plate (Figs. 22-24, Nr), and becomes converted, later on, into the central nervous system. By longitudinal folding of the neural plate a groove is formed
Fig. 21
Fig. 23.
Fig. 24.
Figs. 21 to 24. — Transverse sections across the bodies of Amphioxus embryos, showing the mode of formation of the nervous system and of the meso- blastic somites, x 350. (After Hatschek.)
Pig. 21. — Transverse section across the middle of the back of an embryo of the same age as those shown in Figs. 19 and 20. Fig. 22.— Transverse section across a slightly older embryo, with one pair of mesoblastic somites, and commencing nervous system. Fig. 23. — Transverse section across the same embryo as Fig. 22, but taken rather further back, the section passing through the middle of the first pair of somites. Fig. 24. — Trans* verse section through an einbryo at the time of hatching (cf. Figs. 23 and 24) : the section passes through the middle of the first pair of mesoblastic somites, and shows also the mode of formation of the neural tube. CE, enterocoel or mesoblastic somite. E, epi- blast. Gr, archenteron. H, hypoblast. NP, neural fold. IfP, neural plate.
along its upper surface, and this groove, when roofed over by the
lateral plates or neural folds, becomes the neural canal (Fig. 24).
The neural plate extends back to the blastopore, which, as
already described, is situated on the dorsal surface of the hinder
THE EMERYO AT THE TIME OF HATCHING. 59
end of the embryo (Fig. 19, Br). The lateral plates, or neural folds, of the epiblast extend not merely along the edges of the neural plate, but round the sides and posterior lip of the blasto- pore as well ; and by their fusion in the median plane the blastopore becomes roofed over, so that it no longer opens directly to the exterior, but into the hinder end of the neural canal (c,f. Figs. 19 and 25). The blastopore thus forms a short tubular channel of communication between the neural canal and the archenteron, and to this channel the name neurenteric canal is given (Fig. 25, nt).
It is a curious fact, and one the full meaning of which is not yet determined, that for a time the sole communication between the archenteron, or primitive alimentary canal, and the
Fig. 2
Figs. 25 and 26. — Amphioxus embryos at the time of hatching. x 220. (After Hatschek.)
Fig. 25.— The embryo bisected vertically : the left half is represented, as seen from the inner surface. Fig. 26. — The embryo bisected horizontally : the ventral half is represented, as seen from above. CE, enterocoel or mesoblastic somite. E. epiblast. G, archenteron. H, hypoblast. NF, neural fold. NT, neurenteric canal. PC, polar mesoblast cell.
exterior should be through the central canal of the nervous system. Kowalevsky, who discovered the neurenteric canal in Amphioxus and in the Ascidians, suggested that these relations may possibly be ancestral, and that animals may have existed, or may still exist, in which the nerve-tube fulfilled a non-nervous function, and possibly acted as part of the alimentary canal. Comparative anatomy has not at present, however, given any support to this suggestion.
The closure of the neural tube, by meeting and fusion of the neural folds, proceeds from behind forwards, so that a section
GO AMPHIOXUS.
through the posterior part of an embryo (Fig. 23) will show a more advanced stage in the formation of the nervous system than one taken through the same embryo nearer its anterior end (Fig. 22).
At the time of hatching (Fig. 25), the closure of the neural tube is completed along about a third of the length of the embryo; the anterior opening of the tube, just in front of the reference line nf in the figure, is spoken of as the neuropore.
The mesoblastic somites. During the formation of the neural canal important changes take place in the hypoblast. The flattening of the dorsal surface of the embryo at the completion of the gastrula stage affects the hypoblast as well as the epiblast (cf. Figs. 19 and 21). As the neural plate becomes marked off and depressed, a pair of longitudinal folds of the wall of the archenteron are formed, one along each side, in the angle between its dorsal and lateral walls (Figs. 22 and 23, ce), These folds are at first very inconspicuous, but rapidly become more prominent, and especially so about the time of closure of the neural canal (Fig. 24, ce).
By the formation of these folds the archenteron becomes divided into three portions : a central division (Fig. 24, g), which is the alimentary canal itself, and a pair of lateral slit-like diverticula (Fig. 24, ce), which may be termed enteroccelic cavities, and which later on give rise to the body cavity or coelom of the adult.
The cells composing the walls of these folds are clearly of hypo- blastic origin. In the later stages (cf. Figs. 27, 28, and 29), they separate completely from the wall of the alimentary canal, and are then spoken of as forming a third germinal layer, or mesoblast, situated between the two primary layers, epiblast and hypoblast.
The mesoblastic folds extend the whole length of the embryo ; they are most prominent near its anterior end, and gradually diminish posteriorly, becoming continuous at their hinder ends with the two large polar mesoblast cells (Figs. 25 and 26, p£), which have already been described as present in the posterior lip of the gastrula from an early stage in its formation (Fig. 18).
Soon after their appearance, the mesoblastic folds become divided by transverse constrictions into segments or compart- ments, the mesoblastic somites, arranged in pairs along the sides of the embryo. The anterior pair of somites, which are the first
(
THE EMBRYO AT THE TIME OE HATCHING. 61
to be formed, lie a little way behind the anterior end of the embryo, and the remaining ones are formed in succession from before backwards as the embryo increases in length ; at the time of hatching, two pairs of mesoblastic somites are usually present Figs. 24 and 26, ce).
The notochord. The roof of the archenteron, between the mesoblastic folds, is formed by a band of hypoblast cells lying immediately below the neural plate, and in close contact with this (Figs. 21 to 24). The cells composing this band, up to the time of hatching, differ little if at all from the hypoblast cells of the sides or floor of the archenteron ; but shortly after the time of hatching, they undergo changes and become converted into the notochord, the most important part of the skeleton of Amphioxus.
Condition of the embryo at hatching. At the time of hatching, which occurs about eight hours after fertilisation of the egg, the embryo (Figs. 25 and 26) is ovoid in form, about twice as long as it is wide, and in bulk about equal to the egg from which it was developed (cf. Fig. 14, l). The epiblast is a single layer of short, almost cubical cells, each of which bears a single flagellum, by which the swimming of the embryo is effected. The neural canal is roofed in for about the hinder third of its length ; in front it opens to the exterior by a rather wide aperture, the neuropore ; posteriorly, the neural canal communicates with the archenteron through the neurenteric canal, the former blastopore. The mesoblastic folds are present, and two pairs of mesoblastic somites are already constricted off from their anterior ends.
Immediately after working its way out of the egg membrane the embryo swims to the surface of the water, and enters on the second part of the embryonic period.
Part II. From the hatching of the embryo to the formation of the mouth : a period lasting from about twenty-four to twenty- eight hours (cf. p. 48).
The later stages of embryonic development consist chiefly in further elaboration of the organs which are already established at the time of hatching. The nervous system becomes more complex; the mesoblastic somites increase considerably in number, and undergo important changes whereby the muscular and other systems are formed ; the notochord is definitely established ; and at the close of the period the mouth and first
G2
a .m ni i ox i :s.
gill-slit are formed. The embryo elongates very rapidly, and becoming much narrower and more slender, gradually acquires a shape and proportions resembling those of the adult. During the whole period the embryo is pelagic : swimming is effected at first by the flagella clothing the surface, but towards the close of the period the muscles of the body-walls become definitely established, and the young Amphioxus swims by means of muscular contractions, like the adult.
Although there is a great increase in length during the period, there is little if any change in bulk, and it is doubtful whether the embryo obtains any food from without until the formation of the mouth at the close of the period.
Fig. 27.— Amphioxus embryo shortly after hatching, with five pairs of meso- blastic somites ; seen in optical section from the right side, x 22 1 (After Hatschek.)
E, epiblast. H, hypoblast. NC, neural canal. NF, neural fold. NE, neuropore. NT, neurenteric canal. PC, polar mesoblast cell. SI, first mesoblastic somite of right side. T, archenteron.
In dealing with this period in the developmental history it will be convenient to describe the several systems one by one.
1. The Nervous System.
After hatching of the embryo, the closure of the neural canal, by fusion of the neural folds, proceeds rapidly forwards (Figs. 25 and 27), and soon reaches the anterior border of the first somite, beyond which level the nervous system does not extend.
From the mode of its formation (Figs. 23, 24, 26, and 27), the neural canal is, in its early stages, merely the space between the neural plate and the overlapping lateral plates of epiblast, and has at first no independent roof of its own. The canal is at first wide from side to side, but shallow dorso-ventrally.
THE LATER EMBRYONIC DEVELOPMENT. 63
In the later stages the neural canal deepens, owing to longitudinal folding of the neural plate ; at the same time the cells at the free margins of the plate grow in towards one another from the two sides, and meeting in the median plane complete the wall of the neural canal (Fig. 32).
The nervous system is now a tube (Figs. 30 and 33), with proper walls of its own, extending along the dorsal surface of the embryo. It opens in front to the exterior, at the neuropore, op- posite the anterior border of the first somite; and it communicates posteriorly with the archenteron, through the neurenteric canaL The wall of the tube consists of a single layer of cells, which bear flagella at their inner ends.
The anterior end of the neural tube, close to the neuropore, has, almost from the first, thicker walls than the rest of the tube. This thickening, which affects especially the ventral wall of the tube (Fig. 33), becomes much more marked in the later stages ; partly owing to actual increase in the thick- ness of the wall itself; and partly to a great diminution in the diameter of the hinder part of the tube, as the embryo becomes drawn out into the elongated form characteristic of the later larval condition.
In the ventral wall of the neural tube, opposite the fifth pair of somites, a black pigment spot, possibly a sense organ, appears at about the stage represented in Fig. 33 ; and much later, towards the end of the embryonic period, another pigment spot, the eye, is formed in the anterior wall of the brain swelling (cf. Fig. 36).
2. The Notochord.
The notochord is developed, as already noticed, from the band of hypoblast cells which forms the dorsal wall of the archenteron, and lies between the two lateral mesoblast folds.
Its earliest appearance as a distinct structure is seen in a larva with three pairs of somites, i.e. immediately after the time of hatching ; and the successive stages in its formation are shown in Figs. 28, 29, and 32, ch.
The median plate of hypoblast cells, forming the roof of the archenteron, first becomes marked off, by a difference in mode of arrangement of the cells, from the lateral mesoblast folds, and
<34
AMPHIOXUS.
then grooved ventrally along the median plane (Fig. 28). The ventral groove deepens, and at a stage with five pairs of meso- blastic somites the plate is completely folded on itself, so that its two halves are in contact with each other. The cells of the two halves now begin to grow across the median plane, inter- digitating with one another (Fig. 29, CH), and forming a solid ridge of cells along the mid-dorsal surface of the archenteron. At a slightly later stage, with eight or nine pairs of mesoblastic somites, this ridge begins to separate from the gut wall as a cylindrical rod of cells, the notochord (Fig. 32, CH).
Behind the first somite, i.e. along the greater part of its
MS
Fig. 29.
Figs. 28 and 29.— Transverse sections through Amphioxus embryos shortly after the time of hatching ; showing stages in the formation of the noto- chord and mesoblastic somites, x 435. (After Hatschek.)
Fig. 28. — Embryo with five pairs of somites : transverse section through the middle of the first pair. Fig. 29. — Embryo with six pairs of somites : transverse section through the hinder end of the first pair. CE, enteroccelic poucli or mesoblastic somite. CH, notochord. G, archenteron. MS, mesoblastic somite. IfGr, neural canal. T, mesenteron.
length, the notochord develops from before backwards. Op- posite the first somite the notochord forms more slowly, and is always a little behind the stage reached in the second somite. In front of the first somite the notochord is developed from be- hind forwards, but otherwise in the same manner as in the hinder part, though much more slowly ; towards the close of the em- bryonic period, at the time when the pointed anterior end of the animal is forming, it grows much more rapidly (Fig. 33). This late development of the anterior end of the notochord will be referred to again further on.
THE LATER EMBRYONIC DEVELOPMENT. 65
Opposite the neuropore, and corresponding to the marked thickening in the ventral wall of the neural tube already described, there is a distinct bending of the notochord (Fig. 33), traces of which persist even in the adult animal.
The histological development of the notochord presents some features of interest. The interdigitation of the cells of the two sides, the commencement of which is shown in Fig. 29, proceeds rapidly ; and, at the time of its separation from the gut, the notochord consists (Fig. 32) of four or five rows of cells, arranged horizontally one above another, each cell extending across the whole of its width. Within the notochordal cells numerous small vacuoles now appear ; these vacuoles are, from the first, most abundant in the two middle rows of cells, and in these they increase greatly in size ; so that in its later stages, as in the adult, the notochord consists of a middle series of cells, enormously distended by vacuoles, and covered on its dorsal and ventral surfaces by rows of smaller and comparatively little modified cells.
3. The Mesoblastic Somites.
The mesoblastic ridges, as described above, are a pair of longitudinal folds of the dorso-lateral walls of the archenteron, inclosing slit-like diverticula of the archenteric cavity (Figs. 26, 28). By transverse constrictions these ridges become divided into somites, which, though separated from one another by the constrictions, still retain for a time their communi- cations with the archenteron (Figs. 27, 28).
At the time of hatching, two pairs of these somites are present ; and, as the embryo elongates, other pairs are added in succession from before backwards, the number of pairs of somites present affording a convenient basis for estimating the age of an embryo (Figs. 27, 30, 33).
The anterior somites, which are the first formed, are also the largest, and the remainder decrease in size towards the hinder end of the embryo (Figs. 27, 30) ; the hindmost pair passing into the, as yet, unsegmented mesoblast folds, which end posteriorly in the two polar mesoblast cells (Figs. 30, 31, pc).
At a stage when six pairs of somites are present, the cavities of the anterior ones become constricted off from the
66
A.\rpiiioxi;s.
archenteron, and separate completely from this (Fig. 29). This separation rapidly extends backwards, involving the hinder somites in succession; and the somites now form (Figs. 27, 29) a series of squarish hollow bodies, arranged in a row along each side of the embryo, at the level of the notochord.
The somites are at first small, and lie above or dorsal to the alimentary canal (Fig. 29) ; but they rapidly increase in size, and, extending ventral wards (Figs. 30 and 32), make their way
Fig. 30.
Fig. 31.
FlGS. 30 and 31. — Amphioxus embryos with nine pairs of mesoblastic somites, x 224. (After Hatschek.)
Pig. 30.— Embryo seen in optical section from the right side. Fig. 31.— Embryo seen in horizontal section, at the level of the notochord. CH, notochord. DL, left anterior gut diverticulum. DR., right anterior gut diverticulum. NR, neuropore. NT, neur- enteric canal. PC, polar mesoblast cell. SI, first mesoblastic somite of the right side. S9, ninth mesoblastic somite of the right side. T, mesenteron.
round the sides of the embryo, between the gut wall and the external epiblast, ultimately reaching the mid-ventral line, where the somites of the right and left sides of the body become continuous with one another.
During their earlier stages (Figs. 27, 30), the long axes of the somites lie transversely, or slightly obliquely to the axis of the embryo ; but towards the close of the embryonic period
THE LATER EMBRYONIC DEVELOPMENT.
67
CH
(Fig. 33) they acquire the >-like shape so characteristic of the .adult (Fig. 11, n).
The walls of the somites soon undergo important changes. At the time of separation from the archenteron (Fig. 29, ms), the wall of each somite consists of a single layer of cells, somewhat irregular in shape and size, but showing no marked differences in different parts. As the somites extend down the sides of the body they become somewhat triangular in trans- verse section. In each somite there may now be distinguished (Fig. 32) an outer or parietal wall, next the external epiblast ; a visceral wall, in contact with the hypoblast of the archenteron ; and a notochordal wall, forming the base of the triangle, and in contact with the notochord and the nerve cord. The cells of the parietal and visceral walls are slightly flattened, but show no special peculiarities ; those of the notochordal wall, on the other hand, show marked changes.
Each cell (Fig. 32, ml) is much flattened dorso-ventrally, and elongated in a direction parallel to the axis of the embryo (Fig. 31) ; and is undergoing changes by which it becomes con- verted into a muscle cell or fibre. This differentiation of muscle cells begins at a stage with about nine pair of somites, and proceeds rapidly ; the muscles, at a stage with eleven pairs of somites, beginning to contract and cause lateral undulations of the body. The mass of muscle cells, formed in this way by A^ modification of the notochordal wall of a somite, is called a myotome : the myotomes, being formecl from the somites, are, like these, arranged segmentally from their first appearance ; they increase rapidly in size, and become the great lateral muscles or myotomes of the adult Amphioxus (Fig. 12, x). Each muscle cell extends the whole length of the somite to which it belongs.
In the higher Vertebrates it will be found that the earliest
p 2
Fig. 32.— Transverse section through the middle of an Amphioxus embryo with nine pairs of meso- blastic somites, x 435. (After Hatschek.)
CH, notochord. I, spinal conl. ML, muscle layer. MS, cavity of inesoblastic somite. T, inesenteron.
68 AM HI I ox IS.
muscles to appear in the development of the embryo correspond in mode of formation, and in relations, to the myotomes of Am- pliioxus. The formation of muscles, as indeed of all other tissues, by direct modification of epithelial cells, is a further point of very great and general interest, indicating that the epithelial cell is a more primitive type of structure than muscle, connective tissue, nerve tissue, or any of the other histological elements of which the body of an adult animal is composed.
The cavities of the somites give rise to the coelom or body cavity of the adult. After their separation from the archenteron they are completely closed, and remain so for some time ; the anterior and posterior walls of adjacent somites becoming closely applied to one another, and forming septa which separate the cavities of successive somites from one another (Fig. 31). To- wards the close of the embryonic period, the ventral portions of these septa disappear, so that the somites open into one another ; and the body cavity, which up to this time has been represented by a series of isolated chambers, now becomes continuous from end to end of the animal. The dorsal portions of the somites, however, remain separate from one another throughout life.
The first somite (Fig. 27, s l) is a little distance from the an- terior end of the body : from its anterior and dorsal border, at a stage with about nine pairs of somites, a hollow conical process is given off towards the anterior end of the embryo (Figs. 30, 31) ; the walls of this process undergo changes similar to those de- scribed above as occurring in the body of the somite itself.
At the time of their first appearance the somites are paired ; the two somites of each pair being exactly opposite each other, and the whole embryo being bilaterally symmetrical. At a stage with nine pairs of somites this symmetry becomes disturbed (Fig. 31), the somites of the right side becoming situated a little behind the corresponding ones of the left side, and ultimately alternating with these. This curious lateral asymmetry is preserved in all the later stages, and in the adult animal as well. The fact that the somites are at first symmetrically arranged shows that it is a secondary and not a primitive feature, and the further fact that it appears just at the time when the great lateral muscles are being formed, and are coming into use for swimming, suggests that the explanation of the asymmetry is to be found in some mechanical advantage gained by the alternating arrangement of
THE LATEK EMBRYONIC DEVELOPMENT.
69
the muscles in an animal in which the skeleton is represented merely by an elastic notochorcl.
The development of new somites during the later stages of embryonic life occurs very slowly ; and at the time of the formation of the mouth, marking the close of the period, there are not more than fourteen or fif- teen pairs. The elongation of the body, which is so marked a feature of the later embryonic stages, is due, not so much to addi- tion of new segments, as to lengthening of those al- ready present ; and this lengthening, as shown in Figs. 33 and 34, principally concerns the anterior or oldest somites.
1. The Alimentary Canal.
After separation of the somites and the notochorcl, the archenteron, or, as it is usually termed from this time, the mesenteron, forms a straight tube (Figs. 30 and 33, t), dilated at its anterior end, but narrow and cylindrical along the greater part of its length. It is closed in front, but at its hinder end it com- municates through the neurenteric canal with the neural tube, and so, indirectly, through the neuropore, with the exterior. It is ciliated along its entire length, but no food particles have as yet been observed in it prior to the formation of the mouth.
70 A.MIMIIOXUS.
a. The anterior gut diverticula. At a stage with seven pairs of somites, a pair of lateral diverticula arise from the dilated anterior end of the mesenteron. These are situated (Figs. 30, 31, DLj dr), near the dorsal surface of the mesenteron, just in front of the first pair of somites, and ventral to the anterior prolongations of these somites.
The two diverticula soon separate from the mesenteron, which then shrinks back from the anterior end of the body. They are at first of equal size, but from a stage with about ten pairs of somites, onwards, they develop very unequally.
The right anterior gut diverticulum (Fig. 33, dr) forms a thin- walled sac, which extends forwards so as to occupy a large space at the anterior end of the body, below the notochord ; its walls become flattened epithelial cells, and the space which they inclose may be spoken of as the head-cavity.
The left anterior gut diverticulum (Fig. 33, dl) remains of small size, and forms a spherical thick-walled sac, lying on the left side of the head, just in front of the mesenteron and a little way behind the level of the neuropore ; its wall consists of a single layer of columnar ciliated epithelial cells. Towards the close of the embryonic period it opens to the surface by a small pore on the left side of the head (Fig. 34, dl), and from this time is spoken of as the praeoral pit.
The homologies of these anterior gut diverticula with organs of higher Vertebrates are very uncertain. They are probably to be regarded as parts of the body cavity or ccelom, though it must be admitted that their development differs in important respects from the rest of the ccelom. In the mode of their origin, in their asymmetry, and in the fact that the left diver- ticulum early acquires an opening to the exterior, they resemble the anterior ccelomic diverticula of Balanoglossus, and the enteroctelic outgrowths of Echinoderms, with which they have by some observers been held to correspond.
b. The club-shaped gland. In embryos with nine or ten pairs of somites a shallow transverse groove appears across the floor of the mesenteron, and extending up its sides, opposite the septum between the first and second pairs of somites. The first commencement of this groove is seen in Fig. 30, opposite the ventral end of the first somite, but is not indicated by a refer- ence letter. Towards the end of embryonic life the lips of the
THE LATER EMBRYONIC DEVELOPMENT. 71
groove close to form a tube, which splits off along its whole length from the mesenteron, but remains in close contact with this. The limb of the tube which lies at the right side of the mesen- teron expands slightly to form the club-shaped gland (Fig. 36, GL) ; the rest of the tube forms a slender duct, which passing across the body, under the mesenteron, to its left side (Fig. 36, Gd), acquires an opening to the exterior just below the anterior border of the mouth, as soon as this latter is formed. The further development of the club-shaped gland will be described in the section dealing with the larval stages.
c. The mouth. At the close of the embryonic period, a disc- like thickening of the epiblast forms on the left side of the head, opposite the first somite but ventral to its lower edge. The hypoblast of the mesenteron fuses with this patch of epi- blast, and the mouth is formed as a perforation in the middle of the fused patch. The mouth is at first a minute circular aper- ture, but it rapidly increases in size, and at the end of the embry- onic period is a large oval opening (Fig. 36, o), with a slightly thickened border, on the left side of the head.
d. The first gill-slit. Simultaneously with the formation of the mouth, a slight depression of the hypoblast of the ventral surface of the mesenteron appears, opposite the second pair of somites; this fuses with the epiblast, and then, by perforation, an opening is formed which is the first gill-slit (Fig. 34, l). The perforation is formed from within outwards : the gill-slit is at first very small, and situated in the mid-ventral wall ; but it soon enlarges, and as it does so shifts upwards to the right side of the body (Fig. 36, hk i). Like the mouth, it is bordered by long cilia.
e. The anus. This is formed shortly after the mouth and the first gill-slit (Fig. 34, u). It is at first much nearer the hinder end of the body than in the adult, and is placed slightly to the left of the median plane.
5. The Blood-vessels.
The development of the blood-vessels in Amphioxus has been but very imperfectly studied. The first vessel to appear is said to be the ventral or cardiac aorta, which is developed in a longi- tudinal strip of mesoblast, formed by fusion of the ventral edges of the somites of the two sides along the mid-ventral line,
72 AMPHIOXUS.
and extends along the whole length of the under surface of the intestine. The anterior end of the aorta, on reaching the level of the second somite, turns upwards, and runs obliquely forwards along the right side of the pharynx, passing dorsal to the first gill-cleft, and ending in close relation with the club-shaped gland.
(3. Structure of the Embryo at the Close of the Embryonic Period.
The general appearance of the embryo at this stage is shown in Fig. 34. The embryo has a total length of about 1*3 mm., and is of a glassy transparency in all its parts and organs, owing to the complete absorption of the yolk granules originally present in the egg. It is widest about the level of the mouth, in front of which it tapers rapidly, ending in a sharply pointed snout. The hinder part of the body tapers very gradually, and ends in a thin vertical fin of rather larger size than is shown in the figure.
The embryo swims actively, by alternating contractions of the myotomes of the two sides of the body. Of these myotomes there are fifteen pairs present ; the myotomes of the first pair are opposite each other, those of the next two or three pairs are placed more or less obliquely, and behind the fourth pair the myotomes alternate regularly along the two sides of the body. The first pair of myotomes give off anterior prolongations, which extend along the sides of the notochord to the tip of the snout, and by their contractions bend the snout freely from side to side. Each muscle fibre is formed by elongation of a single cell, and the majority of the fibres show more or less evident transverse striation. The alimentary canal is divided into an anterior, dilated, pharyngeal region, lying opposite the first two myotomes ; and a posterior, cylindrical, intestinal region which extends to the anus. In connection with the pharyngeal region are the mouth, the first gill-slit, and the club-shaped gland ; there is as yet no trace of the liver.
The nervous system consists of a neural tube, with proper walls of its own, extending the whole length of the back of the animal, just above the notochord. The neural tube opens to the exterior at its anterior end through the neuropore, immediately behind which the tube presents a slight dilatation or ' brain.' The posterior end of the neural tube (Fig. 34, ne) bends downwards
STRUCTURE AT CLOSE OF EMBRYONIC PERIOD. 73
round the end of the notochord, and still communicates, though by a very minute aperture, with the hinder end of the intestine. Sense organs are represented by pigment spots in the wall of the neural tube ; and a pair of small filaments, formed of elongated and adherent cilia, and situated on the under sur- face of the body behind the mouth, are very possibly taste organs.
One of the most interesting points to notice is that, up to this stage, all the various parts of the body, the epidermis, the walls of the neural tube and of the alimentary canal, the myo- tomes, &c, all alike consist of single layers of cells, and cells which, at any rate in their earlier stages of development, are of epithelial origin.
II. THE LARVAL PERIOD.
This extends from the formation of the mouth to the critical stage, at which latter date the mouth assumes its median position, and the gill-slits become symmetrically arranged on the two sides of the pharynx. The duration of the period is about three months.
During the larval period, development proceeds far more slowly than in the earlier stages. An interval of about a fort- night is said to elapse between the formation of the first and the second gill-slits ; and the close of the larval period, which indicates a very definite stage in development, is also marked by a pause of considerable duration. The chief events that occur during the larval period are the formation of the gill- slits of both sides of the pharynx, the formation of the endo- style, the development of the atrial cavity, the shifting of the mouth to its adult position, the establishment of the full number of myotomes, together with certain important changes in their relations to other organs, and the disappearance of the club- shaped gland.
Until recently our acquaintance with these stages was very fragmentary, and due entirely to Kowalevsky's careful, but brief and incomplete descriptions. Now, owing to Hatschek's observa- tions on the development of the myotomes, and those of Willey and Lankester on the formation of the gill-slits, atrial cavity, and endostyle, we have far more complete and satisfactory knowledge of the actual course of events, although there are many points that still require investigation.
74
AMPIIIOXUS.
During the larval stages the young Amphioxus leads a pelagic life, and is found most abundantly, not at the surface.
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but at depths of from fifteen to twenty fathoms. At the close of the period, it usually abandons its pelagic life, and adopts the
The actual time, however, at
burrowing habits of the adult.
THE LAKVAL PERIOD. 75
which the larva takes to living in the sand varies greatly in different individuals. At the close of the larval period the larva measures about 3*5 mm. in length.
1. The Gill-slits.
It has already been mentioned, in the general account of the development of Amphioxus, that the gill-slits of the two sides are not formed simultaneously ; those of the left side, which may be termed primary slits, appearing before those of the right side, or secondary slits. The primary slits, of which there are as a rule fourteen, are formed, not on the left side, but in the mid-ventral wall of the pharynx, and, after their formation, shift upwards so as actually to be for a time on the right side of the pharynx. The secondary slits, usually eight in number, are formed at a later stage, along the right side of the pharynx, dorsal to the primary slits. Towards the close of the larval period, as the mouth assumes its median position, the primary slits shift across to their permanent position on the left side : at the same time, by an actual diminution in number, through disappearance of the slits at the two ends of the series, the primary slits become reduced to eight, and the critical stage is reached, at which the primary and secondary slits are equal in number, and symme- trically arranged along the left and right sides of the pharynx respectively.
a. The primary gill-slits, or the gill-slits of the adult left side of the pharynx, are formed in succession from before back- wards. Like the first gill-slit, the development of which has already been described, each of the succeeding primary gill-slits lies at first in the mid-ventral wall, but, with the exception of the hindermost two or three, shifts almost at once to the right side of the pharynx. The full number of primary gill-slits is as a rule fourteen, but varies in different specimens from twelve to fifteen. The slits are at first metamerically arranged, corre- sponding, when they are fourteen in number, to the somites from the second to the fifteenth inclusive ; this metameric arrangement is, however, entirely lost in the later stages of development.
The condition with three fully developed primary gill-slits, and a fourth slit in the act of forming, is shown in Fig. 36 ; and the stage in which all fourteen primary gill-slits are present, in
76
AMPHIOXUS.
Fig. 37. TJie gill-slits are at first wide, window-like apertures in the wall of the pharynx ; and, until the formation of the atrial
Fig. 36.-— The anterior end of an Amphioxus Larva with four primary gill-slits, from the left side. (After Lankester and Willey.) x 200.
CH, notochonl. DL, praeoral pit. ES, endostyle. GD, aperture of duet of club- shaped gland. GrL, club-shaped gland. HK 1, 2, 3, 4, first, second, third, and fourth primary gill-slits. ]STS, spinal cord. O, margin of mouth opening. OC, eye-sp :. P C, sixth myotome of the left side.
cavity, they open directly to the exterior. At a comparatively early stage (Fig. 37), the first primary gill-slit becomes markedly smaller than the succeeding ones.
Wl< 7 MO O HK I
Fig. 37. — The anterior end of an Amphioxus Larva with fourteen primary gill- slits, seen from the right side. (After Willey.) CH, notochord. DL, praeoral pit, ES, endostvle. GL, club-shaped gland. GO, opening from elub-shaped gland into pharynx. HK 1, 7, 14, first, seventh, and fourteenth primary gill-slits. HP 2, 7, thickened patches in which the second and seventh secondary gill-slits will be formed at a slightly later stage. MD, free edge of right metapleural fold. NC, neural canal. NO, anterior dilatation, or ventricle of neural canal. K"S, spinal cord. O, mouth. OC, eye-spot. P 13, septum between thirteenth and fourteenth myotomes.
b. The secondary gill-slits, or the gill-slits of the adult right side of the pharynx, appear later than the primary slits, and in the following manner. At a stage (Fig. 37) when fourteen
THE GILL-SLITS.
77
primary slits are present, of which the hinder three or four already open into the atrial cavity, a longitudinal ridge appears in the right wall of the pharynx, above the primary gill -slits. In this ridge six oval thickenings or enlargements appear simultaneously, formed by fusion of the hypoplastic wall of the pharynx with the external epiblast. These fused patches alternate with the primary gill-slits; the first patch (Fig. 37, HP 2) lying above and between the third and fourth primary slits, and the sixth patch, HP 7, above and between the eighth and ninth primary slits. Each patch now becomes perforated by a minute aperture, which by enlargement becomes one of the secondary gill-slits.
The most anterior of these six slits is usually formed a little later than the remaining five ; and a little later still two more
£0 ES
HPI
HKI
Fig.
38. — The anterior end of an Amphioxus Larva with thirteen primary, and eight secondary gill-slits, seen from the right side. (After Willey.)
CH, hotochord. DF, dorsal fin. DL, praeoral pit. ES, endostyle. GL, club- shaped gland. GO, opening from club-shaped gland into pharynx. HK 1, 7, 13, first, seventh, and thirteenth primary gill-slits. HP 1,6,8, first, sixth, and eighth secondary gill-slits. HT, tongue-bar of the fourth secondary gill-slit. LM, velum. ETC, neural
canal. NO, anterior dilatation, or ventricle of neural canal. MTS, spinal cord. OC, eye-spot. P 7, septum between seventh and eighth myotomes. V, cardiac aorta.
slits are formed in similar fashion, one at each end of the series. In this manner the full number of eight secondary gill-slits is acquired (Fig. 38) ; the first, HP i, lying above and between the second and third primary slits ; and the eighth, HP 8, above and between the ninth and tenth primary slits. A ninth secondary gill-slit is sometimes developed at the hinder end of the series.
c. Further development of the primary and secondary gill- slits. The secondary gill-slits are at first very small, but they rapidly increase in size, extending down the right side of the pharynx ; as they do so, the primary slits move downwards to the ventral wall of the pharynx, and then extend up its left
78
AMPHIOXUS.
wall, finally assuming their permanent position on the left side of the pharynx. During the process of shifting, the primary and secondary slits gradually become equal in size, and of similar shape. From the dorsal border of each slit a small process, the tongue-bar, grows downwards across the slit, dividing it into anterior and posterior portions; these tongue-bars (Fig. 38) appear rather earlier in the secondary than in the primary slits. Of the fourteen primary slits, the first and the fourteenth close up and disappear ; and at slightly later stages the thir- teenth, twelfth, eleventh, and tenth similarly, and in succession, close and disappear (c/. Fig. 39). In this way the primary gill-
-■
HK.I2
HK.I
Fig. 39. — The anterior end of an Amphioxus Larva with twelve primary gill- slits, of which the first and twelfth are disappearing-, and eight secondary gill-slits ; seen from the ventral surface. (After Willey.)
CH, notochord. ES, endostyle. HK 1, first primary gill-slit just before its final disappearance. HK 2, second primary gill-slit. HK 12, twelfth primary gill-slit, in the act of closing, prior to its disappearance. HP 1, 8, first and eighth secondary gill- slits. HT, tongue-bar. LM, velum. OB, buccal cavity. OT, buccal tentacles.
slits become reduced to the same number, eight, as the secon- dary slits, the eight persisting primary slits being the second to the ninth inclusive.
The anterior persisting slits of both series, i.e. the second primary slit and the first secondary slit, differ from the others in their smaller size, and in the fact that they alone do not develop tongue-bars (Fig. 39, HK 2 ; HP 1).
The gill- slits have now reached the condition characteristic of the critical stage. Eight slits are present on each side of the pharynx, alternating with one another as in the adult ; the anterior slit of the right side, i.e. the first secondary slit, HP l, being opposite the interval between the first and second slits of the left side, i.e. the second and third primary slits.
THE GILL-SLITS AND THE ENDOSTYLE. 79
2. The Endostyle.
The endostyle appears at the commencement of the larval period, or towards the close of the embryonic period, as a band of columnar ciliated cells on the right side of the anterior end of the pharynx, immediately in front of the club-shaped gland, and in close contact with this. Its condition at an early stage of the larval period is shown in Fig. 36, es, where it is seen as a broad > -shaped band, formed by modification of the hypoblast cells of the right side of the pharynx, opposite the anterior part of the mouth opening. The apex of the > is directed back- wards ; the upper arm is much shorter than the lower ; and the whole band is divided down its centre by a groove.
In. the later stages (Figs. 37 and 38, es), the endostyle ex- tends backwards, its apex passing behind the duct of the club- shaped gland and making its way between the primary and secondary series of gill-slits. As the critical stage is approached, and the primary gill-slits shift across to the left side, the endostyle (Fig. 39) moves to its permanent position on the mid-ventral wall of the pharynx. At the same time it continues to extend backwards, and at the critical stage has reached to about the level of the fifth gill-slits. During the shifting of its position the two limbs of the >, which were originally upper and lower, become right and left respectively ; and as it extends back- wards along the floor of the pharynx the two limbs become closely applied, and fused together. From the anterior ends of the limbs, a pair of ciliated ridges of epithelial cells extend up the sides of the pharynx, and grow backwards along its dorsal surface to form the epibranchial band of the adult.
3. The Club-shaped Gland.
The early stages in the formation of the club-shaped gland have been already described, p. 70. The gland reaches its maximum development about the commencement of the larval period (Fig. 36), when it consists of a dilated sac, GL, lying on the right side of the pharynx, and continuous with a narrow tubular duct, which passes round the ventral surface of the pharynx and opens to the exterior on the left side, close to the anterior border of the mouth, gd.
The dilated part of the gland soon becomes narrower, and tubular, but according to Willey acquires an opening into the
80 AMPIIIOXI s.
pharynx at its dorsal end. It does not shift its position in any way; 1 nit, about the stage represented in Fig. 38, when the secondary gill-slits are formed, and the primary slits are moving across to the left side, the club-shaped gland begins to atrophy, and by the stage shown in Fig. 39 has disappeared completely.
The function and the morphological meaning of the club- shaped gland are very doubtful. Willey has suggested that it may be the modified first gill-slit of the right side, adducing in support of the suggestion the fact that the first gill-slit of the left side is also a structure which disappears early ; indeed, about the same time as the club-shaped gland itself. It is difficult, however, to understand, if the club-shaped gland is formed from a gill-slit of the right side, why its external opening should be on the left side of the head.
4. The Mouth.
The most striking features about the mouth, at the com- mencement of the larval period, are its position on the left side of the head, and its enormous size. As shown in Fig. 36, the mouth, o, and the first gill-slit, hpi i, with the part of the pharyn- geal cavity between them, form a huge opening, perforating the animal from side to side like the eye of a needle.
During the formation of the primary gill-slits the mouth remains on the left side of the head, and increases considerably in length ; extending, at the close of the stage (Fig. 37, o), from the second to the seventh myotome inclusive.
From the commencement of the formation of the secondary gill-slits the mouth gradually shifts its position, growing round the anterior end of the pharynx, and eventually attaining the median position and the shape characteristic of the adult. The shifting commences with the formation of a groove on the surface of the head, leading from the praeoral pit to the upper and anterior angle of the mouth. By deepening of this groove the mouth opening becomes placed obliquely across the body, and by a continuance of the process, together with growth forwards of its posterior lip, it ultimately becomes median in position. The mouth is relatively much smaller in the adult than in the larva, but not actually so.
The margin of the mouth opening of the larva becomes the velum of the adult, from which the velar tentacles arise as out-
THE MOUTH. 81
growths ; of these, there are four present at the critical stage, the remaining eight being developed later.
5. The Buccal Hood and Buccal Tentacles.
The true mouth of the adult Amphioxus, the development of which has just been described, is the small opening in the velum, or partition separating the buccal cavity from the pharynx (Fig. 11, p. 38).
The buccal cavity itself is formed by a pair of folds of integument, which appear about the time of formation of the secondary gill-slits. The two folds are at first upper and lower respectively ; the upper fold commencing above the praeoral pit, and becoming continuous posteriorly with the upper margin of the mouth ; while the lower fold arises as a ridge along the lower and hinder border of the mouth, extending in front across the ventral surface to the right side.
As the mouth assumes its median position the upper and lower folds increase in size, and form the left and right halves of the buccal hood respectively.
The buccal tentacles appear early, as papilla-like outgrowths from the buccal folds (Fig. 39, ot). They arise at first entirely from the lower, or future right fold, about the time the mouth commences to shift its position, and they do not extend into the left fold until a much later period. The median ventral tentacles are the first to be formed, and the others are added on in suc- cession at either end of the series. Small cartilaginoid skeletal elements are present at the bases of the tentacles from their first appearance, and ultimately give rise to the buccal skeleton.
6. The Praeoral Pit.
At the commencement of the larval period, the praeoral pit, which, it will be remembered, is formed from the left anterior gut diverticulum (p. 70), is a small pit with thick ciliated walls, lying on the left side of the anterior part of the head, above and in front of the mouth, and opening to the exterior by a small aper- ture (Fig. 36, dl). When the mouth commences to shift towards the median plane, a ciliated groove is formed, connecting its upper and anterior angle with the aperture of the praeoral pit ; and as the mouth sinks further and further towards the right side the praeoral pit gradually becomes flattened out (Figs. 37, 38, dl),
G
82 AMPHIOXUS.
its walls becoming ultimately converted into the tract of colum- nar ciliated epithelium, which in the adult Amphioxus lines the posterior part of the buccal cavity.
7. The Atrial Cavity.
The atrial chamber begins to form in larvge which have from nine to ten primary gill-slits, but in which the secondary gill- slits have not commenced to develop. A narrow longitudinal groove appears along the ventral surface of the body of the larva,
Fig. 40. — A diagrammatic transverse section across an Amphiosus Larva with eleven or twelve primary gill- slits, but no secondary ones. (Slightly modified from Lankester and Willey.)
A, aorta. AG, atrial cavity. AP, subatrial fold. CH, notocliord, CM, myoccel. C~N, diverticulum of myoccel lying between notocliord and myotome. CS, splancliiuoccel. CU, cutis layer. DF, cavity of dorsal fin. US, skeletogenous layer. I, spinal cord. MD, metapleural ridge. MP, muscle-fascia layer. ML, rnyotoniio muscle. MV, metapleural canal. TI, intestine. V, subintestinal vessel.
behind the region of the pharynx. The groove is bordered by two folds, which become later the metapleural ridges of the adult (Fig. 40, md) : on reaching the pharyngeal region, the two meta- pleural ridges are deflected towards the right side of the larva, and run forwards one on each side of the row of primary gill-slits. From the inner side of each metapleural ridge a horizontal shelf-like outgrowth, the subatrial fold, arises ; and the two sub-
THE ATRIAL CAVITY.
83
atrial folds meet and fuse, converting the groove into a tube (Fig. 40, ac). This tube, of which the roof is formed by the ventral wall of the body, the sides by the metapleural folds, and the floor by the fused subatrial folds, is the atrial chamber. The formation of the floor of the chamber proceeds from behind forwards. In the larva shown in Fig. 37, in which there are fourteen primary slits, and the secondary slits are just commencing to form, the
Fig. 41. — A diagrammatic transverse section through an advanced Amphioxus Larva with fully formed atrial cavity. (Slightly modified from Lankester and Willey, and from Boveri.)
A, aorta. AC, atrial cavity. AP, floor of atrial cavity, formed by fusion of the subatrial folds. CH, notochord. CM, myocoel. CJX", diverticulum of rnyoooel lying between notochord and myotomic muscle. CS, splanclmoccel. CTJ, cutis layer. DF, cavity of dorsal fin. HS, skeletogenous layer. I, spinal cord. MD, metapleural ridge. MF, muscle-fascia layer. ML, myotomic muscle. MV, metapleural canal. OR, nencing reproductive organs. TI, intestine. V, subintestinal vessel.
atrial tube is completed to about the level of the ninth primary gill-slit ; and at a stage shortly before that shown in Fig. 38 the tube is completed along the whole length of the pharynx. The anterior end of the tube ends blindly, but the posterior end remains open as the atrial pore.
The atrial tube is at first very narrow, and of nearly equal
G 2
84 AMPHioxrs.
diameter along its whole length. Later on, it enlarges very greatly, and, pushing the ventral body-wall before it, en- croaches on the space hitherto occupied by the ccelom, finally extending so far dorsalwards as nearly to surround the ali- mentary canal (Fig. 41, AC; cf. also Figs. 12 and 13).
The primary gill-slits at hrst open directly to the exterior, but, as they lie between the two metapleural folds, they become boxed in on the formation of the floor of the atrial tube, and from this time open into the atrial tube or chamber. The secondary gill- slits, which also lie between the two metapleural folds, very close to the base of the right metapleural fold, are not formed until the floor of the atrial chamber is completed, and consequently open into this chamber from the first.
The metapleural folds are at first solid ridges ; large spaces soon appear in them, which become the metapleural canals of' the adult (Figs. 12, 13, and 41, mv).
8. The Mesoblastic Somites.
At the commencement of the larval period fourteen or fifteen pairs of somites are present ; during the early part of this period the number steadily increases, and, shortly before the appearance of the secondary gill-slits, the full number of somites of the adult animal, which appears to be very generally sixty- one, is attained. The somites formed during the larval period differ from those developed in the embryonic stages in not com- municating with the mesenteron at any time in their formation. In the development of these hinder somites it is probable that the polar mesoblast cells take an important share.
Concerning the further development of the somites some interesting details are given by Hatschek. At the commence- ment of the larval period, i.e. about the time of formation of the mouth, each somite (cf. Figs. 32 and 42) becomes divided into a dorsal portion or proto vertebra, and a ventral portion or lateral plate.
The proto vertebras retain the original segmental arrange- ment, i.e. the cavities of successive protovertebrse remain separate from one another ; but in the ventral portions of the somites, or lateral plates, the septa become absorbed, and the cavities open into one another along the whole length of the body, forming a continuous body cavity or ccelom.
THE MESOBLASTIC SOMITES.
85
The cavity of the protovertebra is spoken of as a myoccel (Fig. 42, cm) ; and at a stage when five primary gill-slits are > present {cf. Fig. 36) the myoccels of each pair of protovertebra) ^)C -communicate with each other above the spinal cord (Fig. 42). The outer or parietal wall of the protovertebra is very thin, and closely applied to the epidermis : it gives rise to the cutis, or connective tissue basis of the skin, and may be spoken of as the cutis layer (Fig. 42, cu). The inner or notochordal wall of the protovertebra, as already noticed (p. 67), thickens very greatly, and, though still remaining only one cell thick, becomes converted into the myotomic muscles (Fig. 42, ML). The lower or visceral wall of the protovertebra, like the parietal wall, is thin, and is in contact with the dorsal wall of the alimentary canal.
The cavity of the lateral plates, or splanchnoccel (Fig. 42, cs), is continuous from end to end of the body, through absorption of the septa between the successive so-
d— ML
CM
FlG. 42.— Diagrammatic trans- verse section across the intestinal region of an Am- phioxus larva with five primary gill-slits : cf. Fig. 3G. (After Hatschek.)
CH, notochord. CM, myoccel. CS, splanchnoccel. CU, cutis layer. EP. epidermis. I, spinal cord. ML, myotomic muscle. TI, intestine. V, subintestinal blood-vessel.
cs
mites ; it is also continuous from side to side across the mid-ventral plane. The walls of the splanch- noccel are thin ; the outer, or parietal layer, is in contact with the ventral epidermis, while the inner or splanchnic layer clothes the sides and ventral wall of the alimentary canal.
In the later stages important changes occur in these rela- tions, and the condition immediately after the completion of the larval period is shown in Fig. 43.
The myoccels now extend ventral wards much further than before, so that the parietal layer of the splanchnoccel (Fig. 43, ) no longer touches the epidermis. The median dorsal and ventral parts of the myoccels have separated off as the compart- ments, df and VF, of the dorsal and ventral fins, which are now prominent structures.
The ventral or splanchnic wall of each myoccel is folded to
8G
AMPHIOXUS.
the
form a pouch, which extends upwards, between the myotome on 1 he outer side, and the notochord and spinal cord on the inner side. The outer wall of this pouch (Fig. 43, mf) becomes
the fascia covering the inner surface of the myotome ; while inner wall of the pouch (Fig. 43, hs) gives rise to the skeletal connective tissue, which invests the notochord and the spinal cord. The cavity of the pouch becomes ultimately obliterated by growth of the connective tissue, except in the anterior three or four segments of the body.
The splanchnoccel (Fig. 43, cs) undergoes but slight modification. It extends further dorsalwards than before, and almost completely sur- rounds the alimentary canal, cut- ting out the myoccel from its former share ; while the myoccel in its turn, owing to its ventral extension, shuts out the splanchnoccelic wall from all contact with the external epi- dermis. The splanchnoccel becomes the body cavity, or ccelom, of the adult.
It is interesting to note that even at this stage, when the larval development is completed, all the parts of the body are, as in the earlier stages already noticed in this respect, made up of epithelial layers, which in each case are but one cell thick ; the complications in various regions being brought about by differences in the shapes of the
cells at different places, together with foldings of the walls of
the several cavities.
The origin of the connective tissue is not determined with
certainty. Hatschek considers that it is at first of a gelatinous
nature, probably formed by excretion from, and between, the
trans - young
FlG. 43. — Diagrammatic verse section across a Amphioxus immediately after the completion of the larval period. The section is taken at a level between the atrial pore and the anus. (After Hatschek.)
A, dorsal aorta. CH, noto- chord. CM, myoccel. CS, splanchnoccel. CU, cutis layer. DF, cavity of dorsal fin. EP, epidermis. US, skeletogenous layer. I, spinal cord. MF, muscle-fascia layer. ML, myo- tomic muscle. V, subintestinal vessel. VF, cavity of ventral fin. (Compare also Figs. 40 and 41.)
THE ASYMMETRY OF THE LARVA. 87
several epithelial layers; any cellular elements it may obtain being derived by migration from these epithelial layers.
9. The Asymmetry of the Larva.
The asymmetry of the larva during its early stages is one of the most striking features in the development of Amphioxus. The fact that, but for the alternation of the myotomes on the two sides of the body, the embryonic stages are symmetrical ; and the further fact that at the close of the larval period the symmetry is regained, indicate that the asymmetry of the earlier larval stages is a secondary or acquired character, and that the explanation of it is probably to be found in peculiari- ties of habit or environment of the larva during these stages.
The cardinal point in the asymmetry of the larva is the lateral position of the mouth, which, coupled with its huge size, is probably sufficient to explain the displacement of the gills of the left side.
Willey has suggested that the lateral position of the mouth is correlated with, or actually due to, the anterior extension of the notochord. The mode of development of this front end of the notochord, and a comparison with other Vertebrates, stronglv suggest that the prolongation forwards in front of all the other organs of the head is a secondary feature, associated not improbably with the burrowing habits of Amphioxus ; and if we assume that the ancestral mouth was, as in the Ascidian tadpoles, dorsal in position, then the forward growth of the notochord would of necessity cause lateral displacement of the mouth. The suggestion is an ingenious one, and may be accepted as at any rate a provisional explanation.
III. THE ADOLESCENT PERIOD.
At the close of the larval period, i.e. at the completion of the critical stage, the young Amphioxus abandons its pelagic habits and burrows into the sand, where it passes the rest of its life ; burying itself upright, with the tail downwards and the buccal hood alone projecting from the sand.
The further development takes place gradually. There is a steady increase in size, but no new myotomes are formed, the full number being present at the critical stage. The gill-slits,
88 AMPHIOXUS.
on the contrary, increase greatly in number, new ones being added on at the hinder end of the series, apparently throughout the life of the animal. Each new gill-slit (Fig. 35, l') becomes divided into two, at an early stage in its development, by the growth downwards of a tongue-bar from its dorsal border, just as in the earlier formed slits. The slits further become divided transversely by the horizontal bars characteristic of the adult (Pig. 35, l). Owing to this increase in number of the gill-slits, without any increase in the number of the myotomes, the corre- spondence between the two sets of structures is speedily lost ; the alimentary canal, and the body generally, each acquiring a metamerism of its own.
The Reproductive Organs.
The reproductive organs are formed by proliferation of the epithelial walls of the septa which divide the successive somites from one another. Each of these septa (cf. Fig. 31) is formed by the coalescence of the posterior and anterior walls of adjacent somites, and consists in the young Amphioxus of a thin connec- tive tissue lamella, clothed on each surface by a single layer of flattened epithelial cells, these latter being really parts of the walls of the proto vertebras.
In young specimens of Amphioxus, of about 5 mm. length, the epithelial layer becomes modified over a very small patch at the outer and lower corner of the septum, in the angle between the parietal wall or cutis layer, and the visceral wall of the protovertebra (cf. Fig. 41, or) : at this spot the cells become cubical or columnar in shape, while over the rest of the septum they remain flattened.
This modification does not occur along the whole length of the body, but is from the first confined to the somites in which the reproductive organs lie in the adult animal ; i.e. the patches of modified epithelium are found on the septa forming the walls of the somites from the tenth to the thirty-sixth inclusive.
The modification affects the cells of both surfaces of each septum, but the cells of the posterior surface are almost from the first of larger size than those of the anterior surface, and, growing much more rapidly than these latter, push the septum forwards, and project into the segment in front of that to which they really belong, as a small stalked knob, to which the cells of
THE ADOLESCENT PERIOD. 89
the anterior surface of the septum form a follicular epithelial investment.
These knobs, each of which is a solid mass of enlarged epithe- lial cells, gradually increase in size, extending forwards until they ultimately occupy the whole length of the segments. The cavities in which they lie, really parts of the myoccel (Fig. 41, cm), widen considerably to allow for this increased size, and in speci- mens of about 15 or 16 mm. length become shut off completely from the rest of the myoccel. Boveri has proposed the term gonotome for this portion of the somite, which is specially connected with the reproductive organs, and which is only found in the somites from the tenth to the thirty-fifth or thirty-sixth, in which these organs lie ; he has further directed attention to the fact that the position at which the reproductive organs appear in Amphioxus, close to the line of separation between myoccel and splanch- noccel, corresponds very nearly to that which they hold, in the earlier stages of their development, in the higher Vertebrates.
List of the more important Publications dealing with the development of Amphioxus.
Boveri, T. : ' Ueber die Bildungsstiitte der Geschlechtsdriisen und die Entste-
hung der Genitalkammern beim Amphioxus.' Anatomischer Anzeiger,
vii. 1892. Hatschek, B. : ' Studien iiber Entwicklung des Amphioxus.' Arbeiten aus
dem Zoologischen Institute der Universitat Wien, iv. 1881.
'Ueber den Schichtenbau von Amphioxus.' Anatomischer Anzeiger,
iii. 1888.
' Die Metamerie des Amphioxus und des Ammoccetes.' Verhand-
lungen der Anatomischen Gesellschaft, 1892. Kowalevsky, A. : ' Entwickelungsgeschichte des Amphioxus lanceolatus.'
Memoires de l'Academie Imperiale des Sciences de Saint-Petersbourg,
viie serie, tome xi. No. 4. 1867.
1 Weitere Studien iiber die Entwicklungsgeschichte des Amphioxus
lanceolatus, nebst einem Beitrage zur Homologie des Nervensystems
der Wurmer und Wirbelthiere.' Archiv fur mikroskopische Anatomie,
xiii. 1876. Lankester, E. Kay : ' Contributions to the Knowledge of Amphioxus lanceo- latus.' Quarterly Journal of Microscopical Science, New Series, xxix.
1889. Lankester, E. Bay, and Willey, A. : * The Development of the Atrial Chamber
of Amphioxus.' Quarterly Journal of Microscopical Science, New
Series, xxxi. 1890. Willey, A. : ' The Later Larval Development of Amphioxus.' Quarterly Journal
of Microscopical Science, New Series, xxxii. 1891. Wilson, E. B. : ' On Multiple and Partial Development in Amphioxus.'
Anatomischer Anzeiger, vii. 1892.
90
CHArTER III.
THE DEVELOPMENT OF THE FROG.
Fkogs belong to the class Amphibia, of which toads, newts, and salamanders are other well-known members, while less familiar examples are afforded by the axolotl of Mexico, the Proteus of the caves of Carniola and Dalmatia, the crypto- branch of Japan, which attains a length of three feet or more, and the curious snake-like Coecilia of tropical countries.
As a group, Amphibia are characterised more especially by the double nature of their breathing organs. When adult, they all have lungs ; but in the early stages of almost all genera, and throughout life in a large number, true gills are present, corresponding in structure and in mode of use to those of fish.
In the frog itself these gills are only present during the early, or tadpole, period of existence ; in the later stages they are replaced functionally by lungs, and in the adult they have disappeared completely. The frog is thus, in the course of its own life history, transformed from a water-breathing to an air- breathing animal ; and, in accordance with the principle of Recapitulation explained in the introductory chapter, this trans- formation is to be interpreted as indicating that frogs are descended from fish-like ancestors, each frog in its own develop- ment repeating the ancestral history.
The frog thus holds a position midway between Fish and the higher Vertebrates ; and as frog's eggs can readily be obtained in large numbers, and the embryos and tadpoles develop well in captivity, the frog becomes a very convenient and instructive form for practical laboratory study.
GENERAL ACCOUNT OF THE DEVELOPMENT OF THE FROG.
Frogs' eggs are laid in water, usually during March or the early part of April.
During the act of oviposition, which may last several days,
GENERAL ACCOUNT OF DEVELOPMENT. 91
the male frog clasps the female firmly, embracing her with his arms ; and as the eggs are passed out from the cloaca of the female into the water, they are fertilised by spermatozoa dis- charged over them by the male.
The eggs, which are very numerous, are small sphnrinnl bodies about 1*75 mm. in diameter; they are invested^ by thin coatings of an albuminous substance, which swell up very greatlym the water, and stick togetherto form the_bulky masses we call frog's spawn. Such spawn consists of a trans- parent_igeh±inous mass, formed by the swoiTen albuminous matter^in which the eggs are embedded ; these latter appear as small spherical bodies, each presenting a black half and a white half.
If a number of hen's eggs were broken into a basin, care being taken not to rupture the yolks, a mass would be produced similar to frog's spawn ; the yellow yolks corresponding to the frog's eggs, and the whites or albuminous investments of the yolks to the gelatinous matrix of the spawn.
The frog's eggs, laid in this way, and fertilised b}^ sperma- tozoa shed over them by the male, begin to develop at once. The rate of development depends very largely on the tempera- ture, and varies within very wide limits, warmth hastening development, and cold retarding it. Freezing of the water in which the eggs are kept merely retards development, and does not__injure the— eggs, provided the eggs themselves are not actually frozen. The times mentioned in this chapter may be taken as representing the average rate of development in this country.
Each egg is at first spherical, and remains so during the early stages of development ; at the close of segmentation it becomes slightly ovoid, and then rapidly increases in length. A transverse constriction appears, separating the head from the trunk, and the tail buds out as a small process from the hinder end of the embryo. The embryo soon becomes fish -like in appearance, the tail growing very rapidly ; two pairs of branching tufts, the external gills, followed shortly by a third pair, grow out from the sides of the neck, and in about a fortnight from the time of laying of the eggs the young tadpoles, now about 7 mm. in length, wriggle their way out of the gelatinous mass of the spawn, and swim freely in the water (Fig. 44, 3, 4).
D2 THE FROG,
At the time of hatching, the cloacal opening is already present; but the tadpole has no mouth, and is dependent for nutrition, as it has been during all the earlier stages, on the granules of food-yolk contained in the egg itself. A horse-shoe shaped sucker is present on the under surface of the head, by which the tadpole attaches itself, at first to the gelatinous mass of the spawn, and later on to weeds or other objects in the water.
A few days after hatching, the mouth appears, bordered by a pair of horny jaws, and fringed with fleshy lips studded with horny papillae. The alimentary canal, which has hitherto been short and wide, rapidly increases in length, becoming tubular and convoluted; the liver and pancreas are formed; and the tadpole feeds eagerly on confervas and other plants, especially on decomposing vegetable matter.
About the time of appearance of the mouth, i.e. shortly after hatching, a series of four slit-like openings, the gill-clefts, appear on each side of the neck, leading from the pharynx to the exterior. The margins of the slits become folded, and form the internal gills ; the external gills at the same time decreasing in size and becoming shrivelled in appearance.
While the internal gills are developing, a fold of skin, the operculum, appears on each side of the head, in front of the gills. The two opercular folds, which soon become continuous with each other across the ventral surface of the head, grow back- wards over the gills so as to inclose them in gill-chambers. Towards the end of the fourth week, the hinder edges of the opercular folds fuse with the body wall along the right side and across the ventral surface of the head. On the left side a spout-like opening remains, which communicates with the gill- chambers of both sides ; through this opening the water, taken in at the mouth for respiration, and passed out through the gill- slits, makes its escape to the exterior (cf. Fig. 83).
During this time the tadpole has been feeding freely, and has greatly increased in size. The body (Fig. 44, 8) is broad and round ; the tail is much larger than before, and forms a powerful swimming organ ; while the sucker on the under surface of the head, though still present, is small, and divided into two separate halves ; and is but little used.
Very shortly afterwards, rudiments of the hind limbs can be seen as a pair of small papillae at the root of the tail, one on
GENERAL ACCOUNT OF DEVELOPMENT
93
each side of the cloacal opening (Fig- 71); the limbs increase steadily in size ; about the seventh week they become divided into joints, and a week or so later the toes appear. The fore limbs arise about the same time as the hind limbs, but are covered by the opercular folds, and hence do not become visible until a later stage (Figs. 84 and 85, la).
Towards the end of the second month the lungs come into use, and the tadpoles, which now have the form shown in Fig. 44, 9 and 10, frequently come to the surface of the water to
Fig. 44.
-Various stages in the development of the Froj (From Brehm's ' Thierleben.')
1, eggs just laid. 2, eggs shortly after laying. 3, tadpole shortly before hatching. 4. tadpoles just hatched. 5 and 6, tadpoles with external gills. 7 and 8, tadpoles with fully formed opercular folds. 9 and 10, tadpoles with wed-developed hind legs, shortly before the metamorphosis. 11, tadpole during the metamorphosis. 12, young frog with tail only partially absorbed.
breathe. The gills begin to degenerate, but for some time respiration is carried on both by the gills and the lungs.
A fortnight or three weeks later a distinct metamorphosis occurs, whereby the tadpole becomes transformed, from the fish- like condition in which it has hitherto been, to the purely air- breathing state characteristic of the adult. The tadpole ceases to feed ; a casting, or ecdysis, of the outer layer of the skin takes place; the horny jaws are thrown off; the large frilled lips shrink up ; the mouth loses its rounded suctorial form and becomes much wider; the^tongue, previously small, increases
94 THE FROG.
considerably in size. The eyes, which as yet have been small, become larger and more prominent. The fore-limbs appear, the left one being pushed through the spout-like aperture of the gill-chamber, and the right one forcing its way through the opercular fold, in which it leaves a ragged hole. The abdomen shrinks ; the stomach and liver enlarge, but the intestine becomes considerably shorter than before, and of smaller dia- meter ; the animal, previously a vegetable feeder, now becomes carnivorous. The gill-clefts close up ; the gills themselves are gradually absorbed ; and important modifications, accompanying the change in the mode of breathing, occur in the blood-vessels of the pharynx. The kidneys undergo considerable changes ; the bladder is formed ; and sexual differentiation is definitely established. The tail, which is still of great length (Fig. 44, n), now begins to shorten, and is soon completely absorbed ; the hind legs lengthen considerably, and the animal leaves the water as a frog.
By preventing tadpoles from breathing air directly, as by placing a wire net an inch or so below the surface of the water in which they are living, the occurrence of the metamorphosis can be indefinitely deferred. Under these conditions tadpoles increase greatly in size, but do not become transformed into frogs.
In the remainder of this chapter the several stages in the development of the tadpole, and the formation of the various organs and systems, will be described in detail.
THE FROG'S EGG. 1. Formation of the Egg.
The early stages in the formation of the eggs cannot be seen in the adult frog, but must be studied in tadpoles.
In tadpoles of about 10 mm. length, shortly after the open- ing of the mouth, a pair of longitudinal ridge-like thickenings of the peritoneum appear along the dorsal surface of the body cavity, close to the root of the mesentery. These genital ridges are found in all tadpoles alike, no difference of sex being esta- blished until a considerably later period.
Each genital ridge is at first due merely to a modification in shape of the peritoneal epithelial cells, which, elsewhere flattened,
THE EGG. 95
become here cubical or slightly columnar. The ridges soon become more prominent, especially at their anterior ends, their growth being due, partly to increase of the epithelial cells by repeated division, the epithelial layer becoming several cells thick ; and partly to ingrowth of an axial core of connective tissue, from the basal membrane of the peritoneum, along which blood-vessels gain access to the ridge. The anterior third of each genital ridge undergoes degenerative changes at an early period (Figs. 85, 86), and ultimately becomes the fat body of the adult ; the posterior two-thirds develop into the reproduc- tive organ, OR.
At an early stage, certain of the epithelial cells of the genital ridge become conspicuous by their larger size and more spherical shape ; these are the primitive ova or gonoblasts. Round each primitive ovum the neighbouring cells become arranged so as to form a capsule or follicle ; the follicles forming distinct projec- tions on the surface of the genital ridge. New primitive ova are formed from the surface epithelium, and also by division of those already present ; they, also, soon become inclosed in follicles formed by the neighbouring cells.
Sexual differentiation appears at the time of the metamor- phosis. In the female, the changes consist essentially in a great increase in the size of the genital ridges, which now become the ovaries, and in the number of the contained follicles ; and in the formation of the permanent ova or eggs. The permanent ova are formed from the primitive ova, but different accounts have been given of the details of the process, and it is possible that they are not the same in all cases. As a rule, each primitive ovum divides rapidly to form a nest of cells, one of which becomes a permanent ovum, while the rest form part of the follicle which surrounds it, and serves for its protection and nutrition. In other cases it is stated that a primitive ovum may become directly converted into a permanent ovum.
The permanent ovum, in whatever manner it is formed, differs from the primitive ovum : — (i) in its much greater size ; (ii) in possessing a very large vesicular nucleus, or germinal vesicle ; and (iii) in containing a number of yolk-granules, im- bedded in the protoplasm of its cell-body.
The egg nucleus, or germinal vesicle, is a spherical capsule, with a diameter of from one-third to half that of the ovum itself.
96 THE FROG.
It consisls of a thick elastic nuclear membrane, apparently per- forated by fine radial pores, and inclosing a watery nuclear fluid ; the latter is traversed by a finely granular protoplasmic network, enlarged at the nodes to form nucleoli, or germinal spots, of which one is usually larger than the others.
The yolk granules are small, sharply defined, spherical or ovoidal, yellowish particles of food-substance, which are elabo- rated by the follicle cells and passed on from them into the ovum. They are confined to the protoplasm of the cell-body, not penetrating into the nucleus. They increase rapidly in number as the egg approaches maturity, and it is to them that the size of the egg as well as its opacity are chiefly due.
When the egg has attained a diameter of about 0*5 mm. an exceedingly thin structureless investment, the vitelline mem- brane, is formed immediately around it, and within the follicle. The mode of origin of the vitelline membrane is not clearly made out, but it seems to be formed from the egg itself rather than from the follicular epithelium.
A little later, and as the egg is approaching its full size, a layer of black pigment appears on its surface ; this is at first irregularly distributed over the whole surface, but, as the egg- ripens, the pigment becomes restricted to one half or hemisphere, and the distinction between the white and black poles of the egg is thus established. The pigment is contained, and appa- rently formed, within the egg itself ; but it is not clear how it is formed, or what purpose it fulfils. The facts, that the pigment is confined to the pole of the egg which develops most rapidly, and that warmth greatly increases the rate of development, suggest that the pigment may facilitate development by pro- moting the absorption of heat.
2. Maturation of the Egg.
Our knowledge of the phenomena accompanying the matura- tion of the frog's egg is based almost entirely on the researches of 0. Schultze, and is still in many respects imperfect. An account of these changes has already been given in the intro- ductory chapter, but will be repeated here in order that the developmental history of the frog may be given as fully as practicable.
The process of ripening or maturation commences in an egg
si;
FORMATION AND MATURATION OF THE EGO. 97
while it is still in the ovary, shortly before it reaches its full size, and the successive stages are shown in Fig. 45.