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.

The whole nucleus shrinks considerably, becoming reduced
less than half its former diameter. This shrinking is accom-
panied by exudation of part of the nuclear fluid, through the
nuclear membrane, into the protoplasm of the cell-body (Fig.
15, A, UH), where it forms a fluid layer. surrounding the nucleus:
t the same time the nuclear membrane becomes wrinkled,
its surface, which was previously smooth, becoming raised into
little wart-like projections, so as to present an appearance some-
thing like a blackberry (Fig. 45, A).

Within the nucleus a number of the larger chromatin
granules, or nucleoli, remain close to the nuclear membrane,
often lying within the wart-like protuberances ; a number of
others, chiefly smaller ones, collect towards the centre, where
they surround a clear region in which lie a number of exceedingly
minute chromatin granules. These latter are at first scattered
irregularly, but soon run together to form moniliform threads of
extreme slenderness, which interlace and unite to form a minute
nuclear skein (Fig. 45, A).

About the time of discharge of the egg from the ovary further
changes occur, which are apparently stimulated by the act of
copulation. The nuclear membrane disappears completely ; and
its contents, the nuclear fluid and nucleoli, become distributed
through the yolk. The only part of the egg nucleus which
persists is the minute nuclear skein : this moves towards the
surface of the egg, and takes up a position at the upper or black
pole of the egg, immediately below its surface (Fig. 45, B) ;
here it lies in a lenticular patch, which is rather more fluid and
more transparent than the rest of the yolk, and is separated
from this by an ill-defined capsule of pigment, prolonged to-
wards the centre of the egg in the manner shown in the figure.

The nuclear skein (Fig. 45, B, uo), now assumes a spindle
form, and lies at first with its long axis tangential to the surface
of the egg. Shortly afterwards the spindle turns so that its axis
becomes radially situated, one of its poles being at the surface
of the egg, and the other directed towards the interior ; it then
divides transversely into two parts, of which one (Fig. 45, C, uo),
remains within the egg, while the other (Fig. 45, C, vn), is
extruded as the first polar body. Shortly before the formation

H

98

THE FKOG.

of the polar body the black pole of the egg becomes slightly
flattened, leaving a space between the egg and the vitelline
membrane (Fig. 45, C) ; this space is occupied at once by a peri-
vitelline fluid, exuded from the egg, and in this fluid the polar

uc

Fio. 45. — Successive stages in the maturation of the egg of the Frog. The
eggs are represented as bisected vertically, x 25. (After O. Schultze.)

A, stage in which the nucleus has commenced to shrink, and the nuclear skein is
formed in its centre. Q, stage in which the nuclear skein has moved to the surface of
the egg, just prior to the formation of the first polar body. C> stage in which the first
polar body has been formed, by division of the nuclear skein, and extruded. D> stage in
which the second polar body has been extruded, and the remaining part of the nuclear
skein, or female pronucleus, has retreated from the surface of the egg, and is about to
unite with the male pronucleus or head of the spermatozoon.

PB, first polar body. PB', second polar body. UP, female pronucleus. TJG-, egg
nucleus, or germinal vesicle. UH, peri -vitelline fluid exuded from germinal vesicle.
UM, male pronucleus. Z, vitelline membrane.

body may be seen as a minute ovoidal white body, usually lying
in a small depression on the surface of the egg (Fig. 45, C, pb).

MATURATION AND FERTILISATION OF THE EGG. 99

The formation of the second polar body in the frog has not
been seen, but there can be little doubt that it is due, as in
other animals, to a further division of the part of the nuclear
spindle which remains within the egg, after extrusion of the first
polar body. According to Schultze, the extrusion of the second
polar body from the egg does not take place until about half
an hour after fertilisation of the egg ; i.e. after the entrance of
the spermatozoon, but before the completion of the act of fer-
tilisation.

The two polar bodies are of about equal size ; they lie freely
on the yolk, in the peri-vitelline fluid, and shift about with this
latter if the eggs are rotated.

3. Laying of the Eggs.

The eggs when ripe are discharged from the ovary, and fall
into the body cavity ; along this they pass forwards, directed
partly by contraction of the muscular body walls, partly by the
action of the cilia of the peritoneum, to the mouths of the ovi-
ducts, which are situated at the extreme anterior end of the
body cavity, opposite the roots of the lungs. Within the first or
thick-walled part of the oviduct the eggs acquire gelatinous
investments secreted by glands in its walls : the terminal part
of each oviduct is a thin-walled pouch, capable of great dis-
tension, within which the eggs accumulate in large numbers.
Finally, at the time of copulation, the eggs are passed out
through the cloacal opening into water, in which the albuminous
investments of the eggs speedily swell up to form the gelatinous
mass of the frog's spawn.

4. Fertilisation of the Egg.

The spermatozoa, after being shed over the spawn by the
male frog, swim actively by means of their long tails, work their
way into the gelatinous mass of the spawn, bore through the
vitelline membranes, and so^penetrate into_the eggs themselves,
which they enter at, or close to, theirupper or black_poles.

A single spermatozoon is sufficient to fertilise an egg, and it
is doubtful whether more than one is ever concerned in the pro-
cess. About an hx)ur_££terJ4ie spermatozoon, has entered, a pig-
mente(L_process may be seen projecting into the egg from the
point of entry (Fig. 45, D), and in the centre of the process a clear
spot. This spot (Fig. 45, D, UM), is' the nucleus of the sperma-

100 THE FBOGh

tozoon, or male_proiuieleiis ; it penetrates farther into the <■
carrying the i pigment with it, and soon meets the female pro-
nucleus, or part of the nuclear skein which remains within the
egg anSer extrusion of the two polar bodies.

The two pronuclei come into close contact with each other,
and, after having increased considerably in size, t fuse together to
form the segmeji^ipjLiiiicletis. This fusion, which occurs about
two and a half hours after the spermatozoon first entered the
egg^ completes Jhe act of fertilisation^

Almost immediately after the spermatozoon enters the egg a
considerable extrusion ofjjeri^yjtelline fluid takes jplace, between
the egg and~tne vitelline membrane (Figs. 45, C and D).
This separates the egg from the vitelline membrane, and greatly
facilitates the rotation of the egg within the membrane ; from
this time, in whatever position the spawn be placed, the black
poles_of-thajeggs_will always, from their less specific gravity, be
irprjermost, and thp, whjtp, polp^ wln'fh are of higher specific
gravity .Q wing to the greater abundance of yolk-granules in them,
will be undermost. The extrusion of the peri- vitelline fluid, and
the consequent separation of the egg from the vitelline membrane,
may possibly serve further to prevent or hinder the entrance of
a second spermatozoon.

THE EARLY STAGES OF DEVELOPMENT OF
THE FROG'S EGG.

1 . Segmentation of the Egg.

Segmentation of the _ frog's i egg is, like that of Amphioxus, a
process of cell-division ; but although the processes in the two
animals are essentially similar, there are important differences
in detail, due to the much larger amount of food-yolk present
in the egg of the frog, and its unequal distribution.

Food-yolk consists of small granules of highly nutritious
matter, imbedded in the substance of the egg ; but although it
forms a store of readily assimilated nutriment, at the expense of
which the developmenT~oT~the embryo can be effected, it must
be remembered that until it has been so assimilated the yolk
granules will be foreign bodies, and, like any other foreign bodies,
will be a hindrance rather than an aid to development. The direct
influence of food-yolk is to mechanically impede the activity of

FERTILISATION AND SEGMENTATION OF THE EGG. 101

the protoplasm in which it is imbedded, acting in exactly the
same way as so many grains of sand or other foreign matter
would do, and actually checking the processes of development.

The frog's egg is a telolecithal egg ; i.e. one in which the food-
yolk is not uniformly distributed throughout the yolk, being
more abundant in the lower or white hemisphere than in the
upper or black one. The passage from one pole to the other is a

Fig. 46.

Fig. 47.

Fig. 48.

!0.

Figs. 46, 47, and 48. Segmentation of the Frog's Egg.

Fig. 46. — The egg just before the completion of the first cleft, by which it is divided
into two equal blastomeres : the egg is represented in vertical section.

Fig. 47. — A surface view of an egg at the completion of the third cleft : the egg is
now divided into eight blastomeres, an upper tier of four small ones, and a lower tier of
four much larger ones.

Fig. 48. — A vertical section of an egg at the same stage as Fig. 47. In the middle,
between the inner ends of the blastomeres, is the commencing segmentation cavity.

gradual, not an abrupt one ; there is no line of demarcation be-
tween the two ; still, the black pole is much less encumbered
with food-yolk than the white or lower pole, and it is to this fact
that the relatively rapid development of the black pole is due.
Very shortly after the completion of the act of fertilisation

102 THE FROG.

and the formation of the segmentation nucleus, this latter loses
its spherical form and becomes spindle-shaped, the yolk granules
at the same time showing a tendency to arrange themselves
along lines radiating outwards from the ends of the spindle.

The nucleus now divides into two halves, which move away
from each other ; the yolk granules tend to aggregate themselves
around the two nuclei, and a thin vertical plate of finely granular
protoplasm, almost free from yolk granules, is left, dividing the
egg into two halves. This plate, which soon becomes pigmented,
splits vertically into two, the split appearing near the centre of
the egg^ and at first not reaching to its surface.

At the upper or black pole of the egg a depression now ap-
pears, at first as a small pit, and then elongating to form a groove,
which rapidly extends all round the egg. The groove deepens,
and, meeting with the split already present in the interior of the
egg (Fig. 46), divides the whole egg into two completely separate
and equal parts, the plane of division corresponding with the
vertical pigmented plane mentioned above.

This first plane of division is stated to correspond to the
median sagittal plane of the future embryo and adult ; i.e. the
two cells into which the egg is divided by the first segmentation
plane are said to correspond respectively to the right and left
halves of the body of the frog.

Each of the two nuclei soon becomes spindle-shaped, and
then divides into two ; and a second cleft is then formed in a
similar manner to the first. This second cleft is also a vertical
one, but in a plane at right angles to the first one ; on its com-
pletion the egg is divided into four similar and equal cells, or
blastomeres.

The third cleft is horizontal, but not equatorial, lying (Fig.
47) much nearer the upper than the lower pole. It divides each
of the four cells or blastomeres into two, an upper smaller and a
lower larger one.

From this stage segmentation proceeds rapidly, but according
to no definite rule, the several cells dividing independently of
one another. Throughout the process the upper cells divide
more rapidly^_and_are consequently always of smaller size
than the lower cells, the latter being hampered by the large
number of yolk-granules they contain : in all cases division of the
cells is preceded by division of their nuclei, as in the earlier stages.

SEGMENTATION OF THE EGG.

103

At the stage represented by Figs. 47 and 48, when eight
cells are present, i.e. on the completion of the third cleft, a
small cavity appears in tlie_ce.ntre of the ^gg-> betweenthe inner
ends of the cells (Fig. 48). This is the segmentation cavity or
blastoccel. From its first appearance it is situated nearer the

I upper than the lower pole of the egg. It is filled with fluid,
and during the later phases of segmentation it increases con-
siderably in size (Figs. 49, 50).
At the close of segmentation the egg has the structure shown
in section in Fig. 50. It is a hollow ball, the same size as the
original ovum, with a small, excentrically-placed cavity, and
ith walls of very unequal thickness. The cells of the upper half

Fig. 49.

Fig. 50.

Fig. 49. — The blastula stage in the development of the Frog's Egg, bisected

vertically, x 20.
Fig. 50. — The Frog s Egg at the close of segmentation, bisected verticallv.

x20.

B, segmentation cavity or blastoccel.

are small, approximately uniform in size, and arranged more or
less definitely in two layers, outer and inner ; while the cells of
the lower half are larger, and much more irregular in shape,
size, and arrangement : furthermore, the superficial cells of the
upper half are deeply pigmented at their outer ends, while those
of the lower half are nearly colourless.

The distinction between upper and lower cells is, however,
not an absolute one, the cells at the equator being intermediate
in all respects between those of the upper and lower poles.

The stage represented in Fig. 49 is the one which corresponds
most closely with the blastula stage of Amphioxus (Fig. 14, vm).
There are, however, important differences between the two. In
the blastula stage of the frog there are fewer component cells ;

104

THE FROG.

the cells differ more markedly from one another in shape and
size ; and the segmentation cavity is much smaller relatively to
the entire ovum, and is excentric instead of central in position.
From the description given above it will be seen that all these
differences may be attributed to the greater amount of food-yolk
present in the frog's egg.

2. The Epiblast.

Of the two kinds of cells of which the egg consists at the
close of segmentation (Fig. 50), the smaller pigmented cells of the
upper half are the epiblast cells, while the larger unpigmented
cells of the lower half, in which the yolk-granules are mainly
contalneH^ may be spoken of as the lower layer cells or yolk-cells.

EN

Fig. 51. — Median sagittal section of a Frog Embryo, showing the spreading
of the epiblast and the commencing formation of the mesenteron. x 25.

B, blastocoel or segmentation cavity. BP, lip of blastopore. EE, outer or epidermic
layer of epiblast. EN, inner or nervous layer of epiblast. Y, lower layer or yolk cells.

The distinction between the two is not an absolute one, the cells
at the equator of the egg being intermediate in all respects
between the epiblast and the yolk-cells. As seen from the
surface, the limit is indicated by the boundary line between the
black and the white areas of the egg, and at the close of seg-
mentation these two areas are approximately equal in extent.
In the succeeding stages the black area increases rapidly at the
expense of the white area (Figs. 51, 52, 54), and in a few hours
the pigmented epiblast cells have covered the whole of the egg

THE EP1BLAST. 105

with the exception of a small circular patch at the lower pole
(Figs. 52, y, and 54 yp), where alone the white yolk-cells come
to the surface.

This extension of the epiblast occurs all round its margin,
and is effected by the addition of cells cut out from the super-
ficial layer of yolk-cells. This superficial layer first becomes
pigmented, and then divides into, (i) a surface stratum of small
epiblast cells, which from the first are similar to the original
epiblast cells, and are added on round their margin ; and (ii) a
deeper mass of larger and non-pigmented yolk-cells.

During this extension of the epiblast, the process of cell
division has been continuing rapidly in all parts of the embryo.
The epiblast now consists of two very definite layers of cells :
an outer or epidermic layer (Fig. 51, ee), formed by a single
stratum of short columnar cells, which are deeply pigmented,
and packed close together side by side ; and an inner or nervous
layer (Fig. 51, en), consisting of smaller, more spherical cells,
less strongly pigmented than those of the epidermic layer, and
arranged two or three deep. The cells that are added on round
the margin of the epiblast, during its spreading, are similar in
shape and size to the epiblast cells derived from the upper pole
of the egg, and, like these, soon become arranged in epidermic
and nervous layers.

3. The Mesenteron.

The alimentary cavity, or mesenteron, is formed as a narrow
slit, opening to the surface at the lower pole of the egg and
extending a certain distance into its interior (Fig. 51, bp). The
slit rapidly deepens, spreading concentrically with the surface of
the egg, and lying near to what will subsequently become the
dorsal surface of the embryo ; it is at first exceedingly shallow,
its two walls being almost in contact (Figs. 52, 53, t) ; but
very shortly, by depression of the lower wall or floor (Figs. 54,
55, 56, t), the cavity becomes of considerable size, and forms
the alimentary tract of the embryo.

This slit-like mesenteron was formerly described as arising
by a process of invagination, the epiblast cells being said to grow
into the interior of the egg to form the wall of the mesenteron
cavity. Later investigations have shown that this description is
incorrect, and that the cavity is formed, not by invagination from

106

THE PROG.

the surface, but by splitting apart of the yolk-cells as described
above, this splitting being preceded by the formation of pigment
in the adjacent surfaces of the cells between which the split is
to appear.

The mesenteric slit appears first as a slightly crescentic
groove on the surface of the egg (Fig. 51, bp), at the margin of
the spreading epiblast, and about midway between the equator
and the lower pole of the egg. It is very conspicuous, because
the pigmented epiblast cells stop sharply at its upper or convex
border, so that the boundary between the epiblast and yolk-

Fig. 52.

-Sagittal section of a Frog Embryo during the formation of
the mesenteron. x 25.

B, blastocoel or segmentation cavity. BP, upper or dorsal lip of blastopore. BP',
lower or ventral lip of blastopore. EE, outer or epidermic layer of epiblast. EN, inner
or nervous layer of epiblast. H, hypoblast. T, mesenteron. Y, yolk-plug.

cells is here an abrupt one, while round the rest of the circum-
ference, as shown on the right-hand side of Fig. 51, the transi-
tion is more gradual.

The groove rapidly extends at its extremities, becoming
semicircular, then horse-shoe shaped, and finally, by meeting of
its limbs, a complete circle. This circular groove separates the
epiblast, which now ends sharply against it round its entire
margin, from a circular patch of yolk-cells (Fig. 52, Y, and
Fig. 58, A), which still remains at the surface of the egg. The

THE MESENTEKON.

107

circular aperture in the epiblast, denned by this groove, is spoken
of as the blastopore, or anus of Rusconi ; and the mass of yolk-
cells which fills up the aperture, as the yolk-plug.

The blastopore lies at first at the lower pole of the egg.
Reference to Figs. 51, 55, and 58 will show that this lower pole
becomes subsequently the hinder or tail end of the embryo, so
that the lips of the blastopore, bp and bp', may be spoken of
as dorsal and ventral respectively.

From the figures, and from the above description, it will be

M-

Fig. 53. — Horizontal section across a Frog Embryo of the same age as that
shown in Fig. 52, the section being taken along a line joining the refer-
ence letters T and B in Fig. 52. x 25.

B, blastocoel or segmentation cavity. EE, outer or epidermic layer of epiblast.
EN, inner or nervous layer of epiblast. H, hypoblast. M, mesoblast. T, mesenteron.
Y", yolk-cells.

seen that the groove which limits the blastopore appears first
at its dorsal margin, bp, and spreads round the sides to the
ventral margin, bp'. The slit extends at first radially inwards,
towards the centre of the egg (Figs. 51, bp, and 52, bp') ; but
along the dorsal surface the slit, after a short radial course
(Fig. 52, bp), turns sharply at right angles (Fig. 52, t), and
spreads forwards concentrically with the surface of the embryo.

The whole embryo, which up to this stage has been spherical,
now begins to elongate, becoming ellipsoidal, with the blastopore

108

THE FROG.

marking the posterior pole (Figs. 54, 55). By an alteration in
the position of the cells of its floor, the mesenteric slit (Fig. 52,
t) becomes widened out into a large cavity (Fig. 54, t) ; the
roof or dorsal wall of which is formed by a well-defined layer of
small cells, arranged three or four deep, and lying in close

Fig. 54. — Sagittal section of a Frog Embryo just before the disappearance of
the segmentation cavity, x 25.

B, blastocoel or segmentation cavity. BP, upper or dorsal lip of the blastopore. BP',
lower or ventral lip of the blastopore. CH, notochortl. EE, outer or epidermic layer
of epiblast. EN, inner or nervous layer of epiblast. H, hypoblast forming dorsal wall
of mesenteron. M, mesoblast. T, mesenteron. Y, yolk-cells. YP, yolk-plug.

contact with the epiblast, while the floor and sides consist of
yolk-cells (Fig. 54, y).

During this change the segmentation cavity, B, gradually
becomes reduced in size, and ultimately disappears altogether.
It can always be distinguished from the mesenteron by the fact
that it lies between the epiblast and the yolk-cells, and that its
wall is therefore formed on one side by epiblast cells only (Figs.
52 and 53, b) ; while the mesenteron, T, always has walls formed
by both epiblast and lower layer cells.

Figs. 52, 54, and 55 show that the segmentation cavity
becomes reduced and obliterated, partly by the growth forwards
of the cells which form the roof of the mesenteron ; and partly
by a shifting in the position of the yolk-cells forming the floor

THE HYPOBLAST AND MESOBLAST. 109

of the mesenteron, which accompanies the elongation of the
embryo and the enlargement of the mesenteric cavity. The
niesenteron and the segmentation cavity may, as shown m
Figs. 52 and 54, communicate with each other for a time during
these changes.

Fig. 55. — Sagittal section of a Frog Embryo after the disappearance of the
segmentation cavity and completion of the mesenteron. x 25.

BP, blastopore. CH, notochord. E, epiblast : the cell outlines and the distinction
between the epidevmic and nervous layers are not shown. H, hypoblast. M, mesoblast.
T, mesenteron. Y, yolk-cells.

4. Formation of the Hypoblast, the Notochord, and the Mesoblast.

During the formation of the mesenteron, the cells forming-
its walls (Figs. 54 and 56) become arranged in two concentric
layers : — an inner layer, the hypoblast, which forms the true wall
of the mesenteron ; and an outer layer, the mesoblast (Fig. 56, m),
which lies between the hypoblast and the epiblast.

The splitting off of the mesoblast commences in the dorso-
lateral walls of the mesenteron, and spreads towards the median
plane, both dorsally and ventrally. Before this splitting reaches
the mid-dorsal plane, a pair of longitudinal clefts appear along
the dorsal surface, by which a median longitudinal rod of cells
(Fig. 56, ch) is cut off from the two laterally placed mesoblast
sheets, M. This rod, CH, remains attached to the hypoblast for a
short time after the mesoblast sheets are completely separated ;
but very shortly afterwards the rod in its turn splits off from
the hypoblast, and becomes the notochord.

110 THE FROG.

The mesoblast (Fig. 56, m) thus arises in the frog as two
lateral sheets of cells, split off from the outer surface of the
hypoblast and yolk-cells. The two sheets very early become
continuous with each other in the mid-ventral plane, but are
separated dorsally by the notochord, which is formed, indepen-
dently, from the hypoblast in the mid-dorsal region.

At intervals along their length, the mesoblast sheets remain
for a time attached to the hypoblast along the dorsal surface of
the mesenteron, not far from the median plane ; and, at these

Fig. 56. — A transverse section through the middle of the length of a Frog
Embryo at about the stage represented in Fig. 55. x 25.

CH, notochord. E, epiblast. HM, pouch-like diverticulum of the hypoblast into
the mesoblast. M, mesoblast. NG, neural groove. NP, neural plate. T, mesenteron.
Y, yolk-cells.

places, slight pouch-like diverticula from the mesenteron (Fig.
56, hm) may be seen extending into the mesoblast, sheets. It
has been suggested by Hertwig that these diverticula are pos-
sibly indications of a mode of origin of the mesoblast as hollow
diverticula from the mesenteron, such as occurs in Amphioxus
(cf. Figs. 24 and 28, ce).

5. The Blastopore and the Primitive Streak.

The blastopore, or anus of Eusconi, has been defined above as
the circular aperture in the epiblast which is filled up by the
yolk-plug (Figs. 52 and 58, A) ; the lip of the blastopore and

THE BLASTOPORE. Ill

the yolk-plug being separated from each other by the narrow
circular groove which leads into the mesenteron. In the im-
mediately succeeding stages the blastopore becomes greatly
reduced in size, though still retaining its circular outline
(cf. Figs. 52, 54, 55). This reduction is effected, not by con-
traction of the whole circumference of the blastopore, but by
a folding together, or concrescence, of its lips in the median
plane, beginning at the lower or ventral margin and proceeding
upwards towards the dorsal margin, the line of fusion being
marked by a faint vertical groove on the surface of the embryo
(cf Fig. 58, A and B).

At the lip of the blastopore, round its entire circumference,
the three germinal layers, epiblast, mesoblast, and hypoblast,
are indistinguishably fused together (Figs. 54, 55) ; the sepa-
ration between the layers first appearing a little distance beyond
the margin of the blastopore. As the lips of the blastopore
meet and unite from below upwards, in the manner described
above, a vertical band is produced by their union, at the hinder
end of the embryo, in which the three germinal layers are fused.
This band is spoken of as the primitive streak ; and the faint
median groove, already described (Fig. 58, B, C), which runs
along it, and marks the line of union of the right and left lips
of the blastopore, is named the primitive groove.

The primitive streak and primitive groove are comparatively
inconspicuous features in the frog embryo, but are much more
prominent in the chick and the rabbit. They are probably
to be regarded as secondary rather than as essential characters,
and as associated with the great distension which the egg has
undergone in consequence of the number of yolk-granules
imbedded in its substance.

The further development of the primitive streak, and the
ultimate fate of the blastopore, will be described in a later part
of this chapter.

The reduction in size of the blastopore, caused by the con-
escence of its lips, gives rise to a corresponding diminution
of the yolk-plug (cf Figs. 52, 54, yp) ; and at the close of the
period now being described this withdraws completely from the
surface of the embryo (Fig. 55).

112 THE FROG.

6. Comparison of the Early Stages in the Development of the
Frog with those of Amphioxus.

The frog's egg is more than 5,000 times the bulk of that of
Amphioxus : this large size is due mainly to the much greater
amount of food-yolk present in the frog's egg, and it is chiefly
owing to this food-yolk that the development of the two forms
is so different. In the earliest stages the differences are less
marked than in the succeeding ones. The first two segmenta-
tion clefts divide the frog's egg in the same way as they do that
of Amphioxus ; the third cleft is in both cases a horizontal one,
but while in Amphioxus it is nearly equatorial, in the frog it
lies much nearer the upper pole. The stage shown for the frog
in Fig. 49 corresponds fairly closely, in essential resj:>ects, with
the blastula stage of Amphioxus ; but from this point the
development of the two forms becomes widely different.

There is no stage in the frog which exactly corresponds to
the gastrula stage in Amphioxus ; for at the stage shown in
Fig. 52, which most nearly approaches to this, both epiblast and
hypoblast are already three or more cells thick, instead of being,
as in Amphioxus, single layers of cells. Moreover, the primitive
digestive cavity of the frog (Fig. 52, t) is formed, not by invagi-
nation, as in Amphioxus, but by a process of splitting, or separa-
tion, among the yolk-cells occupying the interior of the embryo.
The history of development in some allied animals, notably in
the newt, suggests that the process of splitting is a secondary
modification, which has arisen in consequence of the hindrance
offered by the large mass of yolk-cells to the occurrence of
invagination.

The early establishment of the two-layered condition of the
epiblast is another point in which the frog presents a modified
and specialised condition : in the corresponding stages of the
newt the epiblast consists, as in Amphioxus, of a single layer of
cells.

DEVELOPMENT OF THE NERVOUS SYSTEM.

It will be convenient from this point to deal with the several
systems one by one, following each up to its condition in the
adult. The order in which the systems are taken is chiefly a
matter of convenience, but for several reasons the nervous

THE CENTEAL NERVOUS SYSTEM.

113

system is the most suitable to commence with. It is formed
from the epiblast, which is the earliest of the germinal layers to
be definitely established ; it appears at a very early stage ; and
it plays a prominent part, especially in the younger embryos, in
•determining the shape and proportions of the body.

1. General History of the Central Nervous System.

The epiblast of the frog, as already described, consists,
almost from the first, of two layers, the distinction between
which is established before the close of the period of segmenta-
tion. Of these, the upper or epidermic layer is a single stratum
of closely fitted, short columnar or cubical cells ; while the lower
or nervous layer (Figs. 51, 52) consists of spherical or ovoid
•cells, more loosely arranged, and two or three deep : it is from
this lower layer that the nervous system is developed.

The first trace of the nervous system appears at a stage
mediately succeeding that shown in Fig. 55, when the
embryo is ellipsoidal in shape, and the blastopore has become
much reduced, and less conspicuous owing to the yolk-plug
having withdrawn from
the surface.

The dorsal surface of
the embryo now flattens
slightly, and along the
flattened area the deeper
•or nervous layer of the
epiblast thickens to form
the neural plate, a tri-
angular area extending
along the back of the
■embryo, wider in front
but narrowing poste-
riorly towards the blas-
topore. Slightly raised
ridges, the neural folds
(Fig. 57, xf), soon ap-
pear, bordering the neural
plate laterally ; and a shallow neural groove (Figs. 56, 57, ng) is
formed along its dorsal surface in the median line, extending
forwards from the blastopore.

I

NC

YP-

Fnj. 57. — A Frog Embryo at the time of ap-
pearance of the neural folds : seen from
the dorsal surface, x 20.

NF, neural fold : the reference line points
to the junction of the anterior and the left
lateral folds. WGr, neural groove. YP, yolk
plug, greatly reduced in size, but still visible
through the blastopore.

114

THE FROG.

Anteriorly, the two neural folds are connected by a trans-
verse fold (Fig. 57), which runs across the anterior end of the
neural plate, and slightly raises it above the level of the sur-
rounding parts ; while at their hinder ends the two neural folds
are continuous with the lateral lips of the blastopore.

The neural folds rapidly increase in height and thickness :
the groove between them deepens ; and the folds, becoming more
and more prominent (Figs, 58, 59), approach each other, and
finally meet in the median plane and fuse together, converting
the neural groove into a tube.

The neural folds first meet about the junction of the head

Fig. 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.J

A, stage hi which the blastopore is nearly circular, and is occupied by the white
yolk-plug. B. stage in which the lateral lips of the blastopore have met and fused to
'form the primitive streak ; the short vertical line, corresponding to the position of the
blastopore in A, is the primitive groove ; the depression at the upper end of the primi-
tive groove is the greatly reduced blastopore, and the depression at the lower end of the
primitive groove is the commencing proctodasal or anal invagination. Above the blasto-
pore is seen the commencing neural groove, bordered by the neural folds. C, later
in 'Which the neural groove has deepened, while the neural folds are more prominent and
are growing inwards to meet each other. D, stage in which the neural folds have met
and the tail is commencing to form. Both blastopore and proctodasum are still present.
E, later stage, in which the neural tube is completed and the tail has increased in size.
The blastopore lias finally closed, and the black spot below the tail is the proctodeum.

and neck of the embryo; and from this point the fusion extends
rapidly backwards, and more slowly forwards. The point at
which fusion last occurs is a little distance behind the anterior
end of the neural tube, at the spot where the pineal body is
formed later, the part of the tube in front of this point being
roofed in by growth backwards of the anterior or transverse
neural fold seen in Fig. 57.

The neural groove extends back as far as the blastopore
(Figs. 57 and 58, B), and the neural folds, as noticed above,
become continuous at their hinder ends with the lips of the
blastopore. For a short time, after completion of the neural
tube, the blastopore still remains open, communicating, as seen
in Fig. 60, both with the mesenteron and with the cavity of the

THE CENTEAL NEEVOUS SYSTEM.

115

neural tube. Very shortly, however, the fusion, by which the
neural tube is closed, extends further back, so as to involve the
lips of the blastopore, and the external opening of the blastopore
becomes finally closed (Fig. 58, E, and Fig. 61). The communi-
cation between the neural tube and the mesenteron still persists,
however, as a narrow tubular passage, the neurenteric canal
(Fig. 61), passing round the hinder end of the notochord.

FlG. 59. — Transverse section through a Frog Embryo, at a stage corresponding
to Fig. 58, C, and showing the neural folds shortly before they meet each
other to complete the neural tube.

C, ccelom or body cavity. CH, notochord. EE, outer or epidermic layer of epiblast.
EN, inner or nervous layer of epiblast. M, mesoblast. ME, outer or somatopleuric
layer of mesoblast. MH, inner or splanchnopleuric layer of rnesoblast. NC, neural
groove. ND, dorsal root of a spinal nerve. NS, spinal cord. T, mesenteron. "W, liver
diverticulum. Y, yolk.

The neurenteric canal persists only for a very short time.

the immediately succeeding stages the tail begins to

gthen rapidly, carrying the hinder end of the neural tube far

ay from the mesenteron, and the channel of communication

tween the two becomes speedily obliterated. At the time of

ling (Fig. 69, p. 146), the hinder end of the neural tube

i 2

In"

lengt
away
betwi
hatel

11G THE FROG.

curves slightly downwards, round the end of the notochord, but
ends blindly a long distance from the mesenteron. A string of*
cells, connecting the two structures, is at this stage the sole indi-
cation of the former communication between them.

The neural tube, formed in the way described above, by
fusion of the neural folds, soon separates along its entire length
from the external epiblast, and by thickening of its walls and
various histological changes becomes converted into the central
nervous system ; the anterior part forming the brain, and the
posterior part the spinal cord. The lumen or cavity of the
neural tube persists throughout life as the central canal of the

H S N

Y \ XM

L
Fig. 60. — Sagittal section of a Frog Embryo shortly before closure of the
blastopore, and of the same age as the embryo shown in Fig. 58, D. x 30.

B, blastopore. BF, fore-brain. BH, hind-brain. BM, mirl-brain, H, hypoblast.
L, liver. M, mesoblast. MN, mesenteron. ~N, notochord. NC, neurenteric canal.
P, ingrowth of epiblast to form the pituitary body. PD, proctodeum. R, rectal
diverticulum of mesenteron. S, central canal of spinal cord. Y, yolk cells.

spinal cord and the ventricles of the brain (cf. Figs. 60, 61, 64,
and 65).

The further changes undergone by the spinal cord are com-
paratively slight, and will not be described in detail. Almost
from the first (Fig. 70, p. 147), the spinal cord is oval in transverse
section, the central canal being a vertical slit. The layer of
cells lining the central canal, derived (cf. Fig. 59) from the
outer or epidermic layer of the epiblast, remains throughout life
as a layer of columnar ciliated epithelial cells ; while the outer
wall of the neural tube, formed from the deeper or nervous layer

THE CENTRAL NERVOUS SYSTEM.

117

Di

of the epiblast, gives rise directly to the nervous elements, i.e.
to the nerve cells and nerve fibres, of the adult spinal cord. The
histological changes by which the nervous elements are formed
will be described in the chapters dealing with the chick and the
rabbit, in which animals they have been investigated more
completely than in the frog.

The spinal cord extends to the extremity of the tail, which in

e later stages of tadpole life is of great length (Fig. 44, 9, 10, n).

uring the absorption of the tail, at the time of the metamor-
phosis, fully two-thirds of the length of the spinal cord are lost.

I. The Development of the Brain.
The brain is merelv the specialised anterior part of the

Fig. 61. — Sagittal section of a Frog Embryo, shortly after closure of the blas-
topore and formation of the anus, and of the same age as the embryo
shown in Fig. 58, E. x 25.

BF. fore-brain. BH, hind-brain. BM. mid-brain. CH, notochord. M, myo-
blast. NC, cavity of neural tube. NT, neurenteric canal. PN, pineal body. PT,
ingrowth of epiblast which gives rise to the pituitary body. TI, intestinal region of
mesenteron. TP, pharvngeal region of mesenterou. U, proctodasal or cloacal aperture.
W, liver, Y, yolk-cells.

neural tube, and is directly continuous posteriorly with the
spinal cord.

While the spinal cord is straight, or nearly so, the brain is

om its first appearance bent rather sharply, and nearly at

right angles, about the middle of its length ; the axis of the

posterior part being horizontal and continuous with that of the

spinal cord, and the axis of the anterior part vertical. The

118 THE FROG.

whole central nervous system may be compared to a retort (Fig;
GO), the bulb of the retort being formed by the anterior and
vertical part of the brain, bf, and the neck by the posterior
horizontal part of the brain, together with the spinal cord.

This bending of the brain is spoken of as cranial flexure. It
takes place, as shown in Fig. 60, round the anterior end of
the notochord, and is due, in the first instance, to the spherical
shape of the surface of the egg on which the neural plate is
formed. A similar ventral flexure of the hinder end of the
neural tube is present at first, but becomes early obliterated by
the outgrowth of the tail (cf. Fig. 58, B, C, D, E). The ventral
flexure of the brain, round the anterior end of the notochord,
persists throughout life.

\7ery shortly after the closure of the brain-tube is completed,
a slight transverse constriction appears, at the bend between
the horizontal and vertical portions of the brain, and a little
later a second constriction is formed rather further forwards.
By these constrictions the brain (Figs. 60 and 61) becomes
divided into three portions, named fore-brain, mid-brain, and
hind-brain respectively.

The fore-brain (Figs. 60 and 61, bf) is the terminal ver-
tical portion, corresponding to the bulb of the retort ; the mid-
brain, bm, which is the smallest of the three divisions, forms the
angle of the bend, opposite the anterior end of the notochord ;
and the hind-brain, bh, is the horizontal portion, continuous
posteriorly with the spinal cord. This division of the embryonic
brain into three regions, anterior, middle, and posterior, is a
convenient one, as it obtains throughout the higher groups of
Vertebrates, from fishes to mammals, each of the divisions
giving rise to important and characteristic parts of the adult
brain.

The walls of the brain-tube are at first of approximately
uniform thickness in all parts, excepting the roof of the hind-
brain, which from the first is thin. By unequal thickening of
various parts, especially of the sides, and by outgrowths, either
median or paired, with accompanying histological changes, the
adult brain is gradually built up.

In these changes the most important share is taken by the
fore-brain. The fore-brain itself becomes the part known in
the adult as the thalamencephalon; its cavity persisting as the

THE ERA IN.

119

third ventricle ; from it the pineal body and the infundibulum
are developed as median diverticula, dorsal and ventral respec-
tively ; while the optic vesicles and cerebral hemispheres arise
as paired lateral and anterior outgrowths.

The mid-brain undergoes comparatively little change ; from
its roof the optic lobes of the adult are formed.

The hind-brain becomes the medulla oblongata of the adult :
from the roof of its anterior part the cerebellum is formed.

Before considering the development of the several parts of
the brain in detail it will be well to notice the general propor-

Fig. 02.

Fig. 03.

FlG. 62.— The brain of the adult Frog- : dorsal surface, x 4.
Fig. 63. — The brain of the adult Frog : ventral surface, x 4.

C, cerebellum. CH, cerebral hemisphere. CP, choroid plexus of third ventricle.
P, fourth ventricle. IN", infundibulum. M, medulla oblongata. O, olfactory lobe.
OC, optic chiasma. OL, optic lobe. P, stalk of pineal body. PP», pituitary body.
T, thalamencephalon. I, olfactory nerve. II, optic nerve. Ill, third or motor oculi
nerve. IV, fourth nerve. V, fifth or trigeminal nerve. VI, sixth nerve. VII and
VIII, combined root of facial and auditory nerves. IX anil X, combined root of
glossopharyngeal and pneumogastric nerves.

tions and relations of the brain during the successive stages of
its formation. These will be readily understood from comparison
of Figs. 60, 61, 64, 65, and 89.

At the time of the first formation of the brain-tube, before
the hatching of the tadpole (Figs. 60, 61), cranial flexure is very
strongly marked, and the fore-brain, BF, projects far in front of
all other organs of the body. Later on (Figs. 64, 65), both
these relations are changed ; the brain appears to become

120

THE PROG.

straightened out, and it also recedes some distance from the
anterior end of the head.

The straightening of the brain, or rectification of the cranial
flexure as it is sometimes termed, is apparent rather than real,
and is brought about principally by the formation of the cerebral
hemispheres (Figs. G4, C5, bc), which grow forwards from the

Fig. 64. — Sagittal section of the head end of a Tadpole just
before the opening of the mouth.

A, dorsal aorta. BC, vesicle of the hemispheres. BH, hind-brain. BM, mid-brain.
CH, notochord. DS, septum separating stomatodasum and pharynx. IN, ini'undibuluni.
PN, pineal body. PT, pituitary body. RS, sinus venosus. *RT, truncus arteriosus!.
RV, ventricle. TH, thyroid body. TI, intestine. TP, pharynx. TO, plug of
epithelial cells blocking up the oesophagus. "W", liver. "WGr, gall-bladder.

fore-brain, and speedily attain so large a size relatively to the
other parts of the brain as to alter the direction of the axis of the
brain as a whole, and to completely obscure the original flexure,
which really persists throughout life. The receding of the
brain from the anterior end of the head is due to the more
rapid growth of the surrounding parts, and more especially of

THE BEAIN.

121

the face and lips, which causes the brain to take a much less
prominent share in determining the shape of the head.

In describing the development of the brain in detail it will
)e convenient to take the several parts in order, from behind
forwards, commencing with the medulla oblongata.

122 THE FROG.

The medulla oblongata undergoes less change than any
other part of the brain. In the early stages, up to about the
time of formation of the mouth, it is the widest part of the
brain, but afterwards it is exceeded by both the optic lobes and
the cerebral hemispheres. It is continuous posteriorly, without
any line of demarcation, with the spinal cord ; while anteriorly
it is separated from the mid-brain by a well-marked constriction,
deepest dorsally and at the sides.

From the first, the roof of the medulla oblongata is thin ; in
the later stages the sides and floor thicken very considerably,
while the roof (Figs. 65 and 84) widens out and becomes re-
duced to an extremely thin membrane, consisting of a single
layer of pigmented and ciliated epithelial cells, without nervous
elements of any kind.

This thin roof is at first smooth and level ; but about the
time of formation of the mouth opening, i.e. in tadpoles of about
9 mm. length, the roof becomes thrown into folds (Fig. 65, x'),
which become deeper and more pronounced as the tadpole in-
creases in size. Lying on this thin roof, and in very close
contact with it, is a rich network of blood-vessels, the choroid
plexus, which extends between the folds of the roof, and so
appears to hang down into the cavity of the medulla, though
always in reality separated from this by the thin epithelial roof.

The cavity of the medulla oblongata, or fourth ventricle, is
of considerable size : it is wide in front, and tapers gradually
towards its hinder end, where it passes into the central canal of
the spinal cord.

The cerebellum is an inconspicuous structure throughout
the early stages of tadpole life. Up to the time of the opening
of the mouth it can hardly be said to exist (Fig. 64) ; but shortly
after this event it appears as a thickening of the roof of the
fourth ventricle, in the form of a transverse band, immediately
behind the constriction separating the medulla oblongata from
the mid-brain. In the later stages of development it increases
gradually in size (Fig. 89, bl), but even in the adult frog it is very
small as compared with its condition in most other Vertebrates.

The mid-brain does not undergo very great changes. Its
floor remains thin in the actual median plane ; but immediately

THE BRAIN. 123

to the right and left of this the sides become thickened by the
formation of the crura cerebri, two longitudinal bundles of nerve
fibres which connect the mid-brain with the fore-brain. The
roof of the mid-brain is thin in the early stages ; but shortly
after the opening of the mouth the two halves of the roof thicken
considerably, and, bulging upwards, form a pair of rounded swell-
ings, the optic lobes (Fig. 62, ol), separated by a median groove.
The optic lobes continue to increase in size, and about the
time of the metamorphosis become, as in the adult frog, the
widest portion of the brain. The cavity of the mid-brain
persists as a fairly wide passage, the Sylvian aqueduct.

I The thalamencephalon is the original fore-brain of the
embryo; and in connection with it important changes occur.
Its cavity, the third ventricle, is at first large ; but, owing to
thickening of its walls to form the optic thalami, the cavity
becomes early reduced to a vertical cleft, very narrow from side
to side.

The roof of the thalamencephalon is very thin, consisting,
like that of the medulla oblongata, of a single layer of epithelial
cells, devoid of nervous elements. About the middle of its
length, and at the place where the final closure of the neural
tube was effected, the pineal body is formed. This appears
in embryos of about 3 mm. length (Fig. 61, PN) as a median
hollow diverticulum, which at the time of hatching of the
tadpole (Fig. 64, rx), forms a small round knob on the top of
the brain, immediately beneath the surface epiblast. This
grows forwards, and becomes dilated distally. At the time of
opening of the mouth it forms a small rounded vesicle, connected
with the brain by a tubular stalk ; it is of a glistening white
appearance, owing to the presence of small snow-white particles
imbedded in its substance, and stands in this respect in marked
contrast to the rest of the brain, which is pigmented rather
strongly. In tadpoles of 12 mm. length the pineal body itself
is solid (Fig. 65, risT), but its stalk is still tubular. Shortly
after this, on the formation of the skull, the pineal body becomes
cut off from its stalk, and lies outside the skull, just beneath
the skin of the top of the head. It persists throughout the
tadpole stages, but disappears at the time of the metamorphosis.
The stalk of the pineal body persists throughout the life of the

12-4 THE FROG.

frog, retaining its tubular character, and its communication
with the third ventricle.

About the time the roof of the fourth ventricle is becoming
folded, and the choroid plexus established in connection with it.
a similar change is going on in relation with the third ventricle.
Immediately in front of the pineal body, the thin roof of the
thalamencephalon becomes thrown into folds which hang down
into the ventricle (Fig. 65, x). A dense plexus of blood-vessels
lies on the roof and grows in between its folds, giving rise to a
choroid plexus similar to that of the fourth ventricle, but more
restricted in its extent. The vascular plexus on the surface
of the thalamencephalon forms also a dorsally projecting process,
the supra-plexus, with which the distal end of the stalk of the
pineal body is in very close relation (Fig. 89, pn).

The infundibulum (Figs. 64, 65, in) is a depression of the
floor of the thalamencephalon, with which the pituitary body
comes into close relation at an early stage. The infundibulum
is already recognisable at the time of closure of the neural tube :
its hinder wall is in close relation with the anterior end of the
notochord, and it is in fact the infundibular depression (Fig. 61)
which causes the brain to appear to be bent round the end of
the notochord, one of the most striking features of cranial
flexure. The infundibulum is separated from the mid-brain
(Figs. 61, 65) by a deep transverse groove, running across the
ventral surface of the brain, and very conspicuous when the
brain is seen from below. On the appearance of the optic
vesicles, a second transverse groove (Fig. 64) is formed, further
forwards, between the optic vesicles and the infundibulum. The
infundibulum retains the same character and relations through-
out all the later stages of development : it appears as a wide
thin-walled sac, forming a conspicuous projection on the under
surface of the brain, and having the pituitary body in close
relation with it posteriorly (Fig. 89, in).

The pituitary body, although not really a part of the brain,
may conveniently be described here. It arises (Figs. 60, r, and
61, pt) as a plug-like ingrowth of the deeper or nervous layer
of the epiblast, immediately below the anterior end of the brain.
It appears very early, and may be recognised as a slight thicken-
ing of the epiblast even before the neural tube is closed ; and at

THE BRAIN. 125

the time of completion of the tube (Fig. GO) it projects inwards
as a small, solid, tongue-like process beneath the brain, between
this and the dorsal wall of the pharynx.

The projection continues its growth inwards, and expands at
its end into a somewhat flattened mass of cells, which lies imme-
diately beneath the infundibtilum, close to the anterior end of the
uotochord, and which becomes the pituitary body itself (Fig. 64,
it); the rest of the process forms a slender stalk, which connects
the pituitary body with the surface epiblast. About the time of
opening of the mouth, the pituitary body becomes hollow and
separates from the stalk, which atrophies and soon disappears
completely. The pituitary body (Fig. 65, pt), which is now all
that remains of the original ingrowth, acquires close relations
with the hinder end of the infundibulum, which it retains through-
out life (Fig. 63, PB). It becomes partially divided into anterior
and posterior portions, of which the latter forms a complicated
mass of convoluted tubes.

The optic vesicles. From the sides of the fore-brain, about
the time that closure of the neural tube is effected, a pair of
hollow lateral outgrowths, the optic vesicles, arise. Each optic
vesicle soon becomes constricted at its base, so as to form a
bulb, opening by a tubular stalk into the fore-brain. From the
bulbs, the eyes are developed in a manner that will be described
later on ; while the optic stalks form paths along which the fibres
of the optic nerves pass from the eyes to the brain.

About the time of opening of the mouth (Fig. 64), a trans-
verse groove runs across the floor of the fore-brain, in front of the
infundibulum, in, and between this and the vesicle of the hemi-
spheres, bc. This groove is bounded in front and behind by
transverse ridges, and is produced outwards at its two ends
into the tubular optic stalks. At a slightly later stage, about the
time of appearance of the hind limbs (Fig. G5), the stalks be-
come solid along their whole length ; the further changes in
connection with them will be described in the section dealing
with the development of the eye.

The cerebral hemispheres. The hemispheres, although the
largest part of the adult brain (Figs. 62, 63), are' the last to
appear. About the time of hatching of the tadpole, the anterior

12G THE FROG.

end of the fore-brain begins to grow forwards as a median thin-
walled vesicle of the hemispheres ; this steadily increases in size,
but, up to the time of the formation of the mouth, remains un-
divided. At this stage (Fig. 64, bc), it is approximately
spherical, and about equal in size to the mid-brain: its roof and
anterior wall are both extremely thin, but its side walls are much
thickened, so that the central cavity is compressed laterally.

The vesicle continues to increase in size, but remains single
and undivided up to a stage slightly later than that shown in
Fig. 65, when a division between the two hemispheres appears.
This is effected by the roof and anterior wall becoming folded ver-
tically along the median plane ; the fold, which is continuous pos-
teriorly with the choroid plexus of the third ventricle, projects into
the cavity of the vesicle, and partially divides this into right and
left halves, which become the lateral ventricles of the hemispheres.

By further growth forwards of the hemispheres, with thick-
ening of their walls, the proportions of the adult brain are
gradually acquired ; the brain at the time of the metamorphosis
being practically identical with that of' the fully formed frog.
The anterior or distal extremities of the hemispheres become the
olfactory lobes ; these are at first separate from each other, but
ultimately become fused together along their inner surfaces.

3. The Development of the Peripheral Nervous System.

It will be convenient to give first a general description of the
early stages of development of the peripheral nervous system of
the frog, and then to deal separately with the cranial and the
spinal nerves in regard to the later phases of their formation.
There are still many points on which our knowledge of the
development of the nervous system in the frog is imperfect
and unsatisfactory.

The early stages of development of the nerves. The dorsal
roots of the spinal nerves, and the majority of the cranial nerves,
arise in closely similar manner, and at a very early period. The
first commencements are seen in embryos which are still almost
spherical, and in which the neural plate and neural ridges are
just commencing to form, but have not yet begun to fold in to
inclose the neural tube (Fig. 57).

The neural plate is formed, as described above, by thickening

thi

THE PERIPHERAL NERVOUS SYSTEM. 127

of the deeper or nervous layer of the epiblast along the dorsal
surface of the embryo. Along the sides of the neural plate, where
it passes into the unmodified epiblast of the body wall, and on
its inner or deeper surface, opposite to the commencing neural
ridges, a pair of longitudinal bands of epiblast appear. These
are at first merely the lateral edges of the neural plate, but they
soon become separated by lines of demarcation from the neural
plate, and rather later from the epiblast along their outer sides.
In transverse sections they appear as a pair of small triangular
wedges, at the sides of the neural plate, continuous with the
epiblast above, but separated by divisional planes, often indis-
tinctly marked, from the neural plate on the inner side and
the general epiblast on the outer side (c/. Fig. 59, xd).

These bands of epiblast cells, cut out from the inner or
nervous layer of the epiblast, are at first continuous structures,
extending the whole length of the embryo. They are spoken of
as the neural ridges, and from them the dorsal roots of the
spinal nerves, and the majority of the cranial nerves, are derived.

As the neural folds grow upwards to inclose the neural tube,
the neural ridges get carried up with them ; and at a time when
the lips of the tube are about to unite, the neural ridges form a
pair of longitudinal bands (Fig. 59, nd), projecting outwards on
either side from the angle between the external epiblast, EE,
and the wall of the neural tube, NS.

On the closure of the neural tube, the neural ridges separate
completely from the external epiblast, and the ridges of the two
sides become continuous with each other across the median plane,
forming a plate of cells, the neural crest, attached to the dorsal
surface of the brain and spinal cord (Fig. 70, nd).

As the several divisions of the brain are formed, the origi-
nally continuous neural ridge of each side becomes discontinuous,
growing outwards so as to become more prominent at certain
parts of its length, and disappearing in the intervening regions ;
it thus becomes broken up into a series of outgrowths, which are
the rudiments of the nerves.

The neural crest, formed by the fusion of the two neural
dges, and therefore also the nerves into which the crest becomes
t up, are at first connected with the dorsal surface of the
neural tube. The permanent attachments of the nerves to the
sides of the neural tube are acquired at a later stage, by

128 THE FROGh

the growl li of processes from the cells of the nerves into the
substance of the brain, or spinal cord.

Up to this point the development of the cranial and the
spinal nerves is practically the same; the cranial nerves appear
at an earlier stage in the formation of the neural tube than do
the spinal nerves, and are from the first of much larger size
than these latter, but the history of the early stages is essentially
the same in the two cases.

The spinal nerves. After reaching the neural crest stage,
the development of the spinal nerves proceeds for a time very
slowly. The nerve rudiments, after a rather long pause, grow
slowly down between the myotomes and the sides of the spinal
cord. The permanent attachment to the side of the cord is
acquired in the manner described above, by growth of nerve
fibres from the nerve rudiment, or ganglion, into the cord. The
ganglion itself enlarges, and the nerve fibres continue their
course beyond it to form the trunk of the dorsal or sensory root
of the spinal nerve.

The ventral or motor root arises quite independently : the
details of its development have not been determined so accu-
rately in the frog as in other animals, but each ventral root
appears to arise as a number of outgrowths from the lower part
of the side of the spinal cord, which from the first occupy their
permanent positions in regard to the cord, and which very early
become connected distally with the muscles of the body.

The dorsal and ventral roots of each nerve lie close along-
side each other, and become bound together by a common con-
nective-tissue sheath to form the trunk of the spinal nerve, from
which branches soon arise supplying the various parts to which
the nerve is distributed in the adult (cf. Figs. 87 and 88).

The cranial nerves. The development of the cranial nerves
of the frog has not been very thoroughly studied ; and there are
several points on which our knowledge is still in an unsatis-
factory condition. The nerves which are undoubtedly derived
from the neural ridges are the trigeminal, the facial and auditory,
and the sensory branches of the glosso-pharyngeal and pneumo-
gastric ; i.e. the fifth, seventh, eighth, ninth, and tenth cranial
nerves according to the ordinary nomenclature. The olfactory

THE SPINAL AND CEANIAL NEE YES. 129

nerve is perhaps to be added to these. The optic nerve develops
in a very special manner ; and the mode of development of the
third, fourth, and sixth nerves in the frog has not yet been
determined with accuracy.

The trigeminal, facial, glosso-pharyngeal, and pneumogastric
nerves, although arising from the neural ridges in the same way

the dorsal roots of the spinal nerves, yet differ from these, and

je amongst themselves, in certain important features, of
diich the following are the principal : —

i. The nerves in question, in place of growing downwards,
:e the spinal nerves, alongside the central nervous system,
*ow outwards, close to the surface of the embryo, between the
)iblast and the mesoblast.

ii. Each of these four nerves acquires a new connection with
Le surface epiblast some considerable distance beyond the root
origin from the brain, and at about the horizontal level of the
lotochord ; at this place, and at any rate in part from the surface
epiblast itself, the ganglion of the nerve is formed.

iii. The nerves have special relations to the gill-slits, each
nerve dividing into two main branches, which embrace between
them one of the gill-slits.

iv. A special system of cutaneous nerves is developed from
the surface epiblast in connection with these four nerves, form-
ing the lateral line system of nerves.

In dealing with the several cranial nerves individually it will
be convenient to consider them in order from behind forwards.

X. The pneumogastric, vagus, or tenth cranial nerve. This
grows rapidly in the early stages, and soon attains an enormous
size. In embryos of about 3 mm. length (cf. Figs. 58, C, and
59), when the neural folds have not quite met in the hinder part
of the head, and the neural groove is, therefore, still open, the
pneumogastric nerves are already present as a pair of wing-
like expansions of the neural ridges. The root of attachment
of the nerve, in the re-entering angle at the top of the brain,
between the epiblast and the brain wall, is slender ; but the rest of
the nerve is of great thickness. It extends more than half way
down the side of the pharynx, lying between the mesoblast and
the surface epiblast, very close to the latter but distinct from
it along its entire length (cf. Fig. 79, x). The nerves of the
two sides are in some cases unequally developed at this stage.

E

130 THE FROG.

In embryos of about 4 mm. length (cf. Fig. Gl), the nerves
themselves have undergone but little further change. At the
level of the notochord the external epiblast presents, on each
side, a very distinct and localised thickening of its inner or
nervous layer. This thickening projects inwards, and lies very
close to the pneumogastric nerve, a little below the middle of
its length, but as yet the two structures are independent. The
thickening is well marked, and extends horizontally backwards
a little distance beyond the nerve.

At the time of hatching, i.e. in tadpoles of about 7 mm.
length (cf, Figs. 69, 72, and 73), the epiblastic thickening and
the nerve have fused, and together form the ganglion of the
pneumogastric : the horizontal extension backwards of the
thickening, which forms the lateral line nerve, has grown enor-
mously, reaching now almost to the hinder end of the body of the
tadpole. The mode in which this lateral line nerve grows has not
been determined with certainty in the frog ; at its first appear-
ance it is clearly a ridge-like thickening of the inner surface of
the epiblast, but it is difficult to decide whether the extension
backwards, which is effected with great rapidity, is due to a
splitting off from the epiblast, or to growth backwards of a solid
rod of cells from the ganglion of the pneumogastric. Such
evidence as is forthcoming rather favours the latter view. In
transverse sections at this stage (Fig. 82, nl), the lateral line
nerve has the appearance of a solid rod of cells, lying in a groove
along the inner surface of the epiblast, at the level of the lower
part of the spinal cord. The lateral line nerve is of large size
throughout the whole period of tadpole life ; it is present during
the metamorphosis, but disappears completely at its close.
During the later tadpole stages it separates from the skin, and
becomes more deeply placed among the muscles of the body
wall. Besides the main lateral line nerve described above,
other similar cutaneous branches are formed in connection with
the pneumogastric ganglion ; a more slender nerve is developed
nearer the mid-dorsal line ; and a stout nerve runs at first
ventralwards from the ganglion, and then backwards along the
sides of the ventral surface of the abdominal region.

Concerning the further development of the pneumogastric
nerve itself there are some points of interest. The root of
attachment to the brain, which is acquired in the same manner

i

THE CRANIAL NERVES. 131

as that of the dorsal root of a spinal nerve, is from the first of

considerable length horizontally. About the time of opening of

the mouth the root divides into two ; an anterior one, which runs

nearly straight outwards from the brain ; and a posterior one,

which runs very obliquely forwards, to join the anterior root just

before it reaches the ganglion. The ganglion lies immediately

behind the ear; beyond it the nerve divides into a set of

ranches which supply the three hinder branchial clefts, and

set of visceral branches, which run to the heart and alimen-

ry canal. All these branches are well established by the time

he mouth of the tadpole is formed.

IX. The glosso-pharyngeal, or ninth cranial nerve, is formed

om the part of the neural ridge immediately in front of that

om which the pneumogastric nerve is developed, the roots of

he two nerves being at first continuous with each other. The

erve is very similar to the pneumogastric, but of smaller size :

the upper part of its course it lies immediately in front of the

neumogastric ; it then runs forwards and outwards, round the

inder border of the auditory vesicle, to the upper edge of the

rst branchial cleft, where it expands to form the ganglion.

The ganglion, like that of the pneumogastric, is formed in part

from an independently arising thickening of the external epi-

blast, which fuses with the nerve rudiment about the time of

hatching of the tadpole. The ganglion of the glosso-pharyngeal

nerve is separated from that of the pneumogastric by the anterior

cardinal vein. Beyond the ganglion the glosso-pharyngeal

runs downwards, as a slender nerve, along the anterior edge of

the first branchial arch, giving a small prasbranchial branch to

the hyoid arch. All the main branches are present at the time

of opening of the mouth of the tadpole.

VIII. The auditory, or eighth nerve, arises from the neural
crest, in common with the seventh nerve, opposite the middle of
the auditory vesicle ; the two nerves being absolutely continuous
with each other up to the time of formation of the mouth.
The auditory portion of the combined nerve forms a large
ganglionic swelling, which is continuous with the inner wall of
he auditory vesicle from its very earliest appearance. In the
later stages, as the various parts of the ear become differentiated,
the auditory nerve divides into separate branches supplying its
several parts. (Of. Fig. 75, vnL)

K 2

132 THE FROG.

VII. The facial, or seventh nerve, as noticed above, is, in its
early stages, continuous with the auditory nerve. Beyond the
auditory vesicle the facial nerve runs downwards and forwards,
close to the surface epiblast. Shortly before the time of hatching
of the tadpole, the nerve becomes connected with an ingrowth of
the epiblast at the level of the upper border of the notochord,
and at this place the ganglion is formed. Beyond the ganglion
the nerve divides into three principal branches : (i) a small
cutaneous branch which appears to develop in connection with
the epiblastic ingrowth, and to belong to the lateral line series
of nerves ; (ii) a stout post-branchial branch, which runs down-
wards and forwards along the hyoid arch, close to its surface ;
and (iii) a small palatine nerve, which runs forwards in the roof
of the pharynx, not far from the median plane.

VI. The development of the sixth, or abducent nerve, has
not been determined in the frog. From its general relations, and
from what is known concerning its mode of formation in other
animals, it is probably comparable to a ventral root of a spinal
nerve.

V. The trigeminal, or fifth cranial nerve, is the largest of the
whole series ; it lies immediately in front of the facial nerve,
with which it is in close relation from the first.

The trigeminal nerve, like the pneumogastric, early attains a
large size, and in 4 mm. tadpoles (cf. Figs. 58, E, and 61) extends
half way down the side of the pharynx. At or shortly before
this stage, a thickening of the external epiblast occurs at the
level of the upper border of the notochord, immediately behind
the eye, and in front of the auditory vesicle ; this meets and fuses
with the nerve, the two together forming the ganglion. The
thickening of the epiblast extends forwards a short distance in
front of the ganglion, and gives rise to a cutaneous nerve,
similar to the lateral line nerve formed in connection with the
pneumogastric nerve. Shortly after hatching of the tadpole, the
ganglion of the trigeminal nerve recedes somewhat from the
surface, and becomes more deeply placed, though still remaining
connected with the surface by the cutaneous branch.

Before the hatching of the tadpole, the trigeminal nerve
divides distally into ophthalmic and mandibular branches, of
which the former runs horizontally forwards, and the latter down-
wards and forwards, the eye lying in the fork between the two.

i

THE CRANIAL NERVES. 133

At the time of opening of the mouth, in tadpoles of about
9 mm. length, the condition of the trigeminal nerve is as
follows : — The nerve arises on each side, by a single root, from
the side of the medulla oblongata ; and, running downwards and
forwards, expands to form the Gasserian ganglion, which lies
midway between the eye and ear, and immediately in front of
the ganglion of the facial nerve. From the ganglion three
branches arise : (i) a small but well-marked cutaneous branch
runs directly outwards, behind the eye, to the skin, along which
it continues forwards for a short distance, (ii) A large ophthalmic
branch runs horizontally forwards between the eye and the
brain, parallel to the outer margin of the brain, and dorsal to
the optic nerve and to the nose ; in front of the nose it turns
slightly downwards, and ends in branches supplying the skin of
the snout : the hindmost or proximal part of the ophthalmic
nerve is very thick and ganglionic, the distal part is thin, (iii) A
very thick mandibular branch, which is also ganglionic at its
proximal end, runs downwards and forwards below the eye,
close to the ganglion of the facial nerve, but separated from
this by the anterior cardinal vein ; it runs through the jaw
muscles, and ends in the floor of the mouth. From the mandi-
bular branch a slender maxillary branch runs forwards, beneath
the eye, and along the upper jaw to the anterior end of the
head, where it ends in the skin of the upper lip.

IV. The mode of development of the fourth cranial nerve ol
the frog has not been determined.

III. The third cranial nerve is also very imperfectly known.
Its early development has not yet been ascertained. At the
time of opening of the mouth, in tadpoles of about 9 mm.
length, it is present as a slender nerve, arising from the lower part
of the side of the mid-brain, not far from the median plane, and
having already the course and relations of the nerve in the adult.

II. The optic, or second cranial nerve, will be best dealt with
in the description of the development of the eye. The nerve-
fibres arise in connection with the retina, and grow inwards
I along the optic stalk to the brain.
I. The olfactory, or first cranial nerve. The early stages in
the development of the olfactory nerve in the frog have not
been seen ; there are reasons for suspecting that it is developed in
—

134 THE FROG.

also in part from the anterior end of the neural ridge. The
nerve develops early, and is recognisable before the hatching
of the tadpole as a short thick trunk, connecting the side
of the brain with the thickened epithelium of the olfactory
pit.

The nerve remains short, up to the time of opening of the
mouth, or rather up to the time when the cerebral hemispheres
begin to grow forwards. This anterior growth of the cerebral
hemispheres is accompanied, as already noticed, by still more
rapid growth of the anterior part of the head, in consequence of
which the olfactory pits are carried forwards from their original
position at the sides of the brain, and become situated in front
of it. This causes lengthening of the olfactory nerves, and a
change in their direction ; in place of running outwards from
the brain, they now run almost directly forwards : the roots of
the nerves, however, still arise from the ventral surface of the
brain, some distance from its anterior end, as in the adult.

The sympathetic nervous system. The sympathetic system
develops as a series of outgrowths from certain of the cranial, and
from all the spinal nerves. These develop ganglionic swellings,,
which, in the body region (Figs. 84 and 87, NY), lie beneath the
notochord, and alongside the dorsal aorta. At an early stage,
shortly after the formation of the mouth, the several ganglia of
each side become connected together by a longitudinal nerve-
cord, but whether this cord arises independently of the ganglia,
or, as is more probable, by the formation of outgrowths from the
ganglia, has not been definitely determined.

DEVELOPMENT OF THE SENSE ORGANS.

The organs of special sensation, like the nervous system
itself, are developed from the deeper or nervous layer of the
epiblast, and are continuous with their respective nerves from a
very early stage in their formation. The derivation of the
sense organs from the epiblast is explained by the fact that
they are concerned in the appreciation of the presence and
nature of external objects, and are therefore necessarily formed
on the surface of the body. They are in all cases to be regarded
as specially modified parts of the epidermis.

THE SENSE ORGANS.

135

1 . The Nose.

The olfactory organs appear at a very early period of develop-

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Fig. 66. — Diagrammatic horizontal section of a 12 mm. Tadpole, at the time of
appearance of the hind limbs. The plane of section of the right side of
the head is taken at a more dorsal level than that of the left side, x 30.

A, aorta. AC, carotid artery. ACE, pharyngeal artery. ACI, anterior cerebral
artery. AP, pulmonary artery/ AT. cutaneous artery. EF.l, EF.2, EF.3, EF-4,
efferent branchial vessels of first, second, third, and fourth branchial arches. GM,
glomerulus. HC1, first branchial cleft. KA, right segmental duct. KA', hinder
end of left segmental duct. KP, head kidney. KS, nephrostome. LC, laryngeal
chamber. LGr, lung. OC, optic cup. OP, o'lfactory pit. OL, lens. PA, pancreas.
PD, pancreatic duct. TD, duodenum. TI, intestine.

ment, about the time of closure of the brain, as a pair of thicken-
ings of the nervous layer of the epiblast at the anterior end of

136 THE FROG-.

the head, lying at the sides of the fore-brain, and in front of, and
slightly dorsal to, the position in which the mouth will after-
wards appear.

The two layers of epiblast soon lose their distinctness in these
patches ; and a pitting in of the surface, involving both layers,
appears in each of the patches. The pits so formed become the
nasal sacs ; the mouths of the pits forming the nostrils or
anterior nares, and the epiblastic lining of the pits becoming
converted into the olfactory epithelium. The condition of these
pits at the time of hatching of the tadpole is shown from the
surface in Fig. 72, oc, and in horizontal section in Fig. 74, of.

The olfactory pits rapidly deepen (Fig. 66, of), rather by
the upgrowth of folds of skin round their margins than by
depression of the floors of the pits themselves ; the result of
this process being the formation of a pair of deep pits, of which
the inner walls are derived from the original patches of olfactory
epithelium.

A short time after hatching of the tadpole, a solid rod of
epithelial cells is formed by proliferation of the cells of the
floor of each olfactory pit. These rods of cells grow downwards
and inwards towards the roof of the pharynx, meeting and fusing
with this immediately behind the septum between the pharynx
and the stomatodaaum (Fig. 64). Shortly after the mouth open-
ing is established, by perforation of this septum, these rods of
cells become tubular ; and in tadpoles of 12 mm. length, in which
the hind limbs are just appearing, the tubes open into the roof
of the mouth as the posterior nares (Fig. 76, zi).

By further folding of the walls, and by the formation of
caecal outgrowths from each sac, the complicated olfactory laby-
rinth of the adult is developed. A special diverticulum of the
ventral wall of each sac gives rise to the organ of Jacobson.

2. The Eye.

The eye differs from the other sense organs inasmuch as an
accessory part, the lens, is alone formed from the surface
epiblast ; while the sensitive part of the eye, or retina, arises as
an outgrowth from the brain, and thus is only indirectly derived
from the epidermis.

The optic vesicles have already (p. 125) been described as a
pair of hollow outgrowths, which arise from the fore-brain about

THE NOSE AND EYE.

137

ie time that closure of the neural tube is effected ; they project

it wards at right angles to the axis of the head, their outer

rails being in close contact with the epidermis of the sides of

Le head. Each of the vesicles becomes constricted at its base, so

to form a spherical optic bulb, connected with the fore-brain

a hollow tubular stalk. The outer wall of the bulb, which

in contact with the external epidermis, soon becomes flattened,

id then thickens so greatly as almost to obliterate the cavity

the vesicle (Fig. 67, oc).

The lens. About this time a thickening of the inner, or
irvous, layer of the surface epiblast takes place opposite to the

Fig. 67. — Transverse section through the head of a Tadpole of 6£ mm. length,
about the time of hatching : the section passing through the fore-brain
and the developing eyes, x 45.

AC, carotid artery. BP, fore-brain. DS, stomatodasal invagination. NL, cuta-
neous or lateral line branch of the trigeminal nerve. OC, inner wall of optic cup.
OD, outer wall of optic cup. OL, lens. OS, optic stalk. PT, pituitary body.
TP, pharynx. VJ, jugular vein.

centre of each optic vesicle ; this thickening increases rapidly,
and at the time of hatching of the tadpole forms a solid
spherical body projecting inwards from the surface ; this soon
becomes hollow, by breaking down of the cells in its centre, and
then separates from the surface epiblast. It may now be spoken
of as the lens vesicle (Fig. 66, ol) ; in the later stages, after
the formation of the mouth opening, the lens vesicle becomes
solid once more (Fig. 67, ol), mainly through lengthening of
the cells of its inner wall ; and by further increase in size it
becomes the lens of the adult eye.

138 THE FROG.

The optic cup. Partly in consequence of the ingrowth of the
lens vesicle, but mainly through active growth of the walls of
the optic vesicle itself, this latter becomes pitted on its outer
surface, and so converted into a cup (Figs. 66, 67). This optic
cup, as it is termed, has double walls : the inner wall (Figs. 66
and 67, oc) is very thick, and consists of cells arranged three or
four deep ; the outer wall (Fig. 67, od) is thin, and consists of
a single layer of flattened cells, in which pigment is early deve-
loped. (Gf. Fig. 76, oc.)

In the later stages of tadpole life the optic cup slowly
enlarges ; it remains in contact with the lens at its edge or lip,
but elsewhere is separated from this by a space, which becomes
the posterior chamber of the eye, and in which the vitreous body
is formed.

The inner, or thicker, wall of the optic cup gives rise to the
retina ; the molecular and nuclear layers, and the layers of nerve
cells and nerve fibres, being formed by modification of the cells
of the wall itself ; while the rods and cones of the bacillary layer
arise as outgrowths from its outer surface, which grow towards,
and become imbedded in, processes developed from the pig-
mented cells of the outer wall (Fig. 67, od) of the optic cup.

If the mode of development of the brain be called to mind, it
will be seen that the layer of epithelial cells which lines the
cavity, or ventricle, of the fore-brain (Fig. 67, bf) is morpho-
logically equivalent to the outer or epidermic layer of the
surface epiblast, and was originally directly continuous with
this, before closure of the neural groove was effected. As the
optic vesicle is an outgrowth from the fore-brain, the cells lining
its cavity, i.e. the cells lining the space between the inner, oc,
and outer, od, walls of the optic cup, will be of the same nature
as those lining the cavity of the fore-brain itself.

The optic nerve. The fibres of the optic nerve are developed
on the inner surface of the inner layer of the optic cup, i.e.
the surface next to the vitreous body, and grow inwards along
the optic stalk to the brain. It follows from what has been said
in the previous paragraph that this inner surface of the optic
cup is morphologically equivalent to the deeper or nervous layer
of the epidermis, from which we have seen that all the other
nerves are developed, directly or indirectly.

Up to the time of hatching there is no trace of the optic

THE EYE AND EAB. 1S9

nerve-fibres ; but shortly after this period (Fig. 66) certain of
the epithelial cells at the inner surface of the optic cup become
pyriform in shape, forming what are termed neuroblasts. From
the narrower ends of the neuroblasts, nerve-fibres grow out which
spread over the ventral edge of the optic cup, and grow back as
a bundle of nerve-fibres along the ventral and posterior wall of
the optic stalk, and towards the brain. The optic stalk itself ap-
parently takes no direct part in the formation of the nerve-fibres ;
its cavity becomes obliterated shortly after the mouth opening
is established, except at the end next the brain, where the
cavity persists, as the optic recess, throughout life. The rest of
the stalk gradually becomes broken up, as the distance between
the brain and the eye increases with growth of the tadpole.

The optic fibres reach the under surface of the brain shortly
after the mouth opens, and cross over almost at once to the
opposite side of the brain to form the optic chiasma.

The outer coats of the eye, choroid, sclerotic, and cornea, are
formed from the mesoblast surrounding the optic cup.

The eye develops very slowly, and during the greater part
of the tadpole stage of existence is in an imperfect condition ; at
the time of the metamorphosis it moves nearer to the surface,
and becomes a functionally more perfect organ.

3. The Ear.

General account. The ears are developed as a pair of pit-
like invaginations of the deeper or nervous layer of the epiblast,
at the sides of the hind-brain. The invaginations do not involve
the epidermic or outer layer of the epiblast, which is continued
across the mouths of the pits. The auditory pits, therefore, do
not, in the frog, open at any time to the exterior.

The mouths of the pits very early narrow and close, and the

auditory vesicles so formed separate completely from the epiblast,

and lie imbedded in the mesoblast at the sides of the head. By

folding of its walls, and by the ingrowth of septa, the vesicle,

ram being a simple, almost spherical sac, becomes divided up

the complicated auditory vestibule of the adult.

The auditory nerve becomes connected with the inner wall
the vesicle at a very early stage, indeed almost from its first
appearance ; the relations of the nerve to the wall of the vesicle
being essentially similar to those between the other cranial

140 . THE FROG.

nerves and the special patches of epiblast with which they
become fused.

Certain of the accessory organs of hearing, especially the
Eustachian tube and the tympanic cavity, may conveniently be
described here, although they are essentially independent of
the auditory apparatus, and only become secondarily connected
with this.

The early development of the ear. About the time of closure
of the neural groove, the auditory epithelium can be recognised
as a pair of thickened circular patches of the deeper layer of
epiblast, one at each side of the hind-brain, with which patches
the auditory nerves are already continuous.

Soon after closure of the neural tube, in embryos of about
3 mm. length (Fig. 60), each of these patches becomes depressed,
forming a shallow pit, semicircular in transverse section, and
covered at its mouth by the outer layer of epiblast, which is
continued over it without interruption. The pit deepens, and
the mouth gradually closes by ingrowth of its lips. Shortly
before the hatching of the tadpole the closure is completed, and
the auditory vesicle separates from the surface epiblast.

At the time of its separation the vesicle is a closed sac, some-
what pyriform in shape ; its lower or ventral portion being
spherical, and lying opposite the notochord, and its dorsal wall
being prolonged upwards into a short blind diverticulum lying
at the side of the hind-brain. The wall of the vesicle consists
of a single layer of cubical or columnar cells ; those of the
inner wall, with which the auditory nerve is continuous, being
rather more elongated and more deeply pigmented than the rest.

The internal ear or labyrinth. After closure of its mouth the
vesicle increases considerably in size, and becomes further sepa-
rated from the surface by ingrowth of mesoblast between its outer
wall and the external epiblast. Up to the time of the formation
of the mouth it undergoes no further change of importance,
remaining as a spherical sac with a blind dorsal diverticulum.

Shortly after the opening of the mouth, i.e. in tadpoles of
from 10 to 12 mm. in length, the various parts of the internal
ear become gradually differentiated, the chief process by which
the changes are brought about being the formation of septa, by
folding of the wall of the vesicle, which project inwards into
the cavity and partially subdivide it. Mesoblast soon grows in

THE EAE. 141

between the two layers of each fold, the septa thereby acquiring
increased thickness.

The first septum which appears divides the vesicle into its
two main cavities, sacculus and utriculus. It arises in tadpoles
of about 11 mm. length as a fold of the outer wall of the vesicle,
which projects somewhat obliquely across the cavity, dividing it
into an upper and inner division, the utriculus ; and a lower
and outer portion, the sacculus. The septum is at first confined
to the hinder part of the vesicle, but soon extends all round it ;
and, growing inwards, separates the two divisions almost com-
pletely from each other, a very small aperture of communication
alone persisting between them.

From the utriculus, the semicircular canals are formed. Each
canal is really a portion of the utriculus, which becomes partially
shut off from the main cavity by the formation of a septum
along the middle portion of its length ; remaining, however, in
communication with the cavity at each end. Each septum is
formed by two separate folds, which grow towards each other
from opposite sides of the vesicle, meet along their edges, and
fuse to complete the septum (Fig. 75, p. 162). The septum
soon thickens, through the ingrowth of mesoblast between its
layers ; it also elongates, and so causes lengthening of the canal,
which gradually acquires the adult shape and relations.

Of the three semicircular canals, the anterior vertical and
the horizontal are formed simultaneously, and first appear in
tadpoles of about 11 mm. length. The posterior vertical canal
arises in the same way, but at a slightly later stage, in tadpoles
of about 15 mm. length.

The ampullas of the semicircular canals are formed later than
the canals themselves, not as dilatations of the canals, but by
constriction of parts of the utriculus, at the places where the
canals open into it.

The second division of the vesicle, or sacculus, grows down-
wards, and soon acquires the pouch-like character it has in the
adult. From its upper and hinder portions three small bulgings
or pouch-like outgrowths appear, which together form the
cochlea. Of these, the lagena cochleae is the largest and the
earliest to appear, arising in tadpoles of about 15 mm. length;
the pars neglecta appears shortly afterwards, and the pars
basilaris last of all.

142 THE FROG.

The inner wall of the auditory vesicle, facing the brain, is
from the first composed of cells which are more columnar in
shape than those of the rest of the vesicle (Fig. 75, ev) ; and
it is with these elongated cells that the auditory nerve is
connected. As the vesicle grows, and as the septa form, by
which it is divided up into its various portions, the patch of
epithelium with which the nerve is continuous also divides,
giving rise to all the sensory patches present in the adult ear.
Of these there are eight : — one in each of the three ampullao of
the semicircular canals, three in the cochlea, one in the wall of
the sacculus, and one in that of the utriculus.

The dorsally directed diverticulum, to which the pyriform
shape of the vesicle in its early stages is due, persists in the
adult, and undergoes a rather remarkable development.

On the formation of the septum, dividing the vesicle into
sacculus and utriculus, the diverticulum remains in connection
with the inner side of the sacculus. It elongates considerably,
growing upwards close alongside the brain as the recessus
vestibuli (Fig. 75, er). In tadpoles of about 20 mm. length,
the distal blind end of the recessus vestibuli dilates to form a
thin-walled vesicle, lying on the roof of the fourth ventricle;
while the rest of its length forms a narrow tubular duct with
rather thick walls, which connects the dilated end with the
sacculus.

At the time of the metamorphosis the distal thin-walled
dilatation, or saccus endolymphaticus, has increased greatly;
it lies within the skull, between this and the brain, as a large
sac with thin but very vascular walls, covering the roof of the
hind-brain for a considerable length, and extending downwards
along the sides of the brain and beneath its floor as well. The
sacs of the two sides meet, both above and below the brain, and
apparently open into each other; in their cavities abundant
calcareous concretions are found.

The stalk, or ductus endolymphaticus, persists as a narrow
tube, which passes through a hole in the skull wall, and connects
the saccus endolymphaticus with the sacculus of the internal
ear. These relations of the saccus and ductus endolymphaticus
are retained in the adult frog.

In the mesoblast surrounding the internal ear the perilymph
spaces are formed ; and beyond these the cartilaginous and

THE EAR. 143

osseous walls of the auditory capsule are laid down. (Of.
Figs. 75 and 68.)

The accessory auditory apparatus. It will be convenient to
consider here the development of the Eustachian tube, and the
tympanic cavity and membrane, which, though only secondary
parts of the organ of hearing, are exceedingly characteristic of

IMrestrial Vertebrates, as contrasted with the truly aquatic
rertebrates, or Fishes.
The Eustachian tube and tympanic cavity. The details of
evelopment of these parts are not thoroughly determined.

The Eustachian tube appears first in tadpoles of about
25 mm. length, as a solid rod of epithelial cells, running for-
wards from the anterior and dorsal edge of the first branchial
cleft.

At the time of the metamorphosis, when the fore legs are
protruded, the Eustachian tube is a rod of cells with a very ill-
defined lumen, starting from the dorsal and anterior part of the
pharynx, and extending straight forwards beneath the eye ; it
is slightly dilated at its distal end, which lies opposite the
anterior border of the eye.

During the metamorphosis, the Eustachian tube separates
from the pharynx, and divides into a variable number of short
lengths ; these gradually shift backwards to the position occu-
pied by the Eustachian tube in the adult frog ; by the time the
tail of the tadpole is completely absorbed, the several lengths
unite together, and with a diverticulum from the pharynx, to
form the definite Eustachian tube of the adult, which now runs
almost directly outwards beneath the ear. The tympanic cavity
is merely the dilated outer end of this tube, lying just beneath
the surface ; and the layer of skin closing its outer end is the
tympanic membrane. (Cf. Fig. 68, E, D.)

From this account it appears that the tympanic cavity does
not at any period open on the surface of the head ; and it is
doubtful whether the Eustachian tube in the frog has any
definite relation to a gill-cleft. It is very probable that in this,
as in many other features of its embryological history, the frog
Bhows a modified rather than a primitive type of development.

The tympanic cartilage. In tadpoles of about 10 mm.
length, shortly before the fore legs emerge, the tympanic carti-
lage appears as a dense mass of cells, surrounding the anterior

144

THE FROG.

end of the Eustachian tube at a time when this lies below the
eye. During the metamorphosis, this ring of cells preserves its
relation with the outer end of the Eustachian tube, or tympanic

Fig. 68.— A 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. On the right side the section
passes through the tympanic cavity and the columella ; on the left side
through the anterior cornu of the hyoid. The cartilage is dotted, and
the bones, except the columella, represented black.

A, parasphenoid. AS, angulosplenial. B, buccal cavity. C, columella. D. tym-
panic membrane. E, Eustachian tube. F, anterior cornu of the hyoid. FP, fronto-
parietal. Gr, glottis. H, arytenoid cartilage. I, posterior cornu of the hyoid. K.
auditory nerve. L, vestibule of the ear. M, anterior vertical semicircular canal.
N, horizontal semicircular canal. O. pro-otic. P. pterygoid. Q,, quadrate cartilage.
It, quadratojugal. S, squamosal. T, tympanic cartilage. V, vocal cord. X, mid-brain.

cavity, and gradually shifts back with this latter to its adult
position. A bar of cartilage appears in its ventral portion,
which gradually extends at its ends until it forms the complete
annular tympanic cartilage.

The development of the auditory ossicle, or columella (Fig.
68, c), will be described in the section dealing with the develop-
ment of the skull (p. 209).

4. The Cutaneous Sense Organs.

During the tadpole stage, while the animal is leading an
aquatic life, special sense organs in the form of small epidermal
papillae are present, arranged in rows along the body, round the
eyes, and on other parts of the head. They are supplied by the
lateral line series of branches of the trigeminal and pneumo-
gastric nerves, which have already been described (pp. 130, 132) ;
they are lost completely at, or shortly after, the time of the
metamorphosis.

The mouth of the tadpole is also provided with special

THE ExlR AND THE ALB1ENTAEY CANAL. 145

papillae, probably gustatory in function, which are lost at the
time of the transformation to the frog.

DEVELOPMENT OF THE ALIMENTARY CANAL.

1. General Account.

The alimentary canal of the frog, like that of other Verte-

Bates, is developed in three lengths : (i) the mesenteron (Fig.
>, t), which is formed, as already described, by a process of
litting amongst the yolk-cells, and which corresponds to the
mesenteron or gastrula cavity of Amphioxus : the mesenteron
the frog gives rise to almost the whole length of the ali-
entary canal, from the pharynx to the rectum ; and from it
e developed the gill-clefts, the thyroid, the thymus, the lungs,
e liver, the pancreas, and the bladder, (ii) The stomatodaeum
ig. 69, ds) is a pitting in at the anterior end of the body,
m which the mouth opening and buccal cavity are formed,
d in connection with which the lips and teeth are developed,
i) The proctodeum (Fig. 60, pd) is a pocket-like depression
at the hinder end of the body, which gives rise to the anal or
cloacal opening.

The mesenteron. The mode of development of the mesen-
teron, up to the stage shown in Fig. 55, has already been
described. At its first appearance, and throughout the early
stages, the mesenteron has walls of very unequal thickness ; the
roof or dorsal wall (Fig. 56) being thin ; and the floor or ventral
wall being of great thickness, owing to the large size of the yolk-
cells which form it.

After separation of the mesoblast cells as a distinct layer,
and the definite formation of the notochord, this difference
becomes still more marked, the roof of the mesenteron (Fig.
56, t) consisting of a single layer of hypoblast cells, while the
floor is formed by the thick mass of yolk-cells ; at the sides the
transition from the thin roof to the thick floor is a somewhat
abrupt one.

As the central nervous system is formed, and the shape of
the embryo becomes more clearly established, the mesenteron
acquires more definite characters (cf. Figs. 55, T; 60, mn). By
enlargement of its anterior end a wide pharyngeal cavity

146

THE FROG.

(Fig. 61, tp) is formed, of which the floor and sides, as well as
the roof, are formed of a single layer of hypoblast cells. The
hinder or intestinal region of the mesenteron (Fig. 61, ti), has
much the same relations as before, its roof being thin, but its

floor and sides (Fig. 70) of great thickness. The mass of yolk-
cells, forming the floor of the intestinal region, becomes more
compactly arranged and more definitely restricted ; in front it is

THE ALIMENTARY CANAL.

147

sharply marked off from the pharyngeal region by a backwardly
directed diverticulum (Fig. 60, l), which forms the first com-
mencement of the liver ; while at the hinder end of the body, by
withdrawal of the yolk-plug from the surface of the embryo (cf.
Figs. 55 and 60), the posterior limit of the yolk-mass becomes
clearly defined, and the short rectal diverticulum (Fig. 60, k)
opened out.

At the time of hatching of the tadpole (Figs. 69 and 74),
this distinction between a wide, thin-walled pharyngeal region,

Fig. 70. — Transverse section across the middle of the length of a Frog
embryo 3| mm. in length. (Cf. Figs. 58, D, and 60 for other views of
embryos of the same age.) x 52.

CH, notocliord. C J. subnotocliordal rod. CM. myocal. CS, splanchnocoel. E'
epiblast. KB, archinephric duct. M, mesoblast. MS, mesoblastic somite. WD, dorsal
root of spinal nerve. NS, spinal cord. SO, somatopleuric layer of mesoblast. SP,
splanchnopleuric layer of mesoblast T, intestinal region of 'mesenteron. Y, yolk-
cells.

and a narrow, thick-walled intestinal portion is very well
marked, the passage from one region to the other (Fig. 74)
being an abrupt one. Up to this time the alimentary canal has
been perfectly straight, but shortly after hatching, and especially
after the formation of the mouth, the intestinal region elongates
very rapidly ; the food-yolk is speedily absorbed, and the intestine
becomes a long tube, coiled in a characteristic spiral manner, and

148 THE FBOGh

of approximately uniform diameter along its whole length (Fig.
65). Owing to this rapid elongation, and the convolutions into
which it necessarily becomes thrown, the intestine, which al
first is closely attached to the dorsal wall of the body cavity.
immediately beneath the notochord, shifts ventral wards, re-
maining, however, suspended from the mid-dorsal wall of the
body cavity by the mesentery.

At the time of the metamorphosis the alimentary canal
shortens rapidly and very considerably ; and the distinction in
diameter between the stomach, small intestine, and large intes-
tine becomes much more pronounced. During these changes
the entire alimentary canal is in a condition of active inflam-
mation, and no food is taken, nutrition being effected by the
gradual absorption of the tadpole's tail.

The stomatodseum. At the time of hatching (Fig. 69, ds),
the stomatodseum is a well marked though shallow pit on the
under surface of the head ; its floor is in close relation with the
anterior wall of the pharynx, the epiblast of the stomatodseal pit
and the hypoblast of the pharyngeal wall being in contact with
each other, without any intervening mesoblast. From the
dorsal border of the stomatodaeum, the pituitary body (Fig. 69,
pt) projects inwards between the brain and the pharynx.

The stomatodseal pit rapidly deepens, not by depression of
its floor, but by uprising of its walls (Fig. 64), the margins of
which give rise to the lips. The septum between the stomato-
daeum and the pharynx gradually becomes thinner, and in tad-
poles of from 9 to 10 mm. length is perforated; the mouth
opening is thus established, and the pharynx placed in direct
communication with the exterior.

In the later stages the limits of the original stomatodseal
invagination can be fairly accurately determined. In the section
of a 12 mm. tadpole given in Fig. 65 the boundary is indicated
by a difference in the mode of shading employed ; the epiblastic
lining of the stomatodseum is represented by a thick black line,
while the hypoblastic wall of the pharynx is shown by a double,
•cross-hatched line. The posterior nares mark the boundary
between the two regions exactly ; they open (cf. Fig. 76, zi)
into the pharynx immediately behind the septum, so that a line
drawn across the roof of the mouth, through the anterior borders

THE STOMATODJEUM AND PROCTODEUM. 149

of the narial openings, divides the stomatoda?al from the pharyn-
geal portion.

After the mouth opening is established, the lips of the
stomatodaeum grow forwards rapidly, and in connection with
them the powerful horny jaws of the tadpole, by which it crops
its food, are speedily developed (Fig. C5, j).

The proctodeum. The mesenteron, from its first appear-
ance, and throughout the early stages of development, communi-
cates with the exterior through the blastopore (Fig. 60, b). It
also communicates, through the neurenteric canal, XC, with the
central canal of the spinal cord and brain ; this communication
persists for some time after the blastopore has closed (Fig. 61),
but is lost when the tail begins to lengthen (Fig. 69).

The proctoda?al invagination appears as a pit-like depression
at the ventral end of the primitive streak (Figs. 58, b, c, d,
and 60, pd). In embryos of about 4 mm. length (Fig. 61), this
invagination reaches and opens into the rectal portion of the
mesenteron, i.e. the portion which lies posterior to the mass of
yolk-cells.

The closure of the blastopore usually occurs before the anal
perforation is completed ; but it may happen that the two
openings into the mesenteron are present for a time simul-
taneously.

In the frog this proctodasal invagination is a new opening
into the mesenteron, and is not a persistent part of the original
communication of the mesenteron with the exterior, through
the blastopore. If it be borne in mind, however, that the
proctodaeal invagination appears in the primitive streak, and
as an actual deepening of the ventral end of the primitive
groove (Fig. 58, b) ; and further, that the primitive streak is
formed by concrescence of the lips of the blastopore, then the
formation of the proctodseal invagination may be viewed, not as
in entirely independent depression of the surface, but as a
^-opening of the ventral portion of the blastopore. This view
strongly supported by the development of other Amphibians,
some of which the blastopore actually persists as the anus.

It will be noticed that the proctodaeal, or anal, opening is
stablished some time before the embryo hatches, while the
;omatodaeal or mouth opening is not formed until a consider-

150 THE FROG.

ably later period. This early appearance of the proctodasal
opening is perhaps to be associated with the early formation of
the kidneys, which are already present, and have ducts opening
into the hinder end of the mesenteron (Figs. 69, ka, and 74, Kr,
ka), shortly before the time of hatching of the tadpole.

The development of the several regions of the alimentary
canal, and the structures arising in connection with them, will
now be described in more detail, with the exception of the gill-
clefts and gills, which form the subject of the next section of
this chapter.

2. The Lips.

The mouth of the tadpole is very small compared with that
of the frog (cf. Figs. 85 and 86, p. 193). It is surrounded by
prominent frill-like lips, which form a short conical proboscis
(Figs. 83 and 85, LI, lj). The inner surfaces of the lips bear
rows of minute teeth, and at the bottom of the funnel, sepa-
rating the proboscis, or labial cavity, from the buccal cavity, is
the beak, formed by the two powerful horny jaws (Fig. 65, j).

There are two lips, upper and lower, which are continuous
with each other at the angles of the mouth, so as to completely
surround the opening. The upper lip (Figs. 65 and 83, Li)
is a crescentic fold of integument bounding the labial cavity in
front ; it is smaller and less mobile than the lower lip, and
bears along its free edge a row of minute horny teeth. The
lower lip (Figs. 65 and 83, lj) is both longer and deeper than
the upper ; it is also softer and much more mobile. It is
separated behind by a well-marked transverse groove from the
under surface of the head, and is produced at its free edge into
a series of small fleshy papillas. These papillse, which are pro-
bably tactile in function, are more numerous at the angles of
the mouth, where they are arranged in groups.

The inner surfaces of the lips, between their free edges and
the beak, bear transverse ridges or folds, which support along
their crests comb-like rows of minute black horny teeth. Of
these rows, the upper lip, in addition to the row round the
margin already mentioned, has three incomplete rows, interrupted
in the middle by a considerable interval. The lower lip bears
four similar but complete rows of teeth.

Each of these teeth is formed by modification of a single

THE LIPS AND BEAK 151

epithelial cell. In shape it is a hollow cone, produced at its
apex into a spoon-shaped process, notched at its free edge.
These horny epithelial teeth are easily rubbed off during use,
and are speedily replaced by other similar ones formed beneath
them. Each tooth is in fact the top member of a column of
specially modified epithelial cells, imbedded in the general
epithelium of the lip. In each column the deepest cells are
ordinary epithelial cells, scarcely distinguishable from those in
which they are imbedded : the succeeding cells of the column,
nearer the surface, become first flattened, then cup-shaped, and
finally conical, the apex of the cone fitting into the cavity of
the cell next above it.

The deeper cells of the column are soft, and have distinct
nuclei ; nearer the surface the cells have their outer layers con-
verted into horny matter, while their shape gradually approaches
that of the fully formed teeth. The nucleus becomes less dis-
tinct, and finally disappears, as the cornification extends deeper
and deeper into the substance of the cell.

Each tooth is thus formed by cornification of a single epi-
thelial cell, which commences its career in the deeper layer of the
epidermis, at the base of the column, and gradually approaches
the surface through loss of the teeth above it, acquiring, as it
does so, the characters of the fully formed tooth. On reaching
the surface it comes into functional use for a time, and then in
its turn becomes rubbed off and lost.

3. The Beak.

The beak consists of the two jaws, upper and lower, and is
in shape not unlike that of a bird or turtle (Figs. 65 and 71).
Each jaw is a strong, curved band of cornified epithelium, sup-
ported at its base by the labial cartilages (Fig. 90, LU, ll), and
ending at its free surface in a sharp biting edge. The upper or
maxillary jaw (Fig. 65) is longer and less sharply curved ; the
lower or mandibular jaw, which bites behind the upper jaw, is
shorter, stronger, and almost horse-shoe shaped in outline.

The minute structure of the two jaws is the same, each con-
sisting of modified epithelial cells. The cutting edge of the jaw
is formed by a row of horny teeth, very similar to those of the
labial rows, but placed so closely side by side as to form a con-
tinuous blade. Each of these teeth is, as in the case of the labial

152 THE FROG.

teeth, the uppermost of a column of cells, the more deeply plar<<l
members of which are indifferent epithelial cells, but which as
they approach the surface become first flattened, then cupped, and
finally hollow cones fitting into one another. As in the labial
teeth, the hardness is due to cornification of the cells, invading
first the outer surface and ultimately the entire cell. As the
biting edge of the jaw gets worn away by use, it is constantly
renewed by the more deeply placed cells.

The rest of the jaw consists of a dense mass of flattened and
cornified epithelial cells, which become firmly fused together, and
which, like the cells of the cutting edge, are renewed from the
indifferent epithelial cells of the deeper layers. Into this deeper
layer vascular papillae of the dermis project, increasing the
extent of the nutritive surface of the jaw.

At the metamorphosis the horny jaws are cast off, and lost.

4. The Pharynx.

The characteristic feature of the pharynx, both in the tadpole
and in the adult frog, is its great width from side to side (cf.
Figs. 74 and 68) ; and this is acquired, as already described, at
a very early developmental stage. In horizontal section the
pharynx of the tadpole is somewhat lozenge-shaped (Fig. 74),
narrowing rather gradually in front to open into the buccal
cavity, and much more abruptly behind, where it passes back
into the oesophagus.

The roof of the pharynx may be divided into two regions :
an anterior part, clothed by a flattened pavement epithelium,
and bearing taste bulbs and sensory papillae ; and a posterior
part, covered by a ciliated epithelium, and containing numerous
multicellular glands.

The gills, which are the most important structures in con-
nection with the sides of the pharynx, will be described in the
next section (pp. 157 to 163).

The tongue is formed on the floor of the pharynx, but does
not appear until shortly before the metamorphosis; it then
grows rapidly and soon attains its adult shape and proportions
(Fig. 89, tn).

5. The Thyroid Body.

About the time of hatching of the tadpole, or a little earlier,

THE PHARYNX AND THYROID BODY. 153

a short median longitudinal groove appears along the floor of the
pharynx (Fig. 69, th). The groove is shallow anteriorly, but
deepens at its hinder end, where it leads into a small, conical, pit-
like depression of the hypoblast forming the pharyngeal floor,
just in front of the pericardial cavity (Fig. 69, CP.)

At a later stage, shortly before the opening of the mouth,
the median groove is still present. The pit at its hinder end has
deepened slightly, and the hypoblast cells, forming the floor of
the pit, have grown back as a solid rod of cells (Fig. 64, th),
closely connected at its hinder end with the anterior wall of the
pericardium ; this solid rod of cells becomes the thyroid body.

Soon after the mouth opens, the thyroid body separates com-
pletely from the floor of the pharynx, remaining as a solid
rounded mass of pigmented cells, in close contact with the
anterior wall of the pericardium. A little later, in tadpoles of
about 12 mm. length (Fig. 65, th), the thyroid body becomes
divided into right and left halves by the growth downwards of
a median keel from the basihyal cartilage (Fig. 65, hb). The
two halves remain connected by a narrow bridge of cells below
the cartilage for a short time, but soon separate and become the
paired thyroid bodies of the adult frog.

After their separation the thyroid bodies increase consider-
ably in size ; they are at first solid, but the component cells soon
become arranged in strings, which become hollowed out along
their axes, and so form a series of rounded or oval vesicles, which
communicate freely with one another, and are filled with fluid.

The thyroid bodies are very vascular ; they lie in the floor of
the mouth, a short way in front of the glottis, immediately to the
inner sides of the lingual arteries, which supply them, and along
the course of the lingual veins.

6. The (Esophagus.

The oesophagus is formed from the most anterior part of the
narrow or intestinal region of the mesenteron, and leads directly
from the pharynx. It is at first tubular, but in tadpoles of
about 8 mm. length, shortly before the opening of the mouth, the
cavity of the oesophagus becomes completely blocked up, by pro-
liferation of the cells forming its walls (Fig. 64, to). This
solid portion of the oesophagus lies immediately behind the
pharynx, and has a length of about 0-15 mm. The solid con-

154 THE FKo<;.

dition lasts for a little time after the opening of the mouth ; and
then, in tadpoles of about 10J mm. length, the lumen is gradu-
ally re-established, though it is for a time exceedingly narrow.

This blocking up of the oesophagus, which prevents any food
getting into the digestive part of the alimentary canal until some
little time after the mouth opening is established, is a curious
developmental feature ; it occurs also in the chick and in many
other Vertebrates, but its meaning has not yet been explained
satisfactorily.

7. The Lungs.

The lungs arise as a pair of pouch-like diverticula of the
side walls of the oesophagus, shortly before the hatching of the
tadpole ; they are at first exceedingly small, and have strongly
pigmented walls. After hatching, the lungs increase slowly in
size, growing backwards along the sides of the oesophagus ; in
9 mm. tadpoles, at the time when the oesophagus is solid, the
lungs are present as a pair of lateral outgrowths immediately
behind the oesophageal plug (Fig. 64, to), but sometimes arising
from the solid part itself. After the re-opening of the oesophagus,
the part of the ventral wall from which the lung sacs arise be-
comes depressed to form the laryngeal chamber : the mouth of
the depressed portion narrows to form the glottis, and the lungs
themselves rapidly increase in size.

In 12 mm. tadpoles, in which the hind limbs are just ap-
pearing (Figs. 65 and 75), the glottis is a narrow slit-like opening,
guarded in front by a well-developed epiglottis, and leading into
a large laryngeal chamber (Fig. 65, LC), from which the two lungs
arise ; these latter are thin- walled vascular sacs (Fig. 76, LG),
which now reach to the hinder end of the body cavity, lying
along the sides of the alimentary canal.

From their mode of development as outgrowths of the oeso-
phagus, it follows that the lungs are lined by an epithelium
which is of hypoblastic origin ; the connective tissue and vascular
elements of the lung wall are, like those of other parts of the
body, mesoblastic.

8. The Liver.

About the time of first appearance of the nervous system, the
yolk-mass becomes marked off in front by a deep, backwardly
projecting depression (Fig. 60, l), from the thin-walled anterior

THE LUNGS AND LIVER.

155

region of the mesenteron. This depression becomes still more
marked in the later stages (Fig. 69, w) ; and from its anterior
wall the liver is developed.

Fig. 71. — Horizontal section of the head and body of a 12 mm. Tadpole ;
drawn from the dorsal surface, x 27.

AP.l, 2, 3, 4, afferent branchial vessels of first, second, third, find fourth branchial
arches. BR. 3, cartilaginous bar of third branchial arch. EF.l, efferent branchial
vessel of first branchial arch. HB, basihyal cartilage. HY, cartilaginous bar of
hyoid ai-ch. J, jaw. KA, posterior end of archinephric duct, opening into cloaca.
LY, subcutaneous lymphatic spaces. OP, aperture leading from opercular chamber.
RA, right auricle of heart. RB, left auricle. BS, sinus venosus. RT, truncus
arteriosus. TD, duodenum. TI, intestine. VH, posterior vena cava. "W, liver.
"WD, bile duct. WG, gall bladder. Z, commencing hind limb.

This anterior wall becomes early invested by mesoblast on
its outer surface, and in this mesoblast numerous blood-vessels

156 THE FEOG.

of large size are developed. The wall now becomes thrown into
folds (Fig. -64, w) ; the blood-vessels following in between the
folds. By a continuation of this process, accompanied by the
formation of outgrowths from the hypoblast cells, and ingrowth
of the blood-vessels, the liver rapidly increases in size and acquires
the structure shown in Fig. 71, w; consisting of a trabecular
framework of solid rods of hypoblast cells, the meshes of the
framework beiug occupied by the hepatic blood-vessels. As the
liver attains definite shape and increased size, it separates more
distinctly from the intestine, remaining, however, connected
with this by the bile-duct, which is formed by lengthening out
of the original diverticulum from the mesenteron. The gall-
bladder is a lateral outgrowth from the bile-duct ; it develops
at an early period (Fig. 64, Wg), and is of large size during the
whole of tadpole life (Fig. 71, wg).

9. The Pancreas.

The pancreas develops as a pair of hollow outgrowths from
the mesenteron, behind the liver. In the later stages (Fig. 71.
pa), the ducts shift so as to open into the bile-duct instead of, as
at first, directly into the intestine.

The secreting cells of the pancreas, like those of the liver, are
of hypoblastic origin.

10. The Bladder.

The bladder is absent during the greater part of the tadpole
period; but shortly before the metamorphosis it arises as a
median ventral outgrowth from the hinder end of the mesen-
teron, which soon becomes bifid distally (Fig. 89, tb).

11. The Post-anal Gut.

Post-anal gut is the name given to an extension of the hinder
end of the mesenteron into the base of the tail, which appears
as this latter is developed.

The mode of formation of the neurenteric canal as a tubular
communication between the hinder end of the neural canal and
the mesenteron has already been described (cf. Fig. 61, nt). As
the tail lengthens, the notochord and spinal cord grow back-
wards with it, and the neurenteric canal becomes drawn out into
the post-anal gut. This is an evanescent structure, disappearing
completely at a very early stage : at the time of hatching of

THE PANCREAS, POST-ANAL GUT, AND GILLS.

157

the tadpole (Fig. G9), the only trace of the post-anal gut is a
solid cord of cells, running in a slightly irregular course beneath
the notochord, from the hinder end of the spinal cord to the
mesenteron.

DEVELOPMENT OF THE GILL-CLEFTS
AND THE GILLS.

The gills and gill- clefts, which form the main respiratory
apparatus of the tadpole, are developed in connection with the
side walls of the pharynx. The gill-clefts are a series of slit-like

Fig.

Fig. 73.

Fig. 72. — Side view of a Tadpole at the time of hatching, x 16.
Fig. 73. — Ventral view of the same Tadpole.

BR1, external gill of first branchial arch. BR.2, external gill of second branchial
arch. DS, stomatodaeal pit. MT, mesoblastic somites seen through the skin. OC,
olfactory pit. Q,, sucker. U, proctocheal or cloacal aperture.

perforations in these walls, leading from the pharynx to the
exterior ; while the gills themselves are vascular tufts developed
on the gill-arches, i.e. on the parts of the pharyngeal wall between
the successive gill-clefts.

1. The Gill-clefts.

The gill-clefts are formed as vertical, pouch-like foldings of

158

THE FROG.

the side walls of the pharynx (Fig. 74, hm, hc), which grow
outwards towards the exterior. They appear first at a very
early stage, while the blastopore is still open (Fig. 60), and even

Fig. 7

40.

4. — Horizontal section of a Tadpole at the time of hatching

AF, afferent branchial vessel of first branchial arch. BF, fore-brain. BR1, first
branchial arch. BR. 2, second branchial arch. BE.3, third branchial arch. C, body
cavity or coelom. EF, efferent brand lial vessel of first branchial arch. HM, hyo-
mandibular gill-pouch. HY, hyoid arch. IN, infundibulum. KA, archinephric duct
of right side. KA', archinephric duct of left side, seen in section. KP, head kidney
or pronephros. KS, third nephrostome of right pronephros. KS', third nephrostome of
left pronephros, seen in section. OF, olfactory pit. OS, optic stalk. TP, pharyngeal
region of niesenteron. TI, intestinal region of mesentcron. Y, yolk-cells.

before the closure of the neural canal is completed ; they develop
rapidly, reaching the external epiblast, and fusing with it, at an
early stage.

THE GILL- CLEFTS. 159

In tadpoles of o mm. length there are three pairs of gill-
pouches present, which appear almost simultaneously ; and by
the time of hatching of the tadpole two additional pairs are
formed behind these, making five pairs in all.

The condition at this stage is well shown in the horizontal
section given in Fig. 71-. The gill-pouches form vertical
partitions, radiating outwards from the pharynx to the surface
epiblast. Each pouch is formed of a double fold of hypoblast,
the two layers of which are in close contact with each other.
The outer ends of all five pairs of gill-pouches reach the epiblast,
and fuse with its inner or nervous layer.

Of the five pouches of each side, the most anterior one is
the hyomandibular pouch or cleft (Fig. 74, hm), and the succeed-
ing ones are named first, second, third, and fourth branchial
pouches respectively : the hindmost or fourth branchial pouch
(Fig. 74, HC.4) is smaller than the others, and is often imper-
fectly developed at this stage.

The parts of the wall of the pharynx between the successive
gill-pouches are spoken of as the visceral or gill arches. The
arch between the hyomandibular and the first branchial pouches
is named the hyoid arch (Fig. 74, hy) ; and then in succession
come the first branchial arch, br.i : second branchial arch, BR.2,
and third branchial arch, BR.3. Behind the fourth branchial
pouch, HC.4, is an imperfectly defined fourth branchial arch.

The pharynx is widest opposite the first branchial arches ;
and between the pair of fourth branchial arches it passes back
into the narrow oesophagus.

About the time of formation of the mouth, the two hypo-
blasts lamellas, of which each gill-pouch consists, separate from
each other, so as to form a narrow vertical slit, or chink, leading
from the pharynx to the exterior. These slits are the gill-
clefts.

The first clefts to open in this way are the second and third
branchial clefts, i.e. the ones immediately behind the first and
second branchial arches respectively. At a slightly later stage
the first branchial cleft, between the hyoid and first branchial
arches, also opens in a similar way ; and later still the fourth,
or hindmost branchial cleft opens.

The hyomandibalar pouch, although it is in its early stages
exactly like the hinder branchial clefts, and is fused in similar

1G0 THE FROCK

manner with the external epiblast, yet does not open to the
exterior. Shortly before the mouth opening is established, the
hyomandibular gill-pouch separates from the external epiblast
and recedes somewhat from the surface. The two hypoblastic
lamella? separate from each other, so as to form a saccular
diverticulum from the pharynx, and this gradually opens out
into the cavity of the pharynx, and in tadpoles of about 20 mm.
length ceases to be recognisable as a distinct pouch.

The Eustachian tube and tympanic cavity are formed near
to the hyomandibular pouch, but independently of it, and in a
manner which has already been described in the section dealing
with the development of the ear (p. 143).

2. The Gills.

There are two sets of gills in the tadpole, external and
internal respectively ; the former being branching processes pro-
jecting outwards from the first three branchial arches on each
side, while the internal gills are formed later as vascular tufts
on the sides of all four branchial arches. The two sets of gills
differ in some important respects, and it is generally considered
that they are independent series of structures.

The external gills appear shortly before the time of hatching,
as two pairs of small, backwardly directed processes from the first
and second branchial arches. They are at first somewhat conical
in shape, with rounded or very slightly notched borders : the
gill of the first arch overlaps that of the second arch, and is
placed rather more ventrally than this latter.

By the time of hatching (Figs. 72 and 73, br.i, br.2), the
external gills have increased in size. The first one is notched
at its free posterior border into three blunt lobes ; and the
second into two or three similar ones.

In the succeeding stages the external gills grow rapidly, and
the lobes into which they are divided become larger and more
numerous. A third external gill appears on the third branchial
arch of each side (Figs. 73, 74) : it is very small, and is overlapped
and almost concealed by the two anterior gills.

The external gills attain their maximum development about
the time of opening of the mouth. At this stage (Figs. 44, 5,
and 77), they form much-branched plumose tufts, exceeding

'

.THE GILLS. 161

iii length the transverse diameter of the head. Each of the two
anterior gills consists of from five to seven main lobes, decreasing
in size from above downwards; and each main lobe gives off
minor lobes along its posterior border. The third or posterior
gill (Fig. 77) is much smaller than the other two, and only
slightly subdivided.

The external gills are usually carried projecting outwards
and backwards from the head, at an angle of about 45° with the
axis of the body. Each gill has, however, muscles of its own, by
means of which the entire gill or its individual lobes can be
moved freely and independently.

The course of the circulation in the external gills can be
well studied in the living animal. Each main lobe, and each
of its minor lobes, contains two blood-vessels, afferent and
efferent, which lie alongside each other and are directly con-
tinuous at the tip of the lobe ; the afferent vessel being posterior,
and in part ventral to the efferent vessel (Fig. 77, AF and ef).

Before the mouth opens, the opercular folds arise, as a pair of
folds of skin from the posterior edges of the hyoid arches, which
soon become continuous with each other across the ventral
surface of the head. Shortly after the formation of the mouth,
the opercular fold begins to grow back rapidly, covering over the
gills like a hood. The posterior border of the fold fuses with
the body wall along the right side, and across the ventral
surface : on the left side of the body it remains free, and is
prolonged backwards as a short tubular spout (Fig. 71, op),
through which the opercular cavity opens to the exterior.
After completion of the opercular fold the external gills rapidly
shrink up, those of the left side persisting longer than those
of the right side, and often protruding for a time through the
opercular spout.

External gills occur in the adult or in the larval stages of
most, though not of all Amphibians. Their morphological value
has been much discussed, and it is commonly held that they are
to be regarded as secondarily acquired or larval organs, essen-
lly different in their nature to the internal gills.

The internal gills. In tadpoles of from 9 to 1 0 mm. length
the mouth opening is formed, by perforation of the oral septum
(p. 148, and Fig. G4, ds) ; and about the same time the gill-clefts

M

162

THE FKOG.

open to the exterior. Almost directly after the opening out
of the gill-clefts, the internal gills begin to form, as a series
of small papilla3 along their margins, ventral to the external
gills : the tadpole now begins to breathe in the typical fish-like
manner, taking in water at its mouth, and passing it through
the gill-clefts, and so over the internal gills, into the opercular
cavity, from which it escapes by the opercular spout.

Fig. 75. — Transverse section through the head of a 12 mm. Tadpole ; the sec-
tion passing through the auditory organs, the pharynx and internal gills,
the glottis and laryngeal chamber, and the heart, x 40.

A, aorta. AF, afferent blood-vessel of second branchial arch. BH, hind-brain.
BR.1, .2, .3, .4, first, second, third, and fourth branchial arches. CH, notochonl.
CP, pericardia] cavity. EA, anterior vertical semicircular canal EP, efferent blood-
vessel of second branchial arch. EH, horizontal semicircular canal. ER, recessus
vestibuli. EV, vestibule of ear. G-I, internal gills. HC2, second branchial cleft.
LC, laryngeal chamber. LT, glottis. LY, lymphatic space. OP, opercular cavity.
RA. auricle of heart. RV, ventricle. TP, pharynx. V.4, fourth ventricle. X'.
choroid plexus of fourth ventricle. VIII, auditory nerve.

The internal gills rapidly increase in size, and branch so as
to form plumose tufts arranged in a double row along the
ventral half of each of the first three branchial arches, and a

THE GILLS. 1^3

single row along the fourth arch (cf. Figs. 75 and 83). From
their first appearance the internal gills are very vascular, re-
ceiving branches from the afferent and efferent branchial vessels,
which are connected by capillaries in the gill-tufts themselves.

The relations remain much the same up to the time of the
metamorphosis, the gills forming a series of vascular tufts
arranged in double rows along the ventral surfaces of the gill
arches (Fig. 75, Gi), and hanging down into the opercular cavity,
which they in great part fill. The dorsal or pharyngeal borders
of the gill arches develop a complicated system of tooth-like
processes, which form a filtering or straining apparatus, pre-
venting the passage of food from the pharynx through the gill-
clefts. This is still further obviated by a pair of velar plates,
anterior and posterior, on each side of the floor of the pharynx,
which cover over the gill-arches, and separate them from the
pharyngeal cavity ; a rather narrow slit is left between the edges
of the two plates of each pair, for the passage of water from the
mouth to the gill-clefts, for the purpose of respiration.

The disappearance of the gills. Towards the end of the tad-
pole period of existence, large numbers of lymph follicles form on
the inner surface of the opercular membrane ; and at the same
time a great proliferation of epithelial cells takes place from
the epithelium of the opercular membrane, and from the gills
themselves. On the gills the cells become cubical, and then by
rapid division form layers several cells thick. In this way, by
thickening of its walls, the opercular cavity becomes greatly re-
duced in size, and ultimately completely blocked up. The gill-
clefts become closed, by fusion of their walls with one another ;
and the gills themselves, with the branchial cartilages, and the
entire gill apparatus, degenerate and are rapidly absorbed.

Portions of the ventral ends of the gills persist, even in the
adult, as a pair of soft, lymphoid bodies, reddish in colour, which
lie at the sides of the larynx, just behind the thyroid bodies, and
a little further apart than these. They are sometimes spoken
of as tonsils.

Remnants of the dorsal ends of the gills also persist for a time
as a pair of compact lymphoid masses, lying immediately beneath
the skin, and just behind the ears ; they usually disappear in the
course of the second year.

1G4 THE FROG,

3. The Thymus.

The thymus arises in tadpoles of about 8 mm. length,
shortly after hatching, as a pair of epithelial buds from the
wall of the pharynx, opposite the dorsal ends of the first
branchial clefts. Soon after the opening of the mouth, these
buds separate from the epithelium as a pair of solid rounded
bodies, formed of deeply-staining epithelial cells, which lie
imbedded in the roof of the mouth, below the anterior ends of
the auditory vesicles, and between the ganglia of the facial and
glosso-pharyngeal nerves.

In each thymus a distinction early appears between an outer
cortical layer of small deeply pigmented cells, and a central
medullary portion consisting of large pale granular cells. At a
later stage the distinction becomes less evident, owing to the
cortical cells extending inwards through all parts of the
thymus.

The thymus lies behind the quadrate cartilage, and is
carried backwards by the rotation of this cartilage which accom-
panies the widening of the mouth at the time of the metamor-
phosis. The thymus is larger in the tadpole than in the frog,
and undergoes degenerative changes after the metamorphosis.
In the frog it lies behind the ear and the tympanic membrane,
and slightly ventral to these.

Buds similar to those from which the thymus is formed are
developed opposite the dorsal ends of the hyomandibular clefts,
simultaneously with the thymus buds ; and at a slightly later
stage opposite the second and third branchial clefts as well.
These all disappear before the metamorphosis and take no part
in the formation of the adult thymus.

4. The Post -branchial Bodies.

A pair of small diverticula of the floor of the pharynx arise,
in tadpoles of about 8 mm. length, behind the last gill-clefts,
and at the sides of the glottis. These soon separate from
the epithelium as a pair of small vesicular bodies, lined by
cylindrical epithelium ; they disappear shortly after the metamor-
phosis. It is possible that they represent, in a modified form, a
fifth pair of branchial clefts.

THE HEART AND BLOOD-VESSELS. 165

I) KVELOPMENT OF THE HEART AND BLOOD-
VESSELS.

1 . Preliminary Account.

The blood-vessels arise in the mesoblast. Id most parts of
the body of the tadpole they appear first as irregular spaces or
lacunas, formed by separation of the mesoblast cells from one
another. These lacunar spaces are at first independent, but
soon extend so as to meet and open into one another as irregular
channels. The cells surrounding these channels assume more
definite arrangement and character, and in this way the chan-
nels become converted into blood-vessels. The blood corpuscles
are either cells which are inclosed from the first within the
lacunar spaces, or more usually are cells budded off at a later
stage from the walls of the vessels into their cavities.

Each blood corpuscle is a single cell. In the earl}' stages of
development all the blood corpuscles of the frog embryo are alike,
consisting of spherical cells in which are imbedded numerous yolk-
granules. These yolk-granules are gradually used up for the
nutrition of the embryo, and shortly after the hatching of the
tadpole the corpuscles begin to acquire the shape and characters
of the red blood corpuscles in the adult frog.

The chief point of interest in the development of the blood-
vessels of the frog is afforded by the changes which occur during
the transition from the gill-breathing to the lung-breathing condi-
tion.

While the tadpole is breathing by means of gills its circula-
tion is in all essential respects that of a fish. The venous blood,
returned from the body at large, enters the posterior end of the
heart, or sinus venosus : from this it passes into the second or
auricular chamber, thence to the ventricle, and from that to
the truncus arteriosus (Fig. 64). The blood passes through the
several cavities in succession, there being as yet no division
between the right and left sides of the heart.

The truncus arteriosus divides distally into right and left
branches, from each of which four afferent branchial vessels (Fig.
76, af) arise. The four vessels of each side run outwards along
the hinder borders of the four branchial arches, giving off'
along their whole length numerous branches to the gill-tufts

1GG

TJIK FROG.

on these arches. From the gills the blood, now aerated, passes
into the efferent branchial vessels (Fig. 76, ef). These lie

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alongside the afferent branchial vessels, and just in front of them,
but do not communicate with them except through the capillary
loops of the gills.

THE HEART AND BLOOD-VESSELS. 167

The four efferent branchial vessels of each side unite in the
dorsal wall of the pharynx to form the aorta : the two aorta3
are continued forwards to the head as the carotid arteries, while
posteriorly they unite to form the single dorsal aorta, from
which branches arise supplying all parts of the body.

The lungs arise at a very early stage, but are for a long time
extremely small and of little functional importance. Each lung
receives blood from a branch of the fourth efferent branchial
vessel (Fig. 76, ap), and returns it directly to the auricle by a
pulmonary vein, VP. As the tadpole increases in size, and the
lungs become of greater importance, a septum appears, dividing
the auricle into systemic or venous, and pulmonary or arterial
cavities. Simultaneously with this, valves are formed in the
truncus arteriosus, by which the venous and arterial streams of
blood are kept apart to a certain extent.

At the time of the metamorphosis the gill circulation is cut
off, by the establishment of direct communications between the
afferent and efferent branchial vessels, and the pulmonary cir-
culation becomes of much greater importance than before.

2. The Heart.

The heart lies at first (Fig. 69, p. 146) on the under surface
of the head, below the floor of the pharynx, above and slightly
behind the sucker, and immediately in front of the commencing
liver.

In this region the mesoblast, as in the body generally
(Fig. 70, so, Sp), is split into somatic and splanchnic layers,
separated by a distinct space. This space becomes the peri-
cardial cavity ; the outer or somatic layer of mesoblast forming
the wall of the pericardial cavity ; and the inner or splanchnic
layer giving rise to the muscular wall of the heart.

The endothelial lining of the heart is derived from a number
of scattered cells, which appear below the floor of the pharynx,
and which are formed partly, if not entirely, by direct pro-
liferation of the hypoblast cells of the pharyngeal wall and of
the liver (cf. Fig. 69). These cells are at first irregularly ar-
ranged, but soon become disposed so as to form a tubular lining
to the heart, which is for a time closed in front, while its
posterior wall is formed by the anterior surface of the liver
diverticulum (Figs. 61 and 69).

168 THE FROG.

The heart remains attached at its hinder end to the liver,,
and in front to the floor of the pharynx ; but along the resl off
its length it becomes free, and increasing rapidly in length
becomes twisted on itself in a letter S shape. At the same
time, constrictions appear, partially dividing the cavity into
chambers, the first loop of the S forming the auricular, the
second the ventricular portion of the heart ; while the posterior
and anterior limbs become the sinus venosus and truncus
arteriosus respectively (Fig. 64).

At the time of opening of the mouth the heart is still more
markedly twisted on itself, and the successive chambers more
sharply separated from one another; and a little later a septum
grows down from the dorsal wall of the auricle, dividing its
cavity into a small left auricle and a much larger right auricle.

The condition of the heart in tadpoles of 12 mm. length is
shown from the right side in Fig. 76, p. 166 ; in sagittal section
in Fig. 6b, p. 121 ; and in horizontal section in Fig. 71, p. 155.
The sinus venosus, or proximal division of the heart, is a wide
transverse vessel (Fig. 71, RS), which runs across the hinder and
dorsal part of the pericardial cavity, and receives the blood return-
ing from the body generally.

The sinus venosus opens in front by a large round median
aperture into the right auricle (Figs. 65 and 71, Ra). From the
auricle the blood passes through a narrow auriculo-ventricular
aperture (Fig. 65) into the ventricle, which receives also the
blood from the smaller left auricle. The cavity of the ventricle
is much subdivided by muscular trabecular, which, growing
inwards from its walls (Fig. 65 and 75, Rv), branch and unite to
form a kind of sponge work, in the meshes of which lie the
blood corpuscles.

From the ventricle a small aperture leads into the truncus
arteriosus. This latter is divided into proximal and distal parts,
by a pair of valve-like folds, just before the point where it
bifurcates into right and left branches ; of these, the proximal
part, which becomes the pylangium of the adult, is partially
subdivided by a longitudinal fold, which runs along its interior
in a somewhat spiral course. It is difficult to imagine that
these valves can play any part in directing the blood into one
pair of afferent branchial vessels rather than another ; but it is
significant that they should appear just at the time when the

THE HEART. 169

auricular septum is being completed and the lungs are coming
into use.

At the time of the metamorphosis the condition of the heart
is practically that of the adult. The proximal and distal parts
of the truncus arteriosus, or pylangium and synangium, are now
separated by three pocket valves in place of the two simple

I Ives originally present ; the spiral valve of the pylangium is
)re strongly developed than before ; and the synangium is
rided internally into anterior and posterior portions, the
•mer communicating with the first and second pairs of aortic
:hes, and the latter with the third and fourth pairs.
The mode in which the thickening of the wall of the ventricle
effected, by the formation of a muscular reticulum within the
cavity, and not by a simple increase in thickness of the wall
itself, is of interest, inasmuch as it explains why the ventricle of
the frog's heart has no nutrient blood-vessels. The blood within
the ventricle occupies the meshes of the muscular reticulum and
so comes in close contact with all parts of the ventricular walls,
a condition which renders special nutrient vessels unnecessary.

The pericardial cavity, in its early stages, communicates freely
with the general body cavity, of which it is merely the anterior
portion. Owing to the great bulk of the mass of yolk cells, the
body cavity in the abdominal region of the embryo is at first
merely a narrow chink (Fig. 70, CS) ; while the pericardial
cavity is already a space of considerable size.

Later on, and especially as the large veins opening into the
sinus venosus increase in size, the opening between the pericardial
cavity and the general body cavity becomes much reduced ; but
up to the time of the metamorphosis there is free communica-
tion between the two cavities, through a pair of apertures at the
dorsal and posterior border of the pericardial cavity, close to the
sides of the laryngeal chamber.

8. The Branchial Blood-vessels.

The blood-vessels of the pharynx have close relations with
the visceral arches, into which the side walls of the pharynx are
divided by the gill-clefts.

The first vessels to appear in the body, with the exception
of the heart and the great veins opening into it, are the dorsal
aorta\ These arise on each side as a series of isolated lacunar

FIG. 77

Fig.

a AP » m =* En ™

A Ar i l

A 8

Fig. 77. — Diagrammatic figure of the head and anterior part of the body of
a 7 mm. Tadpole, shortly after hatching ; showing the branchial blood-
vessels from the ventral surface. The heart has been removed, x 35.
Fig. 78.— Diagrammatic figure of the same embryo from the right side. The
heart is represented in situ, but the external gills of the first and second
branchial arches have been cut off short at their bases, x 35.

A, dorsal aorta. AB, basilar artery. AC, carotid artery. AF.l, AF.2, AF.3,
afferent branchial vessels of first, second, and third branchial arches. AP, pulmonary
artery. AR, anterior cerebral artery. AT, anterior palatine artery. CA. anterior
commissural vessel. CP, posterior commissural vessel. EP.1, EF.2, EF.3, EF.4,
efferent branchial vessels of first, second, third, and fourth branchial arches. EH,
efferent vessel of hyoid arch. EM, efferent vessel of mandibular arch. GE, external
grills. GM, glomerulus. KA, segmental duct. KP, head kidney or pronephros.
KS1, KS.3, first and third nephrostomes of head kidney. LV.4, efferent lacunar
vessel of fourth branchial arch. RA, auricle. BV, ventricle. RT, truncus arteriosus.
VD, Ouvierian vein. VH, hepatic veins. VK, vein of sucker. VM, mandibular
vein. VY, hyoidean vein.

THE BRANCHIAL BLOOD-VESSELi

171

spaces along the roof of the pharynx, which by opening into
one another form a pair of longitudinal vessels ; these soon
extend backwards along the body, but remain for some time
distinct from each other.

In the four branchial arches, blood-vessels are formed on a
definite plan. In the first and second branchial arches these
vessels appear immediately after the dorsal aorta), and are well
established at the time of hatching ; in the third and fourth bran-

A!"i

IG. "!>. — A diagrammatic transverse section through the head of a 7 mm.
Tadpole, seen from behind ; the section is taken just behind the auditory
vcsieles, and passes through the first branchial arch on each side. On the
right side both the afferent and efferent vessels of this arch are shown ;
on the left side the greater part of the afferent vessel has been removed
in order to expose the efferent vessel more thoroughly, x 50.

A, aorta. AF-1, afferent vessel of first branehial arcli. BH, medulla oblongata.
CH, notochord. CP, pericardial cavity. EF-1, efferent vessel of first branchial arch.
Gr, capillary loop of gill, connecting the afferent and efferent vessels together. Q, sucker.
B,T, truncus arteriosus. TP, pharynx. V, inferior jugular vein. VJ, anterior
cardinal vein. V.4, fourth ventricle. ' X, pnemnogastric nerve.

cliial arches they arise in a similar manner, but at a somewhat
later stage. In the hyoid and mandibular arches, vessels com-
parable to those of the branchial arches appear at an early stage :
these, however, never quite conform to the type seen in the
branchial arches, and early undergo degenerative changes.

a. The vessels of the first branchial arch may conveniently

172 TIIK l-'KOG.

be taken as typical of the series. The vessels proper to the
arch are derived from four factors : (i) an efferent lacunar
vessel, which appears in the mesoblast opposite the base of the
external gill, and may be recognised in tadpoles some little
time before hatching; (ii) an efferent diverticulum from the
dorsal aorta ; (iii) an afferent diverticulum from the truncus
arteriosus ; (iv) an afferent lacunar vessel, which lies opposite
the base of the external gill, immediately behind the efferent
lacunar vessel.

These four vessels appear in the order given above ; they
are at first quite independent of one another ; and, for some time
after their first appearance, the lacunar vessels, afferent and
efferent, have no connection with any other vessels.

The efferent lacunar vessel grows rapidly ; it extends
dorsally until it meets with, and opens into, the efferent diver-
ticulum from the aorta ; and it extends ventrally towards, but
not to meet, the truncus arteriosus. It is widest in the middle
part of its course, opposite the external gill ; and here it becomes
connected with the afferent lacunar vessel by capillary loops in
the substance of the gill. This is the condition reached at the
time of hatching.

Shortly after this, the afferent lacunar vessel and the
diverticulum from the truncus arteriosus grow towards each
other and unite. The circulation in the gill is now definitely
established (Figs. 77 and 78) ; the blood passes from the heart
to the truncus arteriosus, RT, and from this along the afferent
diverticulum and the afferent lacunar vessel, which now form
one continuous afferent branchial vessel, AF, to the gill loops, in
which it becomes aerated ; from the gill loops it passes along
the efferent lacunar vessel and efferent diverticulum, which
form a continuous efferent branchial vessel, EF.i, to the dorsal
aorta.

As the external gill increases in size, and becomes fimbri-
ated or lobed at its margin, the original capillary loops become
lengthened out, and additional ones are developed; but the
afferent and efferent vessels remain connected by capillaries
alone, and it is only by passing through the gill capillaries that
blood can get from the heart to the aorta.

At a later period of tadpole life, shortly after the mouth
opening is established, the internal gills are developed on the

THE BEANCHIAL BLOOD-VESSELS.

173

gill arch as a double row of branching tufts, ventral to the
external gill. Capillary loops soon appear in these tufts, form-
ing a series of capillary connections between the afferent and
efferent branchial vessels, similar to those in the external gill,
but situated more ventrally. At the same time the external
gill diminishes considerably in size.

The next change of importance is the establishment of a

VD AVA. RB

Fig. 80. — Diagrammatic figure of the head of a 12 mm. Tadpole from the
right side, showing the heart and branchial blood-vessels. The capillary
loops of the gills are omitted, x 35.

A, aorta. AB. basilar artery. API, AF.2, AP.4, afferent branchial vessels of
first, second, and fourth branchial arches. AL. lingual artery. AP, pulmonary artery.
AH, anterior cerebral artery. AS, posterior palatine artery. AT, anterior palatine
artery. AIT, cutaneous artery. AY. pharyngeal artery. CA, anterior commissural
vessel, seen in section. CGr, bulb-like dilatation on the lingual artery. CP, posterior
commissural vessel, seen in section. EP.l, EF.2, EP.3, EF.4, efferent branchial
vessels of first, second, third, and fourth branchial arches. GM, glomerulus. EA, right
auricle. RB, left auricle. B/F, truncus arteriosus. RV, ventricle. VD, Cuvierian
vein. VH, hepatic vein. VI, posterior vena cava. VP, pulmonary vein.

direct connection between the afferent and efferent branchial
vessels. The two vessels (Figs. 77 and 79, af.i and ef.i) lie
alongside each other in the arch, the afferent being the posterior
of the two. In tadpoles of about 12 mm. length, in which the
sucker is disappearing, and the hind limbs are present as a pair
of small rounded papillae at the base of the tail, a direct com-
munication is established between the ventral ends of the

171 THE PROG.

afferent and efferent vessels, close to the truncus arteriosus;
(Fig. 80). The precise mode in which the communication is
established will be described in the section dealing with the
development of the carotid gland (p. 181). As the communi-
cation is ventral to the gills, both external and internal, any
blood which passes across it will get from the heart direct to the
aorta, without passing through any part of the gill circu-
lation, i.e. without being aerated ; and the efficiency of the
gill respiration will consequently be impaired in direct propor-
tion to the amount of blood which takes this short cut in
preference to the circuitous route through the gill capillaries.

The subsequent changes in the vessels of the first branchial
arch may conveniently be considered after the vessels of the
other arches have been described.

b. The vessels of the second branchial arch (Figs. 77, 78,
and 80, af.2, EF.2) develop in exactly the same way as those of
the first branchial arch, and almost simultaneously with these.

c. The vessels of the third branchial arch (Figs. 77, 78, and
80, AF.3, EF.3) are formed rather later. They are of smaller size
than those of the first and second branchial arches, but in other
respects are similar to these. The external gill, and the vessels
supplying it, are considerably smaller than those of the two
arches in front.

d. The vessels of the fourth branchial arch (Figs. 77, 78,
and 80, af.4, EF.4) arise still later, but in essentially the same
manner, except that no external gill is formed *on this arch.
The vessels are well established before the mouth opening
appears. From the dorsal end of the efferent vessel of this arch
the pulmonary artery, ap, arises as a diverticulum which grows
backwards along the outer side of the lung (cf. Fig. 76, p. 166).

It should be noticed that the pulmonary artery arises, and
acquires its relations with the lung, before the afferent branchial
vessel of the arch has joined the diverticulum from the truncus
arteriosus ; indeed, before this latter has commenced to develop
(cf Fig. 78). Consequently, the only blood that can at this
stage reach the lung is blood from the dorsal aorta, and not blood
from the heart ; in other words, the lung in the early stages of
its development receives arterial and not venous blood.

The afferent vessels of the fourth branchial arch develop
very late ; and the afferent diverticulum from the truncus

THE BRANCHIAL BLOOD-VESSELS. 175

arteriosus ( Fig. 80, af.4), really a branch from that of the third
arch, does not commence to form until after the mouth of the
tadpole is established.

e. The vessels of the hyoid arch. In the hyoid arch, at an
early stage of development, vessels are present which agree
closely in relations and in arrangement with those of the
branchial arches; but which, after developing up to a certain
point, undergo degenerative changes, and in the later stages of
tadpole existence lose all trace of their original disposition.

In tadpoles of 5 mm. length, not long before hatching, the
hyoid vessels consist of: — (i) an elongated efferent lacunar vessel,
lying parallel to, and in front of, the efferent vessel of the first
branchial arch ; and (ii) a very small diverticulum from the
dorsal aorta, which lies opposite the upper end of the lacunar
vessel, but does not quite meet this.

In newly hatched tadpoles two further changes have oc-
curred:— (i) a small blind diverticulum arises from the truncus
arteriosus, just in front of the diverticulum for the first
branchial arch ; and (ii) the efferent lacunar vessel has become
obliterated about the middle of its length, and so divided into
two separate portions, dorsal and ventral. Of these, the dorsal
one has no communication with any other vessel, although it
lies very close to the diverticulum from the aorta ; while the
ventral portion, which may be spoken of as the hyoidean vein,
opens below into an irregular longitudinal venous sinus lying
just above the sucker.

In tadpoles shortly after hatching (Fig. 78), the diverticulum
from the truncus arteriosus has disappeared, as has also the dorsal
portion of the efferent sinus ; so that the onhr vessels remaining
are the small diverticulum, EH, from the aorta, and the ventral
portion of the efferent sinus, or hyoidean vein. VY, which opens
below into the veins of the sucker, vk. By the time the mouth
opening is established, the diverticulum from the aorta has also
vanished, and the hyoidean vein is the only persistent part of
The series of hyoidean vessels.

It thus appears that in the hyoid arch vessels are developed
which are essentially similar to those of a branchial arch ; the
chief difference being that no afferent lacunar vessel is formed
in the hyoid arch, a difference which may clearly be correlated
with the absence of gills, both external and internal, from the

170 THE FRO0.

arch. The arrangement and mode of development of the vessels
which are actually present in the hycid arch, agree so closely
with those seen in the vessels of the branchial arches as to
strongly suggest that frogs must be descended from ancestors
in which gills were present on the hyoid arch as well as on the
branchial arches.

f. The vessels of the mandibular arch. These appear later
than the vessels of the hyoid arch, and depart even more markedly
from the typical branchial arrangement.

Up to the time of hatching there are no vessels at all in the
mandibular arch. Shortly after hatching there appears in the
lower or ventral part of the arch a lacunar vessel, which lies
parallel to and in front of the similar vessel in the hyoid arch,
and, like this, opens into the venous sinuses above the sucker ; it
may be spoken of as the mandibular vein. There is also present
a very small diverticulum from the dorsal aorta.

A little later (Fig. 78), both these factors have grown con-
siderably. The mandibular vein, VM, has extended dorsalwards,
and the aortic diverticulum, em, ventralwards ; and the two
vessels are now continuous with each other. Shortly after the
mouth opens, the two again separate ; the mandibular vein
gradually shrinks up, as the sucker degenerates, and the aortic
diverticulum grows forwards as the pharyngeal artery of the
adult (Fig. 80, ay).

From the above account it appears that the vessels of the
mandibular arch, though still referable to the type of the
branchial vessels, are even more modified than those of the
hyoid arch ; the afferent lacunar vessel and the diverticulum
from the truncus arteriosus are completely absent, and at no
time have the vessels any connection with the heart.

g. The changes in the branchial vessels at the metamorphosis.
For some time before the metamorphosis the tadpole breathes by
lungs as well as by gills, though the main part of the respiratory
work is performed by the latter.

The condition of the blood-vessels during this period of
double respiration is as follows. The mandibular and hyoid
vessels may be omitted, as, although these are formed on the
type of the branchial vessels, they have no connection with the
heart, and no gills are developed in relation with them.

Gills are present on all four branchial arches, and the

THE BRANCHIAL BLOOD-VESSELS. 177

arrangement of the vessels is practically the same in all. In
each arch (Fig. 80) there are two main vessels, afferent and
efferent, which lie side by side close to each other. Of these,
the afferent vessel is a branch of the truncus arteriosus, and lies
in the posterior part of the arch ; while the efferent vessel
lies immediately in front of the afferent, and opens at its
dorsal end into the aorta. The afferent vessel is confined to
the ventral half of the arch ; while the efferent extends along
its whole length, its ventral termination lying in the floor of
the mouth, close to the origin of the afferent vessel from the
truncus arteriosus.

The afferent and efferent vessels of each arch are connected
together in two ways : (i) by the capillary loops of the gills,
of which the most dorsally placed belong to the external gill,
and the ventral series to the internal gills ; (ii) by each afferent
vessel opening directly into the corresponding efferent vessel,
the communication (Fig. 80) being ventral to the gills, and of
very small size. This direct connection between the afferent
and efferent vessels is present in all four branchial arches, though
its position and relations are not easy to determine. The blood
entering an afferent vessel from the heart has thus two courses
open to it : it may either continue along the afferent vessel,
and pass through the gill capillaries to the efferent vessel, and
so reach the aorta ; or it may pass across at once, through the
aperture of communication, to the efferent vessel, and reach
the aorta without having passed through the gills.

At the commencement of the metamorphosis these direct
communications enlarge, so that an increasing quantity of blood
passes from the heart to the aorta without going through the
gills. The gills thus receive less and less blood, and gradually
diminish in size and in efficiency. Increased work is thereby
thrown on the lungs ; and an increasing supply of blood is
sent to the lungs and skin by enlargement of the pulmonary
and cutaneous arteries, which are branches of the efferent vessel
of the fourth branchial arch (Fig. 80), close to its dorsal end.

By further enlargement of the direct communications
between the afferent and efferent vessels, the definite aortic
arches are formed, leading directly from the heart to the aorta ;
each aortic arch (cf. Figs. 80 and 81) consisting of the basal or
proximal end of the afferent branchial vessel, and the whole

17*

TJIE FKOG.

length of the efferent vessel ; while very nearly the whole length
of the afferent vessel, and all the gill capillaries, disappear
completely.

Very slight changes will now convert the branchial system
of the tadpole to the aortic system of the adult frog. Of the
four aortic arches, the first, in the first branchial arch, becomes
the carotid arch of the frog (Fig. 81, l). The portion of the
dorsal aorta between the points of opening of the first and
second aortic arches remains an open tubular vessel for some
time, and may even retain its lumen in the adult. More usually,
however, the cavity becomes obliterated, and the walls of the

Fig. 81. — Diagrammatic figure of the arterial system of an adult male
Frog, from the right side, x 1.

a, stomach, b, nostril, c, small intestine, ca, carotid artery, eg, carotid gland.
cm, cceliaco-mesenteric artery, en, cutaneous artery, d, large intestine, da, dorsal
aorta. /, femur, h, spleen, ha, hepatic arterj-. i, right lung, la, lingual artery, m,
testis, o, kidney, oa, occipito-vertebral artery, pa, pulmonary artery, r, pelvic
girdle s, sternum, sa, subclavian artery, sc, sciatic artery. I, tongue, ta, truncus
arteriosus, ua, urinogenital arteries, v, ventricle. 1, carotid arch. 2, systemic arch.
3, pulmo-cutaneous arch.

vessel persist merely as a pigmented band, connecting the dorsal!
ends of the carotid and systemic arches with each other. After
the obliteration of this part of the aorta, the blood in the carotid
arch is distributed exclusively to the head.

The second aortic arch, in the second branchial arch of the
tadpole, becomes the systemic arch of the frog (Fig. 81, 2).

The third aortic arch, in the third branchial arch of the tad-
pole, disappears altogether. In young frogs of the first year it
loses its connection with the aorta, and then gradually shortens
up, the distal part becoming a solid cellular cord, and the
proximal or cardiac part retaining for a time its lumen. Before
the end of the first year this vessel has entirely disappeared.

THE AORTIC ARCHES. 179

The fourth aortic arch, in the fourth branchial arch of the
tadpole, becomes the pulmo-cutaneous arch of the frog. It
retains its communication with the aorta for some time after the
dorsal part of the third arch has atrophied ; but before the end
of the first year, the part of the fourth arch above the origin of
the cutaneous artery loses its cavity and becomes solid, so that
the pulmo-cutaneous arch no longer opens into the aorta.

The condition of the aortic arches is now that of the adult
frog (Fig. 81).

Before leaving the branchial circulation, it should be noticed
that in other species of frogs the development of the blood-
vessels differs in some important respects from that described
above for the common English frog, Rana temporaria. In Rana
esculenta, according to Maurer, the efferent lacunar vessel of
each arch becomes connected at its ventral end with the truncus
arteriosus, and at its dorsal end with the aorta, before the afferent
branchial vessel is formed ; so that in this species of frog there
is a stage, prior to the formation of the gills, in which there is
a direct passage from the heart to the aorta. As the gills appear,
the afferent gill-vessel arises as an outgrowth from the ventral
end of the efferent vessel, with which it becomes connected, more
dorsally, through the gill-capillaries. During the time that the
tadpole is breathing by gills, the direct connection between the
ventral ends of the afferent vessel and the efferent vessel is lost,
but it is re-established at the time of the metamorphosis, after
which time the further changes are the same as in Rana tempo-
raria. A similar mode of development has been observed as an
exceptional occurrence in Rana temporaria itself, and there are
reasons for thinking that this plan of development, in which
there is, from the first, direct connection between the heart
and the aorta, through the aortic arches, and in which the whole
gill circulation is secondarily derived from this condition, is
more primitive than the mode of development usually seen in
Rana temporaria. This more primitive type of larval vessels is
comparable to the condition obtaining throughout life in the
branchial blood-vessels of Amphioxus.

:

The Dorsal Aorta and its Branches.

There are at first two aorta?, one on either side of the body.
L'hese appear, as described above, as a number of isolated

K 2

180 TEE FROG.

lacunar spaces in the roof of the pharynx, which run together
to form a pair of continuous vessels. These are widesl apart
opposite the first branchial arches (Fig. 77, A ) ; further bade,
behind the pharyngeal region, they lie close alongside each other,
and soon fuse to form the definite dorsal aorta.

a. The arteries of the head are derived from the anterior ends
of the dorsal aorta3.

The carotid arteries (Fig. 78, ac), which are the direct con-
tinuations forwards of the dorsal aortge, become early connected
by two transverse vessels, the anterior and posterior commissural
vessels (Fig. 77, CA, cp), which, with the carotid arteries them-
selves, form an arterial circle surrounding the infundibulum. In
front of the anterior commissural vessel the carotid arteries are
continued forwards as the anterior cerebral arteries (Figs. 78
and 80, ar).

The basilar arteries (Figs. 78 and 80, ab), appear shortly
before the mouth opens, as a pair of vessels which arise from the
outer ends of the posterior commissural vessel, and run backwards
along the ventral surface of the brain and spinal cord, not far
from the median plane.

The anterior palatine arteries (Figs. 78 and 80, at) arise as
branches from the carotid arteries, just before these enter the
skull, and run forwards in the mucous membrane of the roof of
the mouth as far as the nose ; they appear very shortly after the
tadpole hatches.

The pharyngeal artery (Fig. 80, ay) is formed, as already
described, from the dorsal part of the efferent vessel of the man-
dibular arch (Fig. 78, em). From it the posterior palatine artery
(Fig. 80, as) arises as a small, anteriorly directed branch, which
runs forwards and outwards in the roof of the pharynx.

The lingual artery (Fig. 80, al) is of considerable interest.
It appears shortly before the mouth opens, and in tadpoles of
about 9 mm. length is present on each side of the head as a very
short vessel, lying in the floor of the mouth immediately in front
of the truncus arteriosus. In the middle of its course it presents
a swollen bulb-like dilatation, from which arise : — (i) an anterior
branch, the lingual artery proper, which runs forwards a short
distance, giving off a small thyroid artery to the thyroid body ;
and (ii) a posterior branch, which runs a short distance outwards
and backwards towards the ventral end of the efferent branchial

THE ARTERIES OE THE HEAD. ]81

vessel of the first branchial arch, EF.l, but does not quite meet
this. At this stage, therefore, the lingual artery is a closed
vessel, having no communication with any other blood-vessel.

Slightly later, in tadpoles of about 12 mm. length, the pos-
terior end of the lingual artery and the ventral end of the first
efferent branchial vessel become directly continuous with each
other (Fig. 80). The bulb-like swelling is still present, and it
is immediately dorsal to this that the direct connection between
the afferent and efferent branchial vessels is effected, as described
above. The anterior end of the lingual artery has by this time
grown forwards considerably (Fig. 80), extending to the lower

I' aw and lower lip.
The lingual artery thus arises in the floor of the mouth in-
lependently of any other vessel, but soon acquires connection
viih the ventral end of the first efferent branchial vessel, of
vdiich in the later stages it appears to be a direct continuation.
b. The carotid gland of the frog is formed by elaboration of
the direct passage between the afferent and efferent vessels of
the first branchial arch. At 12 mm., as just noticed, the lingual
artery and the first efferent branchial vessel are continuous ; the
lingual artery has at its base (Fig. 80) a small bulb-like swelling,
CCr, and immediately dorsal to this swelling the afferent and
efferent branchial vessels are in direct communication with each
other. This direct passage is at present a single, and a narrow
one : it traverses a small plate of epithelial cells, which is budded
off from the epithelium of the first branchial cleft, and wedged
in between the afferent and efferent vessels.

In the later stages of development this passage becomes
plexiform, there being now three or four openings into the
afferent vessel, and about the same number into the efferent
vessel, one or more of the latter leading directly into the bulb-
like swelling at the base of the lingual artery.

This plexiform communication forms the carotid gland ; the
history of its formation shows that it is not to be regarded as
a persistent or modified part of a gill, as was formerly held to
be the case, but that it is a specially acquired structure, formed
by elaboration of the direct passage between the afferent and
efferent branchial vessels, a passage which is present in a simpler
form in the hinder arches as well.

Solid epithelial plates, of a similar character to that in which

182 THE I'K'Xi.

the carotid gland is formed, are developed, at a slightly later
stage, between the ventral ends of the afferent and efferent
vessels of the second and third branchial arches, and perhaps
in the fourth arch as well ; and it is by perforation of these
plates that the direct communications between the afferent and
efferent vessels are established. After the metamorphosis the
epithelial plates persist, and even increase in size, forming solid
epithelial bodies, lying in the floor of the mouth, in the angles
between the carotid and systemic, and systemic and pulmo-
cutaneous arches respectively. They acquire connective tissue
capsules, and have been spoken of as accessory thyroid bodies.

c. The hinder part of the dorsal aorta. The union of the two
aorta3 to form the definite dorsal aorta occurs almost immediately
behind the pharyngeal region (Fig. 77).

At, or close to, their point of union each aorta has, at the
time of hatching, a small bulging of its ventral wall. Later on,
these bulgings increase in size and become sacculated, forming
a pair of prominent, pigmented, tliick-walled swellings, the glo-
meruli of the head kidneys (Figs. 77 and 78, GM), which hang
down into the dorsal part of the body cavity, lying opposite the
head kidneys, KP, along almost their entire length.

In their further development, the glomeruli keep pace with
the head kidneys ; they remain large up to about 23 mm., i.e. so
long as the head kidneys remain functional (cf. Figs. 83, 84, and
85) ; but in the later stages, when the head kidneys begin to
degenerate, the glomeruli also become smaller. They are still
present, though of very small size, up to the end of the first
year, but disappear completely in the second year. Certain
further points in connection with the glomeruli will be noted
in the section dealing with the kidneys.

d. The pulmonary arteries arise, as described above, shortly
after the time of hatching (Figs. 77, 78, ap), as diverticula from
the dorsal ends of the efferent vessels of the fourth branchial
arches. As the connection of the afferent vessel of this arch
with the truncus arteriosus is not acquired until some time after
the mouth opening is established, there is a considerable period
during which the supply of blood to the lungs is derived from
the aorta and not from the heart, i.e. is arterial and not venous;
a condition which suggests that the lungs had originally some
function other than respiration to fulfil.

THE ARTERIES AND VEINS, 183

The cutaneous artery (Fig. 80, au) is a branch of the fourth
efferent vessel, which arises very close to the pulmonary artery,
but independently of this, and at a rather later stage.

5. The Veins.

The veins, in the earlier stages of their development, are
chiefly characterised by their large size, and irregular lacunar
character.

a. The vitelline veins are the first veins to be formed in the
body. They appear as irregular lacunas in the mesoblast of the
splanchnopleure, along the sides of the yolk-mass and of the liver
diverticulum, and unite in front to form the sinus venosus or
most posterior part of the heart ; they carry to the heart the
food matter absorbed from the yolk-mass.

About, or shortly after, the time of hatching, the liver diver-
ticulum becomes more definitely bounded, and the vitelline and
hepatic veins become distinct from one another ; later still, by
folding of the wall of the hepatic diverticulum, the hepatic veins
are carried deeply into the substance of the liver (Figs. 64 and
76), as already described in the section dealing with the deve-
lopment of the liver (p. 154).

b. The sinus venosus (Fig. 71, RS) is at first formed merely
by the union of the vitelline veins, but very early becomes a
definite transverse vessel, running across the body in close con-
tact with the anterior wall of the liver, and opening into the
auricular portion of the heart by a large anterior aperture.

c. The anterior cardinal veins are paired, and return blood
from all parts of the head, except the floor of the mouth. Each
is formed by the union, behind the ear, of two principal veins : —
the jugular vein, which returns blood from the brain and dorsal
part of the head ; and the facial vein, which runs more super-
ficially along the side of the head, ventral to the eye and ear.

d. The posterior cardinal veins (Fig. 84, vc) are also paired,
and are in special relation with the head kidneys, which they
completely surround. They are of enormous size during the
early stages of tadpole life, when the head kidneys are function-
ally active, forming vascular networks which occupy the spaces
between the tubules of the head kidneys. Each posterior car-
dinal vein receives somatic veins from the hinder part of the
body wall, and unites, in front, with the anterior cardinal

184 THE I'KOG.

vein, at the level of the hinder border of the pericardial cavity,
to form the Cuvierian vein (Fig. 71). The two Cnvierian veins,
right and left, run almost vertically downwards to open into the
outer ends of the sinus venosus (Fig. 71). The Cuvierian veins
persist as the anterior venae cava3 of the frog.

e. The inferior jugular veins collect the blood from the floor
of the mouth, including the large sinuses above the sucker into
which the mandibular and hyoidean veins open (Fig. 78) : they
then run back in the side walls of the pericardial cavity, a ad open
into the right and left Cuvierian veins respectively, just before
these reach the sinus venosus.

f. The posterior vena cava is a large median vein (Fig. 76,
vi), which develops shortly after the mouth opening is esta-
blished ; it is continuous posteriorly with the hinder portions of
the two posterior cardinal veins, which unite together ; and,
further forwards, it runs in a deep groove along the left side of
the liver (Fig. 71, vh), joins with the hepatic veins, and then
opens into the sinus venosus. The blood from the hinder part
of the body can now return to the heart either along the posterior
cardinal and Cuvierian veins, or else by the posterior vena cava.
In the later stages of tadpole life more and more of the blood
follows the latter course, and the anterior ends of the posterior
cardinal veins gradually diminish in size, and during the meta-
morphosis disappear completely.

g. The renal portal veins are formed by longitudinal anasto-
motic communications between the transverse, or vertebral, veins
of the hinder part of the body ; they are joined posteriorly by the
iliac veins, and with these form the afferent renal system of veins.

h. The anterior abdominal vein is at first paired, and is in
connection , not with the liver, but the heart. The pair of vessels
appear first in the ventral body wall, extending backwards a
short distance from the sinus venosus ; they soon extend further
backwards, and acquire communications with the veins of the
hind legs and of the bladder. At a later stage, the two anterior
abdominal veins unite at their hinder ends, in front of the
bladder, while further forwards the vein of the right side disap-
pears, the left one alone persisting. Later still, the anterior abdo-
minal vein loses its direct communication with the sinus venosus,
and acquires a secondary one with the hepatic portal veins, or
afferent veins of the liver.

THE VEINS AND LYMPHATICS. 185

i. The pulmonary veins (Fig. 70, vp) develop early, but
not quite so soon as the pulmonary arteries. They appear,
about the time of establishment of the mouth opening, as a
series of irregular lacunar spaces along the inner or median
rails of the lungs, and at first do not reach the heart. Very
Lortly afterwards, they are completed along their whole length,
>out the time of formation of the inter-auricular septum in the
sart.

The Lymphatic System.

The most striking point about the lymphatic system in the

[pole stages is the great development of the subcutaneous

iph spaces. In tadpoles of about 20 mm. length, and up-

irds, these form sacs of literally enormous size, filled with a

igulable fluid, and almost completely surrounding the sides

id ventral surface of the body and head (cf. Figs. 71, 75, and 88,

r) ; in the living tadpole they give rise to the appearance of a

semi-transparent border surrounding the head and trunk.

The early development of the lymphatics appears to take
place in the same manner as that of the blood-vessels ; the
lymphatics appearing first as intercellular lacunar spaces, to
which proper walls are formed later by the surrounding tissues.

The lymph-cells appear rather late, after the internal gills
are well established ; according to Maurer, they are derived from
the hypoblast cells of the wall of the mesenteron, and wander
thence into the connective tissue.

The spleen arises as a spherical bud on the mesenteric
artery ; it consists of cells similar to those of the lymphatic
tissues, and, like these, is said to be derived originally from the
hypoblast cells of the mesenteron.

DEVELOPMENT OF THE URINARY AND
REPRODUCTIVE ORGANS.

1. Preliminary Account.

The excretory organs of the tadpole are the head kidneys, a
pair of globular bodies (Fig. 83, kp), imbedded in the dorsal
body-wall, immediately behind the pericardial cavity. Each
head kidney, or pronephros, consists of a convoluted and complex
mass of tubules, with glandular walls, opening into the body

180 THE FROGK

cavity by three ciliated mouths or nephrostouies ; the tubules
are surrounded by, or rather imbedded in, the large posterior
cardinal veins, and it is from the blood in these veins that the
excretory matters are separated. The excretions are carried
away from the head kidneys by a pair of tubes, the segmental
ducts, ka, which run along the dorsal wall of the body to its
hinder end, where they open into the cloaca, or hinder part of
the alimentary canal.

The head kidneys and their ducts are well developed in the
tadpole at the time of hatching ; they subsequently increase
considerably in size, and are the sole excretory organs of the
tadpole during the earlier stages of its existence. About the
time the hind legs appear, the adult kidneys, or Wolffian bodies,
begin to form in the hinder part of the body, as a series of tubes
which grow towards, and open into, the segmental ducts (Fig.
8-3, km) ; these ducts carrying away the excretory matters
separated by both the larval and adult kidneys. The Wolffian
bodies rapidly increase in size and in complexity, especially at
their posterior ends, and by the time of the metamorphosis
(Figs. 85 and 86) have attained considerable dimensions. The
head kidneys at the same time undergo degenerative changes,
and gradually disappear, while the Wolffian bodies, growing still
larger, become the kidneys of the frog.

The genital ducts are formed in close relation with the
segmental ducts, or actually from these ducts ; and the Wolffian
bodies become so closely related to the reproductive organs in
both sexes, that it is desirable to describe the development of
the two systems, urinary and reproductive, together.

2. The Head Kidney and Segmental Duct.

The segmental duct (Fig. 70, kb) first appears, in embryos
of between 3 J and 4 mm. length, as a longitudinal ridge of the
somatopleuric mesoblast, immediately below the ventral borders
of the myotomes, MS. This ridge lies immediately beneath the
epiblast, but is, in the frog, distinct from this at all stages of its
development. It is difficult to determine whether or not it is
solid at its first appearance, but it very soon becomes grooved
along its inner surface (Fig. 70) ; the groove communicating
with the body cavity, and the entire ridge having, in transverse
section, the appearance of a fold of the mesoblast, with a

THE URINARY ORGANS.

187

well-defined ventral or outer lip, and a less distinct dorsal

^»oundary.
In the hinder part of the body the lips of the groove meet,
o as to form a tube, lying between the mesoblast and the epi-
blast ; and this tube is the segmental duct. At the anterior end,
e lips remain separate for a time, so that the duct opens in
nt into the body cavity by a longitudinal slit-like mouth.

MH

Fig. 82.— Transverse section through the body of a Tadpole at the time of
hatching ; the section passing through the second pair of nephrostomes,
and the third pair of myotomes, x 50.

A, aorta. C, coelom, or body cavity. CH, notochord. CJ, subnotochordal rod.
G-M, glomerulus. KP, segmental duct. KS, second nephrostome of left side.
ME, Bomatopleuric layer of mesoblast. MH, splanchnopleoric layer of mesoblast.
ML, myotome. IfL, lateral line branch of pneumogastric nerve. NS> spinal cord.
T, intestinal region of mesenteron. VC, posterior cardinal vein. VH, hepatic vein.
"W, liver diverticulum.

By meeting of the lips at two places this slit becomes divided
into three openings or nephrostomes, lying one behind another,
and immediately below the ventral edges of the second, third,
and fourth myotomes respectively (cf. Figs. 74 and 82).

The anterior end of the duct, and especially the parb between
the second and third nephrostomes, now increases rapidly in
length, becoming twisted on itself; and at the same time
the nephrostomes become drawn out into short tubes. At its

188 THE FROG.

hinder end the duct grows backwards, and, shortly before the
hatching of the tadpole, acquires an opening into the cloaca,

The condition at the time of hatching is shown in Figs. 74
and 82. The head kidney, KP, is a slightly convoluted tube
opening into the body cavity by the three nephrostomes, ks ; of
which the anterior one is at a level slightly ventral to tliat of
the other two. Opposite the nephrostomes, and projecting into
the dilated dorsal portion of the body cavity, is the glomerulus
(Fig. 82, gm), a sacculated diverticulum of the aorta, the develop-
ment of which has been described above. Behind the nephro-
stomes, the segmental duct is continued backwards in an almost
straight course to the hinder end of the body, where it opens
into the cloaca (Fig. 74, ka).

After the tadpole has hatched, the head kidney increases
considerably in size (Figs. 76, 83, and 84). The tubules of which
it consists become more markedly convoluted; and further
complication is caused by the formation of lateral diverticula, of
which there are three principal ones, directed outwards, and
themselves branched. The three nephrostomial openings persist ;
their walls are formed of pigmented cells, bearing long flagella,
directed inwards. The head kidney reaches its full develop-
ment in tadpoles of about 12 mm. length (Figs. 83 and 84).
when it is almost spherical in shape, and formed of an intri-
cately coiled mass of tubes, imbedded in the greatly dilated
anterior cardinal sinus, vc.

In tadpoles of about 20 mm. length, the head kidneys
commence to degenerate. The first step in the process consists
in irregular dilatation of the tubules, more especially of the
blind lateral diverticula. This dilatation may be so great that
a single tubule may nearly equal in diameter the entire head
kidney of the earlier stages ; but as the dilatation of . some
tubules is accompanied by compression of others, the size of the
head kidney as a whole remains practically unaltered.

This irregular dilatation is accompanied by, and perhaps
due to, partial or complete obstruction of the segmental duct
behind the head kidney. The epithelium of the kidney tubules
soon shows degenerative changes, the cells becoming flattened
out in an irregular manner, their outlines indistinct, the cell con-
tents cloudy, and the inner surfaces of the cells ragged. From
this time degeneration proceeds steadily, and the whole organ

THE HEAD KIDNEYS.

189

diminishes in size. In 40 mm. tadpoles (Fig. 85) the head kidneys
are less than half their former size, and the tubules are completely
collapsed. By the time of the metamorphosis, the head kidneys

AL

. 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. The alimentary canal, from the oesophagus
to the rectum, has been removed, x 22.

A, aorta. AF.l, AF.3, afferent branchial vessels of first and third branchial
arches. AL, lingual artery. CGr, dilatation at the base of the lingual artery. EA,
communication between the afferent and efferent vessels of the first branchial arch, by
farther elaboration of which the carotid gland will be formed. EF.l, EF.3, efferent
I iraiicl dal vessels of first and third branchial arches. GM, glomerulus. KA, segmental
duct. KM, Wolffian tubules. KP, head kidney. KS-1, first nephrostome. KS.3,
third nephrostome. LI, upper lip. L J, lower lip. LP, commencing hind limb. OA,
spout-like aperture of opercular cavity. OP, opercular cavity. PS, sinus venosus.
RT, truncus arteriosus. RV, ventricle. TC, cloaca. TO, oesophagus. TR, aperture
of rectal spout.

have almost disappeared (Figs. 86 and 88), a few pigmented and
slightly twisted tubules alone remaining.

All three nephrostomies disappear : the anterior one is the
first to close, and the middle one soon follows ; the third or most

190

THE FROG.

posterior one persists for a short time longer, and then in its
turn closes, loses its connection with the peritoneum, and
disappears. Shortly before the closure of the third nephrostome,
the head kidney separates completely from the segmental duct,
which then ends blindly in front.

The glomerulus of the head kidney (Figs. 83, 84, gm) arises, as

V.4 X

Fig. 84. — Transverse section across a 12 mm. Tadpole ; the section passing-
through the middle of the length of the head kidney, x 44.

A, aorta. AP, pulmonary artery. BH, medulla oblongata. CH, notochoril.
GI, portion of a gill. GM, glomerulus. KP, tubule of head kidney. KS, second
neplirostome. LA, commencing fore limb. LG, root of lungs. NY, sympathetic
ganglion. OP, opercular cavity. TI, intestine. TO, oesophagus. V.4, fourth
ventricle. VC, posterior cardinal vein. VH, posterior vena cava. VP, pulmonary
vein. "W", liver. X, ganglion of spinal nerve. X', thin roof of fourth ventricle.

described above (p. 182), as a sacculated outgrowth from the
ventral and outer wall of the aorta, bulging outwards into
the body cavity opposite the nephrostomes. At the time of
hatching of the tadpole (Fig. 82) the dorsal portion of the body
cavity is practically the only part present : for although the
splitting of the mesoblast extends down the sides of the body to
the ventral surface, yet the two layers, somatic and splanchnic,

.

THE HEAD KIDNEY AND WOLFFIAN BODY. 191

■e in contact with each other, owing to the distension of the
body by the mass of food-yolk, except at the upper or dorsal
angle. The glomerulus, therefore, appears at this stage to lie
in a special cavity, which later on opens into the general body
cavity, as the yolk becomes absorbed and the abdominal viscera
acquire more definite form.

At a later stage still, the part of the body cavity in which
glomerulus lies becomes partially boxed in, by fusion of
e outer wall of the lung with the peritoneal covering of the
ad kidney (cf. Fig. 84). This inclosure is only an incomplete
e, as the part of the coelom lodging the glomerulus still opens
to the general body cavity, both in front of the root of the lung
d behind this.
The position of the glomerulus, opposite the nephrostomes
the head kidney, and the fact that its development, both
ogressive and retrogressive, keeps pace with that of the head
dney, point to the existence of some close physiological
nnection between the two organs, though it is not easy to
agine of what precise nature this connection can be.
The glomerulus, with the part of the body cavity in which it
lies, may be compared to one of the Malpighian bodies of the
adult kidney ; the glomerulus itself corresponding to the
capillary knot of vessels ; the localised part of the ccelom in
which it lies corresponding to the capsule of the Malpighian
body ; and the nephrostomial tube leading from the ccelom
answering to the neck of the Malpighian body.

3. The Wolffian Body.

The development of the Wolffian body, or kidney, commences
in tadpoles of about 10 to 12 mm. length, by the formation of
the Wolffian tubules (Fig. 83, km). These appear as a series
of small, paired masses of mesoblast cells, lying along the inner
sides of the segmental ducts, between these and the aorta. The
Wolffian tubules develop from behind forwards ; the hindmost,
and at first the largest pair, being a short distance in front of
the cloaca, and the most anterior pair being about three segments
behind the head kidneys. They are at first segmentally arranged,
one pair corresponding to each pair of muscle segments or
myotomes ; but this definite arrangement is early lost in the
hinder region.

192 • THE FROG.

The Wolffian tubules are. at first, solid masses of spherical
cells; these arise in the mesoblast of the body wall, and arc for
a time independent of both the peritoneum and the segmental
ducts. The cellular masses soon become elongated into solid
rods; by separation of the component cells along their axes, flic
rods become tubes; and these tubes, growing outwards and
towards the dorsal surface, meet with the segmental ducts and
open into these.

At the opposite end of each tubule, a Malpighian body is
formed, the end of the tubule being dilated into a bulb-] ike
enlargement, which becomes doubled up, to form the Malpighian
capsule, by the ingrowth of a knot of blood-vessels derived from
a branch of the dorsal aorta (cf. Fig. 87, a).

From the neck, or part of the tubule immediately beyond
the Malpighian body, a short solid rod of epithelial cells arises,
which grows towards the peritoneum and fuses with this. By
separation of the cells the rod becomes tubular, and opens at its
outer or peritoneal end into the body cavity. The peritoneal
opening is a nephrostome (Fig. 87, ks), and the tube into which
it leads may be called the nephrostomial tubule.

Although the walls of the nephrostomial tubules are at first
continuous with the necks of the Malpighian bodies, it is very
doubtful whether the nephrostomial tubules ever open into
the Wolffian tubules ; and, shortly after their first appearance,
in tadpoles of 18 to 20 mm. length, the nephrostomial tubules
break away completely from the Wolffian tubules, and acquire
openings at their inner ends into the renal veins, on the ventral
surface of the kidney. This curious arrangement persists through-
out life. On the ventral surface of the kidney of an adult frog
there are as many as two hundred or more of these nephrostomes
present, as funnel-like depressions or mouths : these lead into
short tubes, lined by flagellated epithelial cells, and running
obliquely inwards into the substance of the kidney, where they
end by opening into the renal veins.

The anterior three or four pairs of Wolffian tubules never
complete their development, but early undergo fatty degenera-
tion. The hinder tubules are at first segmentally arranged, but,
owing to the formation of new tubules, soon become much
more numerous than the segments in which they lie. They in-
crease greatly in length, become markedly convoluted, and soon

THE WOLFFIAN BODIES AND DUCTS.

193

become massed together, with the blood-vessels in relation with
them, to form the definite Wolffian bodies or kidneys (Figs. 85,
86, and 87, km).

1 . The Wolffian Ducts or Ureters.

The segmental duct is at first the duct for the head kidney

Fig. 8;

Fig. 86.

Fig. 85. — A 40 mm. Tadpole, dissected from the ventral surface to show the
heart, the branchial vessels, and the urinary and reproductive organs. The
tail, and the alimentary canal, from the oesophagus to the rectum, have
been removed, x 4.
FlG. 86.— A Tailed Frog, during the metamorphosis, dissected from the ventral
surface to show the urinary and reproductive organs. The alimentary
canal has been removed, x 3|.

A. aorta. AF-1. A3T.4. afferent vessels of first and fourth branchial arches. AL.
lingua] artery. OCr» carotid gland. EF.l, EF-4, efferent vessels of first and fourth
branchial arches. F, fat body. GM, glomerulus. KA, segmental duct. KM-
Wolffian body. KP, head kidney. KU. Wolffian duct or ureter. LA, fore limb.
LI, upper lip. L J, lower lip. LP. hind limb. O. mouth. OB., genital ridge. RT-
truncus arteriosus. RV, ventriele. TOs cloaca. TO, oesophagus. TR, aperture of
rectal spout.

alo

one (Fig. 74, ka) ; in the later stages (Figs. 83 and 85), it
serves for both the head kidney and the Wolffian body.

o

194

THE FROG.

During the metamorphosis, and shortly before the complete
disappearance of the tail, the head kidney separates from the
segmental duct. For a short time, a portion of the segmental
duct still persists in front of the Wolffian body ; but this soon

wd / \ \ f

w Va ti vo

FlG. 87. — Transverse section through the hinder part of the body of a Tailed
Frog during the metamorphosis (cf. Fig. 86). On the right side a spinal
nerve is shown, on the left side the vertebral column, x 17.

A, aorta. CH. notochord. F, fat body. IC, centrum of vertebra. IE, neural
arch of vertebra. KA, Wolffian duct. KS, nephrostome. KT, Wolffian body. LGr,
lung. LS, stomach. LY, subcutaneous lymph space. ML, myotome. NC, spinal
cord. ND, dorsal root of spinal nerve. NN, spinal nerve, main trunk. N"V, ventral
root of spinal nerve. NY, sympathetic nerve ganglion. OV, ovary. PA, pancreas.
SN, spleen. TI. intestine. "VA, anterior abdominal vein. VI, posterior vena cava.
VO, hepatic portal vein. VR, renal portal vein. W, liver. WD, bile duct. WG,
gall bladder.

loses its lumen, undergoes degenerative changes, and disappears :
the remaining, or posterior part of the segmental duct, which is
now connected with the Wolffian body alone, is from this time
spoken of as the Wolffian duct or ureter.

The vesicula seminalis is a glandular body developed in con-

THE WOLFFIAN AND MtJLLEJUAN DUCTS.

195

nection with the hinder end of the Wolffian duct, close to the
cloaca. It attains a large size in the male frog, but is absent or
rudimentary in the female.

GM

Fig. 88.— Transverse section through the anterior part of the body of a Tailed
Frog during the metamorphosis (of. Fig. 86). The section passes through
the second spinal nerve on the right side, and on the left side immediately
in front of this nerve, x 14.

A, aorta. AP, pulmonary artery. CH. notochord. CO, coracoid portion of
shoulder girdle. FN, neural arch of vertebra. FT, transverse process of third vertebra.
G-C, glenoid cavity of shoulder girdle. Q-M, glomerulus. HTJ, head of humerus. KS,
third or posterior nephrostome of head kidney. LG, lung. LT, subcutaneous lymph
space. NC, spinal canal. ND, dorsal root of second spinal nerve. ~N~N, dorsal and
ventral branches of second spinal nerve. NS, spinal cord. WV, ventral root of second
spinal nerve. NY, sympathetic nerve ganglion. RS, sinus venosus. SC, scapular
portion of shoulder girdle. SS, supra-scapular portion of shoulder girdle. TO,
oesophagus. VP, pulmonary vein.

The Miillerian Ducts.

Ihe Miillerian ducts form the oviducts of the female frog ;

they are present in the male as well, but are of very small size,
and apparently of no functional importance. The early stages in
the development of the Miillerian ducts are difficult to investi-
gate, and have not yet been determined with absolute certainty ;

o 2

196 THE FROG.

but there is no doubt that the greater part of the length, and
perhaps the whole, of the duct arises independently of the
Wolffian duct; and apparently from a modified strip of perito-
neal epithelium which runs parallel to the Wolffian duct, along
its outer side.

Towards the close of the metamorphosis, and during the
absorption of the tail, a patch of modified peritoneal epithelium
appears on the ventral surface of the now rapidly disappearing
head kidney, below and to the outer side of the nephrostomes.
of which, as a rule, only one persists at this stage (Fig. 88, KS).
This patch of epithelium differs from the general peritoneal epi-
thelium in its cells being columnar instead of squamous in shape.
A longitudinal groove forms on the surface of this patch of
columnar epithelium, to the outer side of the nephrostome ; and,
by fusion of its lips, the groove becomes a tube, opening in front
into the body cavity, and ending blindly behind. This tube
is the first formed part of the Miillerian duct ; it lies very close
to the anterior end of the segmental duct, but it is not clear
that there is any connection between the two ; it is quite in-
dependent of the nephrostome, which closes up and disappears
very shortly after this stage.

The Miillerian duct extends forwards, as an open groove, some
distance in front of the point at which the duct is first formed :
and, by closure of the lips of the groove from behind forwards,
the mouth of the duct is carried forwards, outwards, and down-
wards, round the anterior end of the body cavity, to its adult
position at the root of the lung.

The growth backwards of the Miillerian duct is effected by
a longitudinal band of cells, which appears a little to the outer
side of the Wolffian duct, but quite independent of this, and
apparently derived directly from the peritoneal epithelium. This
band, which is continuous with the hinder end of the Miillerian
duct, ultimately becomes tubular, and acquires posteriorly an
independent opening into the cloaca.

In the male frog the Miillerian duct stops at this stage.
In the female it undergoes further changes by which it becomes
converted into the oviduct. It increases in length, becoming
sinuous, and ultimately convoluted along the greater part of its
length ; its walls thicken greatly, and the epithelium lining them
gives rise to the gelatinous secretion which is poured out over the

THE MULLERIAN DUCTS AND REPRODUCTIVE ORGANS. 197

eggs as they pass down the duct, and which, on reaching' the
water, swells up to form the mass of the frog's spawn. The
hinder end of the oviduct remains thin-walled, but dilates very
greatly, forming a capacious sac in which the eggs accumulate
before being laid.

6. The Reproductive Organs.

The development of the reproductive organs has already

been described (p. 94), but a few further details may be given

lere, more especially with regard to the relation between the

productive organs and the Wolffian bodies, and the develop-

Lent of the fat bodies, or corpora adiposa.

The reproductive organs appear in tadpoles of about 10 mm.

mgth, as a pair of ridge-like folds of the peritoneum, lying

mg the inner borders of the Wolffian bodies, close to the root

the mesentery (Fig. 85, OR, and 87, ov). These genital folds

)on become more conspicuous, and undergo changes which have

already been described in detail, by which the germinal cells

or gonoblasts are formed.

As the Malpighian bodies develop on the tubules of the
Wolffian body, those lying nearest to the genital ridge give off
hollow tubular diverticula, which, arising from the capsules of
the Malpighian bodies, grow towards the median plane, and
into the substance of the genital ridge (cf. Fig. 87), where they
form the so-called tubuliferous tissue.

So far, the processes of development are the same in both
sexes, but from this point differences occur. In the ovary of
the female the tubules of the tubuliferous tissue expand very
greatly, and give rise to the large axial cavities of the ovary,
of which there are usually about fifteen in the adult.

In the male, the outgrowths from the Malpighian capsules
acquire still more intimate relations with the reproductive organs
than in the female, and become the vasa efferentia of the adult,
which convey the spermatozoa from the testis to the kidney.
The Malpighian bodies with which the vasa efferentia are con-
nected are at first perfectly normal ; but later on they undergo
retrogressive changes, and by the end of the first year their
glomeruli have disappeared, and the Malpighian bodies them-
selves merely remain as slight ampulliform enlargements of the
tubules, which have now completely lost their kidney structure

198 THE FROG.

mid function. The tubules of these Malpighian bodies do not
join those of the other parts of the kidney, but open at once
into the terminal or collecting tubules of the Wolffian body, so
thai there is practically a sharp separation of the kidney into two
portions, excretory and reproductive respectively.

7. The Fat Bodies.

The fat bodies, which are conspicuous and characteristic
structures in the adult frog, are formed by fatty degeneration of
the anterior ends of the genital ridges. In tadpoles of about
40 mm. length (Fig. 85) each genital ridge becomes divided by
a slight constriction into two parts of about equal length. Of
these, the posterior part, oh, forms the smooth genital ridge ;
while the anterior part, F, is irregularly notched along its
ventral surface, and is already commencing to undergo fatty
degeneration. In the later stages (Fig. 86, f) the fat bodies
increase greatly in size, and are produced at their margins into
the characteristic finger-like lobes of the adult.

Histologically, the changes by which the fat body is formed
are due to the accumulation of fat within cells, which are at
first indistinguishable from those of the hinder, or reproductive,
part of the genital ridge.

DEVELOPMENT OF THE SKELETON AND
THE TEETH.

1. The Vertebral Column.

The earliest skeletal structure, and for a time the only one,
is the notochord, the development of which, from the mid-dorsal
wall of the mesenteron, has already been described (p. 109).

The notochord (Fig. 61, Ch) is a cylindrical rod, extending
from the blastopore to the pituitary body ; and growing back-
wards along the tail as this is formed (Fig. 69, ch). It consists
of vacuolated cells, filled with fluid : the cells are all alike, and
their nuclei, which at first are conspicuous, disappear in the
later stages. Around the notochord a delicate structureless
elastic sheath is formed at an early period ; this is really
double, consisting of a thicker inner, and an outer thinner layer ;
both layers are homogeneous, and cuticular in nature.

About the time of appearance of the hind limbs as buds, a

THE VERTEBRAL COLUMN. 199

delicate skeletal tube, at first soft, but soon becoming cartilagi-
nous, is formed from the mesoblast surrounding the notochord
(Fig. 84, Ch). This grows upwards at the sides of the spinal
cord as a pair of longitudinal ridges, with which a series of
cartilaginous arches, which appeared at the sides of the spinal
cord at a slightly earlier stage, soon become continuous (Figs.
87, 88).

By the appearance of transverse lines of demarcation, this
.axial skeleton becomes cut up into a series of nine vertebrae,
followed by a posterior unsegmented portion, which later on
gives rise to the urostyle. The vertebrae are cut off in order,
from before backwards, and the division at first involves the
cartilaginous tube alone, the notochord remaining as a continuous
structure until the complete absorption of the tail, at the end of
the metamorphosis (Fig. 89).

Shortly before the metamorphosis, thin rings of bone, slightly
constricted in their middles so as to be hour-glass shaped in
section, are developed in the membrane investing the cartila-
ginous sheath of the notochord ; these from their first appear-
ance correspond with the nine vertebras, to the bony centra of
which they give rise. Like the cartilaginous vertebras, they
develop in order from before backwards.

In the intervertebral regions, between the successive bony
rings, annular thickenings of the cartilaginous sheath occur,
which grow inwards so as to constrict, and ultimately obliterate
the notochord. Each of these intervertebral rings, after the
metamorphosis, becomes divided into anterior and posterior
portions, which fuse with the bony centra of the adjacent
vertebrae, and ossify to form their articular ends.

From the circumference, and from the articular ends of each
vertebra, ossification gradually spreads inwards ; but a small
portion of the notochord persists in the middle of each centrum
for a long time, or even throughout life.

The vertebrae do not lie opposite the muscle segments or
myotomes, but alternate with these ; so that each vertebra is
acted on by two myotomes on each side, one pulling it forwards,
and the other backwards.

The transverse processes arise independently of the corre-
sponding vertebrae, bat very early fuse with these. They extend
along the connective-tissue septa between the successive

200

THE FROG.

myotomes, and very probably correspond to the ribs of other
Vertebrates.

The urostyle is the hindmost part of the axial skeleton, which

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THE VERTEBRAL COLUMN AND THE SKULL. 201

chord : posteriorly, it extends back as a flattened epicliordal
plate above the notochord, and a thick median hypochordal
rod below it. During the shortening of the tail, on the com-
pletion of the metamorphosis, the epicliordal and hypochordal
cartilages extend and meet, so as to completely surround the
notochord. Later still, the cartilage becomes in great part
replaced by bone ; but the hinder end persists as a terminal
plug of cartilage throughout life.

The anterior end of the notochord, imbedded in the base of
the skull, is gradually encroached upon by the cartilage and
bone around it ; and ultimately, in half-grown frogs, is completely
bsorbed.

2. The Skull.

a. The skull of the tadpole. The skull of the tadpole consists
almost entirely of cartilage ; none of the bones of the skull,
with the exception of the parasphenoid, appearing until a short
time before the metamorphosis.

In the cartilaginous skull of the tadpole two main elements
may be distinguished :

(i) The cranium, with the olfactory and auditory capsules,
which may conveniently be taken together.

(ii) The visceral skeleton, which consists of a series of
cartilaginous bars developed in the visceral arches, and en-
circling the pharynx.

Speaking generally, the skull as a whole develops from
before backwards ; the first formed parts being at the anterior
end, in connection with the jaws ; and the hinder end of the
skull being very imperfect in the early stages of tadpole life.

True cartilage does not appear until the tadpole is about 10
to 11 mm. in length; i.e. when the opercular covering of tin1
gills is almost completed, and the hind limbs are visible as
minute buds at the base of the tail. Long before true cartilage
is present, however, the skeletal elements can be readily distin-
guished, as tracts of condensed and modified mesoblast.

The first stage in the histological differentiation, by which
cartilage is formed, consists in a number of mesoblast cells
becoming more closely compacted together ; these cells have
large nuclei and very scanty protoplasm, and are further
distinguished from the surrounding mesoblast cells by having

202 THE FROG.

only a slight tendency to develop processes, and by being
almost completely devoid of yolk-granules.

Fig. 90

RL PN

CH BP* BR.3BR.2 BR.i

Fig. 91.

Fig. 92.

Fig. 90.— The skull of a 12 ram. Tadpole, seen from the right side. The
notochord, the brain, and the entire head are represented in outline, in
order to show the relations of the skull to them, x 30.

Fig. 91. — The same skull from the dorsal surface. The lower jaw, and the
hyoidean and branchial bars are omitted, x 80.

Fig. 92. -The same skull from the ventral surface, x 30.

BB, basi-branchiaL BH. roof of land-brain. BM, roof of mid-brain. BR.I.
BB2, BR.3. BR4, first, second, third, and fourth branchial bars. BS, cerebral
hemisphere. CH, notochord. EC, auditory capsule. HB, basihyal. HO, urohyal.
HQ, articulation of ceratohyal with quadrate. HR, ceratohyal. JL, lower jaw. j~U.
upper jaw. LI, upper lip. L J, lower lip. LL, lower labial cartilage. LIT, upper
labial cartilage. MC, Meckel's cartilage. PKT, pineal body. Q,, quadrate. Q,0,
orbital process of quadrate. Q,P, palato-pterygoid process. Q,R, connection of quadrate
with trabecula. RC. parachordal cartilage.' RL, trabecula cranii. SA, membranous
patch in the outer wall of the auditory capsule, in which the stapes is developed at a
slightly later stage. X. choroid plexus of third ventricle.

In the next stasre the cells become still more closely com-

THE SKULL. 203

pacted, so that the nuclei of adjacent cells lie very close to one
another, while the protoplasm of the several cells becomes fused
into a continuous plasma or matrix. Very slight further changes
convert this into true cartilage.

(i) The cranium or brain case. This, in its fully formed
condition (Fig. 93), is an unsegmented cartilaginous tube, in-
complete dorsally, and inclosing the brain. It is developed in
the following manner.

Shortly after hatching of the tadpole, a pair of longitudinal
p< ils of condensed mesoblast, the trabecule cranii, appear, one
on either side of the anterior end of the brain. These are at
first entirely in front of the notochord (cf. Fig. 64) : their pos-
terior ends lie alongside the fore-brain ; while anteriorly they
are flattened dorso-ventrally, and fused to form a horizontal
plate lying between the two nasal sacs ; the extreme anterior
ends of the trabecule) separate again, and bend downwards into
the upper lip, which they support.

As the tadpole grows, the trabecular rapidly extend back-
wards along the sides of the infundibulum, and soon reach the
anterior end of the notochord ; they continue backwards along-
side the notochord, and in close contact with this, as a pair of
horizontal rods, the parachordals, which with the notochord form
the floor of the brain case.

In tadpoles of 12 mm. length (Figs. 90, 91, and 92) the
trabecules have become cartilaginous ; their extreme anterior
ends chondrify independently as a pair of thin curved plates,
the upper labial cartilages, LU, which support the upper lip.
Behind the labial cartilages the two trabecule remain separate
for a short distance, but soon fuse to form a median horizontal
plate of cartilage, lying between the two nasal sacs, and sup-
porting the extreme anterior end of the brain ; behind this they
are continued backwards as two parallel bars of cartilage, lying
at the sides of, and slightly ventral to the brain ; behind the
infundibulum, and just in front of the anterior end of the noto-
chord (Fig. 91), the two trabecule? are again connected by a
narrow transverse bridge of cartilage, beyond which they continue
backwards, as two narrow parachordal bands, RC, along the sides
of the notochord, to the hinder end of the skull.

In the later tadpole stages the trabecules extend horizontally

204 THE FBOCh

Inwards bo form the floor of the skull, while their outer edges
grow upwards, forming low ridges along the sides of the brain
in the ethmoidal region, m front of the eye, and in the occipital
region behind the ear. Opposite the eyes, the side walls of the
cranium are formed by a pair of cartilaginous plates, which appear
independently, in the membrane at the side's of the brain, and
soon extend ventralwards to meet, and fuse with, the trabeculae.

The side walls and roof of the hinder part of the cranium are
mainly formed by the auditory capsules. At an early period
the mesoblast surrounding each auditory vesicle forms a definite
capsule around it : in this a thin shell of cartilage is formed,
first in the outer wall, but soon spreading round to the floor,
where it becomes continuous with the outer edge of the para-
chordal cartilage (Figs. 70, 90, 91, EC). This cartilaginous
auditory capsule spreads more slowly over the dorsal surface of
the auditory vesicle, and then extends downwards, between the
brain and the ear, to form the side wall of this part of the
skull. Later still, in tadpoles of about 20 mm. length, the
.cartilage of the dorsal borders of the auditory capsules spreads
inwards across the top of the brain case, and so completes its
roof in this region.

The hindmost end, or occipital region of the skull, is formed by
upgrowth of the edges of the parachordal cartilages. This occurs
very slowly, and quite independently of the auditory capsules ; it
is not completed until shortly before the metamorphosis.

In the outer wall of each auditory capsule, a large oval or
circular patch remains unchondrified : this is the fenestra ovalis
(Fig. 90, sa). In the middle of this membranous patch the stapes
appears, in tadpoles of about 16 mm. length, as a cartilaginous
nodule (cf. Fig. 93, sa), the further development of which will
be considered later.

At the anterior end of the cranium, the nasal or olfactory
organs become roofed in by cartilage, which arises as a vertical
crest from the upper surface of the median internasal plate
formed by the trabeculae, and spreads out right and left as a
pair of thin horizontal plates, covering over the olfactory organs,
and forming the cartilaginous olfactory capsules (Fig. 93, of) .

It thns appears that the trabeculae, with their posterior con-
tinuations, the parachordals, give rise to the floor of the cranium
along its whole length ; they also, by upgrowth of their edges,

THE SKULL. 205

form the sides and the roof of the extreme anterior and posterior
ends of the skull. In the orbital region the side walls of the
skull arise independent!}', but grow down to, and fuse with, the
trabecula? ; while the roof remains membranous. Finally, in
the auditory region, both the sides and roof are formed as exten-
sions of the auditory capsules.

|(ii) The visceral skeleton. This consists of a series of cartila-
inous hoops or bars (Figs. 90, 92), developed in the mandibular,
tie hyoidean, and the four branchial arches, which tend to encircle
lie pharynx. Each hoop consists of right and left halves, which
re independent at their dorsal ends, but fused or closely connected
entrally. Like the cranium itself, the visceral skeleton develops
from before backwards.

The mandibular bar is one of the very earliest parts of the
skull to be developed ; it may be recognised at the time of
batching of the tadpole as a curved band of condensed mesoblast,
lying transversely across the floor of the mouth, close to its
anterior end. The outer ends of the bar are enlarged, and pro-
duced upwards into vertical processes, which lie at the sides of
the mouth cavity, and are continuous above with the trabecular
cranii at a level between the nose and eye ; a connection which
afterwards gives rise to the palato-pterygoid bars.

After hatching, the mandibular bar grows rapidly, and under-
goes important changes. It becomes divided on each side into
three parts : — (i) a small anterior segment, which later on becomes
the lower labial cartilage, and which is continuous across the
median plane with its fellow of the opposite side ; (ii) a small
middle segment, which becomes later the basis of the lower jaw,
or Meckel's cartilage : and (iii) a hindmost or quadrate segment,
which is much the largest of the three, and which is connected
with the trabecula by the palato-pterygoid bar mentioned above,
while from its outer side a dorsally directed, leaf-like, orbital
process projects upwards.

On the appearance of cartilage, in tadpoles of about 12 mm.
length, the condition of the mandibular bar is as shown in
Figs. 90, 91, and 92. The hinder part of the bar forms the
quadrate cartilage, Q, a stout horizontal subocular bar, which
lies parallel to the trabecula, but some distance from this, and
along the under and outer surface of the eyeball. The hinder end

206 THE FROG.

of the quadrate turns sharply inwards, QR, and is connected with
i he 1 rabecula just in front of the ear capsule, EC. From the outer
edge of the quadrate the prominent, vertical, orbital process, qo,
projects upwards ; and the anterior end of the quadrate, like its
posterior end, turns sharply inwards, to form the palato-pterygoid
bar, qp, which is fused with the trabecula, between the nose and
the eye. The quadrate may thus be described as a horizontal
bar of cartilage, lying to the outer side of the trabecula, and
connected with this by two struts, one in front of the eye and
one behind it.

The second segment of the mandibular bar, or Meckel's
cartilage, MC, is a short, stout bar of cartilage, which runs
inwards and forwards across the floor of the mouth. The third
segment, or lower labial cartilage, LL, is a much smaller bar at
the inner end of Meckel's cartilage, which meets, or is actually
fused with, its fellow of the other side in the median plane.

The chief further changes that occur in the mandibular bar
during the later periods of tadpole life (cf. Fig. 93) : — are (i) a pro-
gressive diminution in size of the orbital process, qo ; and (ii) the
development of an additional connection between the hinder end
of the quadrate and the auditory capsule, by means of an otic
process (Fig. 93, qe).

At the time of the metamorphosis, very considerable and
important changes take place, which will be described later on.

The hyoid bar appears directly after the mandibular bar, and
immediately behind this. At first the right and left bars are
separate ventrally, but they soon become connected by a median
plate. The hyoid bar thickens rapidly, and at 12 mm. (Figs.
90, 92) forms a broad, somewhat S-shaped plate of cartilage, the
ceratohyal, hr, which lies across the floor of the mouth. The
outer end of the ceratohyal is enlarged, and turned upwards, and
articulates with the ventral surface of the quadrate about the
middle of its length (Fig. 90, hq). The inner ends of the two
ceratohyals are connected, across the floor of the mouth, by a
median basihyal cartilage, hb, the posterior end of which grows
back as a keel-like urohyal process (cf. Fig. 65, hb), which
separates the two halves of the thyroid body from each other.

The branchial bars are a series of four pairs of cartilaginous
rods, developed in the four branchial arches of each side. They
appear in order, from before backwards, as slender curved bands

THE SKULL. 'J07

of cartilage (Fig. 92, BR.i to BR.4). The dorsal ends of the four
rs of each side very early become connected together. The
ntral ends are at first independent, but the ends of the first
ranchial bars early unite to form a broad median basibranchial
ate (Fig. 92, bb), with which the ventral ends of the hinder
anchial bars soon join.
The chief characteristic of the skull of the tadpole is the
ly development, and large size, of the cartilages of the extreme
terior end, in connection with the jaws. The early formation
d great size of the anterior ends of the trabecule, the large
ial cartilages, the early attachment of the mandibular arch to
e anterior end of the trabecula by the palato-pterygoid bar,
e enormous and singularly shaped orbital process of the
adrate, and the massive character of the hyoid arch in the
r of the mouth, are all connected with the need for a firm
pporting skeleton for the horny jaws, and an extensive sur-
e of origin for the muscles by which these jaws are worked.
1 these are characters which concern the tadpole, and not the
and they are all lost, or greatly modified, in the adult
nimal.

b. The skull of the frog. About the time of the metamorphosis,
the skull undergoes important changes, by which the adult
condition and proportions are attained.

A complete cartilaginous floor is formed to the skull, between
the two trabecule. The hinder end of the skull acquires definite
shape : the parachordals increase greatly in size, growing inwards
so as to encroach upon, and ultimately to obliterate the notochord ;
they fuse with the auditory capsules and grow up behind these
to complete the occipital ring. The side walls and roof of the
skull are formed in the manner already described (p. 204). The
nasal capsules are formed by expansion of the anterior ends of
the trabecule ; the upper labial cartilages persisting as the
pro-rhinal cartilages, which bound the anterior nostrils in front.
/V thin cartilaginous shell is formed in the sclerotic coat of
h eye, but remains throughout life free from the skull proper.
In the mandibular arch great modifications occur, associated
with the change from the small ventral mouth of the tadpole to
the wide, slit-like, and terminal mouth of the frog. The anterior
end of the quadrate rotates downwards and backwards, causing
great lengthening of the palato-pterygoid bar (Figs. 90 and 93,

~

208

THE FROG.

qp), which becomes drawn out to form the framework of the
upper jaw. At the same time, Meckel's cartilage, MC, forming
the basis of the lower jaw, becomes also greatly lengthened,
while the lower labial cartilages disappear. At the time of the
metamorphosis, the quadrate still slopes downwards and forwards
(Fig. 93) ; but the rotation continues until the quadrate becomes
vertical, and finally, before the end of the first year, acquires
the inclination downwards and backwards so characteristic of

BR* BB BR.2 HR

Fig. 98. — Skull of a Tailed Frog towards the close of the metamorphosis, from
the right side. The head and eye are represented in outline, x 13.

BB. bagibranohial. BR.2, BR4, second and fourth branchial bars. CL. columella.
EC auditory capsule. HE, ceratohyal. MC, Meckel's car-tilage. OC, outline of eye.
OF, olfactory capsule. OL, outline of lens. ON, foramen for optic nerve. Q.
quadrate. QE. connection of quadrate with auditory capsule. QO, orbital process of
quadrate. QP. palato-pterygoid process. SA, stapes.

the adult (Fig. 94). The orbital process of the quadrate early
becomes inconspicuous (Fig. 93, QO), and finally disappears.

The hyoid bar, which is massive in the tadpole, becomes very
slender in the frog. It gradually elongates, extending dorsal-
wards until it meets, and fuses with, the ventral surface of the
auditory capsule (Fig. 94, b).

The branchial bars become more and more slender as the
gills begin to shrink, and ultimately disappear almost completely.
The basihyal and basibranchial cartilages give rise to the body
of the hyoid (Fig. 94, h) : from the ventral ends of the fourth

THE SKULL.

209

branchial bars, the backwardly directed tliyrolryals (Fig. 94, t)
e formed ; while the rest of the fourth branchial bar, and the
ole of the first three bars disappear.

The fenestra ovalis has been described above as a large

rture, left in the outer and ventral wall of the auditor}'

sule (Fig. 90, sa). This aperture is closed by membrane,

d in this membrane, shortly before the metamorphosis, the

,pes is formed as a disc-like plug of cartilage (Fig. 93, sa).

The columella appears, in tadpoles of about 40 mm. length

cf. Fig 93, cl), as a minute bar of cartilage immediately in

Int of the stapes, and with its outer end directed forwards,
the time of the metamorphosis the columella grows rapidly,

-PM

\MM

FlG. 94. — The skull of an adult Frog, from the right side. >- 2.

A. parasphenoid. AS. angulosplenial. B, anterior cornn of hyoid. C, columella
D, dentary. E, exoccipital. F, nostril. FP, frontoparietal. BE, body of hyoid. L
aperture fox exit of optic nerve. M. maxilla. MM, mento-meckelian. M'. aperture
for exit of fifth ami seventh nerves. N", nasal. O, pro-otic. P, pterygoid. PM, pre-
maxilla. Q,. quadratojugal. It, aperture for exit of ninth and tenth nerves. S.'
squamosal. SE, spheu-etlunoid. T. posterior cornu of hyoid, or thyrohyal.

and becomes rotated outwards, so that its outer end comes into
close relation with the tympanic membrane, while its inner end
fuses with the stapes. At first, the columella lies in the dorsal
wall of the tympanic cavity, but this latter gradually extends
upwards around it, until the columella acquires its adult relations,
and appears to cross the tympanic cavity (cf Fig. 68, C, p. 141).
In its actual development, the columella of the frog shows no sign
of any connection with either the hyoid or mandibular bars, but
comparison with other vertebrates renders it very possible that
the frog is in this, as in so many other respects, in a modified
rather than a primitive condition.

The bones of the skull are of two kinds : — (i) cartilage boner,
which are formed in direct connection with the primary er

'210

THE FROG.

cartilaginous skeleton ; and (ii) membrane bones, which arise
independently of the cartilaginous skeleton, although they may
in their later stages become firmly grafted on to this.

The membrane bones of the skull appear in a connective-
tissue layer, very rich in cells, which is found immediately
outside the cartilaginous cranium. In this connective tissue,
trabeculse and spicules of calcified substance appear, and soon
interlace to form a network ; surrounding the spicules are osteo-
blasts or bone cells, by which the further growth of the framework

is effected, a consistent bony
lamella with imbedded bone cells
being ultimately formed.

The first bone to be developed
is the parasphenoid, which ap-
pears, in tadpoles of about
20 mm. length, in the connective
tissue underlying the floor of the
skull. The exoccipitals, and the
frontals and parietals, which are
at first separate from one another,
soon follow. The premaxilla,
maxilla, dentary, and angulare
are formed at the commencement
of the metamorphosis; and
towards its close the vomer,
palatine, pterygoid, and other
bones appear.
By the time the metamorphosis is completed, and the tail
absorbed, all the bones of the skull are present except the
sphen-ethmoid, which appears rather late, in the course of the
first summer, as a narrow transverse splinter of bone, crossing
the roof of the skull near its anterior end.

Fig. 95.— The skull of an adult
Frog, from the ventral surface.
xlf.

a, parasplienoid. c, columella.
e, exoccipital. f.p, frontoparietal.
in, niaxilla. n, vomer, o, pro-otic.
p, pterygoid, pa, palatine, pm, pre-
maxilla. q, quadrato-jugal. *e,
.splien-ethmoid.

3. The Teeth.

In the frog the teeth are confined to the premaxillas, maxilla?,
and vomers, the lower jaw being edentulous.

The upper jaw bears a single row of small conical teeth
arranged along its inner border, each tooth being attached to
the bone at its base and along its outer surface, and only a very
small part of the tooth projecting freely.

THE TEETH. 211

Each tooth is a hollow cone ; the central or pulp-cavity con-
taining the blood-vessels and nerves. The basal part of the cone
consists of bone, the apical part of dentine, capped at the tip by
a very thin layer of enamel. The teeth are readily lost, and are
replaced by new ones developed below them.

The teeth do not begin to form until the time of the meta-
morphosis. Hound the border of the jaw, a solid ridge-like
ingrowth of the deeper layer of the epidermis takes place into
le underlying connective tissue ; and opposite the edge of this
je a series of small processes, the dentinal papilla?, are formed
the dermis. These papillae, which are very rich in cells,
>w into the epidermal ridge, which thus forms a cap over each
them. The inner lining of each cap, immediately covering
papilla, consists of a single layer of short columnar cells, the
imel cells ; while the rest of the cap, which is three or four
is thick, consists of indifferent cells, the outermost layer of
rich forms a more or less definite capsule.

Of the hard tissues of the tooth, the thin cap of enamel is
•med by calcification of the ends of the enamel cells next to
Le papilla. The dentine is formed by calcification of the sur-
face layer of the papilla itself, the cells of the papilla sending
processes into the dentinal substance while it is forming.

The young tooth now separates from the epithelial ridge,
and moves towards the surface ; it lengthens, by formation of
bony matter at its base, but does not acquire its definite attach-
ment to the bones of the jaw until some time after the comple-
tion of the metamorphosis, usually during the autumn of the first
year. The dermal papilla persists as the pulp of the tooth.

The replacing teeth are developed in precisely similar fashion,
and from the original epidermal ridge. They lie at first to the
inner side of the first row of teeth, but during their development
shift their positions so as to lie directly above these. By further
growth downwards, accompanied by absorption of the bony
bases of the teeth in use, the new teeth move towards the
surface; they often lie for a time partly within the pulp cavities
of their predecessors, and, as these latter fall out, speedily grow
into their places.

The vomerine teeth are straighter than those of the margin
of the jaw, but are otherwise similar to these, both in structure
and in mode of development.

212 tiik EROGh

4. The Appendicular Skeleton.

The limbs arise about the time of completion of the oper-
cular fold, and shortly after the opening of the moutli. i.e. in
tadpoles of about 11 or 12 mm. length.

The hind limbs (Fig. 83, LP) appear as a pair of si null
rounded buds from the ventral surface of the hinder part of the
body wall, at the base of the rectal spout.

The fore limbs (Fig. 84, la) are similar buds, which arisl
about the same time, from the sides of the body wall at
anterior end, opposite the head kidneys. They lie in the dorsal
angle between the body wall and the opercular fold, and, being
covered by this latter, are not visible from the surface.

The limbs grow from the somatopleure alone ; each is a solid
mass of compact mesoblast, covered by a cap of epidermal cells.
which differ in their cubical or columnar shape from the flat-
tened cells of the general surface of the body.

The limbs at first grow slowly. They gradually elongate,
become segmented, and then divided distally into fingers and toes
(Fig. 85, la, lp). Up to the time of the metamorphosis the
hind limbs are small, while the fore limbs remain concealed
within the opercular cavity. During the metamorphosis (Fig.
86), both pairs of limbs grow rapidly, more especially the hind
limbs.

The skeleton of the limbs, and of the limb-girdles by which
the limbs are attached to the body, does not assume definite
form until a short time before the metamorphosis.

a. The pectoral girdle develops as two half-rings of carti-
lage, one on each side of the body, which they encircle at the
level of the second or third vertebra. The dorsal ends of the
half-rings (Fig. 88, ss) lie superficially to the transverse pro-
cesses of the vertebras, ft, and are connected with these by
muscles and ligaments ; the ventral ends, CO. meet each other
in the median plane.

Each half-ring has on its outer surface, rather below the
middle of its length, a cup-shaped depression, the glenoid
cavity, GC, with which the head of the humerus, hu, articu-
lates. The part of the ring above the glenoid cavity is the
scapular portion, the part below it the coracoid portion.

The scapular portion is divided, shortly before the metamor-

THE APPENDICULAR SKELETON. 213

pilosis, into a dorsal blade-like part, the suprascapula, ss, which
remains in great part cartilaginous throughout life ; and a
ventral, more slender, and shaft-like part, the scapula, sc, round
which a ring of bone soon forms.

The coracoid portion is, from the first, split into two diver-
ging processes ; an anterior or pre-coracoid portion, and a pos-
terior or coracoid proper. The ventral ends of the coracoid and
pre-coracoid of each side grow towards each other and meet,
forming a longitudinal band of cartilage, the epi-coracoid ; the
two epi-coracoids lie in close contact with each other in the
median plane, but do not fuse. Along the anterior border of
the pre-coracoid cartilage, a bony rod, the pre-coracoid bone or

•vicle, is formed ; and around the coracoid cartilage a tubular
lath of bone, the coracoid bone, is developed. There is thus
first no trace of a sternum, either as a median or paired
structure.

During and after the metamorphosis further changes occur.
The bones increase in size, especially the scapula and coracoid.
The two epi-coracoid cartilages, in place of merely meeting in
the median plane, overlap each other to a certain extent, the
left epi-coracoid lying dorsal to the right one. From the anterior
ends of the epi-coracoid cartilages a pair of small processes grow
forwards ; these soon fuse to form the omosternum, which rapidly
increases in size. Behind the epi-coracoids, and in close contact
with them, a pair of cartilaginous bands appear, which fuse
together to form a flat median plate of cartilage ; this gives rise
in front to the sternum, round which a ring of bone soon forms,
and behind to the xiphi-sternum, which remains permanently
cartilaginous.

b. The pelvic girdle also consists at first of two half-rings of
cartilage, encircling the hinder part of the trunk. The ventral
ends of the half-rings, which are flattened and expanded, are in
c"ii fact in the median plane, and very early fuse firmly together
to form the pelvic symphysis. The dorsal ends are more slender :
at first they lie free in the muscles of the body wall ; later on
they become connected with the transverse processes of the
ninth or sacral vertebra.

Each half-ring has on its outer surface, close to its ventral
end, a depression, the cotyloid or acetabular cavity, for articula-
tion with the head of the femur. The part above the acetabulum ,

<2U

THE I HOG,

which corresponds will) fche scapular portion of the pectoral
girdle, ossifies as the ilium ; in the part below the acetabulum,
the anterior or pubic- portion remains cartilaginous, while the
posterior portion ossifies as the ischium.

The pelvic girdle is, in its early stages, and until shortly

Fig. 90.

The .skeleton of the Frog, seen from the dorsal surface,
supra-scapula and scapula have been removed.

The left

a, astragalus, c, calcaueum. d, supra-scapula. e, exoccipital. /, femur, fp, fronto-
parietal, g, metacarpals, h. humerus, i, ilium, k, metatarsals. 7, carpus, m, maxilla.
n, nasal, o, pro-otic, p, pterygoid, pm, premaxilla. q, quadrato-jugal. r, radio-ulna.
s, squamosal. »<?, sphen-ethmoid. sv, sacral vertebra, t, tibio-fibula. u, urostyle.

before the metamorphosis, about the same length as the pectoral
girdle ; and, like this, lies at right angles to the vertebral
column. As the hind limbs lengthen, during and after the
metamorphosis, the ventral or acetabular end of the girdle moves
backwards, so that the ilium, which lengthens considerably at

THE APPENDICULAR SKELETON. 215

the same time, now lies, as in the- adult, almost parallel to the
backbone ; and the hip joint is shifted to the extreme hinder
end of the body.

c. The limbs. The skeletal framework of each limb develops
from the proximal towards the distal end ; the humerus and the
femur being the first bones to be differentiated. The radius and
La. and the tibia and fibula, are at first independent of each
Ler, but the cartilages of each pair soon fuse. Of the digits,
two preaxial, radial or tibial, of each limb are established
it ; the others budding out in succession. There is some
lubt as to which of the normal five digits is absent in the
id of the adult frog ; and the embryological evidence, though
>t conclusive, is rather in favour of the view that it is the fifth,
not, as is commonly held, the first digit or pollex that is
i,ntin g.

it of the more important Publications dealing with the Development

of Frogs.

cock, T. : 'On the Development of the Common Frog.' Memoirs of the

Manchester Literary and Philosophical Society, series hi., vol. viii. 1883.

Assheton, R. : ' The Development of the Optic Nerve of Vertebrates.' Quarterly

Journal of Microscopical Science, vol. xxxiv. 1892.
Balfour, F. M. : 'A Treatise on Comparative Embryology,' vol. ii. 1881.
Bambeke, C. van: ' Recherches sur le Developpement du Pelobate brun.' 18(>7.
' Nouvelles Recherches sur l'Embryologie des Batraciens.' Archives
de Biologie, i. 1880.
Barfurth, D. : ' Versuche iiber die Verwandlung der Froschlarven.' Archiv
fur mikroskopische Anatomie, xxix. 1887.

• Die Ruckbildung cles Froschlarvenschwanzes und die sogenannten
Sarkoplasten.' Archiv fiir mikroskopische Anatomie, xxix. 1887.
Bedot, M. : ' Recherches sur le Developpement des Nerfs Spinaux chez les

Tritons.' Archives des Sciences Phys. et Nat., xi. 1881.
Boas, J. E. V. : ' Ueber den Conus arteriosus und die Arterienbogen der Ampbi
bien.' Morphologisches Jahrbuch, vii. 1881.

' Be it rage zur Angiologie der Amphibien.' Morphologisches Jahr-
buch, viii. 1882.
Bourne, A. Gr. : 'On Certain Abnormalities in the Common Frog.' Quarterly

I Journal of Microscopical Science, xxiv. 1884.
?e, E. D. : ' On the Relations of the Ilyoid and Otic Elements of the Skeleton
in the Batrachia.' Journal of Morphology, ii. 1888.
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difterents Ages.' Memoires de l'Academie des Sciences. Paris. 1835.
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^10 THE FROG.

Ecker, A. : ' Ioones Physiological.' Leipzig. 1851-1859.

'The Anatomy of the Frog.' English translation by G. Haslam]

Oxford. 1889.
Krlanger, 11. von: ' Ucber den Blastoporus der anuren Amphibien, sein

Schicksal und seine Beziehungen zum bleibenden After.' Zoolo-

gische Jahrbiicher : Abtheilung fur Anatomie und Ontogenie derThiere,

iv. 1890.
Field, H. F. : ' The Development of the Pronephros and Segmental Duct in

Amphibia.' Bulletin of the Museum of Comparative Anatomy at Har-
vard College, xxi. 1891.
Fiirbringer, M. : ' Zur Entwickelung der Amphibienniere.' Heidelberg. 1877.
Gatehouse, J. W. : ' The Development and Life History of the Tadpole.'

Journal of Microscopical and Natural Science, i. ii. 1888 and 1889.
Goette, A. : ' Die Entwickelungsgeschichte der Unke.' Leipzig. 1875.
Goppert, E. : ' Die Entwicklung und das spiitere Verhalten des Pankreas der

Amphibien.' Morphologisches Jahrbuch, xvii. 1891.
Heron-Royer and C. van Bambeke : ' Le Vestibule de la Bouche chez les

Tetards des Batraciens Anoures d'Europe.' Archives de Biologie, ix.

1889.
Hertwig, O. : ' Ueber das Zahnsystem der Amphibien und seine Bedeutung

fiir die Genese des Skelets der Mundhohle.' Archiv fiir mikroskopische

Anatomie, xi. Supplement. 1874.

• Beitrage zur Kenntniss der Bildung, Bef ruchtung und Theilung des

thierischen Eies.' Morphologisches Jahrbuch, iii. 1877.

' Die Entwicklung des mittleren Keimblattes der Wirbelthiere.'

Jenaische Zeitschrift fiir Naturwissenschaft, xvi. 1883.
Higginbotham, J. : ' Influence of Physical Agents on the Development of the

Tadpole of the Triton and the Frog.' Philosophical Transactions of the

Royal Society. 1 850.
Hinckley, Mary H. : ' Notes on the Development of Rana sylvatica.' Pro-
ceedings of the Boston Society of Natural History. 1882.
Hochstetter, F. : • Beitrage zur vergleichenden Anatomie und Entwickelungs-
geschichte des Venensystems der Amphibien und Fische.' Morpholo-
gisches Jahrbuch, xiii. 1887.
Hoffmann, C. K. : ' Zur Entwicklungsgeschichte der Urogenitalorgane bei den

Anamnia.' Zeitschrift fiir wissenschaftliche Zoologie, xliv. 1886.
Bronn's Klassen und Ordnungen des Thierreichs : Amphibia. 1 873-

1878.
Houssay, F. : ' Etudes d'Embryologie surles Vertebres.' Archives de Zoologie

Experimentale, 1890 ; and Bulletin Scientifique de la France et de la

Belgique, 1891.
Howes and Ridewood : « On the Carpus and Tarsus of the Anura.' Proceedings

of the Zoological Society. 1888.
Huxley, T. H. : Article ' Amphibia.' Encyclopajdia Britannica, 9th edition.

1875.
Kolliker, A. : ' Note sur le Developpement des Tissus chez les Batraciens.'

Annales des Sciences Naturelles. 1840.
Lambotte, M. : 'Observations Anatomiques et Physiologiques sur les Appareils

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Journal of Microscopical Science, xxxiii. 1892.

BIBLIOGRAPHY. 217

Malbranc, M. : ' Von der Seitenlinie und ihren Sinnesorganen bei Amphibien.'

Leipzig. 1875.
Marshall, A. Milnes : ' On Certain Abnormal Conditions of the Reproductive
Organs in the Frog.' Journal of Anatomy and Physiology, xviii. 1881.
'The Frog: an Introduction to Anatomy, Histology, and Embryology.
4th edition. 1891.
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' The Development of the Blood-vessels in the Frog." Studies from
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' Die Kiemen und ihre Gefasse bei Urodelen und Anuren.' Morpho-
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' Die erste Anlage der Milz und das erst e Auftreten von lym-
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1890.
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%21H THE FROG.

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Anatomischer Anzeiger, vii. 1892.

219

Chapter IV.
THE DEVELOPMENT OF THE CHICK.

GENERAL ACCOUNT.
Historical Sketch.

The development of the chick has attracted great attention
account of the ease with which embryos of any desired age
y be obtained, and of the shortness of the period within
ich the embryonic development is completed. Almost all
earlier investigations into the development of animals were
made on chick embryos, and it is round the chick that the most
famous embryological controversies have centred. Even at the
present day, on account of their greajb convenience for laboratory
purposes, chick embryos usually afford the material from which
the student derives his first lessons in practical embryology.

Embryology as a science is barely three centuries old ; the
earliest descriptions and figures of the development of the chick
within the egg, that are of any real value, are contained in two
treatises published by Fabricius, professor at Padua, in 1600 and
1604. Half a century later, Harvey added important details in
his ' Theoria Generationis ; ' and towards the close of the seven-
teenth century, in 1687, Malpighi published the first accounts
of chick embryos based on microscopical examination.

During the eighteenth century facts accumulated rapidly,
but the theories quite outpaced them ; and the current doctrine
throughout the century, supported by many, and notably by
Haller, was that of Preformation, according to which the chick
was stated to be present in the egg at the time it is laid ; all its
its and organs being there from the beginning, but in an
remely minute and unexpanded condition ; the development
the embryo being regarded as a process comparable to the
unfolding and enlargement of the several parts of a bud to form
the perfect flower.

was

=

220 THE CHICK.

This theory of Preformation was vigorously combated by
Caspar Friedrich Wolff, who in 1759, when only twenty-six
years old, published as a thesis for the doctor's degree his theory
of Epigenesis, which offered an entirely new explanation of the
mode of development of the chick and other animals. Wolff
showed conclusively that in the hen's egg, as laid, there is no
trace whatever of the embryo, or of any of its parts or organs ;
and that the formation of the embryo does not commence until
after the egg is laid and incubation has begun. He noted
further, and described accurately, the manner in which the
embryo is formed by folding of the germinal layers or
membranes.

Wolff' was too far ahead of his age, and his conclusions,
though perfectly sound, did not obtain acceptance until towards
the middle of the present century, when their correctness was
demonstrated, not merely for the chick, but for many other
groups of animals as well, by von Baer, Remak, Bischoff,
Kolliker, and others.

Although the chick has thus played a more important part
in the history of embryology than any other animal, it must be
borne in mind that birds are one of the most highly specialised
groups of animals, and that their development is, more particu-
larly in the early stages, very greatly modified. It is practical
convenience alone that justifies the great attention they have
received.

2. The Egg.

The hen's egg is of large size, and ovoid in shape. It con-
sists (Fig. 97) of a calcareous shell, lined by a fibrous shell
membrane ; and inclosing a quantity of a viscid albuminous
fluid, the ' white of the egg,' WA, in the centre of which lies
the ' yolk,' y, a spherical mass of a yellow colour, rather more
than an inch in diameter, and inclosed in an elastic vitelline
membrane, to which the preservation of its shape is due.

Of these parts, the yolk is the egg proper ; it corresponds
to the egg of Amphioxus, or of the frog, and from it the embryo
is developed directly.

The white of the egg corresponds to the investment of the
egg of Amphioxus, which swells up so greatly on reaching the
water, or to the jelly of the frog's spawn. The egg-shell and

GENERAL ACCOUNT.

221

shell membrane are protective envelopes, which are not repre-
sented in the eggs of Amphioxus or of the frog.

The yolk or ovum is, as in other animals, a single cell ; its
great size being due to the enormous quantity of food yolk ac-
cumulated within it, and distending it. As regards the quantity
of food-yolk contained within it, the lien's egg is at the opposite
extreme to that of Amphioxus ; the frog's egg being midway
between the two.

It is in consequence of the abundance of food material

Fig. 97. — The Hen's Egg at the time of laying.

BA, blastoderm. SH. egg shell. SM, shell membrane. SV, air chamber.

WA, white or albumen. WC. chalaza, or twisted cord of denser albumen. Y, yolk.
Z, vitelline membrane.

present in the egg itself, that the chick embryo is enabled to
complete its development within twenty-one days, while the frog
requires three months or more, and Amphioxus an even longer
time. The Amphioxus larva hatches in about eight hours, but in
an extremely immature condition (Figs. 25, 26, p. 59) ; the frog
hatches in about a fortnight, in a form utterly unlike the parent,
and devoid of mouth and limbs (Figs. 72, 73, p. 157) ; the chick
does not leave the egg until the twenty-first day, but is already a
fully developed bird.

A large amount of food-yolk is undoubtedly an advantage,.

222 THE OHICK.

inasmuch as it enables the embryo to develop rapidly and
securely, and frees it from the necessity of obtaining food from
without. However, it has also its disadvantages. Food-yolk is
itself inert, as already noticed in the opening chapter. It is
present in the egg as a number of granules of various shapes
and sizes, embedded in the living protoplasm of the egg ; and the
immediate effect of these inert, inactive, yolk-granules is, not to
aid, but to mechanically impede the processes of development ;
an effect which will necessarily be most marked in the early
stages, when the food-yolk is most abundant. Hence the early
stages of development of the chick, and especially the processes
of segmentation, occur more slowly than those of the frog, and
much more slowly than those of Amphioxus.

Moreover, the amount of food-yolk in the hen's egg is so
great that serious distortion of the shape would occur, were the
whole mass contained within the body of the embryo. To avoid
this difficulty, the yolk, at a very early stage of development,
becomes constricted into two parts, embryonic and vitelline
respectively, which remain connected by a stalk. Of these (cf
Figs. 99 and 100), the embryonic portion, em, is formed from
the part of the egg comparatively free from food-yolk, and be-
comes converted directly into the embryo ; while the vitelline
portion or yolk-sac, YS, which contains the bulk of the food-yolk,
does not give rise directly to any part of the embryo, but forms
a store of nutriment at the expense of which the development
of the embryo is effected.

At first, the embryonic portion is very much smaller than
the vitelline portion or yolk-sac (Fig. 98) ; but, inasmuch as
the embryo grows by absorption of the food-yolk, the yolk-sac
diminishes as the embryo increases in size (cf Figs. 99, 100, 101).
A time comes when the two are about equal in bulk, and in the
later days of incubation the yolk-sac is much smaller than the
embryo. By the twenty-first day of incubation the yolk-sac is
almost completely absorbed, and the chick pecks its way out of
the shell, and hatches.

3. The Embryo.

The hen's egg is fertilised before it is laid, indeed before the
egg-shell is formed, for no spermatozoon could possibly make
its way through the shell. At the time the egg is laid, not only

GENERAL ACCOUNT. 228

has fertilisation been effected, but the egg has already been
developing for a period which varies in different cases, but
amounts on an average to about eighteen hours.

When the egg is laid, development stops. To set it going
again, to start development afresh, all that is necessary is that
the egg should be kept at a temperature about equal to the
blood-heat of the parent bird. This is normally effected by
incubation, the hen sitting on the egg, and so keeping it

rni ; but it may be effected equally well by artificial means.

certain amount of moisture, and free access of air, are neces-
sary to insure normal development. The rate of development

inc

7.

Fig. 98. — The yolk of a Hen's Egg at the thirty-sixth hour from the com-
mencement of incubation. The structure of the embryo at this stage is
shown on a larger scale in Figs. Ill and 112. x §.

AD, area pellucida of tlie blastoderm. AK, area opaca. AV, area vasculosa.
EM, embryo. SM, vitelline membrane. Y, yolk-sac.

varies to a slight extent according to the season of the year,
autumn eggs developing more slowly than spring eggs ; or
according to the temperature, if an artificial incubator is em-
ployed. The length of time the egg takes to travel down the
oviduct, during the whole of which time it is deveknping, varies
considerably, and individual variations may occur from other
causes ; but, as a rule, the chick hatches on the twenty-first
day from the commencement of incubation. The age of an
embryo is always calculated from the commencement of incuba-
tion, or from the time of placing the egg in the incubator ; to
obtain the true age there must be added to this the time
during which the egg was developing, in its passage down the

!24

T1IK CHICK

oviduct, a period averaging, as we have seen, about eighteen
hours.

Owing to the enormous amount of food-yolk, and the me-
chanical hindrance which this offers to the processes of develop-
ment, the entire yolk, i.e. the egg proper, does not divide, but
segmentation is restricted to a small circular patch (Fig. 97, BA),
on the surface of the yolk, which is comparatively free from
yolk-granules, and in which development can readily take
place. This patch, the germinal disc, segments to form the
blastoderm, a membrane composed of cells (Fig. 106), which lies
like an inverted watch-glass on the surface of the yolk. The
blastoderm rapidly increases in diameter, by growth all round

Fig. 99. — The yolk of a Hen's Egg at the end of the third day of incubation.
The structure of the embryo at this stage is shown on a larger scale in
Figs. 113 and 114. x|.

AD. area pellucida of the blastoderm. AK, area opaea. AV, area vasctdosfc
EM. embryo. SM. vitelline membrane.

its margin, and spreads so as to cover more and more of the
surface of the yolk, which it ultimately incloses completely (Figs.
98, 99, 100, 101). Owing, apparently, to its less specific gravity,
the germinal disc, and consequently the embryo, which is formed
from its central part, lies at the top of the egg, and nearest to
the body of the hen, however much the egg be rolled over.

The central part of the blastoderm is thin and translucent,
and is spoken of as the area pellucida (Fig. 98, ad) ; the mar-
ginal portion is thicker and less transparent, and is called the
area opaca, AK ; the inner rim of the area opaca, bordering the
area pellucida, is the seat of an abundant formation of blood-
vessels, and is called in consequence the area vasculosa, AV.

GENERAL ACCOUNT,

225

The first trace of the embryo appears in the centre of the
area pellucida, about the twentieth hour of incubation ; the
formation of the embryo consisting essentially in a process of
folding off, or constriction, of the central part of the area pellu-
cida from the rest of the yolk. By the middle of the second
day the embryo (Fig. 98) measures about 5 mm. in length, and
has acquired definite shape ; the brain, spinal cord, heart, and
-her organs being already established (cf. Figs. Ill and 112).

Fig. 100.— The Hen's Egg at the end of the fifth day of incubation, seen
from the side. The embryo, which naturally lies with its left side on
the yolk-sac and its right side towards the egg-shell, has been lifted up,
in order to show its shape more clearly. The structure of the embryo at
this stage is shown on a larger scale in Figs. 115 and 123. x §.

AN, inner or 'true' amnion. AV, outer margin of area vasculosa. AZ, outer or
'false' amnion, together with the vitelline membrane. EM, embryo. SH, egg-shell.
SM, shell membrane. SV, air chamber. TA, allantois. YS, yolk-sac.

The embryo, at this stage, lies with its dorsal surface towards
the shell, and its ventral surface towards the yolk. The axis of
tht* body is straight, and is usually directed across the axis of
the egg ; the head end of the embryo, in the majority of cases,
pointing away from the observer if th j egg is placed before him
with the broader end to his left. There are, however, great
variations in this respect, and the axis of the embryo may form
almost any angle with that of the egg.

Q

226

Till-: chick

At thirty-six hours, the folding off of the embryo from the
yolk-sac has only made slight progress ; the head of the embryo
(Fig. 112) is lifted up above the yolk-sac by an anterior con-
striction or head fold, but the sides and tail end are as yet only
very imperfectly defined.

By the end of the third day great advance has been made.
The embryo (Fig. 99) has increased considerably in size. In
the head, which has grown much faster than the body, and is
now disproportionately large (Fig. 113), the nose, eye, and
ear, and the several divisions of the brain, are well established.
The head is no longer straight, but is strongly flexed, owing to
the dorsal surface growing much more rapidly than the ventral.
The heart and blood-vessels have acquired definite and charac-
teristic arrangement. The folding off of the embryo from the
yolk-sac has made considerable progress (Fig. 114); the head
and neck are now quite free from the yolk-sac -2 the hinder end
of the embryo is lifted up from the yolk by a definite tail fold
(Fig. 114, tl), and the side walls of the embryo are much more
clearly defined. The yolk-stalk, connecting the embryo with
the yolk-sac, is now a short tube, the diameter of which is about
a third of the length of the embryo. The hinder end of the
embryo still lies with its dorsal surface facing the egg-shell, and
its ventral surface resting on the yolk-sac ; but the head and neck
have rolled over, so as to lie with their left side on the yolk-sac
and their right side towards the egg-shell ; the axis of the body
becoming spirally twisted in consequence (Fig. 113).

(On the fouxtk_day:) the folding off of the embryo makes
further progress, and the yolk-stalk becomes greatly narrowed.
The whole embryo becomes strongly flexed, the dorsal surface
being convex along its entire length. The body, as well as the
head, of the embryo now lies with its left side on the yolk-sac ;
and the rudiments of the limbs have appeared as two pairs of
^small, ill-defined buds from the sides of the body.

By the end of the fifth day the embryo has acquired the
shape and proportions shown in Fig. 100. In the natural condi-
tion, it lies with its left side on the yolk-sac, with which it is
connected by the narrow tubular yolk-stalk. The whole embryo
is strongly flexed, the convex dorsal surface being about four
times the length of the concave ventral surface. .The head is of
relatively enormous size, chiefly owing to the great development

aENEEAL ACCOUNT.

227

of the brain vesicles and of the eyes. The limbs are still small,
hul have increased considerably in size as compared with the
earlier stages, and already show indications of their division into
segments (Fig. 115). From the under surface of the tail of the
embryo a saccular diverticulum, with thin but very vascular
walls, arises as an outgrowth from the alimentary canal : this
is the allantois (Fig. 115, ta), a structure which grows very

Fig. 101.— The Hen's Egg at the end of the ninth day of incubation, seen in
vertical section. The embryo naturally lies with its left side on the yolk-
sac, but has been lifted up in order to show its shape more clearly. * §.
AN, inner or 'true' amnion. HM, hyomandibular cleft. SV air chamber . TA
allantois. "WA, white or albumen. YS, yolk-sac.

rajjidly during the succeeding days, and forms the respiratory
organ of the embryo.

By the end of the ninth day the embryo has grown consi-
derably, and has attained the shape and proportions shown in Fig.
101. The body walls are now definitely formed, and rudiments
of the feathers are already present. The head is still dispropor-
tionately large, and the eyes are of emJrhious size. The beak,
which was absent in the earlier stages, has now grown out from the
front of the face, and at once gives the head a characteristic
avian appearance. The neck is long and slender. The body is
much more bulky than before, largely owing to the great size of

Q 2

228 THE CHICK.

/ the heart and the liver. The limbs have greatly increased in
length ; llieir several segments are well established, and 1 1 1> '
division of the distal ends into fingers and toes is very evident]

~"The white of the egg has almost disappeared, a thick and verjj
viscid mass, WA, alone remaining at the lower surface of the egg.
The yolk-sac, ys, is still large, but its walls are flabby, owing to
the absorption of a large part of its contents as food by the

/embryo. The allantois, TA, has grown enormously, and hn^_

I spread over the back of the embryo, and quite half way round

. the interior of the egg-shell. It lies close to the shell, so that
respiratory interchanges can readily take place, by diffusion
through the porous shell, between the gases of the blood, in the

• vessels of the allantois, and the air outside the egg. In this way

V_the respiration of the embryo is effected.

During the second half of the period of incubation the
changes are of less interest. The young chick steadily increases
in size at the expense of the yolk-sac, and gradually acquires
the proportions and characters which it has on hatching. About
the fourteenth day it shifts its position so as to lie lengthways
in the egg^ rather than across it. On the twentieth or twenty-
first day the yolk-sac is nearly absorbed, and what remains of it
is drawn into the body of the chick, the body walls closing over
it at the umbilicus. The chick thrusts its beak through the
shell membrane into the air chamber at the broader end of the
egg, and for the first time draws air directly into its lungs.
Invigorated in this way, it breaks through the shell, by means of
a hard knob on the tip of its beak, and steps out into the world.

THE EGG.
1 . Formation of the Egg.

In the embryo fowl there are two ovaries, but in the course
of development the right ovary disappears, and in the adult hen
the left ovary is alone present. This is a large, irregularly-
shaped body, suspended in a fold of peritoneum from the dorsal
body-wall, opposite the anterior part of the left kidney. Numerous
ova in different stages of development project from its surface,
varying in size from dust shot up to spherical bodies an inch or
more in diameter.

Of the two oviducts, the right one is rudimentary ; the left

THE EGG. 229

one, which alone is functional, forms in the adult hen a wide
convoluted tube, which commences in front with a long, oblique,
funnel-like mouth, bordered by a fimbriated edge, and lying in
close contact with the ovary. Behind this mouth comes a long,
convoluted, but thin- walled part of the oviduct, and then a
short terminal part with very thick walls, which opens into the

JDaca, and through this to the exterior.
The ovum at the time of its discharge from the ovary
nsists of the yolk alone, inclosed in the vitelline membrane,
le albuminous investment, or ' white of the egg,' is formed
ound the yolk by the walls of the first, or thin-walled, part of the
iduct; and the shell membrane and egg-shell are added while
e egg is in the thick- walled terminal part of the oviduct, just
uefore being laid.

The ovaries can be recognised in chick embryos during the
third day of incubation, as a pair of slightly modified tracts of
the peritoneal epithelium which clothes the dorsal wall of the
body cavity, close to the root of the mesentery. This germinal
epithelium is at first merely a longitudinal strip of peritoneum,
of which the component cells are columnar instead of squamous
in shape. By multiplication of the cells, to form a layer several
cells thick, the strip becomes a prominent ridge. Vascular
connective tissue soon grows in along the axis of this genital
ridge, and renders it still more conspicuous.

Almost from the first, certain of the epithelial cells of the
genital ridge differ from their fellows in their greater size and
more spherical shape, and in possessing nuclei of unusual dimen-
sions ; these larger cells are the primitive ova or gonoblasts.
The primitive ova rapidly increase in size, and move from the
surface, where they all take their origin, into the deeper parts of
the genital ridge ; the smaller, indifferent epithelial cells at the
same time becoming arranged so as to form follicles around them.
The follicular epithelial cells serve to nourish the ova,
drawing nutriment from the blood-vessels of the genital ridge,
and passing it on, probably after elaborating it, into the ovum.
Within the ovum the food matter undergoes further changes, and
is deposited in the form of granules, from which the definite yolk-
granules of the fully-formed egg are finally derived.

During these changes the nucleus of the primitive ovum
increases greatly in size, and acquires a distinctly vesicular

230 TIIK CHICK.

structure, with one or more nucleoli: it is now spoken of as
the germinal vesicle ; and the establishment of the germinal
vesicle, together with the marked increase in size of the ovum,
owing to the accumulation of yolk-granules within it, mark the
conversion of the primitive ovum, which occurs in botli sexes
alike, into the permanent ovum characteristic of the ovary of
the hen bird.

As the egg increases in size it forms a swelling on the sur-
face of the ovary, which rapidly becomes more prominent. A
vitelline membrane is formed round the egg, between it and
the follicular epithelium, and apparently derived from the egg
itself. The follicular epithelium, with the outer wall of the
ovary, form a vascular capsule, investing the egg.

The accumulation of yolk-granules within the egg continues
until this has reached its full size. From an early stage, a
difference may be noticed between white yolk-spheres, and yellow
yolk-spheres ; the former consisting of minute vesicles, each
containing a highly refracting body ; while the latter, which
are apparently derived from the white yolk-spheres, are much
larger bodies, filled with numerous bright, highly refracting
granules.

In the fully-formed egg the white and yellow yolk-spheres
are arranged in a very definite manner. The yellow yolk makes
up the greater part of the bulk of what we call the yolk of the
egg ; the white yolk-spheres forming, (i) a somewhat flask-shaped
plug in the centre of the yolk, with a neck reaching to the
surface at the germinal disc ; (ii) a thin superficial layer invest-
ing the whole exterior of the yolk, immediately below the vitelline
membrane ; (iii) a series of thin concentric shells between the
surface and the central plug, the spaces between the successive
shells being occupied by the yellow yolk.

2. Maturation of the Egg.

The ripening of the egg is accompanied by changes in the
nucleus, which are as yet only imperfectly known.

The nucleus, or germinal vesicle, during the growth of the
egg, is large and vesicular, and occupies a position at or close
to the centre of the egg. As the egg ripens, the nucleus moves
towards the surface, where it lies just beneath the vitelline
membrane, in a small lenticular patch, the germinal disc, which

. TJIE EGG. lJ!31

is comparatively free from yolk-granules. The nuclear mem-
brane disappears ; the chromatin elements form a reticular net-
work, which then becomes distributed through the whole nuclear
substance in the form of very fine granules ; and finally, these
granules run together to form six chromatin rods. The further
changes have not been followed with certainty in the hen's egg ;
and neither the formation nor the extrusion of polar bodies has
as yet been seen.

The egg, which is now ripe, is discharged from the ovary
by rupture of the capsule at its most prominent part. The egg
is received at once into the open mouth of the oviduct, which is
closely applied to the ovary at the time, and then begins its
passage down the oviduct to the exterior.

As it travels along the first or thin-walled part of the oviduct,
the albumen, or 'white of the egg,' is poured out around it from
the walls of the oviduct. The albumen is deposited as a con-
tinuous sheet, which is wrapped spirally round the yolk, owing
to the egg being caused to rotate, in its downward passage, by
spirally arranged folds on the inner wall of the oviduct.

This rotation of the egg causes the spiral twisting of the
cords of denser albumen, at the ends of the egg, which are
spoken of as the chalaza) (Fig. 97, wc).

On reaching the lower part of the oviduct, or c uterus,' the
shell membrane, and finally the shell, are deposited on the outside
of the egg, which is then passed into the cloaca, and laid.

The egg takes about three hours to travel along the thin-
walled part of the oviduct ; in the uterus it remains for a vari-
able time, estimated by different authorities as usually lasting
from twelve to eighteen hours.

o. Fertilisation of the Egg.

The details of the fertilisation of the hen's egg have not yet
been determined. The large size of the egg offers great dif-
ficulties to the investigation of minute changes in connection
with the nucleus, and these difficulties have not yet been sur-
mounted.

All that is known with certainty is that fertilisation is
effected, either in the upper part of the oviduct, or possibly, as
stated by Coste, before the egg leaves the ovary ; so that during
the whole time of the passage down the oviduct development is

232 THE CHICK.

taking place. The spermatozoa are received by the hen some
time before the laying of the eggs, and retain their vitality
and functional activity for about a fortnight.

THE EARLY STAGES OF DEVELOPMENT.

1. Segmentation of the Egg.

Segmentation commences about the time the egg arrives in
the lower, thick- walled part of the oviduct, or uterus ; it is con-
tinued actively during the stay of the egg in the uterus, and is
completed about the time the egg is laid.

Segmentation, as already noticed, does not concern the whole
egg, but is confined to the germinal disc ; and the hen's egg is
therefore spoken of as meroblastic, inasmuch as only a portion
of it takes part in the process of segmentation, in contradistinc-
tion to the holoblastic eggs of Amphioxus and the frog, in
which the entire egg is divided by the first cleft into two equal
parts.

In the hen's egg, segmentation commences with the forma-
tion of a vertical groove or furrow, which runs across the middle
of the germinal disc, but does not quite reach its edge at either
end. This is very shortly followed by a second furrow, crossing
the first one almost at right angles. Four radial furrows soon
appear, about midway between the two first ones ; and then by
cross furrows each segment becomes divided into a central and a
peripheral portion. Additional furrows soon appear, both radial
and concentric, and by these the germinal disc becomes cut up
into a mosaic of segments of irregular shape and size, separated
from one another by the furrows or grooves (Figs. 102, 103).

The segmentation is slightly excentric almost from its first
commencement, the furrows extending nearer to the edge of
the disc, and the segments being smaller, at one side (the lower
side of Fig. 103), which corresponds to the future posterior end
of the embryo ; while at the opposite, or anterior part, of the
germinal disc the furrows stop short further from the edge, and
the segments are of larger size.

Sections of the germinal disc at the stage represented in
Fig. 103 show that, in addition to the vertical furrows by which
the mosaic pattern is produced on the surface, horizontal clefts

SEGMENTATION OF THE EGG. 233

are also forming, by which the segments become completely
isolated from one another, and from the underlying yolk
(Fig. 104, za). These horizontal clefts, like the vertical ones,
appear first in the centre of the germinal disc, and do not reach
its margin until a later stage.

In each segment, or cell as it may now be termed, a nucleus
is present from the first. The precise mode of origin of these
nuclei has not been determined with certainty, but the history
of the segmentation of the egg in Amphioxus, the frog, and other

Fig. 102. Fig. 103.

FlG. 102. — An early stage in the segmentation of the germinal disc of the

Hen's Egg. (After Cos.te, and Duval.) x 10.
Fig. 103. — A later stage, in which the germinal disc has increased in size, and

the segments have, by further division, become smaller and more

numerous. (After Coste, and Duval), x 10.

Both these figures are from eggs taken from the lower part of the

oviduct of the hen.

animals, leaves little doubt that the nuclei of all the cells are
derived, by division, from the single segmentation nucleus of the
fertilised egg.

The result of the process of segmentation, up to the point
shown in Figs. 103 and 104, is the formation of a cap, occupying
the centre of the germinal disc, and consisting of a single layer
of nucleated cells : of these, the central ones, za, are small, and
completely isolated from their neighbours, and from the under-
lying yolk ; while the marginal ones, ZB, are larger, and are
only imperfectly marked off from the yolk, the horizontal clefts
having not yet appeared.

The process of segmentation soon extends into the deeper
part of the germinal disc ; and by a further series of clefts, in
different planes, this deeper part of the disc becomes cut up into

28 I THE (MIK'K.

cells, which from fche firsl are nucleated, and are arranged in a
layer fcwo or three cells deep (Fig. 1.85, zl).

In this way, shortly before the time of laying of the egg, the
germinal disc becomes converted into a cap of cells, spoken of
as the blastoderm (Fig. 105). Of these cells the uppermost or

ZB y' Zf N VL

\ /

FIG. 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. The plane of
section corresponds to a vertical line drawn through the centre of Fig.
103 ; the right-hand end of Fig. 104, which is the future anterior end, cor-
responding to the upper border of Fig. 103 ; and the left-hand end of
Fig. 104 to the lower or posterior border of Fig. 103. (After Duval.) x 25.

N, nucleus Of completed Begment. N7, nucleus of segment not yet completely separated
from the yolk. VL, vacuole. Y, yolk. ZA, completed blastomere. ZB, incompletely
separated blastomere.

most superficial layer (Fig. 105, e), which was the first to be
definitely established, constitutes the epiblast ; it consists of a
single layer of cells, and is separated by a very shallow space,
the blastoccel or segmentation cavity, b, which appears in section

VT. E B ZL N' Y

I Li

1

^^333^05

Fi(x. 105. — Vertical section of the blastoderm and adjacent part of the yolk
of a Hen's Egg towards the close of segmentation. The anterior edge is
to the right, the posterior edge to the left hand. (After Duval.) x 25.

B, blastoccel or segmentation cavity. E, epiblast. ~N', nucleus of blastomere, which
as yet is only Incompletely separated from the yolk. VL, vacuole. Y .yolk. ZL, one

of the lower-layer cells or blastomercs.

as a mere chink or split, from the deeper mass of cells which
may be spoken of collectively as lower layer cells, ZL.

During the rest of the time that the egg stays in the uterus,
while the egg-shell is forming, the process of segmentation
continues actively. The clefts extend to the edge of the germinal
disc, which becomes sharply marked off from the yolk beyond it ;

THE BLASTODERM. 235

and, by rapid division, the cells become of nearly uniform size in
all parts of the blastoderm.

The lower-layer cells become more sharply separated from
the yolk', a space, filled with fluid, appearing beneath them, be-
tween the blastoderm and the yolk. This space, the subger-
minal cavity (Fig. 10G, bv), is sometimes spoken of as the seg-
mentation cavity ; a name, however, which ought to be restricted
to the narrow chink between the epiblast and the lower-layer
cells (Fig. 105, b), which is clearly visible in the early stages,
but becomes practically obliterated before the egg is laid.

Round its margin, new cells are still being cut out of the
yolk, and added on to the blastoderm. Some of the cells which
arise in this way, and lie between the edge of the blastoderm
and the yolk, are markedly larger than any of the others, and
are spoken of as formative cells (Fig. 10(3, zf).

Fig. 100. — Vertical section of the blastoderm and adjacent parts of the yolk
of a Hen's Egg at the time of laying, but before the commencement of
incubation. The anterior edge of the blastoderm is to the right, the
posterior edge to the left side of the figure. (After Duval.) x 25.

BV, subgerrninal cavity. B, epiblast. H, hypoblast. N', nucleus in yolk, round
which a cell will be formed later. Y, yolk. ZF1, formative cell. ZL, lower-layer cells.

2. The Blastoderm.

a. The condition of the blastoderm at the time of laying of
the egg. The actual stage of development reached when the
egg is laid depends on the length of time the egg remains in
the uterns ; and this we have seen is subject to considerable
variation. The following description will apply to an average
case.

Naked-eye examination shows the blastoderm (Fig. 97, BA)
to be a small circular patch, about 3*5 mm. in diameter, on the
surface of the yolk : owing to its less specific gravity, the blasto-
derm is always uppermost, however much the egg be rolled over.
The blastoderm consists of a marginal white rim, the area opaca,
thickest at the posterior edge of the blastoderm (Fig. 106, zf) ;
and a central, circular, and more translucent portion, the area

236 THE CHICK.

pellucida. Beyond the edge of the blastoderm (Fig. 97) the
yolk shows one or more broad concentric bands, alternately
darker and lighter in appearance.

Sections of the blastoderm at this stage (Fig. 106) show
that it consists of two distinct layers of cells, (i) The upper
layer, or epiblast, E, is a continuous membrane, formed of small,
short columnar cells, varying very little in size, and packed
closely together side by side.

(ii) The lower layer consists of cells which are more loosely
arranged, and which vary a good deal in shape and size in
different parts. In the area pellucida, or middle portion of the
blastoderm, they form a thin layer of somewhat flattened cells,
H, only one, or at most two cells in thickness. At the margin of
the blastoderm, or area opaca, the cells become more numerous
and more spherical in shape, forming a thickened rim which rests
on the underlying yolk, and in which the large formative cells,
zf, are found, especially near the posterior margin. In the
yolk, on which the edge of the blastoderm rests, nuclei (Fig.
106, n') are present, round which cells are formed at a later
stage, and added on to the margin of the blastoderm.

Beneath the area pellucida, and separating it from the bed
of yolk, Y, is the subgerminal cavity, bv ; a well-marked space,
filled with fluid.

b. The growth, of the blastoderm. Round the margin of the
blastoderm the epiblast and the lower-layer cells are at first con-
tinuous with each other, but shortly before the laying of the egg
this continuity is lost, except at the posterior border, where, as
shown on the left-hand side of Fig. 106, the two layers are still
continuous with each other at the time the egg is laid.

After incubation has commenced, the blastoderm spreads
rapidly, retaining its circular shape. By the end of the first
day of incubation it is about the size of a sixpence ; and by the
end of the second day it has extended nearly half way round
the egg; after this it proceeds more slowly, the complete
inclosure of the yolk not being effected until about the seven-
teenth day.

In this spreading of the blastoderm (cf. Figs. 98 and 99)
the peripheral part, or area opaca, grows much more rapidly than
the central area pellucida ; the area opaca retains its circular
outline, but the area pellucida (Figs. 98 and 99, ad) very early

THE GERMINAL LAYERS. 237

becomes oval, and then pyriform in shape, the broader end
corresponding to the anterior end of the embryo.

The two layers of the blastoderm grow independently. The
epiblast, after it has become free from the lower layer, extends
slightly beyond this, so that its margin rests directly on the
yolk ; its further spreading is effected mainly by division of the
already formed cells, stimulated, no doubt, by absorption of nutri-
ment from the yolk on which they are lying. The lower-layer
cells, after separation from the epiblast, become directly con-
tinuous at their margin with the yolk, forming a thickened rim,
spoken of as the germinal wall : the extension of the lower-
layer cells is effected principally by the addition of new cells cut
out from the yolk, but partly also by division of the already
formed cells, as in the epiblast.

3. The Hypoblast.

A few hours after the commencement of incubation, the
lower-layer cells undergo important changes, by which the hypo-
blast and mesoblast become established.

In the area pellucida, the majority of the lower-layer cells
become flattened horizontally, and unite at their edges so as to
form a continuous cellular membrane, the hypoblast ; a few
isolated lower-layer cells are left between the epiblast and the
hypoblast, which take part, as will be noticed immediately, in
the formation of the mesoblast.

In the area opaca, or marginal part of the blastoderm, the
differentiation of the hypoblast as a distinct cellular membrane
occurs somewhat later ; and the hypoblast cells of this region,
which are large, and cubical or slightly columnar in shape, differ
markedly from the thin, pavement, hypoblast cells of the area
pellucida.

1. The Primitive Streak.

At the posterior border of the blastoderm, as noticed above,
the fusion of the epiblast and the lower-layer cells persists
longer than it does round the rest of the blastodermic rim ;
and in the egg, at the time of laying, a crescentic opacity is
visible at the posterior edge of the blastoderm, marking this
line of fusion.

As the blastoderm grows, during the earlier hours of incu-

238 THE < hick.

bation, this opacity becomes lengthened out into a linear band,
the primitive streak, which, starting from near the centre of
the blastoderm, extends backwards across the area pellucida
towards its margin. The increase in length of the primitive
streak is effected almost entirely by growth backwards of its
hinder end, the anterior end lengthening very little, if at all.

The area pellucida grows more rapidly in its posterior than
in its anterior part, and from about the fifteenth hour becomes
pyriform in outline. The primitive streak keeps pace with the
growth of the area pellucida ; and about the twentieth hour, when
the area pellucida is markedly pyriform in shape (Fig. 107, ad),

AD.

NP
AK-

PS

FiG. 107. —A diagrammatic figure of the blastoderm of a Hen's Egg about the
twentieth hour of incubation. (In part after Duval.) x 8.

AD, area pellucida : the part left white consists of epiblast and hypoblast alone : in
the hinder part of the pyriform area, covered by the light shading, rnesoblast is present
as well. AK, area opaca. M, dotted line indicating the boundary of the rnesoblast.
NP, neural plate, the first commencement of the central nervous system. PS, primitive
streak.

the primitive streak, PS, forms a well-defined opaque band
stretching about two-thirds of the way across the area pellucida.
The anterior end of the primitive streak is sharply defined ; the
posterior end is less distinct, is often irregularly bent, and
usually dies away a short distance before reaching the edge of
the area pellucida. A shallow median furrow, the primitive
groove, runs along the whole length of the primitive streak.

Transverse sections of the blastoderm (Fig. 108) show that
the primitive streak is formed by proliferation of cells from the
under surface of the epiblast, in the median line. The cells
grow downwards as a solid keel, which spreads out right and
left as a horizontal sheet of cells, ps ; these are spherical in

THE PRIMITIVE STEEAK. 239

shape, rather closely packed together, and situated between the
epiblast, e, and the hypoblast, H.

The primitive streak appears before any trace of the ner-
vous or other systems of the embryo has commenced to form.
The meaning of the primitive streak has been much discussed,
but it is now generally agreed that it corresponds, at any rate
in part, to the lips of the blastopore in the frog, which have
become lengthened out, and fused together; the primitive groove
marking the line of concrescence of the lips of opposite sides of
the blastopore. The anterior end of the primitive streak in the
chick certainly corresponds to the anterior or dorsal lip of the
blastopore in the frog ; but it is not quite clear whether the
entire length of the primitive streak is to be compared to an
elongated and drawn out blastopore, or whether the hinder part

Fig. 108. — Transverse section across the blastoderm of a Hen's Egg about the
twentieth hour of incubation, the section passing through the primitive
streak about the middle of its length (of. Fig. 107). x200.

E, epiblast. H, hypoblast. M. mesoblast. PG. primitiye groove. PS, primitive
streak.

of it is not rather due to the peculiar method of spreading of
the blastoderm, imposed on the chick embryo in consequence of
the distension of the egg by the enormous mass of food-yolk
which it contains.

5. The Mesoblast.

The middle germinal layer, or mesoblast, gives rise in the
chick, as in Amphioxus, in the frog, and in other animals gene-
rally, to all the connective tissue, vascular, muscular, and
skeletal structures, as well as to the urinary and reproductive
organs.

In the chick, the mesoblast cells have a less clearly defined
origin than in Amphioxus or in the frog, and are derived from
three distinct sources.

(i) In the hinder part of the blastoderm, some of the cells

240 THE CHK'K.

of the original lower layer are left, lying between the epiblast
and hypoblast, on the establishment of the latter as a distinct
and continuous membrane; and these cells become mesob
cells (Fig. 108, m).

(ii) In the middle and lateral portions of the area pellucida.
about the time of appearance of the primitive streak, mesoblast
cells are budded off freely from the upper surface of the hypo-
blast, and form a layer between the epiblast and hypoblast in
this region.

(iii) The horizontal sheets of cells (Fig. 108, PS), which
spread out right and left as the wing-like expansions of the
primitive streak, and which, it will be remembered, are of
epiblastic origin, also take part in the formation of the meso-
blast.

As regards the cells themselves, those of groups (i) and (ii)
agree with one another in being usually of an irregular stellate
shape (Fig. 108, m), and in being very loosely arranged. The
origin of these two groups is very similar, though not identical ;
the cells of the first group being derived from the lower-layer
cells, formed by segmentation of the germinal disc ; while those
of the second group arise directly from the hypoblast, after
this is established as a distinct cellular membrane. It is not
possible to draw a sharp line between the two groups, nor to
determine in all cases to which group a given cell belongs.
Speaking generally, the mesoblast of the body of the embryo
itself is derived from group (ii), the cells of group (i) lying almost
entirely in the extra-embryonic parts of the blastoderm.

The cells of group (iii) are derived directly from the epi-
blast, and are therefore of totally different origin to those of
groups (i) and (ii). They also differ from these latter in their
spherical form and more compact arrangement. They are at
first (Fig. 108, PS) sharply marked off from the cells of groups (i)
and (ii), but as the primitive streak spreads laterally, the cells
composing it come into close relation with those of the other
groups, and becoming at the same time less compactly arranged,
and less regular in form, can no longer be distinguished from
those of groups (i) and (ii). The cells of group (iii), or primitive
streak mesoblast cells, lie almost entirely behind the embryo,
and take but little share in its formation.

The mesoblast cells of all three groups soon become con-

THE MESOBLAST.

241

tinuous, forming a sheet of somewhat loosely arranged and
usually stellate cells, which at the twentieth hour of incubation
has a shape and extent indicated by the strong dotted line, M, in
Fig. 107. The two halves of the sheet are continuous with each
other across the median line in the region of the primitive
streak, PS, and behind it ; but in front of the primitive streak,

Fig. 109. — A diagrammatic figure of the blastoderm of a Hen's Egg about the
twenty-fourth hour of incubation. (In part after Duval.) x 8.

AD, area pellucida : the part left white is the proamnion, and consists of epiblast and

hypoblast alone ; in the hinder part of the pyriform area pellucida. covered by the light

ding, mesoblast is present as well. AK, area opaca. BF, commencing fore-brain.

M, dotted line indicating the limit to which the mesoblast has spread. MS, mesoblastic

somite or protovertebra. IfG, neural groove. PS, primitive streak.

For a more exact view of an embryo of this age see Fig. 110.

in the region where the embryo is formed, NP, the two halves
are separated in the middle line by the notochord, the descrip-
tion of which is given on the next page.

In the later stages, as the embryo appears, the mesoblast
sheet spreads rapidly. It does not extend directly in front of
the embryo, but grows forwards as two lateral horns (Fig. 109),
so that for a considerable time there is, immediately in front of
the embryo, a transparent area of the blastoderm, ad, which
consists of epiblast and hypoblast only, without any middle
layer or mesoblast. This area, the proamnion, remains two-
layered until about the middle of the third day of incubation,
when the two lateral horns of mesoblast gradually grow inwards
to meet each other in front of the embryo.

R

242 THE ("HICK.

As shown in Figs. 107 and 109, the mesoblast very early
extends outwards beyond the area pellucida so as to underlie
the inner zone of the area opaca ; this three-layered zone of the
area opaca, represented by the dark shading in Figs. 107 and
109, is known as the area vasculosa, because the blood-vessels
which absorb the yolk and carry it to the embryo are very early
developed in it (cf. Figs. 98, 99, av).

G. The Notochord.

Before the sheet of mesoblast ceils, spoken of above as
group (ii), separates completely from the hypoblast, a distinc-
tion may be noticed in it between a median longitudinal rod of
cells, and two lateral tracts. This median rod is the notochord
(Fig. 117, CH) ; and the cells of which it consists are, from the
first, more closely compacted than those of the lateral tracts.

The notochord sometimes remains attached to the hypoblast
after the lateral mesoblastic sheets have completely separated
from this ; in other specimens the entire sheet of cells separates
as one continuous layer, which then divides into the median
rod, or notochord, and the two lateral mesoblastic tracts.

The notochord of the chick has, accordingly, been described
by some authorities as of hypoblastic, by others as of mesoblastic
origin ; the component cells are, however, in all cases derived
directly from the. hypoblast, and the difference is merely in the
relative times of separation of the notochord from the lateral
sheets of mesoblast, and from the underlying hypoblast respec-
tively.

The notochord lies entirely in the part of the blastoderm
in front of the primitive streak ; its posterior end is, however,
directly continuous with the anterior end of the primitive streak.
Inasmuch as the primitive streak cells are continuous with the
epiblast, and the notochord is, at any rate at first, continuous
with the hypoblast, it follows that the three germinal layers,
epiblast, mesoblast, and hypoblast, are directly continuous and
fused with one another at this point, which marks the hinder
end of the chick embryo, and corresponds to the anterior or
dorsal lip of the blastopore in the frog (cf. Fig. 60, b).

7. The Mesoblastic Somites and the Coelom.

The mesoblast of either side forms at first a continuous sheet
of loosely arranged cells, which in transverse section is somewhat

THE MESOBLASTIC SOMITES. 243

wedge-shaped, being thickest next to the notochord and gradu-
ally thinning as it passes outwards towards the margin of the
blastoderm (cf. Fig. 117, m).

About the twenty-first hour of incubation, the mesoblast
cells become arranged more or less clearly in two layers, upper and
lower, with a slight space between them. This splitting of the
mesoblast, as it is termed, first appears in the part of the meso-
blast beyond the embryo, but soon spreads inwards to the em-
bryonic region, extending almost up to the notochord.

The cavity, formed in this way, by splitting of the mesoblast,
becomes the coslom or body cavity of the chick. Of the two layers
into which the mesoblast is split, the upper or outer is spoken of
as the somatic layer, and the lower or inner as the splanchnic
layer. From a very early period the somatic layer (Fig. 129, me)
becomes closely connected with the surface epiblast, forming with
this the somatopleure or body wall ; while the splanchnic layer
becomes similarly related to the hypoblast, and forms with this the
splanchnopleure or wall of the alimentary canal (Fig. 129, mh).

A body cavity that is formed in this way, by splitting of the
mesoblast into somatic and splanchnic layers, is spoken of as a
schizoccel, in contradistinction to the enterocoel of Amphioxus,
which arises as a series of hollow outgrowths from the enteron
or primitive alimentary canal. Inasmuch as the mesoblast of
the embryo is derived almost entirely from the hypoblast, as
described above, the distinction between an enterocoel and a
schizoccel may be said to consist in this : — in the enterocoel the
mesoblast arises as hollow outgrowths from the hypoblast, which
subsequently become shut off from the gut, while the cavities of
the outgrowths open into one another and become the coelom of
the adult. In the schizoccel, on the other hand, the mesoblast
arises as two solid sheets, budded off from the hypoblast, in
which the coelom is formed at a later stage by splitting of the
sheet into two layers, with a space between them. Of these two
methods of formation of the coelom there can be little doubt that
the enterocoelic is the more primitive one, the schizocoelic the
more modified.

Almost immediately after the splitting of the mesoblast is
effected, about the twenty-second hour, a series of clear trans-
verse lines, really vertical clefts through the mesoblast, appear in
the embryo, extending outwards a short distance each side of the

244 THE chick.

iiotochord ; these are quickly followed by a pair of similar bul
longitudinal clefts, which appear one along each side of
body, a little distance from the middle line. By these clefts
the mesoblast of each side of the body becomes divided into a
vertebral plate, alongside the notochord ; and a lateral plate,
more peripherally placed ; the vertebral plate being further cut
up by the transverse clefts into a series of somewhat cubical |
blocks, the mesoblastic somites or proto-vertebrse (Fig. 110, ms).
The mesoblastic somites appear first in the neck region, and
increase rapidly in number during the last two hours of the
first day, and the following two or three days. One or perhaps
two pairs are formed in front of the pair which appears first ; the
remainder are added on in succession at the hinder end of the
series, as the embryo increases in length. At the twenty-fourth
hour of incubation there are usually five or six pairs present
(Fig. 110, MS); by the thirty-sixth hour (Fig. Ill) these have
increased to about fifteen pairs ; at the end of the second day
there are twenty-seven or twenty-eight pairs, after which date
the further increase takes place more slowly until, during the
fourth day, the full number is established. The increase takes
place in a very regular manner, and the number of somites
present affords a convenient basis for estimating the age, and the
grade of development, of embryos during the earlier stages of
their formation.

The somites extend along the whole length of the neck, trunk,
and tail, but are not formed in the head, in which no segmenta-
tion of the mesoblast occurs. In the case of the first three or four
somites, the splitting of the mesoblast extends up to the notochord
before the somites become marked off from the lateral plates ; and
consequently the cavities of these somites communicate for a time
with the ccelom or cavity of the lateral plate, though this com-
munication is lost as soon as the longitudinal cleft is formed which
separates the vertebral and lateral plates from each other.

The remaining somites, behind the first three or four pairs,
do not communicate at any stage with the ccelom, their cavities
appearing independently, and after the separation of the vertebral
from the lateral plates.

The further stages in the development of the mesoblastic so-
mites will be described in a later section (p. 322).

THE AMNION. 245

8. The Amnion.

The amnion is a fold of the somatoploure which rises up as
a wall all round the embryo, a little distance from it, and, spread-
mo- over its back, forms a thin double membrane between the
embryo and the egg-shell. Though a very characteristic struc-
ture, it is of only secondary importance, and gives rise to no part
of the embryo itself.

The first trace of the amnion appears about the thirty-third
hour, as a small crescentic fold immediately in front of the head
of the embryo. This grows rapidly, and by the thirty-sixth hour
(Figs. 111,112, ax), has extended back over the anterior end of the
head as a transparent cap, formed by a double membranous fold.

This first formed part, or head fold, of the amnion consists at
first of epiblast only, inasmuch as it arises from the proamnion,
or part of the blastoderm immediately in front of the embryo,
into which the meeoblast has not yet spread (cf. Fig. 109).
During the latter part of the second, and the third day, the
mesoblast gradually grows in from the sides, forming a thin
lining to the amnion, which from this time is two-layered.

The head fold of the amnion extends backwards rapidly, and
before the end of the second day covers over the whole of the
head and neck region of the embryo. At the hinder end of the
embryo a similar tail fold is formed during the second day ; and,
a little later, side folds appear, connecting the head and tail folds
together. The embryo is now completely surrounded by the
amnion, which forms a low wall round its sides and tail, and ex-
tends backwards over the head and neck as a thin membranous
cap.

Unlike the head fold, the side and tail folds of the amnion
(cf. Fig. 129, ax) consist from the first of both epiblast and
mesoblast ; i.e. are folds of the somatopleure, beyond the margin
of the embryo.

During the third day the amnion grows rapidly on all sides,
and by the close of the day (Fig. Ill, an, an') has covered
over the whole of the embryo, except a small patch near the
hinder end. During the fourth day the side folds meet each
other over the back of the embryo, which thus becomes com-
pletely covered by the amnion. As the amnion folds meet,
they coalesce, the inner layers of the folds forming a continuous
membrane, the inner or true amnion (Fig. 100, an), which

246 THE CHICK.

closely invests the embryo, and is continuous with the margin
of its body wall (Fig. 129). The outer layers of the amnion
folds also form a continuous membrane, the outer or false amnion
(Fig. 100, az), which lies close beneath the vitelline membrane,
and soon fuses with this, while peripherally it passes into the
layer of somatopleure investing the yolk-sac.

The space between the inner or true amnion and the embryo
is called the cavity of the amnion. It is filled with fluid, and is
at first very small, the true amnion on the fourth and fifth days
investing the embryo very closely (Fig. 100). . During the
following days, owing to accumulation of fluid within it, the
amnionic cavity increases very considerably, forming a water-
bath in which the embryo can move freely in any direction.
During the later stages of incubation, muscle fibres are deve-
loped in the mesoblast of the amnion, which by their contrac-
tions rock the embryo to and fro within the egg.

The space between the inner and outer layers of the amnion
(Fig. 100 AN and az) is, from the mode of formation of the amnion
(Figs. 114, 129), continuous with the ccelomic space which lies
between the two layers of the mesoblast, both within the embryo
and in the extra- embryonic region of the blastoderm. By the
sixth day the splitting of the mesoblast (cf. Fig. 100) has
extended about half-way round the yolk-sac. The further exten-
sion of the splitting takes place much more slowly, and does
not reach the lower pole of the yolk-sac until within a few days
of the time of hatching.

About the tenth day(c/. Fig. 101), when the splitting of the
mesoblast has extended about three-fourths of the way round
the yolk-sac, a circular fold of somatopleure arises from near
its* ventral edge, and grows over the dense mass of albumen,
WA, at the lower surface of the egg, inclosing this in much the
same way as the amnion incloses the embryo at an earlier stage,
and aiding in the absorption of this mass of albumen.

The formation of an amnion is a very characteristic feature
in the development of the three higher groups of Vertebrates —
Reptiles, Birds, and Mammals. These same three groups are
also characterised by the presence, during the later stages of
development, of an allantois, which plays an important part in the
respiration of the embryo, and, in mammals, in its nutrition as
well. The two structures, amnion and allantois, are associated

THE AMNION. 247

to this extent, that. the space between the two layers of the
amnion gives the allantois a ready opportunity for free and
rapid growth, and enables it to obtain a position close to the
inner surface of the egg-shell (Figs. 100, 101, ta), where its
respiratory efficiency is greatest.

It would, however, not be right to regard the amnion as
merely a provision to insure free growth of the allantois, for
this would not explain how the amnion originated in the first
instance ; and it must be remembered that all the characteristic
stages in the development of the amnion are completed while
the allantois is still in a very rudimentary condition. The
amnion has probably to be explained quite irrespectively of the
allantois.

The most satisfactory explanation of the formation of the
amnion is that it is due, in the first instance, not to uprising
of a fold of somatopleure, but to depression of the embryo
into the yolk-sac ; the sinking of the embryo being due partly
to its own weight, partly to the downward growth of the
front part of the head caused by cranial flexure ; and perhaps
in part to the resistance of the vitelline membrane, aided by the
liquefaction of the yolk as this becomes absorbed for the
nourishment of the embryo. The main purpose effected by the
depression of the embryo is to remove it from the danger of
pressure against the egg-shell, a consideration which has more
weight in the case of Keptiles, in which group the amnion was
first acquired, and in which the yolk often completely fills the
egg-shell, than in their descendants, the Birds.

THE DEVELOPMENT OF THE NERVOUS SYSTEM.

1. General Account.

The development of the nervous system of the chick is
effected in practically the same manner as that of the frog.
About the nineteenth or twentieth hour, almost immediately
after the notochord has appeared, the epiblast in front of the
primitive streak becomes thickened along the median line to
form the neural plate (Fig. 107, NP).

During the next four or five hours, the anterior part of the
area pellucida grows rapidly (Fig. 109) : the neural plate lengthens
with it, and soon becomes considerably longer, and more promi-

248

THE CHICK.

nent, than the primitive streak. A longitudinal neural groove
(Fig. 117, NG) forms along its dorsal surface; this is at first
shallow, but rapidly deepens by uprising of its borders as a pair
of longitudinal ridges, the neural folds.

At their hinder ends (Fig. HO), the two neural folds diverge
from each other, and embrace between them the anterior end of
the primitive streak. In front, the neural folds rapidly inert

Fig. 110. — A Chick Embryo at the twenty-fourth hour of incubation ; seen from
the dorsal surface. Cf. Fig. 109, which shows the relations of an embryo
of this age to the blastoderm, x 20.

AD, margin of area pellucida. HD, head of embryo. MS, mesoblastic somite or
protovertebra. NF, neural fold. NGr, neural groove. PS, primitive streak. W,
vitelline vein.

in height, the neural groove between them becoming deeper in
consequence. About the end of the first day (Fig. 110, NF), the
two neural folds meet, in the region of the future hind-brain,
converting the open groove into the closed neural tube (cf.
Fig. 118); and this closure of the tube rapidly extends both
forwards and backwards.

THE NERVOUS SYSTEM. 249

The anterior end of the head of the embryo is lifted up above
the blastoderm by the head fold (cf. Fig. 112) ; and the neural
folds are continued round this uplifted head to its under surface
(Fig. 110), where they become continuous with each other in the
median plane. A transverse section across the extreme anterior
end of an embryo at this stage (Fig. 110) will cut the projecting
neural folds, but no other part of the embryo, and will consist of
two completely separate halves.

By the middle of the second day (Fig. Ill) the neural folds
have met and fused, so as to complete the neural tube, along the
whole length of the brain region; the last point to close being
in the position afterwards occupied by the pineal body. The
fusion has also extended backwards along the greater part of the
region of the spinal cord, but at the hinder end of the embryo
the two neural folds are still a little distance apart.

2. The Spinal Cord.

The spinal cord, in the earlier stages of its development, is
oval in transverse section (Fig. 129, NS) : its roof and floor, in the
mid-dorsal and mid -ventral planes, remain thin ; but its side wails
thicken, so as to reduce the central cavity to a narrow vertical slit.

In the side walls of the spinal cord a distinction is present,
almost from the first, between (i) an inner layer of columnar
ciliated epithelial cells, lining the central canal ; and (ii) the
cells composing the rest of the thickness of the wall. These
latter apparently do not give rise to either nerve-cells or nerve -
fibres, but become modified to form a supporting framework to
the cord. The cells of this second group are from the first
radially arranged, and during the second day they branch at
their outer ends, the branches anastomosing with those of adja-
cent cells to form a delicate reticular framework.

In the meshes of this reticulum certain other cells, the
neuroblasts, appear during the third day ; these are apparently
derived, by direct modification, from certain of the columnar
epithelial cells lining the central canal, which migrate outwards
into the reticulum. Each neuroblast is at first bipolar, having
a shorter process, directed inwards towards the central canal ;
and a longer process which is directed outwards, and which by
further growth becomes the axis cylinder of a nerve fibre. The
axis cylinders thread their way through the meshes of the

250 THE CHICK.

reticulum and reach the surface of the spina! cord, where some
leave it to form the roots of the spinal nerves, while others run
longitudinally along its outer surface to form the layer of white
matter of the spinal cord.

From the third, or fourth, to about the tenth day, this
process of development of neuroblasts and of nerve fibres pro-
ceeds rapidly. The neuroblasts become the nerve cells of the
spinal cord, the first cells to be established being those of the
ventral cornua : their inner processes disappear, and from the
bodies of the cells fine branching protoplasmic outgrowths arise
at a later stage, which anastomose with those of neighbouring
cells. As the nerve fibres increase in number, the layer of white
matter on the surface of the spinal cord necessarily gains in thick-
ness, and the spinal cord rapidly approaches the shape charac-
teristic of it in the adult.

The central cavity of the spinal cord is at first a narrow
vertical cleft (Fig. 129). The side walls of the dorsal half of
this cleft come in contact with each other and fuse, so as to
obliterate the cavity; the ventral half of the cleft persists
throughout life as the central canal of the spinal cord.

Of the two longitudinal fissures of the adult spinal cord, the
ventral fissure is a median groove left between the ventral
columns of white matter, as these increase in thickness ; it may
be recognised on the sixth or seventh day, and by the tenth day
is a conspicuous feature in transverse sections of the spinal cord.

The dorsal fissure is formed in quite different fashion. The
white matter of the dorsal surface grows down into the spinal
cord, about the ninth day, as a pair of vertical plates ; these are
at first separated by a thin median lamina of grey matter ; and
it is by absorption of this median lamina that the dorsal fissure
is formed. The absorption is a gradual one, and for some time
the fissure remains bridged across by slender fibres, derived from
the grey matter.

The neurenteric passages. In the floor of the neural canal,
at the hinder end of the body, two or three pit-like depressions
appear in the early stages of development, which, although they
are usually incomplete, and only rarely open into the mesenteron,
still appear to be homologous with the neurenteric passage in
Amphioxus or in the frog.

THE SPINAL CORD.

251

Three of these depressions have been observed in chick em-
bryos. They appear in succession ; the first one shortly before

Fig. in.

Fig. 112.

Fig. ill.

Fig. 112.

-A Chick Embryo at the thirty-sixth hour of incubation ; seen from

the dorsal surface, x 20.
-A median longitudinal, or sagittal, section of a Chick Embryo at
the thirty-sixth hour of incubation, x 20.

AN, head fold of the amnion. BF, fore-brain. BH, hind-brain. BM, mid-brain.
BO, optic vesicle. CH, notochord. CP, pericardial cavity. EI, auditory pit. GF,
fore-gnt, or anterior portion of the mesenteron. H, hypoblast. MS, mesobiastic somite
or protovertebra. NS, spinal cord. NT, neurenteric canal. PS. primitive streak.
R.V. ventricular portion of the heart. SO, somatopleure. SP, splanchnopleure. TA,
allantois. W, vitelline veins.

the end of the first day; the second one (Fig. 112, NT) about
the middle of the second day ; and the third one in the course
of the third day.

252 THE CHICK.

They are all three blind pockets, extending somewhat
obliquely from the floor of the hinder end of the neural tube
into a fused mass of cells just behind the notochord : this mass
is really the anterior end of the primitive streak, and therefore
corresponds to the anterior lip of the blastopore in the frog
(of. Fig. 60).

3. The Brain.

The general history of the development of the brain in the
chick is very closely similar to that already described in the frog.

At the commencement of the second day, and before actual
fusion of the neural folds has taken place at any part of their
length, the neural canal becomes dilated at its anterior end to
form the anterior cerebral vesicle or fore-brain (Fig. Ill, bf),
from which the optic vesicles, bo, arise almost at once as lateral
outgrowths. Immediately behind the fore-brain, and separated
from it by a slight constriction, is a second and rather smaller
dilatation, the middle cerebral vesicle or mid-brain, BM.

The part of the brain behind the mid-brain, about half its
entire length, is the hind-brain, bh; this consists of a series
of vesicles, separated by slight constrictions, decreasing in size
from before backwards, and passing without any limiting
boundary into the spinal cord posteriorly. The vesicles of the
hind-brain vary considerably in different specimens ; they are
usually four or five in number, of which the two anterior ones,
at any rate, appear to possess considerable constancy. Their
mode of development, and their relations to the nerves and
other structures, strongly suggest that they are each equivalent
to a single vesicle, such as the mid-brain.

By the middle of the second day (Figs. Ill and 112) the
brain is closed, by fusion of the neural folds, along its entire
length ; the point where the folds last meet being at the summit
of the fore-brain, in the position subsequently held by the pineal
body.

The walls of the brain are at first of nearly uniform thick-
ness in all parts ; and transverse sections of the brain are
approximately circular in outline at all parts of its length.

In the following account the several parts of the brain will
be considered in order from behind forwards, and the leading
points in their development described.

THE BRAIN.

-258

tThe medulla oblongata is formed from the hind-brain, the
ntral canal of this part of the brain becoming the fourth
ventricle of the adult.

Fig. 113.— A Chick Embryo at the end of the third day of incubation. Owing
to the twisting of the fore part of the embryo, the head and neck are seen
from the right side, and the hinder part of the body from the dorsal
surface. The amnion has been removed. (Cf. Fig. 99.) x 20.

A, dorsal aorta. Al, first or mandibular aortic arch. A3, third aortic arch, in the
first branchial arch. AC, carotid artery. AV, vitelline artery. BF, thalamencephalon
or fore-brain. BH medulla oblongata. BL, cerebellum. BM, mid-brain. BS,
cerebral hemisphere. EI, auditory vesicle. HC1, first branchial cleft. HC2, second
branchial cleft. HM, hyo-mandibular cleft. MS,mesoblastic somite or protovertebra.
US, spinal cord. OC. optic cup. OF. olfactory pit. OL, lens. PK", pineal body.
BA, auricle of heart. BT, truncus arteriosus. BV, ventricle of heart. VV, vitelline
veins.

The two arrows and crosses indicate the plane along which the section shown in
Fig. 124 is taken.

The walls of the medulla oblongata are at first of nearly
equal thickness all round ; but before the end of the second day

254 THE (MUCK.

(Fig. 121, bh) the dorsal wall or roof becomes very much
thinner than the sides and floor. In the later stages this
difference becomes increasingly marked ; and before the end of
the third day (Figs. 113 and 114) the roof, which is now very
wide, becomes reduced to a single layer of epithelial cells, entirely
devoid of nervous matter ; a condition in which it remains
throughout life. This thin roof soon becomes thrown into folds,
which appear about the seventh clay, and rapidly increase in depth,
hanging down into the cavity of the medulla. Between the
layers of these folds a network of vessels, which early appears on
the outer surface of the roof, grows in to form the choroid plexus
of the fourth ventricle (Fig. 11G, xb).

The division of the hind-brain into a series of vesicles, which
is very noticeable about the thirty-sixth hour (Fig. Ill), becomes
less evident as the side walls thicken, through the formation of
the white nervous matter ; and from the middle of the third day
onwards it is barely perceptible.

The cerebellum is developed from the roof of the anterior
vesicle of the hind-brain, immediately behind the well-marked
constriction which separates the hind-brain from the mid-brain.

It appears towards the end of the second day, as a slightly
marked transverse thickening of the roof of the hind-brain ; it
becomes more conspicuous during the third, fourth, and follow-
ing days (Figs. 113, 114, and 115, bl), but remains as a simple
transverse band until a comparatively late stage of develop-
ment.

About the eighth day, the cerebellum (Fig. 116, bl) becomes
doubled transversely on itself; and at the same time it thickens
considerably, its outer surface becoming slightly folded. From
this time it steadily increases in thickness, and by further
folding of its surface becomes more complicated in structure ;
but up to about the sixteenth day it lies completely behind the
optic lobes.

During the last few days of incubation the cerebellum
enlarges considerably, growing forwards over the top of the
mid-brain and between the optic lobes : by the time of hatching
it has almost met the cerebral hemispheres, and has acquired
the shape and proportions characteristic of the cerebellum in
the adult bird.

THE BRAIN.

255

The fact that the cerebellum remains for so long a time in
the condition of a mere transverse thickening of the roof of the
medulla oblongata, becomes of considerable interest when it is

BM

Fid. 114. — A median longitudinal, or sagittal, section through a Chick Embryo
at the end of the third day of incubation. The amnion is represented by
a dotted line. (Cf. Fig. 113.) x 20.

A, dorsal aorta. AN, head fold of the amnion. AN7, tail fold of the amnion. AV,
vitelline artery. BH, fourth ventricle, or cavity of medulla oblongata. BL, cerebellum.
BM, cavity of mid-brain, the future Sylvian aqueduct. BS, lateral ventricle, or cavity
of the cerebral hemisphere. CH, notochord. DP, proctodaaal pit. GrH, hind-gut,
or posterior portion of the mesenteron. TN, infundibulum. LG-, lung. ITS, central
canal of spinal cord. O, mouth. PIT, pineal body. PT. pituitary body. RS, sinus
venosus. B,T, truncus arteriosus. B,V. ventricle. SP, splanchnopleure. TA,
allantois. TH, thyroid body. TL, tail. TO, oesophagus. TP, pharynx.

256 THE CHICK.

borne in mind thai this is the condition in which it remains!
throughout life in the frog, and in many fish.

The mid-brain undergoes comparatively slight changes. Urn
to the end of the fourth day it is approximately spherical in i
shape; and, owing to its great size and the position which, I
through cranial flexure, it occupies at the apex of the head, it
plays a prominent part in determining the shape of the embryo
(Figs. 113 and 115, em).

On the fifth day, the optic lobes begin to grow out as a pair
of rounded swellings from the roof of the mid-brain, separated by
a median longitudinal groove. These steadily increase in size
during the following days ; up to the sixteenth day they remain
in close contact with each other, but during the last few days of
incubation they become pushed apart by the forward growth of
the cerebellum, and take up the position at the sides of the brain
characteristic of the optic lobes in the adult bird.

The floor of the mid-brain, and the sides, ventral to the
optic lobes, become greatly thickened by the formation of the
crura cerebri. The cavity of the mid-brain becomes greatly
reduced by this thickening of its floor and sides, and forms the
Sylvian aqueduct of the adult.

The thalamencephalon is formed from the original anterior
cerebral vesicle, or fore-brain (Fig. Ill, BF).

The roof and floor of the thalamencephalon remain thin
throughout life, but the sides thicken very greatly to form the
optic thalami, reducing the central cavity to a narrow vertical
cleft, the third ventricle of the adult (c/. Fig. 116, bf).

The anterior wall of the thalamencephalon forms a thin and
narrowband, the lamina terminalis (Fig. 116, bt), which lies
between the roots of the two cerebral hemispheres: in con-
nection with this, the anterior commissure is developed as a
narrow transverse band of nerve fibres, running across between
the basal parts of the hemispheres.

The roof of the thalamencephalon, like that of the fourth
ventricle, becomes early reduced, along the greater part of its
length, to a single layer of epithelial cells, devoid of nervous
elements. About the middle of its length, the pineal body arises,
at the commencement of the third day, as a hollow, rounded,

THE BRAIN.

257

median diverticulum : this is at first directed slightly backwards.
but by the end of the third day becomes inclined forwards (Figs.
113 and 1 11, pn), and lies close beneath the external epiblast.
In the later stages, the pineal body increases in size, becomes
dilated at its distal end, and gives off a number of branching

NNI

Fig. 115. — A Chick Embryo at the end of the fifth day of incubation, seen from
the right side. The amnion has been removed, x 20.

BL, cerebellum. BM, optic lobe, formed from the mid-brain. EI, auditory
vesicle. HY, hyoid arch. LA, fore-limb or wing. LP, hind- limb or leg. IfE,

I rlion of first spinal nerve. NM, commissure connecting first and second spinal
ganglia. OC, eye. OH, choroidal fissure. RV, ventricle of heart. TA, allantois.

IYK, yolk-stalk, cut short. I, olfactory nerve. Ill, third nerve, or motor oculi. V,
fifth or trigeminal nerve. V, ophthalmic branch of trigeminal nerve. VII, seventh or
facial nerve. VIII, eighth or auditory nerve. IX, ninth or glossopharyngeal nerve.
X, tenth or pneumogastric nerve. X', visceral branch of pneumogastric nerve. X'',
commissure connecting pneumogastric nerve with the ganglion of the first spinal nerve.
bular diverticula. Its condition on the eighth day is shown in
g. 116, PN.
In front of the pineal body the roof of the thalamencephalon
very thin, and becomes thrown into folds which hang down
to the ventricle : between the layers of these folds numerous

258

THE CHICK.

blood-vessels penetrate, to form the choroid plexus of the third
ventricle (Fig. 11G, xa).

Immediately behind the stalk of the pineal body, the posterior
commissure is developed in the roof of the thalamencephalon, as

lN PN XA

TO LR RC ES LT BB PT' HB HR

Fig. 116. — 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 incuba-
tion, x 10.

BB, basibrancbial cartilage. BF, third ventricle, or cavity of the thalamencephalon.
BL, cerebellum. BM, Sylvian aqueduct, or cavity of the mid-brain. BS, lateral
ventricle, or cavity of the cerebral hemisphere. BT, lamina terniinalis. BY, olfactory
lobe of the cerebral hemisphere. CH, notochord. ES, aperture of Eustachian tube.
ET, ruesethinoid cartilage. PE, rudimentary feather. HB, basihyal cartilage. HR,
ceratohyal. IN, infundibulum. K, epithelial knob on beak. LB, trachea. LT,
glottis. MC, Meckel's cartilage. NS, spinal cord. PN, pineal body. PT, pituitary
body. PT', stalk of pituitary body. RC, parachordal cartilage. TN, tongue. TO,
oesophagus. VI, neural arch of first or atlas vertebra. V2, centrum of second or axis
vertebra. XA, choroid plexus of third ventricle. XB, choroid plexus of fourth
ventricle. I, notch in mesethmoid cartilage for olfactory nerve. II, optic chiasma.

a transverse band of nerve fibres connecting the two optic
thalami; it is shown, though not lettered, in Fig. 116.

The floor of the thalamencephalon is depressed ventralwards
to form the infundibulum, which lies very close to the ante-
rior end of the notochord, and early acquires intimate relations
with the pituitary body. The infundibulum is already present

THE BRAIN. 259

on the second day (Fig. 112) : during the third and following
days it becomes much more clearly defined (Figs. 114, 123,
in); and about the eighth day (Fig. 116, in), a pocket-like
diverticulum arises from its floor, which is directed backwards,
and becomes wedged in between the anterior end of the noto-
chord and the pituitary body.

In front of the infundibulum the floor of the thalamen-
cephalon becomes greatly thickened, in the later stages, by the
development of the optic chiasma (Fig. 11G, n).

The pituitary body, though not really a part of the brain, is
so intimately connected with this that it may conveniently be
described here.

The pituitary body appears, towards the end of the second
day, as a pocket-like diverticulum of the anterior angle of the
stomatodaaum, or mouth invagination (cf. Fig. 114, pt) ; it lies
wedged in between the anterior end of the mesenteron and the
floor of the infundibulum, and its blind extremity is in close
contact with the anterior end of the notochord.

On the formation of the mouth perforation, which places the
stomatodasum in communication with the mesenteron, the
pituitary body (Figs. 114, 123, pt) persists as a diverticulum
from the roof of the mouth, with the same relations as before
to the infundibulum and to the notochord.

During the succeeding days, while the face is being esta-
blished and the beak is growing forwards prominently, the
pituitary body, retaining its relations with the brain and the
notochord, becomes left further and further back in the roof of
the mouth.

At the eighth day its position and relations are shown in
Fig. 116. The upper blind end, pt, has given off a number of
branching tubular diverticula, which together form a rounded
vascular mass, lying immediately below the infundibulum, IN,
and in the pituitary foramen at the base of the skull, between
the trabecule cranii. The stalk of the pituitary body is still
present as a narrow tube, pt', which opens into the roof of the
mouth in the median plane, opposite the glottis, lt, and just
in front of the opening of the Eustachian tubes, es. By the
twelfth day the stalk has become a solid rod of cells, and the
communication between the pituitary body and the mouth is
finally cut off.

s 2

260 tiik cine!;.

The optic vesicles arise, early on the second day, as a pair 01
lateral outgrowths from the fore-brain (Fig. HI, bo). They give
rise, as in the frog, to the retina and the retinal pigment of the
eye, and their developmental changes will be described in the!
section dealing with the formation of the eye (p. 275).

The cerebral hemispheres. About the middle of the second
day, the fore-brain (Fig. Ill, bf) begins to grow forwards, in front
of the optic vesicles, as an anterior, median outgrowth, the vesicle
of the hemispheres. At the same time cranial flexure becomes
pronounced (Fig. 112), owing to the dorsal surface of the head
growing faster than the ventral surface ; the axis of the brain
becoming a curved instead of a straight line, and. the fore-brain
being carried round to the ventral surface of the head. The
curvature of the brain progresses rapidly ; the fore-brain (Fig.
113) becoming placed at right angles to the rest of the brain,
and the mid-brain growing forwards so as to lie at the extreme
anterior end of the head.

The vesicle of the hemispheres grows rapidly, both in length
and width : during the third day the paired cerebral hemispheres
arise from its anterior end as thin-walled outgrowths, separated
by a median furrow. The hemispheres (Figs. 113, 115, and
123, bs) enlarge rapidly, growing upwards and forwards, and
forming a pair of prominent rounded swellings at the anterior end
of the head, very conspicuous in embryos of the third to the seventh
or eighth day. From the ventral surface of their anterior ends
the olfactory nerves arise at a very early stage.

From the eighth day onwards the hemispheres, though still
increasing in size, become less conspicuous from the surface, owing
to the forward growth of the face, and especially of the beak, which
elongates rapidly and completely alters the shape of the head
(Fig. 116). As the beak extends forwards, the anterior ends of the
hemispheres, from which the olfactory nerves arise, grow out as a
pair of small hollow buds, the olfactory lobes (Fig. 116, by), from
the ends of which the olfactory nerves run forwards to the nose.
The walls of the hemispheres are at first thin ; in the later
stages they thicken considerably, especially on the outer side of
their hinder ends, where they form the corpora striata. The
cavities of the hemispheres persist throughout life as the lateral
ventricles of the brain, which retain their communication with

THE PERIPHERAL NERVOUS SYSTEM. 261

the third ventricle, or cavity of the fore-brain, through a pair of
narrow apertures, the foramina of Monro.

4. The Peripheral Nervous System.

a. General Account. The nerves, both cranial and spinal,
which compose the peripheral nervous system are entirely of
epiblastic origin, and develop in a manner closely similar to that
already described in the frog.

The nerves fall under two categories : —

(i) The ganglionated nerves. These arise directly from the
inner surface of the epiblast, as a pair of longitudinal neural
ridges, along the margins of the neural plate. They appear before
the neural tube is closed (Fig. 117, ma), and by the folding of
its walls to complete the tube they get carried on to its dorsal
surface, where they form a pair of bands (Fig. 118, NA), projecting

Fig. 117.— Transverse section across the body of a Chick Embryo at the twenty -
fourth hour of incubation. (Cf. Fig. 110.) x 200.

CH, notoohord. E. epiblast. H, hypoblast. M, mesoblast. MA. commencing
neural ridge. WGr, neural groove. NP, neural plate.

outwards from the angles between the external epiblast and the
walls of the neural tube. On the completion of the neural tube
by fusion of its lips, the neural ridges separate from the surface
epiblast, but remain in close contact with the dorsal surface of
the tube (Fig. 119, nb).

I The neural ridges are at first continuous structures, from
hich the nerve ganglia arise as paired outgrowths ; these grow
ipidly, extending outwards and downwards, and acquire their
^rmanent roots of attachment by outgrowth of nerve fibres
om the ganglion cells into the brain or spinal cord.
To this category belong the fifth, the seventh and eighth, the
sensory roots of the ninth and tenth, with perhaps one or two
of the other cranial nerves; and the dorsal or sensory roots
of the spinal nerves.

262 THE CHICK.

(ii) The non-ganglionated nerves. These arise as direct out-
growths from the nerve cells of the brain or spinal cord. The
nerves of this category develop at a rather later period than
those of the former one ; they are all motor in function, and to
them belong the sixth, and perhaps some of the other cranial
nerves, and the ventral or motor roots of the spinal nerves.

Certain of the cranial nerves cannot at present be referred
with certainty to either category ; but in their cases our know-
ledge of the developmental history is incomplete, and further
research is necessary before any definite statement can be made
concerning their real nature.

With regard to the nerves definitely included in the first
category, a distinction must be made between the cranial and
the spinal nerves, similar to that already described in the frog.
The cranial nerves, in their growth outwards, lie at first very
superficially, just beneath the external epiblast. Near their
distal ends they early acquire connection with localised thick-
enings of the external epiblast, situated about the horizontal
level of the notochord, and just above the dorsal borders of the
gill-clefts. From these thickened patches of epiblast, which are
probably to be regarded as sense organs, cells are budded off'
into the nerves, which appear to take a direct part in their
further development.

The spinal nerves, on the other hand, are from the first
more deeply situated. They lie between the spinal cord and the
muscle plates (Fig. 124, ne), and do not acquire the connec-
tions with the external epiblast which are characteristic of the
cranial nerves.

b. The Cranial Nerves. The first trace of the cranial nerves
appears, in the chick, in the region of the mid-brain, about the
twenty-second hour. At this stage, slightly younger than that
shown in Figs. 110 and 118, the neural folds have nearly met, in
the region of the head and neck, but have not yet coalesced at
any part of their length ; while in the body region the central
nervous system is still a widely open groove. Only one or two
pairs of mesoblastic somites are as yet present.

At the lips of the neural groove there is on either side a
ridge-like outgrowth of epiblast cells, from the angle between
the external epiblast and the wall of the neural canal. This

THE CRANIAL NERVES.

263

outgrowth (Fig. 118, NA), which consists of cells more spherical
in shape than those of the surface epiblast, or of the brain-
wall, appears first in the region of the mid-brain, but rapidly
extends both forwards and backwards : forwards as far as the
anterior part of the fore-brain ; backwards along the whole
length of the hind-brain, and a certain distance down the
spinal cord.

These outgrowths (Fig. 118, na) are the neural ridges. As
they arise before the lips of the neural canal have met, the
neural ridges of the two sides are at first completely independent
of each other. A few hours later, when closure of the neural

NA~

Fig. 118. — Transverse section across the head of a Chick Embryo at he twenty-
fourth hour of incubation, passing through the region of the mid-brain.
(Cf. Fig. 110.) x 100.

B, cavity of the mid-brain. CH, notochord, not yet separated from the hypoblastie
wall of the pharynx. E, epiblast. H, hypoblast. NA, neural ridge. RT, commencing
heart. TP, pharynx.

canal is effected, the neural ridges separate completely from the
external epiblast, but remain closely attached to the brain ; the
ridges of the two sides at the same time coalescing with each
other to form a continuous longitudinal band, the neural crest
(Fig. 119, in b), extending along the dorsal surface of the brain.

Almost from its first appearance, and before the neural
tube is closed, the neural crest becomes more prominent at
certain places. These more prominent parts form paired
outgrowths of the crest, and are situated opposite the widest
parts of the cerebral vesicles. They are the rudiments of the
cranial nerves (Fig. 119, nb), while the intervening narrower
parts of the crest (Fig. 120, nb), opposite the constrictions

ZCA

THE CHICK.

between the cerebral vesicles, form commissural bands, which
for a time connect together the successive pairs of nerve
outgrowths.

In a typical cranial nerve, such as the facial or glossopharyn-
geal, the further changes are as follows. The nerve rudiment
rapidly extends outwards, lying close beneath the external
epiblast, but independent of this. Opposite the nerve, but at
some little distance beyond the brain, and about the horizontal
level of the notochord, a proliferation of the cells of the external
epiblast takes place, forming a small, inwardly projecting knob.

vv

Fig.' 119. — Transverse section across the head of a Chick Embryo at the forty-
third hour of incubation. The section is taken immediately behind the
auditory pits and the heart, and passes through the rudiments of the glosso-
pharyngeal nerves, x 100.

A, aorta. BH, cavity of hind-brain. CH, notochord. E, epiblast. H, hypoblast.

ME, somatouleuric layer of mesoblast. MH, splanchnoplcnric layer of niesoblast.
NTS, neural crest : the part shown in the figure gives rise later on to the ganglia oftTO
glossopharyngeal nerves. VV, vitelline vein.

The nerve soon comes in contact with this knob, and fuses with
it, close to its distal end. Cells are budded off from the knob
into the nerve, which thus becomes reinforced from the epiblast.
The exact fate of these cells is uncertain, but it is probable that
they take part in the formation of the ganglionic thickening on
the nerve.

The inner or proximal end of the nerve thins rapidly, and
loses its connection with the dorsal surface of the brain, a con-
nection which from the first has been one rather of close contact
than of actual continuity. A little way beyond this point, how-
ever, the nerve acquires its permanent attachment to the brain,

THE CRANIAL NERVES.

265

about half way down its side ; this attachment being effected by
the outgrowth of processes from the cells of the nerve, into the
substance of the brain.

This attachment is acquired by the seventh nerve about the
end of the second, or early in the third day (Fig. 121, vn).
Owing to the part of the brain dorsal to the nerve growing
more rapidly than its ventral part, the root of attachment of the
nerve becomes apparently shifted further downwards, towards the

Fig. 120. — Transverse section across the head of a Chick Embryo at the fort}--
third hour of incubation. The section passes through the commencing
auditory pits, and through the heart. x 100.

A, aorta. BH. cavity of hind-brain. CH, notochord. EI, commencing auditory
pit. H. hypoblast. NB, neural crest : the section passes through the narrow commis-
sural part of the crest, which connects the rudiments of the facial and auditory nerves
with those of the glossopharyngeal nerves. RE, endothelial lining of heart. RM,
muscular wall of heart. TP, pharynx.

ventral surface of the brain, and by the end of the third day has
acquired the position characteristic of the nerve-root in the adult.
At the same time, changes occur in the trunk of the nerve.
Owing to intrusion of mesoblast between the surface epiblast
and the nerve, the latter becomes more deeply placed than in
e early stages. The connection with the sensory patch of the
rface epiblast persists, but becomes drawn out, as the nerve
recedes from the surface, into a cutaneous branch of greater or
less length. Beyond the origin of this cutaneous branch the
nerve continues its growth, and by the end of the third day, or

'266 THE CIIKK.

early on the fourth day, its main branches of distribution become
definitely established.

These branches, in the case of the seventh or facial nerve,
are closely connected with the hyo-mandibular cleft. They
consist of a large hyoidean or post-branchial branch (Fig. 1 1 5,
vn), which runs along the hyoid arch ; and a smaller mandi-
bular or prebranchial branch, which runs forwards over the
dorsal end of the hyo-mandibular cleft, and then downwards a
short distance along the mandibular arch.

The above account will apply to any one of what may be
termed the typical cranial nerves. It will now be convenient to
take the several cranial nerves one by one, and note the chief
points in their individual development.

I. The olfactory, or first cranial nerve. Our knowledge of
the development of the olfactory nerve in the chick is still
incomplete in some respects. At the twenty-ninth hour the
neural ridges extend forwards along the brain as far as the
anterior end of the fore-brain, i.e. in front of the optic vesicles
(cf. Fig. 111). There are reasons for thinking that it is from
the anterior ends of the neural ridges that the olfactory nerves
are, at any rate in part, developed ; but the point has not been
proved by actual observation.

At the fiftieth hour, before the paired cerebral hemispheres
have commenced to appear, the olfactory nerves may be recog-
nised as a pair of short outgrowths, arising from the dorsal
surface of the unpaired vesicle of the hemispheres, and running
downwards and outwards towards a pair of slightly thickened
patches of epiblast, on the under surface of the head, which form
the earliest rudiments of the olfactory pits.

During the third day the cerebral hemispheres arise. These
are, from the first, situated dorsally to the roots of the olfactory
nerves; and, growing rapidly forwards and upwards (Fig. 113),
they appear to drive the olfactory nerves down to the base or
ventral surface of the brain. By the further growth of the
cerebral hemispheres the original unpaired vesicle of the hemi-
spheres becomes obliterated, or rather absorbed into the hemi-
spheres, and the olfactory nerves from this time arise directly
from the hemispheres. During the third day the olfactory
pits deepen rapidly, and the distal ends of the olfactory nerves

THE CRANIAL NERVES. 267

become continuous with the olfactory epithelium. The mode in
which this connection is acquired is closely similar to that in
which the typical cranial nerve acquires connection with the
sensory patch of the surface epiblast, and it has been suggested,
with much reason, that the olfactory epithelium may be homo-
logous with one of these sensory patches.

The condition of the olfactory nerve at the end of the fifth
day is shown in Fig. 115, I. The nerve, which is still very short,
runs downwards and backwards from the under surface of the
hemisphere to the olfactory pit.

On the seventh day, as already noticed, the beak begins to
form ; and during this and the following days it grows forwards
with great rapidity. The olfactory sacs become imbedded in the
sides of the beak (Fig. 131, ok), and are carried forwards with
the beak as it lengthens. This causes a change in the direction
and in the relations of the olfactory nerves, which, previously
quiescent and inactive, have now to elongate rapidly, in order
to maintain the connection between the olfactory organs and the
brain. This elongation is effected mainly by growth of the
nerves themselves, but partly, as already explained, by pulling
out of the anterior ends of the hemispheres, from which the
olfactory nerves arise, to form the olfactory lobes (Fig. 116, by).

It is very possible, therefore, though not yet proved, that
the olfactory nerve is really comparable to a typical cranial
nerve, such as the facial, in which the sensory cutaneous branch
is the only one developed.

II. The optic, or second cranial nerve. The optic nerves in
the chick are very generally described as being formed directly
from the constricted necks, or stalks, of the optic vesicles, which
connect these with the brain. If this be correct, the optic nerve
is in no way comparable with the other nerves, cranial or spinal,
but must be contrasted with all of these as being formed by
direct modification of part of the brain wTalls.

There are, however, strong grounds for suspecting that, as
in the frog (p. 139), the fibres of the optic nerve really arise in
the retina, and grow inwards to the brain; the optic stalk afford-
ing the path along which they grow, but not itself taking any
direct part in their formation.

The neural ridges, as already described, extend forwards

268 THE chick.

along fche whole length of the fore-brain, but they do not appear
to take any part in the development of the optic nerves.

III. The motor oculi, or third cranial nerve. The third nerve
is the only one which, in the adult bird, arises from the mid-brain.
The neural ridges appear first of all on the top of the mid-brain,
and early attain a great size in that position (Fig. 118, NA), but
it is not yet clear what happens to these ridges in the later
stages. It is possible that they take part in the formation of
the third nerve, but this has not been proved to be the case.

The actual date of the first appearance of the third nerve
has not been determined. About the middle of the third day
it is clearly visible as a nerve of rather large size (Fig. 124,
in), arising from the base of the mid-brain, not far from the
middle line, and running backwards and downwards towards
the hinder border of the eye.

By the fifth day (Fig. 115, in), the third nerve has the
characteristic course of the adult nerve, arising from* the floor
of the mid-brain and running downwards and backwards im-
mediately behind the eye.

There are strong reasons for regarding the third nerve as
corresponding to at any rate a part of a typical cranial nerve,
but until its early development is more clearly ascertained it is
impossible to speak definitely with regard to it. Its origin
from the base of the brain, close to the median plane, its dis-
tribution to muscles, and the fact that its root in the early stages
(Fig. 115, in) is multiple, have led most investigators to compare
it with the ventral root of a spinal nerve rather than with the
dorsal root.

The ciliary ganglion is stated to be formed in the chick in
connection with a knob-like thickening of the surface epiblast,
similar to the sensory patch of a typical cranial nerve.

IV. The fourth cranial nerve. The fourth nerve in the
adult is peculiar, inasmuch as it is the only nerve which
arises from the dorsal surface of the brain, and also, so far as is
known, the only nerve which arises from a constriction between
two brain vesicles instead of from the middle of a vesicle.

In a chick embryo of the fifth day the fourth nerve is easily
recognised. It is very slender, but has already the course and

THE CRANIAL NERVES. 269

relations characteristic of the nerve in the adult bird. Its
development in the chick is unknown.

V. The trigeminal, or fifth cranial nerve. The trigeminal
nerve arises from the neural ridge on the first or most anterior of
the vesicles of the hind -brain, and its development accords exactly
with that of a typical cranial nerve as described above. The
ganglion of the trigeminal nerve, or Gasserian ganglion, is
formed mainly from a portion of the neural ridge, reinforced
from an independently arising knob of the surface epiblast. The
permanent attachment of the nerve to the side of the hind-brain
is acquired at the commencement of the third day ; and about
the same time the nerve divides distally into ophthalmic and
mandibular branches, of which the former (cf. Fig. 115, v') runs
forwards along the inner side of the eyeball to the front of the
head, while the latter, V, runs downwards and backwards in the
mandibular arch. From the mandibular nerve, the maxillary
nerve arises on the third day as a branch (cf. Fig. 115), which
runs forwards in the maxillary arch or upper jaw.

The development of the motor root of the trigeminal nerve
in the chick has not been determined satisfactorily, and it is not
yet certain whether this is a part of the original nerve, or whether,
as seems more probable, it arises independently as an outgrowth
from the brain itself.

VI. The sixth cranial nerve. The sixth nerve is of a very
different nature to the trigeminal or facial nerves, and in its
mode of origin and relations agrees more closely than any of the
other cranial nerves with the ventral or motor root of a spinal
nerve.

It appears during the fourth day, arising from the base of the
hind-brain, near the median plane, by a number of very slender
rootlets, the most anterior of which is on a level with the hinder
part of the root of the trigeminal nerve, and the most posterior
one opposite the root of the facial nerve. The rootlets unite
together to form a slender nerve, which runs forwards below the
base of the brain to the external rectus muscle of the eyeball, in
which it ends.

VII. The facial, or seventh cranial nerve arises from the
neural crest on the top of the second vesicle of the hind-brain ;

'270 THE CHICK.

its development has already been described as that of a typical
cranial nerve.

VIII. The auditory, or eighth cranial nerve (Fig. 11 ~>, vmj
is, in the chick, continuous with the facial nerve from its first
appearance. It is a short stout nerve, which at a very early
period, about the fiftieth hour, comes in contact with the auditory
epithelium, and fuses with this. The subsequent development
of the nerve consists mainly in its division, distally, into
branches supplying the several special patches of the auditory
epithelium, and will be described more fully in the section
dealing with the development of the ear.

So far as the chick is concerned, there appears to be no
reason for separating the facial and auditory nerves from each
other. The two together make up a typical cranial nerve, of
which the auditory nerve represents the cutaneous branch,
greatly hypertrophied in consequence of the large size and
importance of the sensory patch, i.e. the internal ear, which it
supplies.

IX. The glosso -pharyngeal, or ninth cranial nerve (Figs. 115,
IX, and 119, nb), is at first continuous with the pneumogastric
or tenth nerve, a single elongated strip of the neural ridge on
the roof of the hind-brain, immediately behind the ear, giving
origin to both these nerves. The strip divides, before the end of
the second day, into an anterior or glosso-pharyngeal portion,
and a posterior or pneumogastric portion.

The glosso-pharyngeal develops as a typical cranial nerve ;
it early acquires connection with a sensory patch of the surface
epiblast, and its main stem, beyond this point, runs downwards
along the first branchial arch (Fig. 115). The root of attach-
ment of the nerve to the brain early becomes multiple, consisting
of four or five small rootlets, which spread out in a fan-like
manner on entering the brain. The multiple character of the
roots of the glosso-pharyngeal nerve is of interest, as showing
that the similarly multiple nature of the roots of the third nerve
is not incompatible with a possible origin of this latter from the
neural ridge.

X. The pneumogastric, or tenth cranial nerve (Fig. 115, x)
arises from the posterior part of the outgrowth from the neural

THE CRANIAL AND SPINAL NERVES. 271

ridge, common to it and the glossopharyngeal nerve. At first the
pneumogastric is, if anything, the smaller of the two nerves,
but it soon becomes distinctly the larger. Like the glosso-
pharyngeal nerve, it early acquires multiple roots, the most
anterior of which is directly continuous with the hindmost of
the roots of the glossopharyngeal nerve, without entering the
brain.

Beyond the roots of origin, the main stem of the pneumo-
gastric nerve runs downwards and backwards, parallel to the
glosso-pharyngeal nerve; it expands into a large fusiform gan-
glion, from which branches are given off to the second and third
branchial arches, as well as large branches to the heart, lungs,
and intestines.

From the hindmost root of origin of the pneumogastric nerve
from the brain, a long commissural branch (Fig. 115, x") runs
backwards along the side of the medulla oblongata, and is con-
tinuous posteriorly with the ganglion of the first spinal nerve.
This commissural branch is derived from the part of the neural
ridge between the pneumogastric and first spinal nerves.

The mode of development of the spinal accessory or eleventh
cranial nerve, and of the hypoglossal or twelfth cranial nerve,
has not been satisfactorily determined in the chick. The hypo-
glossal nerve has, from the first, the relations characteristic of
the ventral roots of the spinal nerves ; though whether it corre-
sponds to one, or to more than one, of such roots is not determined
with certainty.

It is interesting to note that the definite relations of the
fifth, seventh, ninth, and tenth cranial nerves to the visceral
arches are as characteristically shown in an embryo chick of the
fifth day (Fig. 1 1 5) as they are throughout life in a typical water-
breathing Vertebrate such as a dogfish.

c. The Spinal Nerves.

I The dorsal roots of the spinal nerves develop, as already
ticed, in a manner practically identical with the typical
mial nerves. Their first appearance is almost simultaneous
th that of the cranial nerves ; they may be recognised in
ibryos in which the first two or three pairs of mesoblastic

272 THE chick,

eve

somites are alone present (rf. Fig. 110), and sometimes
prior to the definite formation of any of the somites.

In the anterior part of the spinal cord the neural ridged
appear, just as in the brain, as cellular proliferations from the re-
entering angles between the external epiblast and the lips of the
neural plate, which latter have already grown in towards each
other a certain distance. The neural ridges of the spinal cord
are directly continuous with those of the brain, and from them
the ganglia of the spinal nerves are derived as paired outgrowths.
The spinal ganglia differ, however, from the cranial ganglia in
not acquiring any distal connection with sensory patches of the
epiblast, and in being from the first much more deeply situated,
growing downwards close alongside the spinal cord, between
this and the muscle-plates (Fig. 121, ne).

During the third day, the spinal ganglia acquire their definite
attachments to the sides of the spinal cord, these being effected by
the outgrowth of nerve fibres from the inner sides of the ganglia
into the cord. The parts of the ganglia above, or dorsal to,
the points of attachment persist for some time as small pointed
processes, but soon become inconspicuous, and are finally absorbed
into the ganglia.

The spinal ganglia are of considerable width, more than
half the width of the somites to which they belong (Fig. 115, ne),
so that the intervals between successive ganglia are distinctly
less than the width of the ganglia themselves. The ganglia lie,
from the first, opposite the anterior parts of the somites to which
they belong.

The ganglia of the anterior part of the body are connected
together by short commissural bands (Fig. 115, nm), situated at
the same level as the attachments of the ganglia to the spinal cord ;
and the most anterior spinal ganglion, as already noticed, is con-
nected with the hindmost root of the pneumogastric nerve by a
similar but much longer commissure (Fig. 115, x"). These com-
missures appear to be formed from the parts of the originally
continuous neural ridge which are left between the successive
ganglion outgrowths ; they are well developed and conspicuous
structures on the fourth and fifth days, but after the latter date
are difficult to detect.

The spinal nerves of the hinder part of the cord develop in
slightly different fashion to those of the anterior part. They appear

THE SPINAL NERVES. 273

at a slightly later date, but at a relatively earlier stage in the for-
mation of the spinal cord. At a time when the neural canal has
hardly commenced to form, and the neural plate is only very
slightly folded on itself in the middle line, the nerve rudiments
may be recognised in transverse sections (Fig. 117, ma), as small
conical masses of cells, cut out from the deeper part of the epiblast,
at the edges of the neural plate. As the neural folds rise up, and
grow in towards each other, the nerve rudiments are carried up
with the folds to the dorsal surface of the spinal cord, and then
complete their development in the manner already described.

In the posterior half, or so, of the body there appears to be no
continuous neural ridge developed, the nerve, rudiments being,
from the first, independent outgrowths. There are consequently
no longitudinal commissures connecting these hinder nerves,
similar to those in the anterior part of the body (Fig. 115).
These commissures disappear in the anterior part of the body
shortly after the fifth day, and it is possible that their absence in
the hinder part of the body is to be explained as due to abbrevia-
tion of the developmental history, by omission of this stage.

The ventral roots of the spinal nerves arise later than the
dorsal roots, daring the latter part of the third day. They appeal*
as small outgrowths from the lower part of the sides of the spinal
cord, and from the first occupy the position held by them in the
adult. This position is indicated, before the actual appear-
ance of the root, by a slight convergence of the cells at the side
of the cord ; and the nerve root is apparently formed by the direct
outgrowth of processes from these cells, which, passing out from
the side of the spinal cord, become the axis cylinders of the nerve
fibres.

Each ventral root arises by a number of separate rootlets,
which leave the spinal cord in a longitudinal series, the total
length of a root being about equal to half that of a somite ; the
root lies opposite the anterior half of the somite, and vertically
below the corresponding dorsal root.

Towards the end of the third day (Fig. 124), the ventral
roots, growing downwards and outwards, meet the dorsal roots,
and with these form the trunks of the spinal nerves. Beyond
the place of union of the roots the nerves continue their growth
outwards and downwards, lying along the inner surfaces of

T

I

274 THE CHICK.

the muscle plates. By the end of the fourth day the nerves have
doubled in length, and the primary dorsal and ventral divisions
are already established, each division including fibres from both
the dorsal and ventral roots.

In the part of the body between the fore and hind limbs
(cf. Fig. 115), the main branches of the nerves run in the body
wall or somatopleure. In the segments opposite the limbs,
the nerves enter the limbs and divide into dorsal and ventral
branches, which unite with the corresponding branches of the
nerves in front of, or behind, them to form broad plates of nerve
fibres, from which the individual nerves of the adult limb arise.

d. The Sympathetic Nervous System. The origin of the
sympathetic nervous system in the chick has been much debated,
and is not yet satisfactorily determined. The most trustworthy
observations are to the effect that the sympathetic nervous system
arises at an early stage, the third or fourth day, as a series of
outgrowths from the spinal nerves, apparently derived directly
from the spinal ganglia. These grow inwards, at a ^level im-
mediately above the cardinal veins, and close to the dorsal
aorta : at their ends are ganglionic enlargements, the nerve-
cells of which are apparently derived, by direct migration, from
the spinal ganglia. These ganglionic enlargements soon become
connected, along each side of the body, by longitudinal com-
missures, apparently formed by outgrowths of nerve-fibres from
the ganglia themselves.

If this account is correct, the sympathetic nervous system
of the chick is to be regarded merely as a specialised part of
the spinal nervous system.

DEVELOPMENT OF THE SENSE ORGANS.

The general history of the development of the sense organs
in the chick is very similar to that already described in the
frog. In all cases the essential part of the organ, the actual
sensitive surface itself, is derived directly or indirectly from the
epiblast or epidermis.

1. The Nose.

The olfactory organs appear, about the fiftieth hour, as a pair
of thickened patches of the external epiblast on the under

THE NOSE AND EYE. 275

surface of the fore part of the head ; these soon become depressed,
forming pits (Fig. 113, of), with the bottoms of which the
olfactory nerves very early become connected (p. 266).

The mouths of the olfactory pits narrow, and become slit-
like, but remain open throughout life as the external nostrils
(Figs. 125, 126, ok).

The epithelial lining of each olfactory pit becomes thrown
into folds, to increase its surface ; and gives rise directly to the
olfactory epithelium, or Schneiderian membrane, of the adult
nose.

The posterior narial passage is a secondary formation ; it
appears at first as a groove on the under surface of the head,
leading from the edge of the olfactory pit to the anterior and
outer angle of the stomatodasum. This groove is well marked
on the fourth day, its inner lip being formed by the fronto-nasal
process (Fig. 125, fp), or median part of the face, between the two
olfactory pits ; and its outer lip being formed by the maxillary
arch (Fig. 125, Mx), or rudiment of the upper jaw.

On the fifth day (Fig. 125) the olfactory groove deepens,
and its inner and outer lips, formed by the fronto-nasal process
and maxillary arch respectively, meet and coalesce, so as to
convert the groove into a tube, leading from the olfactory pit
to the mouth. This tube is the posterior narial passage ; it at
first opens into the anterior end of the mouth cavity, immediately
behind the upper lip ; but as the mouth elongates, by growth
forwards of the beak, a horizontal shelf-like partition is formed
at the anterior end of the upper jaw on either side. By fusion
in the median plane, the two horizontal partitions form the
palatal septum, which stretches across the anterior part of the
mouth, separating the olfactory or nasal region above from the
buccal cavity below, and shifting backwards the communication
between the posterior nostrils and the mouth.

2. The Eye.

As in the frog, and in Vertebrates generally, the retina or
essential part of the eye is formed from the optic vesicle, while
the lens is an independent invagination of the surface epiblast.

The optic vesicles arise, at the commencement of the second
day, as a pair of hollow lateral outgrowths from the fore-brain ;
they grow rapidly, and attain some size before the lips of the

T 2

276

THE CHICK.

neural folds fuse to complete the neural tube. The optic vesicles
at first stand out at right augles to the head, but they soon
become constricted at their bases, and directed somewhat down-
wards and backwards (Fig. Ill, bo). These constrictions rapidly
deepen, so that by the end of the second day the optic vesicles

OL

Fig. 121. — Transverse section across the head of a Chick Embryo at the forty-
eighth hour of incubation. The section is taken along a line corresponding
to one joining the reference letters ei and ol in the three-day embryo shown
in Fig. 113. Owing to the cranial flexure, both fore-brain and mid-brain
are cut by the section. The right side of the section is slightly anterior in
position to the left side, x 60.

A, aoi-ta. AC, carotid artery. BF. cavity of fore- brain. BH, cavity of hind-brain.
CH, notoohord. EI, auditory pit. HM, hyo-mandibular cleft. MN, mandibular
arch. OC, cavity of optic cup. OL. invagination of epiblast to form the lens. OS.
optic stalk. PT, pituitary body. TP, pharynx. VII, facial nerve.

are connected with the floor of the fore-brain by narrow tubular
stalks (Fig. 121, os).

Towards the end of the second day a circular patch of the
external epiblast, opposite the outer wall of each optic vesicle,
becomes thickened, and shortly afterwards pitted in to form the
vesicle of the lens (Fig. 121, ol). The formation of this pit
is accompanied by an infolding of the outer wall of the optic

THE EYE.

277

vesicle, which thus becomes doubled on itself to form the optic
cup (Fig. 121, oc).

The lens. The pitting-in of the epiblast, to form the lens,
rapidly deepens ; the lips of the pit close in, and unite, so as to
convert the pit into a closed sac, the lens vesicle, which separates
completely from the external epiblast during the third day.
After this separation, the outer wall of the lens vesicle remains
thin, and is formed of a single layer of flattened epithelial cells ;
the inner wall thickens rapidly, by elongation of its component

Fig. 122. — Transverse section across the fore-brain and eye of a Chick Embryo
at the sixtieth hour of incubation. On the right side the section passes
through the optic stalk ; on the left side it passes just behind the stalk.
x45.

BP, cavity of fore-brjtin. BS, cavity of commencing cerebral hemisphere. MX-
maxillary arch. OC, inner wall of optic cup. OD, outer wall of optic cup. OL,
lens. O'S, optic stalk.

cells (Fig. 122, ol) ; and by the fourth day it comes in contact
with the outer wall, so as to obliterate the cavity of the vesicle
entirely. From the epithelial cells of this thickened inner wall
the whole of the substance of the adult lens is derived. The
outer, thin wall of the lens vesicle becomes the epithelial lining
of the lens capsule ; while the lens capsule itself is apparently a
cuticular membrane excreted by the epithelial cells of the lens
vesicle.

The optic cup. In the optic cup important changes occur.
The two layers of the cup soon come in contact with each other

278 THE CHICK.

(Fig. 122, oc, od), and by the end of the third day the origin,! I
cavity of the optic vesicle is practically obliterated. The
whole cup grows rapidly ; its lip remains in contact with the
margin of the lens the whole way round, except at one point on
the under surface of the cup, below the reference line, OL, in
Fig. 122, where a small chink is left between the lens and the lip
of the cup. As the optic cup increases in size, this chink becomes
lengthened out into a slit, the choroidal fissure (Figs. 113,
115, and 125, oh), through which the mesoblast of the head
gains admittance into the cavity of the cup.

From the wall of the optic cup the retina is developed, while
the mesoblast which grows into the cavity of the cup, through the
choroidal fissure, gives rise to the vitreous body. The choroid
and sclerotic coats of the eye are formed from the mesoblast
outside the optic cup, and the cornea from mesoblast which
grows in between the lens and the surface epiblast.

The exact mode of formation of the choroidal fissure is
difficult to determine. The first step in the doubling up of the
optic vesicle to form the optic cup (Fig. 121) is intimately
associated with the ingrowth of the surface epiblast to form the
lens vesicle, and is perhaps due, in part, to mechanical in-pushing
by this latter : the later stages of the doubling up, however,
concern the optic cup alone, and must be regarded as due to
unequal rates of growth of different parts of the wall of the cup.
This unequal rate of growth in different directions probably
plays an important, or even predominant, part in the formation
of the choroidal fissure. The formation of the choroidal fissure
has been recently shown to be closely associated with the
growth of the fibres of the optic nerves ; these fibres passing
through the choroidal fissure on their way from the retina
towards the brain.

The choroidal fissure only remains open for a short time.
About the sixth day its lips come in contact, and very shortly
afterwards they fuse together, so as to complete the closure of
the optic cup ; by the ninth day all trace of the fissure has dis-
appeared.

The retina is formed directly from the wall of the optic cup.
Of the two layers of which the doubled-up wall of the cup
consists, the inner (Fig. 122, oc) is from the first much the
thicker. It consists, on the third day, of elongated nucleated

THE EYE. 279

cells arranged side by side, and vertically to the surface. From
the fourth day onwards it increases rapidly in thickness, and by a
series of histological changes which have not yet been determined
very accurately, it becomes converted into the several layers of
the retina. The inner layers of the retina are the first to be
established, and the last elements formed are the rods and
cones ; these latter growing outwards as processes from the
outer nuclear layer, which until the appearance of the rods and
cones is the outermost layer of the retina.

The outer wall of the optic cup (Fig. 122, od) is, from the
first, much thinner than the inner wall. By the middle of the
fourth day it is reduced to a single layer of flattened cells, which
soon become pigmented, and ultimately give rise to the layer of
special pigmented cells which lie in close contact with the outer
ends of the rods and cones.

It appears, therefore, that_yia_wJiole_of_the sensory part of
the retina is derived from the inner layer of the optic cup. It
is worthy of notice that the rods and cones, the only elements
of the retina directly sensitive to light, are the las_tparts to be
formed.

The optic nerve is very commonly said to be formed from the
optic stalk ; but it is more probable that it arises in the chick,
as it is known to do in the frog (p. 138), by the formation of
processes from cells in the retina, which grow inwards along the
optic stalk to the brain, and become the fibres of the optic nerve.

The iris. The marginal part of the optic cup, nearest to the
lens, does not become converted into the retina, but undergoes
changes of a different character, the boundary between the
retinal and non-retinal parts being indicated by the ora serrata.
The inner and outer walls of this marginal, or non-retinal, part
of the cup coalesce completely, and become pigmented through-
out their whole thickness. They become closely connected with
the choroid coat, on their outer surface ; and the combined choroid
and retina grow forwards, in front of the lens, to form the iris,
which reduces the mouth of the optic cup to a comparatively
narrow aperture, the pupil.

The pecten arises on the fifth day as a lamellar process of
mesoblast, which grows into the cavity of the optic cup through
the choroidal fissure, close to the optic nerve. It early becomes
very vascular; about the tenth day it becomes folded in the

280 THE CHICK.

fan-like manner characteristic of the adult ; and toward the c
of incubation it becomes densely pigmented.

The cornea is formed from mesoblast, which grows in between
the lens and the surface epiblast, at first as a ring, but soon
becoming a continuous layer across the front of the eye. It is
at first structureless, but cells from the mesoblast round its
edge soon grow inwards into its substance to form the corneal
corpuscles. These corpuscles are confined to the middle layer
of the thickness of the cornea, the outer and inner surfaces
remaining structureless as the anterior and posterior elastic
membranes of the cornea respectively. The surface layer of
epiblast persists as the conjunctival epithelium.

The anterior chamber of the eye forms as a space between
the cornea and the lens ; and in it a watery fluid, the aqueous
humour, soon collects.

The accessory organs of the eye. The eyelids are folds of the
integument round the eye : there are three of them, an upper
and a lower eyelid, and the third eyelid or nictitating membrane
(Fig. 126, CD), which arises on the inner or nasal side of the
eye. The lacrymal glands are solid ingrowths of the conjunctival
epithelium, which appear on the eighth day. The lacrymal duct
is also at first solid ; it appears as a ridge of epidermis, along
the line of the lacrymal groove, extending from the eye to the
olfactory pit (Fig. 125). This ridge sinks into the mesoblast,
and soon splits off from the epiblast along the greater part of its
length, but remains attached at its ends to the lower eyelid and
to the wall of the olfactory pit respectively. About the twelfth
day it acquires a central lumen, and becomes the tubular duct.

3. The Ear.

The ears appear, about the middle of the second day, as a pair
of shallow depressions of the external epiblast at the sides of the
hind-brain, just in front of the first pair of mesoblastic somites
(Figs. Ill and 120, El). The pits rapidly deepen (Fig. 121, El) ;
their mouths narrow, and by the end of the third day become com-
pletely closed, the pits thus becoming vesicles imbedded in the
mesoblast at the sides of the head (Fig. 113, ei). By a series of
changes very similar to those already described in the frog, the
vesicle gives rise to the various parts of the membranous laby-
rinth of the ear ; the epiblastic wall forming the epithelial lining of

THE EAR, AND ALIMENTARY CANAL. 281

the labyrinth, and becoming specially developed at certain places,
to form the auditory epithelium. The auditory nerve, as noticed
above, very early comes in contact with the anterior and inner
wall of the auditory vesicle, fusing completely with this by the
fiftieth hour. This fused patch, by division and subsequent separa-
tion of the several portions, gives rise to all the special patches
of auditory epithelium present in the adult labyrinth.

The accessory organs of hearing. The development of the
Eustachian tube, tympanic cavity, and tympanic membrane
will be described in the section dealing with the development of
the pharynx and gill-clefts (p. 283). The development of the
columella, or auditory ossicle, will be described with the skeleton
(p. 330).

■

DEVELOPMENT OF THE ALIMENTARY CANAL.

General Account.

The alimentary canal of the chick, like that of the frog, is
developed in three portions, of independent origin, and of very
unequal length.

(i) The stomatodseum, or mouth invagination, is formed by a
pitting-in of the epiblast at the anterior end of the alimentary
tract ; from it the anterior part of the buccal cavity, and the
pituitary body are developed.

(ii) The mesenteron gives rise to almost the entire length
of the alimentary canal, from the hinder part of the buccal cavity
to the cloaca ; and from it the lungs, liver, pancreas, and other
important structures arise as outgrowths. The mesenteron is
the tubular cavity formed within the embryo, as the result of the
process of folding or constriction by which the embryo becomes
pinched off from the yolk-sac (Figs. 112,1 14, and 123). It is lined
by hypoblast along its whole length. Owing to the mode of its
formation, it communicates freely, through the yolk-stalk, with
the yolk-sac ; and, so long as the yolk-stalk remains tubular, the
mesenteron may be described as consisting of three lengths : — the
fore-gut (Figs. 112, gf, and 114, tp, to), which is the part included
in the head-fold, and has complete roof, sides, and floor : the mid-
gut (Figs. 114 and 123, ys), which opens into the yolk-stalk, and
which therefore has roof and sides, but no floor ; and the hind-gut
(Figs. 114 and 123, Gh), which is the part included in the tail-fold,
and has, like the fore-gut, complete roof, sides, and floor. As the

282

THE CHICK.

constriction of the embryo from the yolk-sac proceeds, the fore-
gut and hind-gut lengthen at the expense of the mid-gut; and
after about the seventh day, when the yolk-stalk is reduced to a

Fig. 123. — A median longitudinal, or sagittal, section through a Chick Embryo
at the end of the fifth day of incubation ; the section is taken strictly in the
median plane, except as regards the Wolffian body and kidney, which are
introduced in the figure in order that their relations to the alimentary
canal may be shown. The optic lobe, cerebral hemisphere, and optic stalk
of the left side are shown in perspective. The amnion has been removed,
and the allantois and yolk-stalk cut short close to the embryo. (Compare
Figs. 100 and 115 for surface views of embryos of the same age.) x 12.
A, dorsal aorta. BP, third ventricle, or cavity of thalamencephalon. BH, fourth

ventricle, or cavity of medulla oblongata. BL, cerebellum. BM, cavity of mid-brain.

BS, cavitv of the vesicle of the hemispheres. CH, notochord. G-H, hind-gut. G-T,

mid-got. IK" , infundibulum. KC, Wolffian duct. KD, ureter. KM, Wolffian body.

KT, kidney. LGr, lung. MW, mandibular arch. !N"S, cavity of spinal cord. PN,

pineal body. PT, pituitary body. RT, truncus arteriosus. RV, ventricle of heart.

TA, stalk of allantois, cut short. TC, cloaca. TH, thyroid body. TP, pharynx.

TS. stomach. W, liver. "WD, bile duct. YS, yolk-stalk, cut short.

very narrow tube, and the walls of the mesenteron are complete
along its whole length, the mid-gut, as a distinct portion of the
alimentary tract, ceases to exist.

THE ALIMENTARY CANAL. 283

(iii) The proctodeum is a barely perceptible pitting-in of
the epiblast, at the hinder end of the alimentary tract, which
forms the anal or cloacal aperture.

Up to the end of the fourth day the alimentary canal is nearly
straight ; but from this time it grows more rapidly than the part
of the body in which it lies, and soon becomes markedly convo-
luted ; it retains its connection with the mid-dorsal wall of the
body cavity by means of the mesentery.

The several regions of the alimentary canal, and the various
organs formed in connection with it, will now be taken in order ;
and the more important points in their developmental history
described.

2. The Pharynx.

Almost from the first, there is a great difference between the
anterior or pharyngeal portion of the mesenteron, which is shallow
dorso-ventrally but very wide from side to side (Fig. 118, tp) ; and
the hinder part, from the oesophagus to the cloaca, which is narrow
and cylindrical.

Towards the end of the second day, pouch-like folds of hypo-
blast grow out in pairs from the sides of the pharynx, towards
the surface. These correspond exactly, in their relations and their
mode of formation, to the gill-pouches of the tadpole ; and like
these, they grow outwards until they meet the external epiblast,
with which they fuse. At a slightly later stage, the fused patches
of epiblast and hypoblast become perforated to form the gill-clefts,
which place the gill pouches, and therefore the pharynx, in direct
communication with the exterior.

Of these gill-pouches, four are formed on each side of the
neck, and are developed in order from before backwards. The
most anterior one is the hyomandibular gill-pouch (Fig. 124,
hm) ; and the succeeding three are the first, second, and third
branchial pouches respectively.

The parts of the side walls of the pharynx between the suc-
cessive gill-pouches are spoken of as the visceral arches ; their
boundaries are indicated on the surface of the neck by grooves,
marking the lines along which the hypoblastic walls of the gill-
pouches meet and fuse with the external epiblast, as shown on
the right-hand side of Fig. 124. The first or most anterior of
these visceral arches is the mandibular arch (Figs. 124 and 125,

284

THE CHICK.

mn), which forms the basis of the lower jaw. The second and
widest arch is the hyoid arch, HY ; and behind this come the first,
second, and third branchial arches, the hindmost or third branchial
arch being immediately behind the last or third branchial cleft.
These visceral arches, and the gill-pouches separating them
from one another, correspond exactly with the similarly named

— UL

Fig. 124. — A section through the head of a Chick Embryo at the end of the
third day of incubation, the section being taken along a plane indicated
by the two arrows and crosses in Fig. 113, p. 253. The right side of the
section is at a level slightly dorsal to that of the left side, x 30.

A, dorsal aorta. Al, first aortic arch, in the mandibular arch. A2, second aortic
arch, in the hyoid arch. A3, third aortic arch, in the first branchial arch. AC, carotid
artery. AI, internal carotid artery. BM, cavity of mid-brain. BB.1, first branchial
arch. CH, notochord. HM, hyomandibular cleft. HY, hyoid arch. MN, mandi-
bular arch. MP, muscle plate. NE, ganglion of spinal nerve. ITS, spinal cord.
TP, pharynx. VB, anterior cardinal vein.

structures in the tadpole ; the sole difference of importance being
that in the chick no gills are, at any period, developed in con-
nection with them. The fact that these structures, which are
only intelligible through their association with aquatic respira-
tion, are present in the early developmental stages of the chick,
must be held to prove the descent of birds from aquatic, gill-
breathing ancestors.

THE VISCERAL ARCHES AND CLEFTS. 285

The hyomandibular cleft opens to the exterior in the latter
part of the third day (Figs. 113 and 124, hm) : it remains open
until about the end of the fourth day, when its walls come in
contact, and the cleft becomes closed. The first branchial cleft,
between the hyoid and first branchial arches, opens a little
later, early on the fourth day ; and closes again during the fifth
dav. The second branchial cleft, between the first and second
branchial arches, is open only for a short time during the fifth
day : and the third branchial cleft does not open to the exterior
at any time.

It is stated by some observers that none of the visceral clefts
in the chick open to the exterior at any stage, but the real con-
dition appears to be as described above ; it is possible that indi-
vidual variations occur in respect to the dates of opening of the
clefts, and the times during which they remain open.

The tympano-Eustachian passage. The branchial clefts close
up and disappear completely at an early stage ; but the most
anterior, or hyomandibular, cleft appears to persist, and to give
rise directly to the tympano-Eustachian passage of the adult bird.
The cleft becomes closed at its outer end, about the end of the
fourth day, by a fold of skin, which becomes directly the tympanic
membrane. From the gill-pouch, on the inner side of the tym-
panic membrane, the tympanic cavity and Eustachian passage
are formed ; while the external auditory meatus is built up as a
short tubular passage on the outer side of the tympanic mem-
brane (Fig. 126, hm). The Eustachian passages of the two sides
unite at their inner ends, and open into the mouth by a median
aperture (Fig. 116, es), nearly opposite the glottis.

According to some observers, the hyomandibular pouch does
not open to the exterior at any period in the chick ; and the
tympanic membrane is formed directly from the thin double
layer, consisting of both epiblast and hypoblast, which closes the
pouch at its outer end ; a layer of mesoblast growing in between
the epiblast and hypoblast, which persist as the epithelial
layers of the outer and inner surfaces of the tympanic membrane
respectively. The whole history of the development of these parts
stands in need of renewed and thorough investigation.

The thyroid body arises, towards the end of the second day,
as a median longitudinal groove in the floor of the pharynx.

286 THE CHICK.

opposite the first pair of branchial arches. The hinder end of
the groove deepens during the third day to form a pit (Fig. 114,
th). The walls of this pit soon join together, obliterating the
cavity and giving rise to a solid plug of hypoblastic epithelium
(Fig. 123, th). About the end of the fifth, or early part of the
sixth day this plug separates from the floor of the throat as a
solid body, composed of epithelial cells, which lies embedded in
the mesoblast, immediately in front of the truncus arteriosus.

The thyroid body soon becomes bilobed, and the lobes
branch out as solid strings of cells, which later on become
tubular. A sheath of vascular connective tissue early forms
around the lobes, which, as development proceeds, gradually shift
backwards along the neck to their adult position.

A pair of solid bodies, formed of epithelial cells, which separate
from the hypoblast immediately behind the third branchial
pouches, and take up a position at the sides of the larynx, are
sometimes spoken of as accessory thyroid bodies.

The thymus arises, on each side, as a couple of epithelial buds
from the walls of the second and third branchial pouches. The
buds soon separate from the surface, and, the two buds of each
side fusing together, give rise to a pair of elongated rod-like
bodies, lying along the sides of the neck close to the carotid
arteries.

The tongue is formed as an outgrowth from the floor of the
pharynx, opposite the hyoid and first branchial arches. It first
becomes conspicuous about the sixth day, and by the eighth or
ninth day (Fig. 116, tn) has attained a definite shape. It is
formed behind the boundary line between the pharynx and
stomatodasum, and its epithelium is therefore of hypoblastic
origin.

3. The Stomatodaeum.

The stomatodaeum, or mouth invagination, is formed by
pitting-in of the ventral wall of the pharynx from the exterior.

From the time of its first formation the ventral wall of the
pharynx, in front of the heart, is very thin (cf. Fig. 112). On the
appearance of the visceral arches, as thickenings of the side walls
of the pharynx, this thin-walled area on its ventral surface

THE STOMATODvEUM. 287

becomes more clearly defined, as a slightly depressed, transversely
elongated patch, bordered by a thickened rim, which is formed
partly by the ventral ends of the anterior visceral arches, and
partly by the under surface of the head itself.

By further thickening of this rim, the depression which it
surrounds becomes deepened ; and the pit formed in this way,
rather by building up of its walls than by lowering of its floor,
becomes the stomatodasum.

Towards the end of the third day the floor of the stomatodaaal
)it thins away and becomes perforated, placing the pharynx for

-NS

Fig. 125. — The head of an Embryo Chick at the end of the fifth day of incuba-
tion ; seen from below. Compare Fig. 115 for a view of an embryo of the
same age from the side, x 8.

BR', 'first branchial arch. BS, cerebral hemisphere. CH, note-chord. DS.
month. FP, fronto-nasal process. HM, hyoniandibular cleft. HY, hyoid arch.
MN, mandibular arch. MX, maxillary arch. NS, spinal cord, seen in section where
the neck has been cut across. OC, eye. OH, choroidal fissure. OK, olfactory pit.
OL, lens.

the first time in direct communication with the exterior, and
forming the permanent mouth opening (Fig. 114, o).

The Face. After the definite formation of the mouth opening,
the borders of the stomatodseal pit continue to develop, and
gradually give rise to the beak and the anterior part of the face of
the bird. At the end of the fifth day the mouth opening (Fig.
125, ds) is oblong in shape. Its anterior border is formed by
the fronto-nasal process, FP, a broad plate, notched in the median
line, and forming, at this stage, the under surface of the head.
The posterior border of the mouth opening is formed by the
ventral ends of the mandibular arches, mn, which meet each

288

T1IK (MUCK.

other in the median plane at the chin ; and the sides of the
opening are bounded by the maxillary arches, MX, which grow
forwards from the mandibular arches to meet the outer angles
of the fronto-nasal process.

The olfactory pits, OK, lie just beyond the anterior and outer
angles of the mouth : the inner border of each pit is formed
by the side of the fronto-nasal process, or inner nasal process ;

MX

Fig. 126. — The head of an Embryo Chick at the end of the seventh day of
incubation ; seen from below, x 8.

BS, cerebral hemisphere. CD, third eyelid, or nictitating membrane. CH,
notochord, seen in section where the neck has been cut across. DS, mouth. HM.
external auditory meatus. MN. mandibular arch. MX, maxillary arch. WS, spinal
cord, seen in section. OC, eyeball. O J, epithelial knob on tip of beak. OK, external
nostril. OL, lens.

the outer border is formed by a strip of the side of the head
between the olfactory pit and the eye, which is spoken of as the
outer nasal process.

Between the outer nasal process and the maxillary arch there
is a slight depression, the lacrymal groove, which runs from the
under surface of the eye to the outer border of the olfactory pit.
Between the inner nasal process, or wing of the fronto-nasal
process, and the anterior end of the maxillary arch there is a
more conspicuous depression, the nasal groove, which becomes

THE FACE. 289

converted by fusion of its lips, as described on p. 275, into the
posterior narial passage.

By the seventh day (Fig. 126) the parts of the face begin

to assume more definite form. The mouth opening, ds, is

more slit-like, and its boundaries are more clearly defined. The

fronto-nasal process is narrower, and has begun to grow forwards

the upper beak, on the tip of which the small epithelial knob,

, which is used for breaking the egg-shell at the time of

itching, is already present. The maxillary arches have fused

ith the sides of the fronto-nasal process ; the nasal grooves are

inverted into the narial passages, and the lacrymal grooves

ive disappeared.

The two mandibular arches, mn, have fused in the median
me to complete the lower jaw, which is already beginning to
>w forwards as the lower beak. Finally, the external nostrils,
:, have narrowed very considerably, and have acquired the slit-
re form characteristic of them in the adult.

The pituitary body (Fig. 114, pt) is a pocket-like diverticulum
from the anterior angle of the stomatodseum, which appears
towards the end of the second day, and which early acquires its
characteristic relations with the infundibulum, and with the
anterior end of the notochord. Its development has already
been described in the section dealing with the brain (p. 259).

4. The (Esophagus.

Immediately behind the pharynx the alimentary canal sud-
denly narrows, becoming a very slender tube, the oesophagus,
which runs back in a perfectly straight course through the neck
(Figs. 114, to, and 123).

The oesophagus is at first very short ; but. as the neck
lengthens, the oesophagus grows rapidly, to keep pace with this.
A curious point with regard to the oesophagus is that for a
time, commencing about the middle of the sixth day, and

Iiting for two or three days, the lumen is completely lost, the
iophagus becoming solid along the greater part of its length,
little later, about the ninth day, the lumen is gradually
established, from below upwards.
This temporary obliteration of the cavity of the oesophagus
the chick is perhaps to be associated with the rapid lengthening
ich the neck and the oesophagus are undergoing at this period ;

290 THE CHICK.

but the fact that a similar solidification of the oesophagus occurs
in dogfish, frogs, reptiles, and mammals, as well as in birds.
renders it possible that it has some further and deeper signifi-
cation, not yet determined.

5. The Stomach and Intestine.

Up to the end of the fifth day (Fig. 123), the alimentary
canal remains almost straight, except for a slight, ventrally
directed loop, gt, at the place where the yolk-stalk, ys, arises,
connecting the intestine with the yolk-sac.

The stomach is recognisable as a slight, fusiform dilatation.
TS, about the end of the fifth day ; during the sixth day the
gizzard becomes evident, as a thick-walled dilatation of the distal
end of the stomach, which grows rapidly, and by the twelfth day
has attained a great size.

From the sixth day onwards, the intestine lengthens rapidly ;
growth occurring most markedly at two parts of its length, and
giving rise to two loops, both of which are directed ventralwards.
Of these, the proximal or duodenal loop is formed from the part of
the intestine immediately beyond the gizzard. The distal, or
vitelline loop, which is much the longer of the two, is formed
by elongation of the two limbs of the V-shaped loop which is
already present on the fifth day, and from the angle of which
the yolk-stalk arises (Fig. 123, ys).

Between the duodenal and vitelline loops there is a part of
the intestine which undergoes hardly any elongation at all, but
remains throughout life closely attached to the dorsal surface
of the body cavity; it corresponds to the point in Fig. 123 im-
mediately beyond the opening of the bile-duct, WD, where the
intestine bends ventralwards to form the proximal limb of the
vitelline loop.

The further development of the intestine consists chiefly in
great elongation of the vitelline loop, which gives rise to the
whole length of the small intestine, beyond the duodenum. Both
limbs of the loop lengthen very rapidly, and become twisted
somewhat spirally. Up to about the seventeenth day the vitelline
loop lies almost entirely in the yolk-stalk, and therefore outside
the body of the embryo ; about the eighteenth day the greater
part of the loop becomes withdrawn into the body, and acquires
the convolutions characteristic of the adult.

THE STOMACH AND INTESTINE. 291

The rectum, or terminal part of the intestine, grows very
slowly, and remains nearly straight throughout the whole period
of development. The boundary between the small intestine and
the rectum is marked by the two rectal diverticula, which appear
as a pair of small pouch-like outgrowths (Fig. 123, gh) about
the end of the fifth day ; these grow rapidly, and by the eighth
or ninth day have attained a considerable length. The rectum
itself remains short ; in the later days of incubation it dilates
very greatly, and shortly before the time of hatching the bursa
Fabricii arises as a dorsal outgrowth from its distal end.

The mesentery. The alimentary canal, along its whole length,
is at first closely attached to the dorsal wall of the body cavity,
immediately below the notochord.

The pharynx, or most anterior division of the alimentary
canal, retains these relations throughout life. The oesophagus
shifts ventralwards to a slight extent, owing to the intrusion of
mesoblast between it and the notochord. Further back the
ventral shifting is much more marked ; and the whole intestinal
region, from the stomach to the rectum, becomes situated some
distance ventral to the notochord, remaining, however, connected
with the dorsal wall of the body cavity by a vertical, laterally
compressed sheet of mesoblast, the mesentery.

An exception to this statement must be made with regard to
the short portion of the intestine between the duodenal and
vitelline loops, which, as already noticed, remains in close con-
nection with the dorsal body wall throughout life.

As the duodenal and vitelline loops of the intestine lengthen,
the mesentery grows, keeping pace with them, and becoming
still further reduced in thickness ; it ultimately forms a thin sheet,
consisting of two epithelial layers, derived from the peritoneum,
and inclosing between them a very thin layer of mesoblast, along
which the blood-vessels run to and from the alimentary canal.

The terminal part of the alimentary canal, or rectum, like
the anterior part, remains closely connected with the dorsal body
wall throughout life, the mesentery in this region only attaining
a comparatively slight development.

6. The Proctodeum.

The proctodeum is a slight depression of the skin at the
hinder end of the body, beneath the tail (Fig. 123). It develops

U 2

292 THE CHICK.

very late, and does not open into the rectum until about the
fifteenth day. The proctodeum in the chick is very shallow,
and gives rise only to the outermost portion of the adult cloaca,
and to the actual external opening.

7. The Lungs.

The lungs arise, during the third day, as a pair of small
hollow outgrowths from the ventral surface of the anterior end of
the oesophagus. By lateral constriction, the ventral part of the
oesophagus, from which the lungs arise, becomes separated off as
a median chamber (Fig. 114, Lg) : this lies ventral to the oeso-
phagus, and opens in front into the hinder end of the pharynx ;
while from its hinder end the lungs extend backwards as poste-
riorly directed outgrowths.

The lungs, after their first appearance, rapidly increase in
size ; they give off secondary diverticula, which branch again
and again ; and from the finest branches arborescent outgrowths
arise at right angles, which become the ultimate spongy sub-
stance of the lungs.

The air sacs, which are structures very characteristic of birds,
appear about the eighth day as thin- walled saccular diverticula
from the hinder edges of the lungs ; the abdominal air sacs are
in the earlier stages the best developed.

The trachea (Fig. 116, lr) is formed by elongation of the
median laryngeal tube, as the neck lengthens and the lungs
gradually shift backwards into the thorax.

From the mode of development of the lungs, as outgrowths
from the alimentary canal, it follows that their lining epithe-
lium, including the minutest passages, and that of the air sacs
as well, is of hypoblastic origin : the rest of the thickness of the
lung walls, including all the blood-vessels, is mesoblastic.

The lungs contain no air, and are not used for breathing,
until immediately before the time of hatching ; when the chick,
breaking through the shell membrane into the air chamber
at the larger end of the egg (Fig. 101, sv), draws air into its
lungs for the first time, and, invigorated by the act, proceeds to
peck its way out of the shell.

8. The Liver.

The liver arises, about the middle of the third day, as a
tubular diverticulum from the posterior end of the fore-gut, in

sa

1

THE LUNGS AND LIVER. 293

the angle between the two vitelline veins, and immediately behind
their point of union. A second diverticulum arises from the
same spot almost directly afterwards ; it is similar to the first,
but of rather smaller size. Both these diverticula have hypo-
blasts walls, with thin mesoblastic investments.

Towards the latter part of the third day, as the folding off of
the embryo from the yolk-sac proceeds, the liver diverticula
are found to arise definitely from the part of the mesenteron
which will later become the duodenum. At the same time they
come into very close relation with a large median vein, the
meatus venosus, which is formed by the union of the right and
left vitelline veins behind the heart (ef. Fig. 128, VE).

The two liver diverticula lie one at each side of the meatus
venosns, and in very close contact with this. The hypoblastic
cells forming the walls of the diverticula now begin to proliferate
freely, growing out as solid strands of cells, which form an
irregular reticulum closely surrounding the meatus venosus;
the meshes of the reticulum being occupied by capillary
blood-vessels, which develop in the mesoblast, and early acquire
connection with the meatus venosus itself.

These processes proceed rapidly during the fourth and fifth
days, and by the end of the fifth day (Figs. 123 and 128) the liver
is an organ of considerable size, consisting of a network of solid
rods of hypoblast cells, which branch and anastomose freely in
all directions ; the meshes of the network being occupied by
blood-vessels, which penetrate all parts of the liver, and are in
free communication with the meatus venosus, round which the
liver is formed.

The liver continues to grow rapidly, and by the tenth day
is the largest organ in the abdominal cavity. The trabecular net-
work of hypoblast cells becomes the liver parenchyma ; the
tubular diverticula from the duodenum branch out freely in the
substance of the liver, and become the two bile ducts of the
idult bird ; while the gall bladder arises on the fifth day as a
accular outgrowth from the right or larger of the two primary
verticula.

The early formation of the liver in the chick, and its large
ize during the greater part of the developmental history,
indicate that it must be of considerable functional importance
during embryonic life. Its relation to the blood system, and

294 THE CHICK.

especially the fact that it intercepts the blood returning from
the yolk-sac to the heart, suggest that its chief purpose is con-
I nected with the elaboration of the food material which is obtained
from the yolk-sac, and at the expense of which the nutrition of
the embryo is effected.

9. The Pancreas.

The pancreas arises, rather later than the liver, as a tubular
outgrowth from the duodenum, just beyond the two liver diver-
ticula, from which secondary outgrowths arise in much the
same manner as in the liver itself. A second diverticulum
arises from the duodenum about the eighth day, and gives rise
to the greater part of the adult pancreas ; and at a later period
a third diverticulum is formed. The three diverticula persist as
the three pancreatic ducts of the adult bird, while the three lobes
of the pancreas, with which they are connected, soon fuse indis-
tinguishably with one another.

10. The Allantois.

The allantois is really an appendage of the alimentary canal,
arising as an outgrowth of its ventral wall, in front of the
cloaca ; it is therefore lined by hypoblast, like all other out-
growths of the mesenteron, while the rest of the thickness of its
wall is formed by the splanchnopleuric mesoblast.

The allantois of the chick is homologous with the bladder of
the frog (Fig. 89, tb). It differs mainly from this in the fact
that, while arising in the same manner, it is not confined within
the body of the embryo, but, growing rapidly, passes out beyond
this as a thin-walled vascular sac (Figs. 100 and 101, ta), which
spreads out in close contact with the inner surface of the egg-
shell, and acts as the respiratory organ of the embryo during
the greater part of its development.

In the chick the allantois commences to form about the
middle of the second day. At this time the tail fold is not yet
established, so that the allantois (Fig. 112, ta) appears at first
as a pocket-like fold of the splanchnopleure, lying a short way
behind the embryo, and with its cavity opening ventralwards.

On the formation of the tail fold, early on the third day, the
part of the splanchnopleure from which the allantois arises
becomes doubled forwards under the embryo to form the ventral

THE ALLANTOIS. 295

wall of the gut, and the allantois now appears as a saccular
depression of the ventral wall of the hind-gat (Fig. 1 1 4, ta).

During the third day the allantois increases considerably in
size, projecting downwards and forwards, as a hollow, thick-
walled bud from the ventral surface of the hind-gut, into the
body cavity, or space between the somatic and splanchnic layers
of the mesoblast.

During the fourth day, by its further growth, the allantois
passes out beyond the embryo, and turns up, along its right side,
into the space between the two layers of the amnion, which, from
the mode of formation of the amnion, is directly continuous
with the body cavity of the embryo (ef. Fig. 114).

On the fifth and following days the allantois grows rapidly ;
from the first it is very vascular, and the blood-vessels now
increase greatly in size ; the arteries, which lie in its super-
ficial layer, are derived directly from the aorta (Fig. 128, aa) ;
while the veins, va, which lie in its inner or deeper layer,
join the vitelline veins from the yolk-sac, and, passing through
the liver, reach the heart.

By the seventh or eighth day (Fig. 101, ta), the allantois
has spread all round the upper half of the egg^ covering over
the embryo, and extending half way round the yolk-sac as well.
It is still saccular, and its cavity contains fluid. Its outer wall
lies in very close contact with the outer layer of the amnion, or
false amnion, and soon fuses with this completely, so that from this
time the allantois lies in close contact with the shell membrane.

In its further growth the allantois does not follow the yolk-
sac ; but, keeping close to the egg-shell, and carrying the somato-
pleure before it, it extends so as gradually to inclose the mass
of the white, WA, which still remains on the under surface and
near the smaller end of the egg. The allantois, about the six-
teenth day, completely incloses this plug of white or albumen,
and from this time the absorption of the plug proceeds rapidly,
the albumen being apparently carried by the allantoic vessels to
the embryo, and aiding in its nutrition.

Towards the close of incubation deposits of urates occur in
the cavity of the allantois, indicating that it serves as a recep-
tacle for the excretory matters formed within the embryo itself,
as well as a respiratory organ in the more restricted sense of the
term.

296 THE ('HICK.

Shortly before the time of hatching, the allantoic ve
become constricted, by the closure of the body walls at the
umbilicus. The allantois itself shrivels up, and is cast off as 1 lie
chick works its wav out of the shell.

DEVELOPMENT OF THE HEART AND BLOOD-
VESSELS.

1. Preliminary Account.

The general arrangement of the vascular system during
embryonic life is strikingly similar to that of the tadpole. The
heart is at first a straight, and later a twisted tube, lying beneath
the pharynx, and driving the blood through a series of paired
aortic arches (Fig. 128) to the dorsal aortse, which distribute it
to all parts of the body. From the body generally, and from the
Wolffian bodies, the blood is returned by anterior and posterior
cardinal veins on each side ; these unite to form the Cuvierian
veins, or anterior vense cavae, which open into the sinus venosus
or posterior end of the heart. From the alimentary canal the
blood is returned by the mesenteric or hepatic portal vein, which,
passing through the liver, joins the posterior vena cava, the vein
through which the blood is returned to the heart from the
kidneys and other organs.

The chief differences between the chick and the frog as
regards the arrangement of the blood-vessels are : (i) that the
chick embryo has no gills, either external or internal, and there-
fore possesses no vessels corresponding to the gill loops of the
tadpole ; and (ii) that in the chick the vessels connected with the
yolk-sac and with the allantois, both of which are structures out-
side the embryo itself, are enormously developed. These blood-
vessels, vitelline and allantoic, are in direct connection with the
vessels of the embryo : the afferent vessels, i.e. the vitelline and
allantoic arteries, being branches of the dorsal aorta ; while the
efferent vessels, the vitelline and allantoic veins, on entering the
embryo, join the mesenteric veins and run, through the liver, to
the heart.

Throughout the greater part of the period of incubation, the
vitelline and the allantoic vessels are of very large size ; and
inasmuch as the returning vessels, the vitelline and allantoic
veins, bring to the embryo food matter from the yolk-sac, and

THE BLOOD-VESSELS. 297

oxygen from the allantois, it follows that the blood entering the
heart by the posterior vena cava is arterial, and not venous, in
character. The right understanding of the peculiarities in tlie
circulation in the chick during embryonic life is mainly depen-
dent on a full appreciation of this fact.

The aortic arches of the chick embryo undergo changes very
similar to those which occur in. the frog ; the arches disap-

>earing in part, and in part becoming modified into the arterial
system of the adult. As in the frog, the pulmonary arteries are

>ranches from the hindmost pair of aortic arches.

Histological development of the blood-vessels. The blood-
essels appear in the vascular area before they are formed in
he embryo itself, and the mode of their development is easier
determine in the former situation.

Shortly before the end of the first day, when two or three
airs of mesoblastic somites are present in the embryo, a
number of outgrowths from the upper surface of the hypoblast
appear round the inner margin of the area opaca. These
branch freely, and unite with one another to form a network,
lying between the rnesoblast and the hypoblast : the strands of
the network are solid ; they contain numerous nuclei, but cell
outlines are difficult or impossible to determine in them.
Within the strands, vacuoles soon appear at intervals : these
enlarge rapidly, and, running into one another, convert the solid
network into a system of anastomosing tubules with nucleated
walls. These tubules are capillary blood-vessels; they are filled
with fluid, but contain no blood corpuscles until a later stage.

This vascular network spreads rapidly, extending outwards
as the vascular area widens, and inwards across the area
pellucida to the embryo, which it invades on the second day.
From their first appearance the vessels of the embryo are con-
tinuous with those of the area pellucida ; but it is not quite clear
how far they arise in situ, or how far by intrusion of vessels

I from the area pellucida.
This network of blood-vessels lies below the rnesoblast, be-
tween this and the hypoblast ; it is connected at places with the
hypoblast, from which it arises in the first instance, but it is
quite independent of the rnesoblast. If this appears to contradict
the general rule according to which the blood-vessels are derived

298 THE chick.

from mesoblast, it should be remembered that the whole of the
mesoblast in the chick, with the exception of the primitive streak
mesoblast, is of hypoblastic origin ; and the facts with regard to
the formation of the blood-vessels might therefore be expressed
by saying that the blood-vessels separate from the hypoblast at a
stage later than that at which the other mesoblastic structures
are formed from it. It is better, however, to accept the facts as
they stand; namely, that in the chick many of the blood-vessels
are derived directly and independently from the hypoblast ; and
to bear in mind that the middle germinal layer, or mesoblast, can-
not be regarded as in any sense equivalent to either of the two
primary germinal layers, epiblast and hypoblast ; the term
' mesoblast ' being used to include a number of very diverse
structures, most if not all of which owe their ultimate origin
to either hypoblast or epiblast.

The hypoblastic vascular network, formed in the way de-
scribed above, gives rise directly to the capillaries and to the
endothelial lining of the larger blood-vessels. The connective
tissue and muscular walls of these latter are derived indepen-
dently from the mesoblast, which grows round and envelopes
them.

The blood-vessels within the embryo are at first, like those of
the area pellucida and area vasculosa, reticular in their arrange-
ment. The definite arteries and veins are formed by straightening
and enlargement of certain of the strands of the network, with
disappearance of other portions ; the dorsal aorta?, for instance,
arising early in the second day, in embryos with eight pairs of
mesoblastic somites, as a pair of longitudinal trunks, lying along
the outer and ventral borders of the somites, between the meso-
blast and hypoblast, and communicating freely along the hinder
part of their course with the reticular network of the area
pellucida.

2. The Heart.

The heart is formed on the under surface of the fore-gut, at
the commencement of the second day. It consists at first of two
longitudinal vessels, which, though closely applied in the median
plane, are for a time quite distinct, but which soon fuse to form
a single median tube.

The walls of the heart consist, as in the frog, of an outer

THE HEART.

299

muscular coat (Fig. 120, km), formed by the splanchnopleuric
layer of mesoblast ; and an inner endothelial lining, he, con-
cerning the origin of which it is difficult to speak with certainty,
but which appears, like that of the other blood-vessels, to be
derived from the hypoblast.

The muscular wall (Fig. 120, em) is at first incomplete dor-
sally, but, after the two halves of the heart have united, the mus-
cular walls grow in towards the median plane, above the heart,

!' L-J. I1

Fig. 127. — The anterior end of a Chick Embryo at the thirty-sixth hour of in-
cubation, removed from the yolk-sac, and seen from the ventral surface.
(Compare Figs. Ill and 112 for other views of an embryo of the same age.)
x30.

Al, first aortic arch, in the mandibular arch. BF, fore-brain. BO, optic vesicle.
EI. auditory pit. HE, posterior limit of head fold of somatopleure (compare Fig. 112).

HH, posterior limit of head fold of splanchnopleure, and posterior boundary of fore-gut.
MS, ' mesoblast ic somite or protovertebra. ET, truncus arteriosus. RV, ventricular
portion of heart. W, vitelline vein, cut short.

Imd coalesce so as to complete its wall. The endothelial tubes
)f the two halves of the heart remain distinct, though closely
ipposed, for some time after the muscular walls have coalesced,
but ultimately they, also, become continuous. Between the
muscular and endothelial walls there is at first a considerable
space, filled with a mucous substance (Fig. 120).

800 THE CHICK.

straight tube, lying below the fore-gut, and closely attached to
its ventral surface, and with its walls consisting of an outer
muscular tube and an inner endothelial tube.

The posterior end of the heart is, from the first, continuous
with, or rather is formed by the union of, two large vessels, the
vitelline veins (Fig. 127, vv), which collect and return to the
embryo the blood from the network of capillaries in the area '
pellucida and area opaca (cf. Fig. 98).

The heart is at first very short ; but as the head fold becomes
deeper, constricting the embryo more and more markedly from
the yolk-sac, the vitelline veins remain at the edge of the fold,
and so get carried back with it, causing thereby lengthening of
the heart.

The heart begins to beat very shortly after its first formation,
and before any distinct histological differentiation into muscle
and nerve cells can be distinguished.

On its first formation, the heart is attached along its whole
length to the under surface of the fore-gut. It remains attached
at its two ends, but about the thirty-third hour becomes free
along the middle portion of its length (Fig. 112, rv) ; and,
growing more rapidly than the parts to which it is attached,
becomes thrown into a loop (Fig. 127), with the convexity
towards the right, and the concavity towards the left side of
the embryo.

The loop, continuing to lengthen, projects downwards and
backwards, so that the whole heart, towards the end of the second
day, becomes twisted obliquely, into a letter S shape. Starting
from the point of union of the vitelline veins, the heart runs for-
wards a certain distance, then makes a sharp bend downwards,
backwards, and to the right side ; then, making a second, equally
sharp bend (cf Fig. 113, rv) upwards, forwards, and to the left,
reaches the median plane again, and is attached to the under
surface of the pharynx opposite the first two pairs of gill-clefts.

The posterior end of the heart, into which the vitelline veins
open, may be called the venous end of the heart ; and the
anterior end, the arterial end. The first bend or loop (Fig.
1 13, ra) marks the auricular part of the heart ; and the second
bend, RV, the ventricular part. The forwardly directed portion
of the heart, in front of the second bend, is the truncus arteri-

THE HEART. 801

During the third day (Fig. 113), the heart increases con-
siderably in size ; the S-like twisting becomes still more pro-
nounced than before ; and constrictions appear, separating the
several chambers of the heart from one another.

On the fourth day, the auricular portion of the heart becomes
widened laterally, and marked off by a sharp constriction from
the ventricular portion, which, in its turn, is separated by a
distinct though less pronounced constriction from the truncus
arteriosus.

The most important event, however, that happens during
the fourth day, so far as the heart is concerned, is the first
appearance of the partitions by which the right and left sides of
the heart become separated from each other. Up to the fourth
day the heart is a single and continuous, though twisted tube,
without any division whatever into right and left sides. The
blood enters at the posterior or venous end of the heart, and
passing through the several cavities in succession passes out in
front, through the truncus arteriosus, into the aortic arches.

The internal division of the heart, into right and left sides,
is effected by three septa or partitions, which appear within the
cavity of the heart, and which arise perfectly independently of
one another : (i) the interauricular septum, which divides the
auricular chamber into the right and left auricles; (ii) the
interventricular septum, which divides the ventricular chamber
into the right and left ventricles ; (iii) the septum of the truncus
arteriosus, which divides the truncus arteriosus, or terminal
chamber of the heart, into right and left halves. Of these septa,
the first two commence to form on the fourth day ; the third, or
septum of the truncus arteriosus, does not arise until the fifth
day.

Concerning the relative times of appearance of the inter-
auricular and interventricular septa, there is some discrepancy
in the published accounts. It is commonly stated that the
interventricular septum develops the earlier of the two, but
according to Masius it is the interauricular septum which is
the first to be formed.

The interauricular septum appears, during the fourth day, as
a septum projecting into the auricular chamber from its anterior
and dorsal wall; it lies between the apertures of the sinus

302 THE (muck.

•101'

venosus and the pulmonary vein, and ends in a free poster
edge. Of the two cavities into which it partially divides fche
auricular chamber, the left auricle is for a time much the larger
of the two.

The interventricular septum also appears during the fourth
day, as a crescentic partition which arises from the ventral wall
of the apex of the ventricular chamber, and gradually extends
across towards the dorsal wall. It divides the ventricular
chamber somewhat obliquely, and as yet imperfectly, into a left
and more dorsally placed cavity, and a right and more ventrally
placed one. The position of the septum is indicated by a slight
groove on the surface of the heart.

On the fifth day the interventricular septum is completed,
but the interauricular septum remains imperfect throughout the
whole period of development, up to the time of hatching.

The septum of the truncus arteriosus appears on this day as a
longitudiual fold, corresponding exactly to the similar one in the
frog. The fold commences near the distal end of the truncus
arteriosus, between the fourth and fifth pairs of aortic arches,
and grows backwards with a somewhat spiral course, dividing the
cavity of the truncus arteriosus into right and left halves. By
the end of the fifth day this longitudinal septum has grown
back to the base of the truncus arteriosus, and now meets with
the upper edge of the interventricular septum and fuses with
this. The eifect of this fusion is that the right ventricle now
communicates with the right division of the truncus arteriosus,
and through this with the hindmost or fifth pair of aortic arches
alone ; while the left ventricle communicates, through the left
division of the truncus arteriosus, with the anterior pairs of
aortic arches, but no longer with the fifth or hindmost pair.

Before the completion of the septum of the truncus arteriosus
three semilunar valves are formed at the base of each of the
divisions of the truncus arteriosus, between these and the
ventricles.

During the sixth day the shape of the heart as a whole
approaches much more closely to that of the adult ; the apex of
the heart becomes more pointed, and the auricular appendices
more prominent.

THE HEART AND ARTERIES. 303

Up to about the twelfth day the interauricular septum
remains very imperfect, and there is free communication between
the two auricles through a large aperture, the foramen ovale.
About the twelfth day this communication narrows considerably.

On the sixteenth day the Eustachian valve is formed as a
fold projecting into the right auricle, between the openings of the
posterior vena cava and the right anterior vena cava. Up to
this time the blood from both these vessels has passed from the
right auricle, through the foramen ovale, into the left auricle and
so to the left ventricle. The effect of the Eustachian valve is to
direct the blood from the right anterior vena cava into the
right ventricle, while still allowing the blood from the posterior
vena cava to pass through the foramen ovale into the left
auricle. From this time the two auricles are about equal in
size.

Shortly before hatching, the foramen ovale becomes partially
blocked up by a membranous, valve-like fold ; the completion
of this stoppage is effected shortly after the time of hatching,
from which time the structure of the heart is practically that of
the adult bird.

The thickening of the ventricular wall, which is a marked
feature of the later stages of development, is effected by inwardly
projecting ridges of the muscular wall, which ultimately form a
system of anastomosing muscular trabecule, from which, by
further thickening, the columns earner and musculi papillares
are developed.

The thickening of the wall of the auricles is effected in
similar fashion, but is not carried to so great an extent.

The wall of the truncus arteriosus thickens by simple increase
in the thickness of the muscular and other layers composing it.

3. The Arteries.

a. The Aortic Arches. The truncus arteriosus divides right
and left, as in the frog, into the aortic arches, which run round
the sides of the pharynx to its dorsal surface ; here they open
into the dorsal aorta?, by which the blood is carried all over the
body of the embryo, as well as to the yolk-sac and the allantois
(cf. Figs. 113 and 128).

The aortic arches of the chick are developed in order, from
before backwards. The first, or most anterior pair (Fig. 127, ai),

304

THE CHICK.

is formed early on the second day, and lies opposite the ante-
rior end of the fore-gut. The remaining arches are formed in
succession behind the first pair.

By the end of the second day a second pair is present, and

VY

I I \

Fig. 128. — A diagrammatic figure showing the arrangement of the blood-
vessels in a Chick Embryo at the end of the fifth day of incubation. The
amnion has been removed, and the vitelline vessels cut short a little
distance from the embryo, x 12.

A, dorsal aorta. A3, A4, A5, third, fourth, and fifth aortic arches of the right side,
lying in the first, second, and third branchial arches respectively. AA, allantoic artery.
AB, basilar artery. AC, carotid artery. AH, caudal artery, the terminal portion of
the dorsal aorta. AL, lingual arter}-. AP, pulmonary artery. AV, vitelline artery.
EI, auditory vesicle. BA, right auricle. BS, sinus venosus. BT, truncus arteriosus.
BV, right ventricle. TA, allantois. VA, allantoic vein. VB, anterior cardinal vein.
VC, posterior cardinal vein. VD, Cuvierian vein. VE, meatus venosus. VH, efferent
hepatic vessel. VI, posterior vena cava. VJ, jugular vein. VO, afferent hepatic
vessel. W, vitelline vein.

by the commencement of the third day a third pair appears still
further back (Fig. 113 A3).

On the establishment of the visceral clefts and arches, the

THE AORTIC ARCHES. 305

aortic arches acquire definite relations with the latter ; the first,
or most anterior aortic arch of each side (Figs. 113 and 124, ai),
lying in the first or mandibular arch ; the second aortic arch,
A2, lying in the hyoid arch ; and the third aortic arch, A3, in
the first branchial arch. During the fourth day, fourth and
fifth pairs of aortic arches (Fig. 128, A4, as) appear in the
second and third branchial arches.

There are thus altogether five pairs of aortic arches in the
chick, corresponding to the five anterior of the six pairs present
in the tadpole. These arches, however, differ from those of the
tadpole inasmuch as (i) they never have any gills developed in
connection with them ; (ii) they form from their first appearance
direct connections between the truncus arteriosus and the aorta,
there being no separation into afferent and efferent vessels. A
further difference lies in the fact that the aortic arches in the
mandibular and hyoid arches are complete in the chick, while
they never become so in the tadpole.

The condition of the aortic arches in the chick is comparable
to that of a frog after the metamorphosis ; but it is at present
a matter of doubt whether this indicates an entire omission of
the earlier stages in the chick, owing to the absence of gills ;
or whether the continuous aortic arch from heart to aorta does
not rather represent a still earlier ancestral condition, prior to
the acquisition of gills.

From the five pairs of aortic arches of the embryo the adult
arterial system is derived, in the following manner.

During the fourth, or fourth and fifth days, the first or
mandibular, and the second or hyoidean aortic arches disappear
along the middle portions of their lengths ; their ventral and
dorsal portions persist, the ventral portions remaining of com-
paratively small size as the lingual or mandibular arteries
(Fig. 128, al); while the dorsal portion, which is much larger,
extends forwards into the head as the carotid artery (Fig. 128,
ac), which divides into internal and external carotid arteries,
supplying the brain and face respectively.

By the end of the fifth day the ventricular septum is com-
pleted, and has fused with the longitudinal or spiral septum of
the truncus arteriosus (cf. p. 302). This latter septum arises,
in front, between the roots of the fourth and fifth pairs of aortic

X

306 THE CHICK.

r

arches, and divides the truncus arteriosus in such manner that
the right division of the truncus arteriosus, and consequently Y
the right ventricle, from which this division arises, sends all its |
blood into the fifth pair of aortic arches ; while the left ventricle
and left division of the truncus arteriosus conduct blood to the
third and fourth pairs of arches. It follows from this that the
supply of blood to the head and anterior part of the body is
derived from the left ventricle ; while the right ventricle supplies
the whole of the body behind the heart, as well as the yolk-sac
and allantois.

About the seventh day the two divisions of the truncus
arteriosus separate completely from each, other at their bases ;
the right branch, or pulmonary trunk, remaining in connection
with the right ventricle and the fifth pair of aortic arches ; and
the left branch, or systemic trunk, with the left ventricle and
the third and fourth pairs of aortic arches.

The part of the aorta connecting the dorsal ends of the third
and fourth pairs of aortic arches (Fig. 128) becomes very slender,
and is finally obliterated altogether, while the branch of the
truncus arteriosus from which the third aortic arch arises elon-
gates very considerably, and carries this arch forwards some
distance in front of the next, or fourth aortic arch. The subclavian
arteries, supplying the fore-limbs, arise from the ventral ends of
the third aortic arches, during the third day ; they grow back-
wards, lying ventral to the other vessels, and reach the limbs
during the fourth or fifth day.

The fourth pair of aortic arches is, from the fifth day
onwards, much the largest of the three persistent pairs. The
arches of the two sides of the body are at first of equal size ; but
the arch of the right side soon becomes much larger than that
of the left side ; and the latter ultimately becomes obliterated
along the greater part of its length, while the arch of the right
side persists as the arch of the aorta in the adult bird.

The pulmonary arteries appear, in the walls of the lungs,
about the middle of the third day, before the two hinder pairs
of aortic arches are formed. On the appearance of the fifth pair
of aortic arches the pulmonary arteries (Fig. 128, ap) become
connected with their ventral ends. Each fifth aortic arch thus
consists of two parts : a proximal part running from the truncus
arteriosus, i.e. from the right ventricle, to the lung ; and a distal

THE ARTERIES. 307

or dorsal part which connects the root of the pulmonary artery
with the aorta. This distal portion is spoken of as the ductus
Botalli, or ductus arteriosus ; and so long as it remains open the
blood from the right ventricle can avoid the lung circulation,
and pass to the aorta direct. Shortly after the hatching of the
chick, however, the ductus Botalli shrivels up, and its cavity
becomes obliterated ; from this time the fifth arch communi-
cates with the lungs alone, and all the blood from the right
ventricle must pass through the pulmonary capillaries.

b. The Dorsal Aortae and their branches. The two dorsal
aortae are at first separate along their whole length, running
parallel to the notochord and some little distance from the
median plane. Each aorta is, from the first, continuous along its
outer side at several places with the vascular reticulum of the
area pellucida and area vasculosa. By coalescence of the vessels
of the reticulum at one place, with shrinking and disappearance
of the reticulum in front of and behind this spot, the definite
vitelline artery of each side is formed (Figs. 113 and 128, av) :
this is a large vessel running outwards from the aorta, between
the splanchnopleuric mesoblast and the hypoblast, and passing out
beyond the embryo to open distally into the vascular reticulum
of the area vasculosa.

Before the end of the second day the two aortae have met and
fused for a short distance along the middle part of their course,
separating again towards their hinder ends, and giving off at
intervals along their length small arteries to supply the various
parts of the body.

By the fourth day the union of the two aortae has extended
much further back than before, and involves the part from which
the vitelline arteries arise. The two vitelline arteries have
themselves coalesced at their proximal ends, and now spring
from the aorta as a single trunk, which divides almost at once
lto the right and left vitelline arteries, of which the left one is
Luch the larger.

The allantoic, or, as they are often called, umbilical arteries

Tig. 128, aa), arise from the aortas just beyond their point of

furcation, and run outwards to the allantois. The left allantoic

;ery is usually the larger of the two from the first, and becomes

Itimately the sole one, the right allantoic artery disappearing.

x 2

308 THE CTTICK.

4. The Veins.

The chief peculiarities in the veins of the chick, as distin-
guished from those of the tadpole, consist in the large size and
great importance of the vitelline and allantoic veins, which return
to the embryo the blood from the yolk-sac and the allantois
respectively ; the blood of the vitelline veins being laden with
food matter absorbed from the yolk, while that of the allantoic
veins is charged with oxygen, and freed from its excess of
carbonic acid.

The blood is returned to the heart by three chief veins, as
in the tadpole : (i) and (ii) the right and left Cuvierian veins (Fig.
128, vd), which return blood from the head and body of the
embryo, and which afterwards become the right and left anterior
vense cavse : and (iii) the meatus venosus (Fig. 128, ve), a
median posterior vein, which is formed in the first instance by
the union of the right and left vitelline veins, VV ; is joined a
little later by the allantoic veins, VA ; and, later still, receives in
addition the posterior vena cava, vi. At the time of hatching
of the chick the vitelline and allantoic veins disappear, or
become comparatively insignificant vessels, and the posterior vena
cava acquires its adult relations.

a. The System of the Anterior Venae Cavse.

Towards the end of the second day a pair of longitudinal
vessels, the anterior cardinal veins (ef. Fig. 128, vb), are formed
in the mesoblast of the sides of the head, slightly ventral to the
level of the notochord. They collect the blood from the sides of
the head, and carry it backwards.

A similar pair of vessels, the posterior cardinal veins, vc,
appear about the same time in the trunk of the embryo. They
return the blood from the hinder part of the body, and more
especially from the Wolffian bodies or embryonic kidneys, as soon
as these are formed.

The anterior and posterior cardinal veins of each side unite,
opposite the heart, to form a short transverse vessel, the Cuvierian
vein (Fig. 128, vd), sometimes called the ductus Cuvieri, which
opens into the sinus venosus, RS, or most posterior division of
the heart.

Throughout the earlier stages of development the two
cardinal veins return the blood from practically the whole of

THE VEINS. 309

the body of the embryo, excepting the alimentary canal and the
liver. The anterior cardinal veins persist throughout life as
the jugular veins, and are joined, at an early stage by pectoral
veins from the wing, and vertebral veins from the head and neck.
The posterior cardinal veins remain of large size so long as the
Wolffian bodies are in functional activity; but when these
begin to diminish in size, on the appearance of the permanent-
kidneys, the posterior cardinal veins also shrink up, and ulti-
mately disappear.

The Cuvierian veins, as already noticed, persist and become
the anterior venas cavas of the adult bird.

b. The System of the Posterior Vena Cava.

The meatus venosus. The heart, as already described, is
formed by the union of the right and left vitelline veins (Fig.
127, vv), which lie along the edge of the fold of the splanchno-
pleure, marking the posterior limit of the fore-gut (cf. Fig. 112).

The vitelline veins remain at the edge of the splanchno-
pleuric fold, and consequently travel backwards with this fold as
the embryo becomes more definitely constricted from the yolk-
sac. This shifting backwards of the point of meeting of the
right and left vitelline veins causes, first, a lengthening of the
heart ; and then, after the heart is definitely established, the
formation of a median vitelline vein lying posteriorly to the
heart, between the hinder end of the heart and the point of
meeting of the two vitelline veins.

This median vitelline vein may be divided into an anterior
part or sinus venosus (Fig. 128, rs), which really forms the
posterior chamber of the heart and receives the Cuvierian veins ;
and a posterior part or meatus venosus, ve.

The meatus venosus has, from the first, very close relations
with the liver. The two primary liver diverticula lie one on
each side of it, and as the liver increases in size it completely
surrounds the vein. Blood-vessels appear in the substance of
the liver mass, and soon acquire openings into the meatus
venosus. At first these vessels are very irregularly disposed,
but by about the fifth day a definite arrangement can be made
out. The meatus venosus on entering the liver gives off afferent
hepatic vessels (Fig. 128, vo), which open into the capillary

310 THE CHICK.

plexus of the liver substance ; from this plexus, efferent hepatic
vessels (Fig. 128, vii) arise, which open into the meatus venosus
shortly before this emerges from the anterior end of the liver.
Thus, while the main stream of blood, entering the liver from
the vitelline veins, passes along the meatus venosus direct to
the heart, a small part of it is diverted through the capillary
system of the liver, joining the meatus venosus again further
on in its course.

The part of the meatus venosus in the substance of the liver,
between the openings of the afferent and efferent hepatic vessels,
is usually spoken of as the ductus venosus.

The vitelline veins return to the embryo the blood from the
capillary network of the vascular area of the blastoderm (cf. Fig.
99, av). From the greater part of the area vasculosa the blood
is returned directly by the right or left vitelline veins ; but from
the more peripheral part of the area the blood is collected by a
circular marginal or terminal vein, which runs round the outer
edge of the area vasculosa, and from which anterior and poste-
rior veins carry the blood to the vitelline veins.

Of the two vitelline veins, the left is, almost from the first,
larger than the right. After the embryo has turned so as to lie
on its left side, the difference between the two veins becomes
more pronounced, and ultimately the right one disappears.

The allantoic veins. As soon as the allantois is definitely
formed, on the fourth day, two allantoic veins are developed, re-
turning the blood from it to the embryo. These unite on entering
the embryo to form a single allantoic vein, which runs forwards in
the splanchnopleure of the left side to join the left vitelline vein.
With growth of the allantois, the allantoic vessels increase greatly
in size, but their relations remain practically the same. In the
earlier stages (Fig. 128), the vitelline vein is much larger than
the allantoic ; but later on, as the yolk becomes smaller through
absorption of its contents, while the allantois continues to in-
crease, the proportions are reversed, and the allantoic vein
becomes the larger of the two.

The mesenteric vein is formed by the union of the veins
which return the blood from the intestine of the embr3ro. It

THE VEINS. 311

appears during the fourth day, and is at first very small, but
it becomes larger and more important as the alimentary canal
lengthens.

The mesenteric vein joins the vitelline vein just before this
reaches the liver. The blood entering the liver by the meatus
venosus is thus derived from three sources : (i) the vitelline vein
returns blood, highly charged with nutritious matter, from the
yolk-sac ; (ii) the allantoic vein returns blood from the allan-
tois, rich in oxygen and freed from carbonic acid ; (iii) the
mesenteric vein brings venous blood from the walls of the ali-
mentary canal of the embryo. Of these, the vitelline and allantoic
veins are, during the period of incubation, much larger than the
mesenteric vein ; but inasmuch as the two former vessels return
blood from parts outside the embryo, they become obliterated at
the time of hatching ; while the mesenteric vein, which alone
persists of the three, retains its relations with the liver, and
becomes the hepatic portal vein of the adult bird.

The posterior vena cava appears about the fourth day (cf.
Fig. 128, Vl). It arises in the mesoblast between the hinder
ends of the Wolffian bodies, and runs forwards in the median
line, ventral to the aorta ; at its anterior end it joins the meatus
venosus, as this emerges from the liver, and just before it reaches
the heart.

As the permanent kidneys become definitely established, the
posterior vena cava acquires special relations with them, and
increases in size as they develop. At first (Fig. 128), the pos-
terior vena cava appears as a comparatively unimportant tribu-
tary of the meatus venosus, but with growth of the embryo, and
especially with the enlargement of the hind limbs, it steadily
increases in size, and towards the close of incubation becomes
the larger vessel of the two.

After the posterior vena cava has attained some size the
hepatic efferent veins shift so as to open into it directly, instead
of into the meatus venosus, and it is from this time that the
vena cava becomes a larger and more important vessel than the
meatus venosus.

•j. The Course of the Circulation.

It will render the description of the development of the
blood-vessels easier to follow if a connected account is given of

312 THE thick.

the course of the circulation on the third day, and during the
later stages of incubation.

a. The circulation at the end of the third day (Fig. 113). The
heart, at the end of the third day, is a single twisted tube, slightly
constricted at intervals which mark the boundaries of the succes-
sive chambers, but with no trace of a division into right and left
sides. The blood enters the hinder end of the heart by the two
vitelline veins, and passes out in front through the truncus
arteriosus ; this at once splits into right and left branches, each
of which again divides into the three anterior aortic arches, which
encircle the pharynx and open dorsally into the aortse. The two
aortge are widely separate in the head, but approach each other
further back, and are fused for a short distance in the body.
Behind this point they separate again, and each aorta gives
off a large vitelline artery (Fig. 113, av), which, passing out
beyond the embryo, opens into the capillary network of the
area vasculosa ; from this network the blood is collected again
by vessels which unite to form the vitelline veins, and so
returns once more to the heart.

The great purpose of the circulation at this stage is to
insure the absorption of nutriment from the yolk-sac, and its
conveyance along definite channels to the embryo. The
vascular system of the embryo itself is as yet only imperfectly
formed, and there is no special respiratory organ.

b. The circulation during the latter half of the period of incuba-
tion. The heart is now fully formed. The sinus venosus has be-
come absorbed into the right auricle, of which it now forms part :
the auricular septum is still incomplete, the large foramen ovale
allowing blood to pass freely from the right auricle to the left
auricle. The ventricular septum is complete ; and the truncus
arteriosus is divided into two entirely separate vessels, of
which one, the pulmonary trunk, arises from the right ventricle,
and the other or systemic trunk from the left ventricle.

Three pairs of aortic arches are present, but these are the
third, fourth, and fifth of the complete series, the first and
second having disappeared along the greater part of their length.
The systemic trunk, arising from the left ventricle, leads to the
third and fourth pairs of aortic arches, and through these to the

THE COURSE OF THE CIRCULATION. 313

head and fore-limbs. The pulmonary trunk, arising from the
right ventricle, leads to the fifth pair of aortic arches, which arc
directly continuous with the dorsal aorta of the body of the
embryo, and from which also the small pulmonary arteries arise.
From the aorta a vitelline artery carries blood to the yolk-sac ;
and a still larger allantoic artery runs from the aorta to the
allantoic .

The blood is brought back to the heart by three great veins : —
the right and left anterior venae cava?, and the posterior vena cava.
The right and left anterior venae cavae return venous blood from
the head and tore-limbs of the embryo. The posterior vena cava
returns blood from the hinder part of the body, the hind limbs,
and the kidneys ; just before reaching the heart it is joined by
the ductus venosus, which returns the blood from the yolk-sac,
from the allantois, and from the alimentary canal of the embryo,
by the vitelline, allantoic, and mesenteric veins respectively. The
blood in the vitelline vein is arterial as regards nutrient matter;
the blood in the allantoic vein is arterial as regards its gaseous
components ; and the blood in the mesenteric vein is venous.
The blood in the posterior vena cava is venous as regards
nutriment, and as regards gaseous components, but, having just
passed through the kidneys, is arterial as regards freedom from
nitrogenous excretory matters.

The blood brought to the heart by the posterior vena cava
may therefore be spoken of as arterial, and stands in this
respect in marked contrast to the venous blood brought to the
heart by the right and left anterior vena? cava?.

All three vena? cava? open into the right auricle of the
heart ; but, owing to the position and direction of the open-
ing, and to the Eustachian valve, the arterial blood from the
posterior vena cava is directed at once through the foramen
ovale into the left auricle, while the venous blood from the right
and left anterior vena? cava? remains in the right auricle. The
right auricle is thus filled with venous blood, and the left auricle
with arterial blood.

I On contraction of the auricles, the blood they contain is
iven into the ventricles, so that the right ventricle will be
led with venous, and the left with arterial blood.
The left ventricle drives its arterial blood along the svstemic

314 THE CHICK.

to the head and fore-limbs ; while the right ventricle force-
venous blood through the pulmonary trunk and the fifth pair of
aortic arches into the dorsal aorta, from which part goes to
supply the body and hind limbs of the embryo, and part, in the
earlier stages by far the larger part, passes out along the vitelline
and allantoic arteries to the yolk-sac and allantois, where it
takes up nutriment and oxygen.

The enormously disproportionate size of the head and anterior
part of the embryo and the stunted condition of the hinder
part during the earlier stages are to be ascribed, at any rate in
part, to this arterial supply of the anterior half as contrasted
with the venous supply of the posterior half of the embryo.

c. The changes in the circulation on hatching. The changes
which occur in the blood-vessels at or about the time of hatching
of the chick are comparatively slight, but suffice to convert the
circulation into that of the adult bird. The more important of
these changes are the following : —

(i) Closure of the ductus arteriosus of each side : i.e. oblite-
ration of the distal parts of the fifth pair of aortic arches, beyond
the points of origin of the pulmonary arteries (cf. Fig. 128, A5).
The effect of this change is that the blood from the right ventricle
can no longer pass into the aorta, but is sent entirely, through
the pulmonary arteries, to the lungs.

(ii) Obliteration of the vitelline and allantoic veins. This is
a necessary consequence of the complete absorption of the yolk-
sac, and the casting off of the allantois. Its effect is to reduce the
blood supply of the liver to the venous blood brought by the
mesenteric vein, or, as it may now be called, the hepatic portal
vein.

(iii) Closure of the ductus venosus. The effect of this change
is to render it impossible for the blood brought to the liver to
pass straight through it to the heart. All the blood entering the
liver by the portal vein must now pass through the capillaries of
the liver in order to get to the posterior vena cava, and so to the
heart. The short cut by which the liver capillaries could pre-
viously be avoided is now stopped.

THE COURSE OF THE CIRCULATION. 315

(iv) Closure of the foramen ovale. This is not completed
until some little time after hatching. Its effect is to absolutely
prevent the passage of blood from the right to the left auricle.
From the time of its completion, the blood brought to the heart
by all three venae cavae is discharged into the right auricle,
and passes from this into the right ventricle ; while the only
blood entering the left auricle is that brought to it from the
lungs by the pulmonary veins.

On the completion of these changes the circulation becomes
that of the adult bird. The arterial and venous streams of blood
are kept quite distinct, and the so-called double circulation is
completely established.

DEVELOPMENT OF THE URINARY ORGANS.

1. General Account.

The urinary organs of the chick, while agreeing in their
general relations and mode of development with those of the
frog, yet present considerable and important points of differ-
ence.

The head kidneys, which in the young tadpole are of large
size, and for a considerable time are the sole excretory organs
present, are in the chick extremely rudimentary structures,
which appear later than the Wolffian bodies, and disappear
again almost at once.

The Wolffian bodies of the chick are developed early : they
soon attain a large size, and form the functional excretory organs
during embryonic life. Shortly after the time of hatching, they
lose their kidney structure and excretory function completely,
though parts of them persist as accessory portions of the repro-
ductive apparatus.

The Wolffian and Mullerian ducts develop independently in
the chick, at any rate so far as their anterior ends are concerned.
As in the frog, the Mullerian duct becomes the oviduct of the
female, while the vas deferens of the male is formed from the
Wolffian duct,

2. The Wolffian Duct.

The Wolffian duct, which is the first part of the kidney
systein to be developed, appears early in the second day, in

316* . THE CHICK.

embryos with about eight pairs of mesoblastic somites (cf. Pig.
110). The duct arises on either side as a ridge-like projection
of the mesoblast, immediately beneath the dorsal epiblast, and
a little to the outer side of the mesoblastic somites. At the
stage mentioned, it lies opposite the three hindmost of the eight
somites.

As the embryo grows, the ridge lengthens, extending slowly
forwards, and more rapidly backwards. It also becomes free
from the mesoblast ; and in embryos with fourteen pairs of
somites, i.e. of about the thirty-sixth hour (cf. Fig. Ill), the
Wolffian duct is present along each side of the body as a solid
rod of cells, lying free between the epiblast and the mesoblast,
and extending from the fourth to the fourteenth somite.

The rod soon becomes tubular, by acquiring a central lumen ;
and at the same time its position becomes changed, the meso-
blast growing rapidly, and spreading over the duct, between
it and the epiblast. The Wolffian duct (Fig. 129, kc) conse-
quently gets driven down into the mesoblast, where it lies, about
the level of the dorsal aorfcae, imbedded in the mass of mesoblast,
sometimes spoken of as the intermediate cell mass, which projects
into the dorsal part of the body cavity as a longitudinal ridge
between the somatopleuric and splanchnopleuric folds.

The Wolffian duct continues its growth backwards during
the succeeding stages, and towards the end of the fourth day
(cf. Fig. 123, kc) reaches the cloaca, or terminal dilatation of the
alimentary canal, and opens into its dorsal surface.

3. The Wolffian Body.

The Wolffian body is developed entirely from mesoblast, and
agrees closely in its structure and mode of development with
that of the frog. In its fully formed condition (Fig. 123, km) it
consists of a complicated mass of convoluted Wolffian tubules,
each of which commences with a Malpighian body, and opens at
its other end into the Wolffian duct.

The Wolffian body extends along the greater part of the
length of the dorsal body-wall, as far back as the thirtieth
somite. Its anterior end is imperfectly developed, or even
rudimentary from the first, but from about the sixteenth to the
thirtieth somite the Wolffian body remains of large size almost
up to the time of hatching.

THE WOLFFIAN DUCT AND WOLFFIAN BODY.

317

In their mode of development the Wolffian tubules present
certain differences in the anterior and posterior parts of the
Wolffian body respectively, and must be described separately.

In front of about the sixteenth somite, a number of small
funnel-like depressions of the peritoneal epithelium appear,
towards the end of the second day (Fig. 129, ks), below and a
little to the inner side of the Wolffian duct. The bottoms of these
funnels are in connection with slightly twisted cellular cords,
which form in the mesoblast. These cords soon become tubular.

Fig. 129. — A transverse section across the body of a Chick Embryo at the
forty- eighth hour of incubation, x 150.

A, aorta. AN, amnion. C, body cavity or cceloin. CH, notochord. CM, myoccel,
or cavity of rnesoblastic somite. H, hypoblast forming roof of mid-gut. KC, Wolffian'
duct. KS, nephrostome. ME, somatopleuric layer of mesoblast. MH, splanchno-
pleuric layer of mesoblast. MP, muscle plate. WE, rudiment of spinal ganglion.
NS, spinal cord. OE, genital epithelium. VC. posterior cardinal vein. VV, vitelline
vein.

and acquire communications at one end with the funnel-like
depressions, and at the other end with the Wolffian duct ; so that
the peritoneal funnels, or nephrostomes, now lead from the ccelom
through these Wolffian tubules into the Wolffian duct. The
nephrostomes, and the Wolffian tubules into which they open, are
usually from the first rather more numerous than the somites in
which they lie.

The anterior Wolffian tubules are imperfect from the first,
and soon undergo degenerative changes. The mouths of the
nephrostomes first dilate greatly. A vascular process, or glo-

/ft

318 THE CHICK.

merulus, then arises from the side wall of each tubule, and pro-
jects into its cavity; by further enlargement, the glomeruli
grow out through the expanded mouths of the tubules, or nephro-
stomies, so as to hang freely into the body cavity. These changes
are probably of a degenerative character, for shortly afterwards
both the glomeruli and the tubules disappear completely.
Further back in the body, but still in front of the sixteenth
somite, the Wolffian tubules develop differently ; the nephro-
stomies close, the tubules separate from the peritoneum, and
then become dilated to form Malpighian bodies, into which little
vascular tufts or glomeruli, derived from the aorta, soon pene-
trate.

In the posterior part of the Wolffian body, from about the
sixteenth to the thirtieth somites, there are no nephrostomes.
The Wolffian tubules in this region have no connection with the
peritoneal epithelium, but arise from the first in the mesoblast,
appearing as oval vesicles, which by elongation become the
Wolffian tubules. These acquire openings into the Wolffian duct,
and dilate at their opposite ends to form Malpighian bodies.
After the first-formed tubules are completed, others arise in the
same manner, and usually nearer the dorsal surface. Partly
owing to this increase in the number of the Wolffian tubules,
and partly owing to each tubule increasing greatly in length
and becoming much convoluted, the Wolffian body soon attains
a considerable size, causing a marked ridge-like projection of
the intermediate cell mass into the body cavity, along each side
of the mid-dorsal line.

4. The Head Kidney and the Miillerian Duct.

Towards the end of the fourth day, three pit-like involutions
of the peritoneal epithelium appear, one behind another, close to
the outer side of the Wolffian duct, and three or four somites
behind its anterior end. A ridge-like thickening of the peritoneal
epithelium connects the three pits of each side with one another,
and grows backwards behind the third pit as a solid rod of cells,
lying along the outer side of the Wolffian duct, and very close
to this.

This rod soon becomes tubular, ending blindly behind, but
opening in front into the body cavity through the three pits.
These three pits form the head-kidney of the chick embryo, and

vv

:

THE KIDNEYS, 319

the tube into which they open is the commencement of the
Miillerian duct.

Towards the end of the fifth day the two hinder pits close
up and disappear. The anterior pit persists, and forms the
peritoneal opening of the Mtillerian duct or oviduct. The
Miillerian duct itself grows rapidly backwards ; it lies in close
contact with the outer wall of the Wolffian duct, and in its
hinder part appears to be formed from cells derived from the
wall of the Wolffian duct.

About the end of the sixth day, the Miillerian duct has grown
backwards as far as the cloaca. It remains blind at its hinder
end in the male, but in the female opens, at a later stage, into
the cloaca.

5. The Permanent Kidney, or Metanephros ; and the Ureter.

The cloacal opening of the Wolffian duct (Fig. 123, tc) is
opposite the thirty-fourth somite, while the hinder end of the
Wolffian body itself does not extend behind the thirtieth somite.
The permanent kidney is formed in the mesoblast immediately
behind the Wolffian body, and is at first confined to the somites
from the thirty-first to the thirty-fourth inclusive, i.e. to the
somites between the hinder end of the Wolffian body and the
cloacal opening of the Wolffian duct.

Towards the end of the fourth day^the ureter arises on each
Bide (cf. Fig. 123, kd) as a forwardly directed diverticulum from
the dorsal surface of the hinder end of the Wolffian duct. From
the ureter lateral outgrowths arise, which become continuous
with masses of cells in the mesoblast around it ; and from these
cells the tubules of the kidney, with their Malpighian bodies,
are formed at a later stage. The kidney is at first very small as
compared with the Wolffian body, but towards the close of incu-
bation it increases very considerably in size, growing forwards
dorsal to the Wolffian body. The ureters, which at first open
into the dorsal wall of the cloaca, through the hinder ends of the
Wolffian ducts (Fig. 123), acquire independent openings into
e cloaca about the sixth day.

The Genital Ducts.

a. In the male, or cock bird, the Miillerian ducts, although
they reach the cloaca, never open into it. In the later stages they

320 THE CHICK.

undergo degenerative changes, and ultimately they become
almost completely obliterated on both sides of the body.

The Wolffian body aborts in great part: a portion of it.
however, persists as the epididymis ; while outgrowths from
the Malpighian bodies of this portion penetrate into the testis.
and become the vasa efferentia. The Wolffian duct persists
as the vas deferens, becoming bent on itself into short trans-
verse folds along almost its entire length, in the manner so fre-
quently seen in the male genital duct both of Vertebrates and of
Invertebrates.

b. In the female, or hen bird, the Wolffian body atrophies, a
small part alone persisting as the parovarium, a body lying in
the mesentery between the ovary and the kidney, and in which
the tubular structure of the Wolffian body remains recognisable
even in the adult bird. The, Wolffian duct disappears.

The Miillerian duct of the right side, like the right ovary,
disappears, though traces of it may persist in the adult. The
left Mtillerian duct becomes the oviduct : its peritoneal opening
becomes the fimbriated mouth of the oviduct, and its walls
thicken greatly, especially at the cloacal end.

7. The Supra-renal Bodies.

The development of the supra-renal or adrenal bodies is not
very satisfactorily determined. Each consists of two parts,
cortical and medullary. Of these, it is generally agreed that the
medullary portion arises as a part of the sympathetic nervous
system. The cortical portion is developed from groups of cells
which appear, about the end of the fourth day, in the mesoblast
along the inner side of the Wolffian body, between this and the
aorta ; and which increase during the fifth and sixth days so as
to form cell masses of considerable size. On the seventh day
these masses become closely attached to the Malpighian capsules
of the Wolffian body, but whether this is to be interpreted as
indicating any essential connection between the two bodies is
very doubtful.

THE CCELOM. 321

THE DEVELOPMENT OF THE BODY CAVITY AND
MUSCULAR SYSTEM.

1. The Body Cavity or Ccelom.

The body cavity or ccelom is formed, as described on p. 243,
towards the end of the first clay, by the splitting of the meso-
blast, or rather by rearrangement of the mesoblast cells into
two layers, upper or somatic and lower or splanchnic respec-
tively ; the body cavity being the narrow chink-like space
between the two layers.

In the later stages, the body cavity enlarges considerably, by
further separation of its walls. It extends the whole length of
the body, but does not reach into the head, stopping at the
first mesoblastic somite or proto vertebra.

The two halves, right and left, of the body cavity are at first
separate ; but as the ventral body-wall of the embryo is com-
pleted, by constriction of the embryo from the yolk, the two
halves meet and open into each other across the mid-ventral
plane. From the mode of its formation, the body cavity is
directly continuous with the space between the two layers,
inner and outer, of the amnion (Figs. 112 and 129, an) ; and
it is owing to this continuity that the allantois is able to pass
out beyond the limits of the embryo, and spread over its back
(Figs. 100 and 101).

The mesoblast cells lining the body cavity become the peri-
toneal epithelium ; from this epithelium the genital organs,
i.e. the ovary or testis, are developed • and from it, or in close
relation with it, the tubules of the Wolffian body are formed.

2. The Pericardial Cavity.

There is at first no separate pericardial cavity ; the heart
lying in the anterior part of the general body cavity, ventral to
the oesophagus and pharynx (Fig. 112). About the end of
the second day a septum begins to form, which shuts off the
ventral portion of the body cavity, in which the heart lies, at
first partially and ultimately completely from the general body
cavity ; and this shut-off portion becomes the pericardial cavity.
The septum is formed in the following manner.

Opposite the hinder end of the heart the Cuvierian veins

Y

822 THE CHICK.

cross the body cavity transversely, in order to reacli the sinus
venosus from the somatopleure ; and it is from the walls of the
Cuvierian veins that the pericardial septum arises, as a thin
sheet of connective tissue which grows forwards and upwards
to the under surface of the fore-gut, and obliquely downward-
and backwards to the ventral body wall, which it meets a little
way behind the heart.

The lungs lie in two pocket-like diverticula of the body-
cavity which extend forwards along the sides of the oesophagus
(cf. Fig. 163). These pleural cavities at first lie dorsal to the
pericardial cavity, but, as they gradually enlarge to make room
for the lungs, they spread ventralwards over the pericardial
cavity, between it and the body walls. The pleural cavities
in the bird remain throughout life continuous at their hinder
ends with the general body cavity.

3. The Muscular System.

The mesoblastic somites or proto- vertebras (cf p. 244) are
paired, cubical blocks of mesoblast, arranged in series along the
sides of the spinal cord, and formed by transverse division of
the vertebral plates of the mesoblast (Figs. 110, 111, 113, ms).

The somites are at first hollow ; their cavities are to be
regarded as parts of the ccelom ; and in the case of the anterior
three or four somites the cavities are for a time actually con-
tinuous with the general body cavity. The walls of the somites
are at first of nearly uniform thickness on all sides, but during
the second and following days they thicken very unequally, and
undergo further changes, by which they give rise to the greater
part of the voluntary muscles of the trunk, and to the elements
from which the vertebral column is developed. These changes
are effected as follows.

During the second and third days (Fig. 129) the ventral
walls of the somites thicken very considerably, so that the
whole embryo is increased greatly in depth, from the dorsal to the
ventral surface, and the cavities of the somites, CM, become
situated, not in their centres, but close to their dorsal surfaces.
The dorsal part of each somite, inclosing the cavity, or myocoel,
CM, now separates off from the underlying mesoblast, as the
muscle-plate, mp : the walls of the muscle-plate consist of cells of
an epithelial character, closely packed side by side, and contrast

THE MESOBLASTIC SOMITES. 823

strongly with the loosely arranged stellate cells of the ventral
part of the somite.

The cells forming the deeper, or ventral, walls of the muscle-
plates become elongated parallel to the axis of the embryo, and
very soon become converted into a system of longitudinal muscle
fibres. In this way a broad band of longitudinal muscle fibres
is formed, stretching along the whole length of each side of the
body of the embryo, each band being divided transversely, by
the original protovertebral lines of segmentation, into paired
muscle segments or myotomes. These muscle bands in the
chick correspond to the great longitudinal body muscles of
Amphioxus or of Fishes ; and from them a great part of the
voluntary muscular system is developed.

The muscle-plates at first lie nearly horizontally, their inner
borders being at a level slightly dorsal to their outer edges.
As the body of the embryo increases in thickness, and assumes
more definite shape, the muscle-plates become placed more and
more obliquely (cf. Fig. 129) ; and by the end of the third
day (Fig. 124, mp) they lie almost vertically, the original inner
border becoming dorsal, and the original outer border ventral
in position.

The outer, or ventral, edge of each muscle-plate rapidly ex-
tends into the somatopleure, and by the fifth day has spread half
way down to the ventral surface of the embryo ; the greater part
of its component cells become converted into fusiform muscle-
cells, from which the muscles of the back and trunk are deve-
loped. At the dorsal and ventral borders of the muscle-plates
the cells retain their epithelial character so long as the plate
continues to grow.

In the segments opposite the limb buds (Fig. 115), the
muscle-plates stop at the bases of the limbs ; the muscles of the
limbs themselves are formed independently of the muscle-
plates, about the fifth or sixth day, an arrangement which is
probably to be regarded as a modification of a more primitive
one by which the musculature of the limbs is derived directly
from that of the body.

324 THE 'IIK'K.

THE DEVELOPMENT OF THE SKELETON.

The skeleton of the chick, like that of the tadpole, consista
in its earliest stages of densely packed mesoblast cells, which
soon give rise, by the formation of an intercellular matrix, to the
primary or cartilaginous skeleton. This, in the adult, is almost
completely replaced by the secondary or bony skeleton, but
persists in places, especially in the skull.

As in the frog, bone may appear either in direct relation
with the pre-existing cartilage, or independently of it, and
hence a distinction may be drawn between cartilage-bones and
membrane-bones ; the latter, which appear independently of the
cartilage, being almost confined to the head.

1. The Vertebral Column.

The vertebral column is formed from the ventral portions of
the mesoblastic somites or prot overt ebrae. After the muscle-
plates have separated off, the remaining or ventral portions of
the somites of each pair, which consist of indifferent and rather
loosely compacted mesoblast-cells, grow inwards towards the
median plane, and meet both above and below the spinal cord,
and below the notochord as well ; so that by the end of the
third day (Fig. 124) both the spinal cord and the notochord
have mesoblastic investments, which during the fourth day
increase considerably in thickness.

Early on the fifth day, the transverse lines of demarcation
between the successive pairs of somites disappear, the mesoblast
becoming a continuous mass the whole length of the body, and
forming continuous investments to the notochord and the spinal
cord. This fusion only concerns the ventral portions of the
somites, the muscle-plates retaining their original distinctness.

In the course of the fifth day, the mesoblast immediately
around the notochord becomes cartilaginous, forming a continu-
ous unsegmented cartilaginous tube, ensheathing the notochord
along the whole length of the body. At the sides of the spinal
cord, paired cartilaginous bars appear, which soon fuse with
the cartilaginous investment of the notochord, and become the
neural arches of the vertebrse.

A little later, but before the close of the fifth day, further
histological changes occur in the cartilaginous tube surrounding

THE SKELETON.

325

Fig. 130.— The left half of the skeleton of the Common Fowl. The skull,
vertebral column, and sternum are bisected in the median plane. (From
Marshall and Hurst.)

A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle. CO,
coracoiil. CR, cervical rib. Cl. first cervical vertebra. FE, femur. HC, hypo-
cleidium. HTJ, humerus. HY. hyoid. IF, ilio-sciatic foramen. IL, ilium. IS,
ischium. L, lacrymal bone. MC3^ metacarpal bone of third digit. MW, mandible.
MS, metosteon. MT, tarso- metatarsus. MTl, metatarsal bone of first digit. N,
nasal bone. OP, optic foramen. P, premaxilla. PB, pubes. PL, palatine bone.
PY, pygosfcyle. R, radius. RC, radial carpal bone. S, keel of the sternum. SC,
scapula. T, tlbio-tarsns. TH4, fourth thoracic vertebra. U, ulna. UC, ulnar
carpal bone, u ±* , uncinate process of rib. Z, infra-orbital bar.

1, 2, 3, 4, the first, second, third, and fourth digits.

326 THE CHICK.

the notochord. Opposite the places of attachment of the neural
arches, the matrix becomes more abundant, and the cartilage
cells fewer; while between the successive neural arches tin-
matrix remains comparatively scanty, and the cartilage cells
more numerous. In this way the cartilaginous tube round the
notochord, while still remaining a continuous unsegmented
structure, becomes marked into alternate vertebral and inter-
vertebral rings, the vertebral rings being the parts to which the
neural arches are attached, and in which the cartilage is of a
more mature type ; and the intervertebral rings being the parts
between successive neural arches, in which the cartilage remains
of a more embryonic character.

Each intervertebral ring, about the end of the fifth day,
divides into two portions, anterior and posterior, which attach
themselves to the vertebral rings in front of, and behind, them
respectively. By this division the originally continuous carti-
laginous sheath of the notochord becomes cut up into a series of
segments ; each segment consisting of a vertebral ring^ fused
with the posterior and anterior halves of successive inter-
vertebral rings, and fused also with a cartilaginpi^ nPinval arch.
The segments so formed become the adult vertebras (Fig. 116).

The original vertebral rings, and the neural arches, lie oppo-
site the intervals between successive pairs of muscle-plates ;
while the intervertebral rings lie opposite the muscle-plates
themselves. As the division into vertebras takes place across
the centres of the intervertebral rings, it follows that the
planes of division between the vertebras do not coincide with the
planes of division between the muscle-plates, i.e. with the
original protovertebral planes of division, but are midway
between these. Hence the division, or segmentation, of the
vertebral column has been spoken of as secondary or permanent
segmentation, in contrast to the primary or protovertebral
segmentation which is retained by the muscle-plates or
myotomes.

It must be borne in mind, however, that the permanent
segmentation is the only one ever shown by the skeletal elements
themselves. The primary segmentation is essentially a divi-
sion into myotomes or muscle segments, and occurs at a time
when the notochord is the only skeletal structure present. The
cartilaginous skeleton is at its first appearance unsegmented, and

THE VEETEBKAL COLUMN. 327

the only segmentation it ever shows is the permanent or ' secon-
dary ' one.

The reason why the permanent or vertebral segmentation
alternates with the primary or myotomic segmentation is probably
to be found in mechanical considerations. The longitudinal muscle
fibres of the myotomes are attached at their ends to the verte-
bras or to their processes, and the strain on the axial skeleton, and
consequent tendency to lateral bending, caused by alternate con-
tractions of the muscles of the two sides of the body, will be great-
est, not opposite the attachments of the muscles, but midway be-
tween these ; and it is at these midway points that the interver-
tebral joints are formed when the axial skeleton becomes too
rigid to allow of free bending of the body, without segmentation.
The mechanical advantage of the arrangement, by which each
vertebra is acted on by two myotomes on each side, one pulling
it forwards and one backwards, is sufficiently clear ; and the actual
segmentation is probably due, in the first instance, to the direct
action of the muscles themselves, causing bending, and subse-
quently jointing, of the originally continuous cartilaginous tube
at points midway between the attachments of the muscles.

Up to the sixth or seventh day (cf. Fig. 116, Cfi), the notochord
remains of full size and nearly uniform diameter ; but from this
time it becomes gradually encroached on by the vertebral centra
surrounding it, which grow inwards, constricting it, and causing
its gradual absorption and final disappearance. Ossification of the
vertebras begins about the twelfth day, in the centrum of the
second or third cervical vertebra, and gradually extends back-
wards along the column; the neural arches ossify rather later
than the centra, and independently of them ; each having two
centres of ossification.

At an early stage, about the seventh day, the true centrum
of the first, or atlas, vertebra separates from the outer ring, and
becomes attached to the second, or axis vertebra, as its odontoid
process. The atlas and axis vertebrae have no rib elements, but
these are present in the remaining cervical vertebras ; they lie
to the outer sides of the vertebral arteries, and they are from
the first continuous with the centra of the vertebras to which
they belong, except in the case of the hindmost two or three
cervical vertebras, in which the rib elements are for a time
independent.

328 THE chick.

In ;i seren-day chick embryo there are forty-five vertebral
present, of which the hindmost five or six fuse at a later stage to
form the pygostyle.

Of the ' sacral ' vertebras, the first four have ribs, which in t In-
first are long, and in the remaining three are much shorter. The
fifth, sixth, seventh, and eighth vertebras have no ribs; the ninth
and tenth have ribs, and are for this reason regarded by many
zoologists as the true sacral vertebras. The remaining five, or
' urosacral,' vertebras have no rib elements. Up to the seventh
day these latter are quite distinct from the ilia, which stop at the
tenth vertebra of the sacral series : in the later stages the ilia
gradually extend further backwards, and ultimately overlap and
fuse with all five ' urosacral ' vertebras.

2. The Skull.

The skull of the chick consists of the same morphological
elements as that of the frog, viz. :

(i) The cranium, or brain case.

(ii) The sense capsules, olfactory and auditory.

(iii) The visceral skeleton.

The sense capsules and the cranium are, however, so closely
united from their earliest appearance that it will be convenient
to describe them together.

The main factors of the skull may be recognised, in the form
of tracts of condensed mesoblast, as early as the fourth day, but
it" is not until the sixth day that cartilage is definitely established.

a. The Cartilaginous Skull.

(i) and (ii) The Cranium and the Sense Capsules.

At the end of the sixth day, when cartilage first appears in
definite form, the structure of the skull is as follows (cf. Figs.
116 and 123). The notochord extends forwards in the median
plane, beneath the brain, as far as the hinder end of the pituitary
body, where it stops. At the sides of the notochord, and in close
contact with it, are a pair of horizontal cartilaginous plates, the
parachordal plates, which, with the notochord, focm a broad floor
to the hinder part of the skull, underlying the hind- and mid-
brains. Imbedded in the parachordal plates, and continuous
with them from their first appearance, are the cartilaginous
auditory capsules, inclosing the auditory organs.

THE SKILL,

32f>

In front of the notochord, the parachordals are continued for-
wards as a pair of short and rather slender rods, the trabecule
cranii: these lie at the sides of the pituitary body, and unite in
front of this to form the ethmoidal plate, which underlies and
supports the fore-brain.

By the eighth day (Figs. 131 and 116) important changes

SF P° 0M

A*- QJ 3B SR

Fig. 131.— The skull of a Chick Embryo at the end of the eighth day of incu-
bation; seen from the right side. The head and eye are represented in

outline.

10.

AN", angulare. AE, articular portion of mandibular bar. BB, basi-branchial
cartilage. BK, cerato-branchial cartilage. CL, columella. EH, external or hori-
zontal semicircular canal. EP, posterior vertical semicircular canal. ET. mesethmoid
cartilage. FE, fenestra ovalis. HE, ceratoliyal cartilage. MC, Meckel's cartilage.
OC, occipital condyle. OK, slit-like aperture of olfactory capsule. OL, outline of
lens. ON, outline of eyeball. PG, pterygoid. Q, quadrate cartilage. QJ,quadrato-
jugal. RL, trabecula cranii. SE, pre-'splienoidal region. SE, ali-sphenoidal region.
SL, supra-occipital region. SR. supra-angular.

ha

:

have occurred in the skull, mainly associated with the growth

rwarcls of the beak.
At the hinder end of the skull the two parachordal cartilages

ig. 116, rc) have united, above and below the notochord, to form
the basilar plate ; and the sides of the basilar plate, including the
auditory capsules which are fused with them, have grown up-

330 THE chick.

wards to form the side walls of the skull (Fig. 131); the ex-
occipital, ali-sphenoidal, and orbito-sphenoidal regirmg being
already established.

In front of the pituitary body the ethmoidal plate (Fig. 131,
et) has grown enormously ; it extends forwards to the tip of the
beak, and is fused in front with the cartilaginous capsules of the
olfactory organs, OK. From the dorsal surface of the ethmoidal
plate, along its whole length, a huge vertical crest, the interorbital
plate, has arisen, which supports the fore part of the brain (Fig.
116) along its upper edge, and is notched in front for the
passage of the olfactory nerves, I.

(iii) The Visceral Skeleton.

In the chick embryo, cartilaginous elements, corresponding
to the cartilaginous bars of the tadpole's skull, are developed
in the mandibular, hyoidean, and first branchial arches.

The mandibular arch. In the mandibular arch two cartilages
appear, proximal and distal respectively, which are from the first
independent. The proximal, or dorsal, one (Fig. 131, q) is the
quadrate cartilage, a stout tri-radiate cartilage of which the
longest arm, or otic process, is directed backwards, and articu-
lates with the auditory or periotic capsule ; while the ventral,
and stoutest limb furnishes the articular surface for the mandible.
The distal cartilage of the mandibular arch is a slender rod,
Meckel's cartilage, mc, which forms the basis of the lower jaw ;
its hinder end, All, which articulates with the quadrate, is ex-
panded and thickened.

The hyoid arch. The bar of cartilage belonging to the hyoid
arch is imperfect or absent along the greater part of its length,
its dorsal and ventral ends alone being present. The uppermost
or dorsal end is believed to be represented by the columella
(Fig. 131, Cl), a slender rod of cartilage which very early fuses
with the stapes, a small plug of cartilage formed in the membrane
closing the fenestra ovalis. The ventral ...end of the hyoid bar
forms the cerato-hyal, or lesser cornu of the hyoid, hr ; and the
median element of the hyoid, or basihyal, appears also to belong
to this arch.

The first branchial arch. In the ventral part of the first
branchial arch a slender cartilaginous bar, the cerato-branchial, or
greater cornu of the hyoid (Fig. 131, BK),is formed ; and in the

THE SKULL

331

mid-ventral plane, where the arches of the two sides meet, a
median basibranchial cartilage, BB, is developed.

In the hinder branchial arches no skeletal elements are
formed in the chick.

b. The Osseous or Bony Skull.

(i) The cartilage-bones developed in connection with the skull
are as follows :

From the parachordal cartilages are formed the basi-occipital,
ex-occipitals, and supra-occipital.

OF SF IS .

Fig. 182.— The skull of the Fowl, from the right side. (From Marshall and

Hurst.)
A, articular surface of the mandible. AT, anterior tympanic recess, leading to
Eustachian tube. B, pterygoid. O, occipital condyle. D, palatine. E, rostrum. F,
mandibular foramen. FO, fenestra oralis. FR, 'fenestra rotunda. FZ, zygomatic
process of frontal hone. Q-, supra-angular. H, dentary. IS, inter-orbital septum. J,
jugal. L. lacrymal. M. maxilla, MP, maxillo-palatine process of maxilla. M\ nasal.
OF, optic foramen. PM, premaxilla. FT, posterior tympanic recess. Q,. quadrate.
QJ, quadrato-jugal. SF, olfactory foramen. SZ, zygomatic process of squamosal
TF, foramen for fifth nerve.

From the periotic capsules are formed the prootics, epiotics,
and opisthotics.

From the trabecules cranii, the ethmoidal cartilages and the
olfactory capsules, are formed the alisphenoids, orbitosphenoids,
presphenoid, and mesethmoid.

In the mandibular arch of each side are formed the quadrate,
and the articulare.

In the hyoid arch are formed the columella, the ceratohyal,
and basihyal.

In the first branchial arch are formed the ceratobranchial,
and basibranchial.

332 THE ('HICK.

(ii) The membrane-bones are less closely connected with thg
cartilaginous skull than are the cartilage-bones, and can only be
grouped somewhat arbitrarily according to the primary divisions
of the cartilaginous skull.

In connection with the cranium and the sense capsules are
formed the parietals, squamosals, frontals, lacrymals, nasals,
vomer, basitemporal, and parasphenoid.

In connection with the upper jaw are formed the ptery-
goids, palatines, quadratojugals, jugals, maxilla?, and premaxilla'.

In connection with the lower jaw are formed on each side the
dentary, angular e, supra-angulare, and splenial.

3. The Pectoral Girdle and Sternum.

The sternum develops as two separate halves, apparently
formed by fusion of the ventral ends of the ribs, which meet
and unite in the median plane on the ninth or tenth day. Both
halves contribute to the formation of the keel, which is formed
by fusion of their adjacent edges. The keel is very small until
towards the close of development. The manubrium of the sternum
is formed rather later, and is apparently a secondary outgrowth.

The sternum shows, in its development, evidence of having
been originally of greater length, and associated with a larger
number of ribs than in the adult fowl. In embryos of the
sixth day the two hindmost cervical ribs are attached to the
sternum ; on the seventh day they have lost their sternal attach-
ments, but are still greatly elongated. At the hinder end of the
thoracic series there is, during the sixth and seventh days, a
rudiment of an eighth rib, which disappears shortly afterwards.

In the shoulder girdle, the scapula and coracoid are almost at
right angles to each other on the seventh day, the scapula being
long and blade-like, and the coracoid short and stout. At the
beginning of the sixth day the scapula and coracoid are con-
tinuous with each other, but before the end of the day they
separate.

The clavicles are membrane-bones ; the median part of the
furcula has been compared to an interclavicle, but the embryo-
logical evidence does not support this view.

4. The Fore-limb or Wing.

The humerus, and the radius and ulna, present no points of
special importance in their development ; but the carpus and the

THE PECTORAL GIRDLE AND FORE-LIMB. 333

maims show peculiar modifications in all birds, and are of much
greater interest (Fig. 130).

a. The carpus. On the seventh day the carpus consists of: —
(i) a proximal row of two cartilages, of which the larger one is
situated opposite the end of the radius, and is regarded by
Parker as corresponding to the radiale and intermedium of the
typical carpus ; while the smaller one, placed opposite the end
of the ulna, is commonly regarded as the ulnare, but, according
to Parker, corresponds to the ulnare and centrale of the typical
carpus, (ii) A distal row of two cartilages, a larger one on the
radial side and a smaller one on the ulnar side. A little later,
about the tenth day, the radial cartilage divides into two, giving
three—cartilages in the distal row of the carpus, of which the
middle one is the largest.

The carpus remains in this condition until some time after
the hatching of the chick. The two proximal carpals persist as
the two free carpals of the adult bird ; they begin to ossify in
chicks about five weeks after hatching. The distal carpals
remain free for some time, but ultimately, in chicks eight or
nine months old, they unite with the metacarpals.

b. The digits. In the manus of the fowl there are three well-
formed digits, which at first are quite independent of one
another, and which correspond to the three radial digits, pollex,
index, and medius, of the typical Vertebrate manus. The fourth
digit is rather doubtfully represented by a small rudiment.

On the seventh day the first three metacarpals are well-
developed cartilaginous rods, completely separate from one an-
other, and from the carpus. The first metacarpal is short ; the
second long and thick ; the third about the same length as the
second, but much thinner. The first digit or pollex has two
short phalanges ; the second digit or index has three ; and the
third digit or medius has two, of which the terminal one ulti-
mately disappears.

During the tenth day a small nodule of cartilage, the pre-
pollex, appears on the radial side of the first metacarpal, with
which it ultimately fuses. Another small bar of cartilage ap-
pears, about the same time, on the outer side of the third meta-
carpal, at its proximal end ; this, which perhaps represents the
fourth. -metacarpal, remains distinct until some time after hatch-
ing, ultimately fusing with the base of the third metacarpal.

334 THE CHICK.

The further changes in the i nanus are effected very slowly.
The three metacarpals begin to ossify about the tenth or twelfth
day, but remain distinct from one another until about a month
after hatching, when they begin slowly to unite together ; the
fusion of the metacarpals with the distal row of carpals does not
occur until some time later.

5. The Pelvic Girdle.

The pelvic girdle, about the sixth day, consists of a somewhat
squarish plate on either side of the body, the central part of
which is at first directly continuous with the femur. The dorsal
border of the plate corresponds with the iliac region, which at
this stage does not extend over more than about three somites,
differing in this respect very markedly from its condition in the
adult. From the ventral and anterior border of the plate two
processes, prepubic and pubic, project downwards and forwards ;
and from the ventral and posterior border a broad ischiatic pro-
cess projects downwards and inwards. -

During the seventh day, the femur becomes separated off
from the hip girdle ; the ilium extends rapidly backwards along
the vertebral column, and more slowly forwards ; the ischium
grows backwards ; and the pubis also begins to grow backwards as
a slender bar, lying parallel to the ventral border of the ischium,
and a little way below this. The prepubic process is relatively
much less conspicuous than in the earlier stages.

During the later stages of development these changes
become more and more marked, and the pelvis gradually acquires
its adult shape. The ilium elongates, both anteriorly and
posteriorly, becoming ultimately attached to no less than fifteen
vertebrge. The ischium also lengthens greatly : its hinder end
becomes expanded, and, shortly before the time of hatching, fuses
with the posterior end of the ilium, to complete the boundary of
the ilio-sciatic foramen. The pubes elongates still more than the
ischium, and forms a long slender rod, lying parallel to the ven-
tral edge of the ischium and projecting backwards some distance
beyond this. The prepubic process, which, both in its cartilagi-
nous condition and when ossified, appears to belong to the ilium
rather than to the pubes, forms in the adult a small blunt
process, projecting from the anterior and ventral border of the
acetabulum .

THE PELVIC GIRDLE AND HIND-LIMB. 335-

6. The Hind-limb or Leg.

The femur, and the tibia and fibula, present no special
points of interest in their development.

The tarsus consists, on the seventh day, of a proximal row of
two tarsal cartilages, of which one represents the tibiale and
intermedium, and the other the fibulare of the typical tarsus;
and a single distal cartilage, which shows indications of its for-
mation from three centres. At a later stage the proximal tarsal
cartilages fuse together ; and about the fourteenth or fifteenth
day the proximal tarsals fuse with the distal end of the tibia to
form the t|bio^l#r£us, while the distal tarsal cartilage fuses with
the metatarsals to form the tarso-metatarsus.

In the pes, the first digit, or hallux, is represented by a short
metatarsal, of which the proximal end is never present ; and two
phalanges. The second, third, and fourth digits are approximately
equal in size, having well-formed metatarsals, and three, four,
and five phalanges respectively. The fifth digit is represented
by a small nodule of cartilage on the outer, or fibular, side of the
proximal end of the fourth metatarsal.

The three fully-developed metatarsals, i.e. the second, third,
and fourth, remain distinct, though closely apposed, until about
the beginning of the third week of incubation, when they fuse
with one another and with the distal tarsal cartilage.

DEVELOPMENT OF THE FEATHERS.

The feathers are formed by special modification of the epi-
dermal coverings of papilla?, which appear as projections of the
skin about the eighth day of incubation.

Each of the permanent feathers is preceded by an embryonic
or down feather, the mode of development of which is as follows.
About the eighth day small conical projections of the skin appear,
the feather papillae, each consisting of a central core of vas-
cular connective tissue, covered by a cap of epidermis. As the
papilla? increase in height, their bases become depressed below
the general surface of the skin. This depression is most marked
on the surface of the papilla? towards the tail end of the embryo,
and gives rise to the characteristic backward slant of the feathers.
The epidermal cap of each papilla consists of a superficial,
or epitrichial, layer of flattened pavement cells, and a deeper or

836 THE chick.

Malpighian layer of shorfc cylindrical or cubical cells. At first
fche epidermal cap is of uniform thickness all over the papilla;
but, as the papilla lengthens, the inner surface of the epithelium
thickens along certain lines, so as to form ridges projecting into
the papilla. The thickening is due to the formation of a third
or intermediary layer of cells, spherical or polygonal in shape,
and lying between the epitrichial and Malpighian layers ; and
it is from the ridges formed in this way that the feather is
developed.

The thickest and best marked of the ridges runs longitudi-
nally, along the upper or anterior surface of the papilla, from base
to apex ; while the other ridges, which develop in order from
the apex towards the base of the papilla, arise from the sides of
this main ridge, and run very obliquely round the papilla to its
lower or posterior surface, the ridges of the two sides not quite
meeting on the lower surface of the papilla.

In each ridge, which is thus a solid rod of epithelial cells,
the outer cells become elongated and cornified, while the central
or axial cells remain comparatively soft. The vascular connec-
tive tissue, forming the core of the papilla, now shrinks away
from its apex. The outer, or epitrichial, layer of the epidermis,
which merely forms a sheath inclosing the papilla, is cast off;
and the epithelial ridges or rods, which now alone remain, spread
out to form the feather ; the main longitudinal ridge becoming
the shaft, and the diverging lateral ridges the barbs ; while
minor or tertiary ridges, which arise from the barbs, give rise to
the barbules.

Towards the base of the papilla the epithelial ridges die out,
and the entire epithelial investment of the papilla, including
both epitrichial and Malpighian layers, becomes converted, by
cornification of its cells, into the quill of the feather, which
remains open at its lower end for the admission of blood-vessels.

At the lower end of the shaft, immediately above the quill,
the main epithelial ridge widens, and the two sides bend inwards
towards each other, and ultimately meet to form a tube, which is
continuous below with the quill, and the upper aperture of which
persists as the superior umbilicus of the feather.

The down feathers do not involve the entire length of the
papillae, but only their distal or apical portions. The basal
portions of the papillae sink deeply below the skin ; a second set

List of the more important Publications dealing with the
develoi~yment of the Chick.

THE FEATHERS. 337

of epithelial ridges is formed on them, and gives rise to the
permanent feathers, the development of which is essentially
similar to that of the down feathers. The permanent feathers,
as they are developed, gradually make their way to the surface,
replacing the down feathers, the nutrition of which is cut off by
further narrowing of the opening, or inferior umbilicus, at the
base of the quill.

Feathers do not develop uniformly over the entire surface of
the body, but along certain definite lines or tracts spoken of as
pterylia, and separated from one another by naked areas or
apteria ; the actual arrangement of these feathered and feather-
less patches varying considerably in different groups of birds.

Von Baer, K. E. : • Ueber Entwickelungsgeschichte der Thiere.' Konigsberg,

1828, 1837. •
Balfour, F. M. : 'The Development and Growth of the Layers of the Blasto-
derm ; and on the Disappearance of the Primitive Groove in the Embryo

Chick.' Quarterly Journal of Microscopical Science, vol. xiii. 1873.
'A Treatise on Comparative Embryology.' 1880-81.
Balfour, F. M., and Sedgwick, A. : 'On the Existence of a Head-kidney in the

Embryo Chick, and on Certain Points in the Development of the

Miillerian Duct.' Quarterly Journal of Microscopical Science, vol. xix.

1879.
Balfour, F. M., and Deighton, F. : 'A Renewed Study of the Germinal Layers

of the Chick.' Quarterly Journal of Microscopical Science, vol. xxii. 1882.
Beard, J. : ' The Development of the Peripheral Nervous System of Vertebrates.'

Quarterly Journal of Microscopical Science, vol. xxix. 1888.
Brandt, A. : ' Ueber den Zusammenhang der Glandula suprarenalis mit dem

Parovarium resp. der Epididymis bei Hiihnern.' Biologisches Central-

blatt, ix. 1889.
Budge, A. : ' Untersuchungen uber die Entwickelung des Lymphsystems beim

Huhnerembryo.' Archiv fiir Anatomie und Entwickelungsgeschichte.

1887.
Cajal, R. : 'A quelle Epoque apparaissent les Expansions des Cellules Nerveuses

de la Moelle Epiniere du Poulet ? ' Anatomischer Anzeiger, v. 1880.
Cazin, M. : ' Recherches Anatomiques, Histologiques, et Embryologiques sur

l'Appareil Gastrique des Oiseaux.' Annales des Sciences Naturelles,

serie 7, tome iv. ] 888.
orin, G., et Berarcl, E. : ' Contributions a l'Etude des Matieres Albuminoides

du Blanc d'ffiuf.' Archives de Biologie, ix. 1889.
te, M. : ' Histoire General e et Particuliere du Developpement des Corps

Organises.' Paris, 1847-59.
vies, H. R. : * Die Entwicklung der Feder und ihre Beziehungen zu anderen

Integumentgebilden.' Morphologisches Jahrbuch, xv. 1889.

Z

338 THE CHICK.

Dexter, S. : ' The Somites and Ccelome in the Chick.' Anatomischer Anzeiger,

vi. 1891.
Duval, M. : ■ Etudes Histologiques et Morphologiques sur les Annexes des

Embryons d'Oiseau.' Journal de l'Anatomie et de la Physiologie, xx.

1884.

* De la Formation du Blastoderme dans l'OEuf d'Oiseau.' Annales

des Sciences Naturelles, serie 6, tome xviii. 1885.
' Atlas d'Embryologie.' Paris. 1889.
Fabricius ab Aquapendente : ' De Formato Fcetu,' 1600. « De Formatione

Foetus,' 1604. The earliest descriptions, with figures, of the Develop-
ment of the Chick and other Vertebrates.
Felix, W. : • Zur Entwickelungsgeschichte der Vorniere des Hiihnchens.'

Anatomischer Anzeiger, v. 1890.
Foster, M., and Balfour, F. M. : 'The Elements of Embryology.' Second

edition, by Sedgwick and Heape. 1883.
Gadow, H. : 'On the Modifications of the First and Second Visceral Arches,

with especial Reference to the Homologies of the Auditory Ossicles.'

Philosophical Transactions, vol. 179. 1888.
Gasser, E. : ' Der Primitivstreifen bei Vogelembryonen.' Marburg, 1878.

' Beitrage zur Kenntnis der Vogelkeimscheibe.' Archiv fur Anatomie

und Entwickelungsgeschichte.' 1882.
Gerlach, L. : ' Ueber die entodermale Entstehungsweise der Chorda dorsalis.'

Biologisches Centralblatt, i. 1881.
Golowine, E. : ' Sur le Developpement du Systeme Ganglionnaire chez le

Poulet.' Anatomischer Anzeiger, v. 1890.
Holl, M. : ' Ueber die Reifung der Eizelle des Huhns.' ' Sitzungsberichte d. k.

Akademie d. Wiss. in Wien,' xcix. 1890.
Johnson, Alice : ' On the Development of the Pelvic Girdle and Skeleton of

the Hind Limb in the Chick.' Quarterly Journal of Microscopical

Science, xxiii. 1883.
Kastschenko, N. : ' Das Schlundspaltengebiet des Hiihnchens/ Archiv fur

Anatomie und Entwickelungsgeschichte. 1887.
Kolliker, A. : ' Entwicklungsgeschichte des Menschen und der hoheren Thiere.'

Leipzig, 1879.
Kowalevsky, R. : ' Die Bildung der Urinogenitalanlage (des Wolff 'schen Ganges)

bei Huhnerembryonen.' Warsaw, 1875.
Lahousse, E. : ' Recherches sur l'Ontogenese du Cervelet.' Archives de Bio-

logie, viii. 1888.
Liessner, E. : ' Untersuchungen betreffend die Entwicklung der Kiemenspalten

bei Vertretern der drei oberen Wirbelthierklassen.' Sitzungsberichte

d. Nat.-Gesellschaft. Dorpat. Band viii. 1887.
Lindsay, Beatrice : ' On the Avian Sternum.' Proceedings of the Zoological

Society of London. 1885.
Mackay, T. J. : ' The Development of the Branchial Arterial Arches in Birds,

with special Reference to the Origin of the Subclavians and Carotids.'

Philosophical Transactions, vol. 179. 1888.
Mall, F. P. : ' Entwickelung der Branchialbogen und Spalten des Hiihnchens.'

Archiv fiir Anatomie und Entwickelungsgeschichte. 1887.

•Development of the Eustachian Tube, Middle Ear, Tympanic

Membrane, and Meatus of the Chick.' Studies from the Biological

Laboratory of the Johns Hopkins University, vol. iv. 1888.

BIBLIOGRAPHY. 339

Marshall, A. Milnes : ' On the Early Stages of Development of the Nerves in

Birds.' Journal of Anatomy and Physiology, vol. xi. 1877.

' The Development of the Cranial Nerves in the Chick.' Quarterly

Journal of Microscopical Science, xviii. 1878.

1 The Morphology of the Vertebrate Olfactory Organ.' Quarterly

Journal of Microscopical Science, xix. 1879.
Masius, J. : ' Quelques Notes sur le Developpement du Coeur chez le Poulet.'

Archives de Biologie, ix. 1889.
Mehnert, E. : ' Untersuchungen iiber die Entwickelung des Os pelvis der

Vogel.' Morphologisches Jahrbuch, xiii. 1887.
Meuron, P. de : ' Sur le Developpement de l'CEsophage.' Comptes Rendus,

tome 102. 1886.

'Recherches sur le Developpement du Thymus et de la Glande

Thyro'ide.' Geneve, 1886.
Mihalkovics, G. V. von : ' Untersuchungen iiber die Entwicklung des Harn-

und Geschlechtsapparates der Amnioten.' Internat. Monatsschr. Anat.

Histol., Bd. ii. 1885.
Moldenhauer, W. : ' Die Entwicklung des mittleren und des ausseren Ohres.'

Morphologisches Jharbuch, iii. 1877.
Onodi, A. D. : ' Ueber die Entwickelung des sympathischen Nervens}rstems.'

Archiv fiir mikroskopische Anatomie, xxvi. 1885.
Pander, C. : ' Beitriige zur Entwicklungsgeschichte des Hiihnchens im Eie.'

Wiirzburg. 1817.
Parker, W. K. : ' On the Structure and Development of the Skull of the

Common Fowl.' Philosophical Transactions. 1869.

• On the Structure and Development of the Bird's Skull.' Trans-
actions of the Linnean Society, series 2, Zoology, vol. i. 1875.

' On the Structure and Development of the Wing in the Common
Fowl.' Philosophical Transactions, 1888.

' On the Morphology of the Gallinacere.' Transactions of the
Linnean Society, series 2, Zoology, vol. v. 1 891.

• On the Vertebral Chain of Birds.' Proceedings of the Royal
Society, vol. xliii. 1888.

Rathke, H. : ' Abhandlungen zur Bildung und Entwicklungsgeschichte des

Menschen und der Thiere.' Leipzig, 1833.
Ravn, E. : ' Ueber die mesoclermfreie Stelle in der Keimscheibe des Hiihner-

embryos.' Archiv fiir Anatomie und Entwickelungsgeschichte. 1886.
Remak, R. : ' Untersuchungen iiber die Entwickelung der Wirbelthiere.'

Berlin, 1855.
Sedgwick, A. : ' Development of the Kidney in its Relation to the'Wolffian

Body in the Chick.' Quarterly Journal of Microscopical Science, xx.

1880.

' On the Development of the Structure known as the Glomerulus of

the Head-kidney in the Chick.' Quarterly Journal of Microscopical

I Science, xx. 1880.
' On the Early Development of the Anterior Part of the Wolffian
Duct and Body in the Chick, together with some Remarks on the
Excretory System of the Vertebrata.' Quarterly Journal of Micro-
scopical Science, xxi. 1881.
on, R. : ' Die indifferente Anlage der Keimdriisen beim Hiihnchen und
z 2

340 THE CHICK.

ilirc Differenzirung zum Hodcn.' Jenaische Zeitsclirift f tir Naturwissen-

schaft, Band xxxi. 1887.
Shore, T. W. : ' Notes on the Origin of the Liver.' Journal of Anatomy and

Physiology, vol. xxv. 1891.
Shore, T. W., and Pickering, J. W. : ' The Proamnion and Amnion in the

Chick.' Journal of Anatomy and Physiology, vol. xxiii. 1889.
Studer, T. : ' Beitriige zur Entwicklungsgeschichte d. Feder.' Zeitsclirift fur

wissenschaftliche Zoologie,' xxx. 1878.
Uskow, N. : ' Die Blutgefiisskeime undderen Entwickelung bei einem Hiihner-

embryo.' Memoires de l'Academie Imperiale des Sciences de St.

Petersbourg, serie 7, tome xxxv. 1887.
Vialleton, L. : * D6veloppement des Aortes chez l'Embryon du Poulet.' Journal

de l'Anatomie et de la Physiologie, xxviii. 1892.
Wijhe, J. W. van : ' Ueber Somiten und Nerven im Kopfe von Vogel- und

Reptilien-embryonen.' Zoologischer Anzeiger, ix. 1886.
Wolff, C. F. : 'Theoria Generationis.' Halle. 1759.

4 De Formatione Intestinorum.' Halle. 1768-G9.

341

Chapter V.
THE DEVELOPMENT OF THE RABBIT.

PRELIMINARY ACCOUNT.

The rabbit may conveniently be taken as a typical member of
the Mammalia, the highest group of Vertebrates.

Mammals differ notably from the animals dealt with in the
previous chapter in being viviparous; that is, in bringing forth
their young alive, instead of laying eggs. This difference, though
at first sight a striking one, is really of but secondary importance ;
and the actual processes of development of the Mammal are
effected in the same way as in the oviparous Vertebrates.

Amphioxus and the frog lay their eggs in water, and the eggs
are not even fertilised until they have left the body of the mother.
In the Bird the eggs are fertilised as they leave the ovary and
enter the oviduct, and the earliest stages of development are
effected during the passage of the egg down the oviduct ; the egg
at the time of laying having already been developing for eighteen
hours or more. However, although development, in the case of
the bird, commences while the egg is still within the parent, it
is only the earliest phases — segmentation of the egg and the
establishment of the germinal layers — that are effected in this
position ; the formation and development of the embryo itself not
commencing until after the egg has been laid.

In the Mammal, on the other hand, the ovum is retained
within the oviduct and uterus of the mother for a very much
longer time ; and the whole embryonic development is completed
before leaving the parent ; the young Mammal at the time of
birth being fully formed, and of considerable size, though not
yet fit for independent existence.

The rabbit embryo, like that of other animals, is developed
from an ovum or egg, which, as in other animals, is a single

342 THE RABBIT.

nucleated cell, of epithelial origin. This ovum, after being
fertilised by a spermatozoon from the male, undergoes cell
division or segmentation, which, as in the bird, is effected during
the passage of the ovum along the oviduct. On reaching the
uterus, which is simply the distal, enlarged end of the oviduct,
the ovum halts, becomes fixed to the wall of the uterus, and there
remains during all the further stages of development, up to the
time of birth.

The actual details of development of the rabbit embryo, the
mode of formation of its various organs, and even the propor-
tions of the several parts and the order of their appearance, are
essentially the same as in the chick. The most noteworthy
point of difference is the exceeding slowness with which the
earlier stages are passed through in the rabbit.

The entire period of development of the rabbit, from the
discharge of the ovum from the ovary, to the birth of the
young animal, occupies thirty days. Of this time the first three
days are spent by the egg in passing down the oviduct, and in
undergoing segmentation ; and it is not until the seventh day
that the first trace of the embryo appears ; the rabbit's ovum
at the end of the seventh day being in a condition closely
corresponding to that of the chick about the end of the first day
of incubation (cf. Figs. 107 and 143). From this time, develop-
ment proceeds much more rapidly, at about the same rate as
in the chick, a twelve-day rabbit embryo (Fig. 161) correspond-
ing closely in structure, and in actual size, with a five-day chick
embryo (Fig. 115).

The eggs of the three animals described in the previous
chapters differ very greatly in size. That of Amphioxus has a
diameter of only Ol mm. ; the frog's egg measures about
1-75 mm. ; and the yolk, or true ovum, of the hen's egg about
30 mm. ; i.e. in round numbers the frog's egg is 5,000 times,
and the hen's egg 27,000,000 times the bulk of that of Amphi-
oxus. It has been already shown that the dimensions of the
egg are chiefly governed by the amount of food-yolk which it
contains, and that the size of the young animal on leaving the
egg is directly dependent on the amount of this food material ;
the tadpole on hatching being far larger than the Amphioxus
larva, while the chick on leaving the egg vastly exceeds the
tadpole in bulk.

PRELIMINARY ACCOUNT. 343

From the large size of the young rabbit at birth it might be
inferred that the rabbit's ova or eggs were large. This, however,
is not the case ; the egg of the rabbit is really very small indeed,
and, at the time it leaves the ovary, measures only 0*116 mm. in
diameter, i.e. is practically the same size as the egg of Amphi-
oxus, and only a fifteenth the diameter of the frog's egg.

It is clear that this very small egg could not develop into an
animal the size of a rabbit at birth unless it received a very
plentiful supply of nutriment from without ; and inasmuch as
the whole increase in bulk takes place while the embryo is
within the uterus, it follows that there must be some arrange-
ment in the uterus for supplying it with food.

This is effected by means of a special organ known as the
placenta, in which blood-vessels, derived from the embryo and
from the mother respectively, lie side by side in very close and
extensive contact with one another. The two sets of blood-vessels,
foetal and maternal, are distinct, but interchange of fluid and
gaseous contents takes place readily, by diffusion through their
thin walls, and in this way the blood of the embryo receives
nutrient matter from the blood of the mother during the whole
period of gestation.

The placenta is formed practically by the allantois, which
develops as an outgrowth from the alimentary canal of the
embryo, and which at an early period becomes closely attached
to the wall of the uterus of the mother. The blood-vessels of
the allantois are, as in the chick, directly continuous with those
of the embryo ; and, by the outgrowth of vascular processes
of the allantois into the walls of the uterus, the vessels of the
embryo are brought into intimate relation with the large,
dilated, and very thin-walled blood-vessels of the uterus itself.

Although the eggs of the rabbit, like those of nearly all
other Mammals, are very small indeed, yet in the mode of their
development they agree in several respects with the large eggs
of the bird, rather than with the smaller ones of the frog, or of
Amphioxus. A large yolk-sac, for instance, is formed from the
rabbit's ovum, although it contains no food material whatever ;
the embryo appears in the middle of the blastoderm, and there
is a well-marked primitive streak present ; while the formation
of the amnion and allantois are further points of resemblance
with the chick, and of difference from the frog or Amphioxus.

344 THE RABBIT.

These and other features in the development of Maim nals, which
will be described more fully later on in this chapter, are best
explained by supposing that existing Mammals are descended
from forms in which a greater amount of yolk was present, and
in which the eggs were consequently of larger size ; and that,
in accordance with the Recapitulation Theory, existing Mammals
have consequently an inherited tendency to develop after the
manner of forms with large eggs. The lowest group of
Mammals now living, the Monotremes of the Australian region,
afford strong evidence of the truth of this view, as, unlike other
Mammals, they are oviparous, laying eggs about the size of
olives, and closely resembling the eggs of many Reptiles.

The young rabbit at birth has not yet completed its develop-
ment. Its eyes are closed, like a kitten's ; and it is quite
incapable of obtaining food for itself, being dependent for
a time on the supply of milk afforded it by the mammary
glands of the mother. Using the word gestation for the whole
period of development, from the first appearance of the embryo
to the time when the young animal becomes capable of indepen-
dent existence, two stages may be distinguished in it: — (i)
uterine or placental gestation, which comprises the period prior
to birth, during which the embryo is nourished by osmotic
interchanges between its blood and that of the mother; and
(ii) mammary gestation, which embraces the period after birth,
during which the young animal is nourished by the milk yielded
by the mother.

The relative lengths of these two periods vary greatly in
different Mammals. In the Marsupials, a lowly organised group,
the uterine gestation is very short, the young animal, as in
the kangaroo or opossum, being born at an early stage, of small
size, and very imperfectly developed ; while in the higher Mam-
mals the period of uterine gestation is a much longer one, and
the young animals at birth are of larger size, and more com-
pletely formed.

THE EGG.
1 . Formation of the Egg.

To study the mode of formation of the ova in the rabbit it
is necessary, as in the chick and frog, to take, not adult animals
nor even new-born young, but embryos at an early stage of
development.

THE EGG. 845

In a rabbit embryo of about the eleventh day, a pair of
ridge-like thickenings of the peritoneal epithelium appear, close
to the mesentery, and along the inner sides of the Wolffian
bodies. These genital ridges (Fig. 165, gr) constitute the earliest
stage in the development of the ovaries, and are at first caused
merely by an alteration in the shape of the epithelial cells of the
peritoneum, which, elsewhere flattened, become here columnar.

The genital ridges soon become more prominent, partly
through an increase in the thickness of the epithelium, which
becomes two or three cells deep ; and partly through the in-
growth into each ridge of an axial core of connective tissue.

The genital ridge (Fig. 165) lies very close to the Wolffian
body ; and, about the fourteenth day, solid columns of cells grow
out from the Malpighian bodies of the anterior end of the
Wolffian body into the ridge. These columns of cells form
what is called the tubuliferous tissue of the ovary ; in the early
stages they occupy almost the whole of the axial part of the
genital ridge, but as development proceeds they gradually
become less conspicuous, and withdraw more or less completely
from the ridge. They have nothing whatever to do with the
formation of the ova.

In a rabbit embryo of the eighteenth day the genital ridges
have grown considerably, and project into the body cavity as a
pair of longitudinal folds, close to the attachment of the
mesentery to the dorsal body wall. Each ridge is covered by
the germinal epithelium, which consists of two or three layers of
cells, the outermost of which are columnar in shape, while the
more deeply placed ones are more or less spherical. The central
part, or core, of the genital ridge is made up almost entirely of
the tubuliferous tissue, with numerous blood-vessels and a small
amount of connective tissue.

By the twenty-second day the germinal epithelium has
increased considerably in thickness ; and among the cells of
which it consists are some of larger size than the rest, these
larger cells being the primitive ova. By the twenty-eighth day,
i.e. about two days before birth, the genital ridges are still
larger ; the germinal epithelium covering their surface is much
thicker than before, and its deeper layers are honeycombed, or
broken up into irregular columns, by ingrowth of the vascular
connective tissue from below.

846 THE BABBIT.

A thin layer of connective tissue, the tunica albuginea,
extends parallel to the surface of the ovary, and divides the
germinal epithelium into two almost completely separate layers ;
a thin outer layer of columnar cells, investing the outer surface
of the ovary ; and a much thicker, deeper layer of spherical cells,
broken up into nests by the connective tissue partitions. In the
germinal epithelium, and more especially in its outer layer, the
primitive ova are conspicuous, as individual epithelial cells,
much larger than their neighbours, and usually spherical or
polygonal in shape, with large granular nuclei. The tubuliferous
tissue is still present along the axis of the ovary, but now
occupies a relatively much smaller space than before.

The permanent ova. Four or five days after birth of the
young rabbit, the germinal epithelium undergoes further
changes, marking the establishment of sexuality, and the con-
version of the genital ridge of the embryo into the definite
ovary.

The changes consist essentially in the formation of per-
manent ova from the primitive ova, and occur in the rabbit in
much the same way as in the chick or tadpole. The nucleus,
or germinal vesicle, increases in size and becomes vesicular,
acquiring a very distinct nuclear membrane ; the nuclear con-
tents collect to one spot, where they form a granular mass,
from which, by branching, a definite reticulum is established ;
and, finally, one or more of the nodal points of the reticulum
enlarge, to form the nucleoli or germinal spots.

The other cells of the germinal epithelium have small nuclei^
and soon arrange themselves in a more or less definite manner,
so as to form follicles surrounding the permanent ova (Fig.
133, ga).

At first (Fig. 133, oz), each nest of epithelial cells may con-
tain several permanent ova, but as development proceeds, and as
the follicle cells become more definitely arranged around the
ova, the nests are broken up by further ingrowth of the vascular
connective tissue, and the separate follicles isolated from one
another.

Although it appears to be the rule in the rabbit's ovary that
each primitive ovum should become a permanent ovum, yet this
is by no means always the case. Sometimes two or more primitive

THE PERMANENT OVA.

347

ova fuse together to form a bi-nuclear or poly-nuclear mass, in
which all the nuclei but one may disappear, the fused mass
with the remaining nucleus becoming a permanent ovum ; while
in other cases it is stated that after a follicle has been formed
round the fused mass, the entire mass, with the follicle, may
divide into two or more permanent ova.

The development of the permanent ova proceeds from the
surface of the ovary towards its deeper parts. In young

Fig. 133.— Section through part of the ovary of an adult Rabbit. The section
is taken vertical to the surface of the ovary, and shows one fully formed
Graafian follicle, and others in various stages of development, x 50.

GrA, follicle cells surrounding an ovum. GrB, outer layer of Graafian follicle, or
• tunica granulosa.' GC, inner layer of Graafian follicle, or ' discus proligerus.' GK,
cavity of Graafian follicle. OE, outer layer of columnar epithelial cells, investing the
ovary. OW. ovum. OY, primitive ovum. OZ, nests of epithelial cells derived from
the deeper layers of the genital epithelium.

rabbits, about a week after birth, the surface epithelium of the
ovary contains numerous primitive ova in process of formation ;
deeper down, beneath the tunica albuginea, are nests of epi-
thelial cells in which are permanent ova in the early stages of
their formation, surrounded by imperfectly formed follicles ;
while still lower, in the deepest parts of the germinal epithelium,
the nests are in many places broken up into isolated follicles,
each containing a large and well-formed permanent ovum.

348 THE RABBIT.

2. The Graafian Follicles.

Each follicle consists, at first, of a single layer of cells
surrounding an ovum, the cells being derived directly from
the germinal epithelium, and being therefore morphologically
comparable with the ova themselves. Very shortly, each follicle
becomes two-layered (Fig. 133, ga), the second layer appeaiin-j
within the first one, between this and the ovum. As to the
origin of this second layer of follicular cells opinions differ ; it
is generally held to be formed by division of the cells of the
originally single layer ; but some investigators maintain that it
is derived from the ovum itself.

Immediately around the ovum, between it and the inner
layer of follicle cells, a thick non-cellular layer, with faint radial
striations, the zona radiata, is formed, apparently as a cuticular
secretion from either the ovum or the follicle cells. Yolk-
granules, elaborated by the follicle cells, accumulate within the
ovum, which consequently increases in size.

The follicle cells increase rapidly by cell-division, so that the
follicle soon becomes several cells thick. The outer layer of
cells now grows much more rapidly than the inner layer, so that
a space, somewhat crescentic in section, and filled with fluid,
appears between the two layers (Fig. 133).

By further growth of the two layers, the fully-formed
Graafian follicle (Fig. 133, gk) is established. This consists of,
(i) an outer layer of follicular cells, GB, arranged three or four
cells deep, and invested by the vascular connective tissue of the
ovary ; and (ii) an inner layer of similar cells, GC, which closely
invests the ovum, ow. This inner layer is attached to the inner
surface of the follicle, with which it is also connected by
irregular radiating strands of follicular cells, well shown in
Fig. 133. The cavity of the Graafian follicle is filled with fluid.

The riper egg-follicles lie at first, as noticed above, in the
deeper parts of the ovary ; but as the Graafian follicles enlarge
they gradually extend nearer and nearer to the surface, and
when fully formed cause rounded projections on the surface of
the ovary (Fig. 133). This enlargement of the Graafian follicles,
with accompanying ripening of the ova, occurs successively in
different parts of the ovary, so that there are never more than a
limited number, half a dozen or so, of ripe Graafian follicles at
any one time in a given ovary.

THE GRAAFIAN FOLLICLES. 349

Having reached its full size, the Graafian follicle ruptures at
its most prominent part, and the ovum is discharged on the
surface of the ovary, from which it is normally taken up at once
by the fimbriated mouth of the oviduct. After discharge of the
ovum, the walls of the Graafian follicle undergo a series of curious
changes, which will be more fully described in the chapter deal-
ing with the human embryo, and which result in the formation
of the corpus luteum, a body which disappears early if the ovum
is not fertilised ; but which, if the ovum is fertilised and develops
into an embryo, persists in the ovary during the whole period of
development, and is even recognisable at the time of birth of the
young rabbit.

Of the two layers of the Graafian follicle, the outer (Fig. 133,
gb) is sometimes spoken of as the tunica granulosa ; and the
inner, GC, as the discus proligerus ; these names, however, and
especially the latter one, are badly chosen, and it will be well if
they drop out of use altogether.

The ovum, surrounded by the inner layer of the Graafian

i follicle, may be attached to any part of the outer layer of the

! follicle : it not uncommonly lies at the side nearest the surface

of the ovary, but it may occur at the opposite or deepest part of

the follicle, or at any other part of its inner surface.

The meaning of the Graafian follicle has been much debated :
the most probable explanation seems to be that it is in some
way associated with the great diminution in size which there is
strong reason for thinking that the ovum has undergone during
the evolution of the existing types of Mammals.

3. Maturation of the Egg.

The nucleus of the ovum is at first centrally placed : but
some time before the Graafian follicle reaches its full development,
the nucleus moves towards the surface of the ovum. The exact
changes that then occur have not been determined with certainty
in the case of the rabbit : so far as they are known, they agree
closely with those already described in the case of the frog.

A thin, homogeneous vitelline membrane is formed within
the zona radiata, and apparently from the egg itself: the nucleus
of the egg becomes inconspicuous ; the yolk retracts slightly
from the vitelline membrane, and the first polar body is extruded
from the egg.

350

THE RABBIT.

At this stage the egg is liberated, by rupture of the Graafian
follicle, and is taken up by the mouth of the oviduct. 11 is
invested by the thin vitelline membrane, outside which is the
much thicker zona radiata. More or fewer of the cells of the

Fig. 134.

Fig. 135.

Fig. 134. — A fully formed ovum of a Rabbit, shortly before its discharge from

the ovary. (After Bischoff .) x 200.

Fig. 135. — A Rabbit's ovum, from the upper end of the oviduct, after extrusion

of the two polar bodies. (After Bischoff.) x 200.

MO, spermatozoon. N", nucleus or germinal vesicle. NIT, nucleolus or germinal
spot. PB, polar bodies. Z, zona radiata.

inner layer of the follicle usually remain adhering to the zona
radiata.

After entering the oviduct, but before fertilisation is effected,
a second polar body, apparently not more than half the size of
the first one, is extruded from the egg (Fig. 135, pb).

4. Ovulation.

Throughout the warmer part of the year, there is a periodi-
cally recurring ripening and discharge of ova from the ovaries of
the doe rabbit. From April to July this periodic discharge,
which is spoken of as ovulation, occurs regularly, and at monthly
intervals : after July it usually takes place with less regularity.

The total period occupied in the development of the young
rabbit, from fertilisation of the egg to the time of birth, is thirty
days : that is to say, the total period of development is in the
rabbit of the same length as the interval between two succes-
sive acts of ovulation.

The ovary of the doe rabbit, at the time she gives birth to
young, usually contains fully formed Graafian follicles, with ripe
ova ready for discharge. As a rule the doe is impregnated by

OVULATION AND FERTILISATION. 351

the buck immediately after giving birth to young ; and at a
period, estimated by different observers at from eight to twelve
hours after impregnation, the ova are liberated from the ripe
follicles.

At each period of ovulation, from three to nine ova are as a
rule discharged from each ovary ; the several ova being set free,
not absolutely at the same moment, but within a veiy short
time of one another.

Although ovulation, or the discharge of ova from the ovaries,
usually occurs a few hours after impregnation, and is probably
stimulated by this, it should be regarded as an essentially inde-
pendent act, a point of view that will be more fully considered
in the next chapter.

5. Fertilisation.

Fertilisation appears to occur in the rabbit, as a rule, from
eight to twelve hours after copulation , the interval being due,
not to the time taken by the spermatozoa to travel up the uterus
and oviduct, for this is effected, according to Hensen, in from a
quarter of an hour to two hours ; but to the fact that the dis-
charge of ova from the ovary does not take place until eight to
twelve hours after copulation.

The act of fertilisation is effected, as a rule, directly after the
eggs enter the oviduct : and, in eggs taken from the upper part
of the oviduct, spermatozoa may be seen in considerable numbers
imbedded in the zona radiata, or lying in the space between the
vitelline membrane and the egg, formed by the shrinking of the
latter.

The details of the process of fertilisation have not been accu-
rately determined in the rabbit. Fusion of a spermatozoon with
the female pronucleus has been seen by Van Beneden ; and there
is no reason for supposing the process to differ from what is
known to occur in other animals.

THE EARLY STAGES OF DEVELOPMENT.

It will be convenient to deal, in the present section, with the
mges undergone by the egg up to the end of the seventh day.
Ang the first three days the egg is travelling down the ovi-
duct, and passing through the stages of segmentation ; at the end

352 THE RABBIT.

of the third day it passes into the uterus, and undergoes further
changes, consisting chiefly in the establishment of the germinal
layers, and in preparations for the attachment of the ovum to
the uterus. Up to the end of the seventh day the ovum lies
freely in the uterus, and there is no trace of the embryo,
which does not commence to form until the early part of the
eighth day.

In estimating the age of rabbit ova, or embryos, it is custo-
mary to date from the time of copulation, which can always be
determined with precision ; and this method of computation will
be adopted here. As the eggs are not discharged from the ovary,
or fertilised, until from eight to twelve hours after this event, the
actual time during which developmental changes have been pro-
ceeding will be less than the stated periods by this amount.

1. Segmentation of the Egg.

Segmentation is effected while the egg is travelling down
the oviduct towards the uterus. During its passage it becomes
surrounded by a thick layer of albumen, formed of concentric
layers secreted by the walls of the oviduct. The egg itself, on
entering the uterus at the close of segmentation, is practically
the same size as the unfertilised egg, in • reality slightly
smaller than this ; but owing to the layer of albumen, which
may be thicker than the diameter of the egg itself, it appears on
a superficial examination to have increased considerably in size.

Segmentation commences, according to Van Beneden, some
ten or twelve hours after fertilisation is effected, i.e. from
eighteen to twenty-four hours after copulation, and is con-
tinued during the next two days. About the seventieth hour,
or the end of the third day from the time of copulation, segmen-
tation is completed, and the ovum enters the uterus.

In segmentation, the first cleft (Fig. 136) divides the egg into
two ovoid cells, which are nearly, but according to Van Beneden
not absolutely, equal in size.

After a pause of about four hours, each of these cells again
divides into two, giving in all four cells, which from the first are
approximately spherical in shape. Each of these four then
divides, giving eight in all, of which the four derived from the
smaller of the first two cells are slightly smaller than the other
four.

SEGMENTATION OF THE EGG.

353

The larger cells now become grouped together in the centre,
while the smaller cells form a cap lying on these, and partially in-
closing them. In the later stages, the smaller outer cells divide
rather more rapidly than the larger cells, and inclose these more
completely ; and at the close of segmentation, about the seventieth
hour, when the ovum passes from the oviduct into the uterus, it
consists of a central solid mass of rather larger and more granular

Fig. 136.

Fig. 137.

Fig. 136.— A Rabbit's Ovum from the middle of the length of t lie. oviduct,
about twenty-two hours after copulation, showing division of the ovum
into two cells. (After Bischoff.) x 200.

CB, blastomere or segmentation cell. MO, spermatozoon Imbedded in the zona
radiata. W, nucleus. Z, zona radiata.

Fig. 137. — A Babbit's Ovum from the lower end of the oviduct, about the
middle of the third day ; showing the morula stage, shortly before the
completion of segmentation. (After Bischoff.) x 200.

cells (Fig. 138, CD), almost completely surrounded by a layer of
rather smaller and more transparent cells, slightly flattened at
their outer ends (Fig. 138, cc) ; the larger cells being visible on
the surface at one spot only.

In the size of the eggs there is a close agreement between
the rabbit and Amphioxus ; the rabbit's ovum measuring on an
average 01 16 mm. in diameter, and that of Amphioxus measuring
0*104 mm. The two eggs agree also in undergoing complete
or holoblastic segmentation, and in the blastomeres, or cells
formed by segmentation, differing very little from one another
in size.

The comparison, however, must not be pushed too far. The
actual arrangement of the cells is entirely different in the two
cases : the rabbit's ovum does not pass through a gastrula stage
(cf. Figs. 15 and 16) ; and there is no stage in the development
of Amphioxus similar to that represented for the rabbit in Fig.
138.

A A

y

#

354

THE RABBIT.

In the spreading of the smaller cells over the larger ones, thej
rabbit and the frog appear to agree ; but the details of the pr< m

r

A"

Fig. 138.

Fig. 130.

Fig. 138. — A Eabbit's Ovum seventy hours after copulation, taken from the
lower end of the oviduct just before entering the uterus, and showing the
condition at the close of segmentation. (After Van Beneden.) x 200.

FlG. 139.— A Rabbit's Ovum seventy-five hours after copulation, taken from the
uterus, and showing the first stage in the formation of the blastodermic
vesicle. (After Van Beneden.) x 200.

CC, outer layer of cells. CD, inner mass of cells. CV, cavity of blastodermic vesicle.

have not been accurately determined in the rabbit's ovum, and
it is doubtful how far the correspondence is a real one.

2. The Blastodermic Vesicle.

On entering the uterus, at the end of the third day, the ovum
has the structure shown in Fig. 138 and described above. It is
spherical in shape, with a diameter averaging 0*09 mm., i.e. is
slightly smaller than the unfertilised egg. It is surrounded by
the vitelline membrane and zona radiata as before ; and outside
the latter are the concentric layers of albumen, which are de-
posited round the egg during its passage along the oviduct, and
which have a total thickness of about 0*1 mm.

From three to nine ova are usually discharged from the ovar}T
at each period of ovulation. These enter the uterus almost
simultaneously, and at first lie close together at its proximal
end. As development proceeds they gradually become spread out
along the uterus, at approximately equal intervals ; each ovum
lying in a special dilatation of the uterus, to the wall of which
it becomes attached during the eighth day.

Very shortly after the egg enters the uterus, and in some
cases before it leaves the oviduct, the smaller outer cells grow

THE BLASTODERMIC VESICLE.

355

completely round the larger inner cells, which from this time
they surround on all sides.

The outer layer of cells now begins to grow rapidly ; the
central or inner cells remaining attached to the outer layer at one
spot, but becoming separated from it at all other parts. By about
the seventy-fifth hour, i.e. four or five hours after entering the
uterus, the ovum has acquired the structure shown in Fig. 139 :
the outer layer of cells, CC, forms a hollow ball, about 0-12 mm.
in diameter, to the inner surface of which the mass of inner cells,

\y

Fig. 140. — Section of the blastodermic vessel of a Rabbit at the end of the
fourth day. (After Van Beneden.) x 250.

CC, outer layer of cell?. CD. inner lenticular mass of cells. CV, cavity of the

blastodermic vesicle.

CD, is attached at one spot, the rest of the cavity of the ball, cv,
between the outer and inner cells, being filled with fluid.

The growth of the ball, or blastodermic vesicle, as it is now
termed, proceeds rapidly ; and by the end of the fourth day, i.e.
about twenty-four hours after entering the uterus, the structure
and proportions are as represented in Fig. 140. The vesicle is
still spherical, measuring on an average about 0*28 mm. in
diameter. It consists of an outer wall of flattened polygonal

A A 2

356 THE BABBIT.

cells, CC, formed from the smaller, outer cells of the previous
stages, to the inner surface of which is attached at one pole
the mass of inner cells, CD. This mass of inner cells is now
flattened out into a lenticular shape ; thicker and more compact
in the middle, where the cells are two or three deep, and poly-
gonal from mutual pressure ; and thinning towards its margins,
where the cells are in a single layer, less closely apposed to one
another, and irregular or even amoeboid in form. The vitelline
membrane is no longer recognisable : the zona radiata is still
present, but, like the outer albuminous investment, is greatly
reduced in thickness.

During the fifth and sixth days, the blastodermic vesicle
remains spherical or nearly so in shape, and continues to in-
crease rapidly in size. By the end of the fifth day it measures
about 1:5 mm. in diameter; and by the end of the sixth day, 3
to 35 mm.

On the seventh day it becomes ellipsoidal in shape, and by
the end of this day (Fig. 143) it measures from 4"5 to 5 mm. in
length by 3*5 to 4 mm. in width. Up to the end of the seventh
day the several blastodermic vesicles lie quite freely within the
uterus, but become gradually spaced out along this, and take
up the positions they will retain during the remainder of their
development.

The measurements given above must be regarded as ap-
proximate only ; the several blastodermic vesicles in the same
uterus vary within certain limits, those lowest down being the
largest and most advanced in development ; and one or two at
the proximal end of the uterus, nearest to the oviduct, being
almost invariably smaller and less developed than the others.

3. The Germinal Layers.

During the fifth, sixth, and seventh days, important changes
occur in the structure of the wall of the blastodermic vesicle,
leading to the establishment of the three germinal layers,
epiblast, hypoblast, and mesoblast, from which the several parts
of the embryo are formed.

These changes more especially concern the part of the wall
of the vesicle to which the lenticular mass of inner cells (Fig.
140, CD) is attached ; and to this part, which at the end of the
fourth day is the only portion in which the wall of the vesicle is

THE TERMINAL LAYERS. 357

more than one cell thick, the name embryonal area may be
given, as it is from the central portion of this that the embryo is
developed.

The mode of formation of the germinal layers in the rabbit
has been very differently described by different observers, and
there are several points, even of primary importance, that are as
yet imperfectly understood. The following description is based
on the independent observations of Rauber and of Kolliker,
which appear to be the most exact ; but the account, though
consistent in itself, makes it very difficult to establish any com-
parison between the mode of formation of the germinal layers
in the rabbit and that occurring in other Vertebrates, or even in
other Mammals; and it seems not at all improbable that further
investigation may necessitate considerable modification in the
interpretation to be put upon the appearances described.

The great length of time that is occupied in the process, as
compared with the chick or frog, for example, is remarkable,
and may perhaps be taken as an indication that the actual
mode of development is a much modified one.

The fourth day. At the end of the fourth day, as already
described, the wall of the blastodermic vesicle is one cell thick,
except in the embryonal area, where cells of two kinds are
present (Fig. 140).

The fifth day. Daring the fifth day, the cells of the outer
layer become thinner and larger ; they also increase in number,
by division, as the blastodermic vesicle grows larger. In the
embryonal area the cells of the outer layer have the same
characters as in other parts of the vesicle.

The granular cells forming the inner layer of the embryonal
area, on the other hand, undergo important changes. They
increase in number by repeated division ; they become smaller
in size ; and they extend further round the interior of the
vesicle. But the most important change is that they become
arranged in two layers : — (i) an upper layer of cells with large
nuclei, rather wider than they are long, and closely fitted
together at their edges so that the outlines of the cells are
difficult to determine ; (ii) a lower layer of very thin flat pave-
ment cells, similar to those of the outer layer of the vesicle, but

358 THE RABBIT.

slightly smaller; this lower layer extends at its margin some
distance beyond the edge of the Upper or thicker layer.

Three regions may, therefore, be distinguished in the wall
of the blastodermic vesicle of the rabbit on the fifth day.

(i) The embryonal area is a circular patch about 048 mm.
in diameter, in which three layers of cells are present (Fig. 141) ;
an upper layer, CC, of thin pavement cells ; a middle layer, E,
of much larger, almost cubical cells; and a lower layer, H, of
thin pavement cells, very similar to those of the upper layer.
Each of these three layers is one cell thick ; and the three-
layered condition is brought about by the splitting of the inner
mass of cells of the fourth day (Fig. 140, CD) into two, which
become respectively the middle and lower layers of the fifth day.

(ii) Surrounding the embryonal area is a border, varying
in width in different specimens, in which the wall of the vesicle
consists of two layers, as seen at the margin of Fig. 141. These
two layers correspond to the upper and lower layers of the three
present in the embryonal area ; and the two-layered condition
is brought about by the lower or innermost layer, H, extending
beyond the margin of the embryonal area.

(iii) All the rest of the blastodermic vesicle, at this stage
about four-fifths of the whole periphery, consists of a single
layer of cells, the outermost layer, CC, of the embryonal area
(cf. Fig. 140).

With regard to the ultimate fate of these layers, it may be
mentioned at once that in the embryonal area, according to
Rauber and Kolliker, the uppermost layer of cells, CC, often
spoken of as Rauber' s layer, disappears altogether ; the middle
layer, E, becomes the epiblast ; and the lower layer, H, becomes
the hypoblast ; so that, according to these observers, both epi-
blast and hypoblast are formed from the inner mass of cells of
the fourth day.

This interpretation involves very considerable difficulties,
and will not improbably require revision. The disappearance of
Rauber's layer from the embryonal area, and its persistence as
the outer wall of the vesicle in other parts, together with the
derivation of both epiblast and hypoblast from the original inner
layer of cells, are difficult to reconcile with the course of develop-
ment in other Mammals; and further investigation is much
needed on these points.

THE GERMINAL LAYERS.

359

The sixth day. By the end of the sixth clay, the blastodermic
vesicle has a diameter of 3 to 3*5 mm., and the embryonal area,
which is still approximately circular in outline, measures 0*75
mm. across.

In the embryonal area the upper layer of cells, or Rauber's
layer, is thinner than before, and very difficult to recognise in

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»jtions. The middle layer of cells, or epiblast (cf. Pig. 141, e),
rather thicker than before, owing to a change in the shape of
the individual cells, which are now columnar in place of being
, cubical. The lower layer, or hypoblast, consists, as before, of a

860

THE KAI3B1T.

single layer of flattened pavement cells, thickened in their
centres by the nuclei, but very thin at their margins.

Beyond the embryonal area, the lower layer, or hypoblast, has
extended further round the vesicle than before, so that it now
lines about a third of the entire vesicle ; the wall of the remain-
ing two-thirds still consists of a single layer of flattened cells,
continuous with those forming Rauber's layer.

The seventh day. During the seventh day, and often before
the close of the sixth, the blastodermic vesicle loses its spherical

AG

Fig. 144.

Fig. 143.

Fig. 143. — The blastodermic vesicle of a Rabbit at the end of the seventh
day, seen from above. (Modified from Kolliker.) x 12.
AD. embryonal area. AG. wall of blastodermic vesicle. M. dot-ted line indicating
the boundary of the meaoblast, PS. primitive streak.

Fig. 144.— The embryonal area of a Rabbit at the middle of the eighth da v.
(Modified from Kolliker.) x 12.

NF, neural fold. "NG-. neural groove. PG. primitive groove, PS. primitive
streak.

shape, and becomes ellipsoidal (Fig. 143). The average dimen-
sions of the entire vesicle at the end of the seventh day are
from 4*5 to 5 mm. in length, by 3'5 to 4 mm. in width; but
individual specimens may considerably exceed these limits.

The embryonal area (Fig. 143, ad) is now distinctly pyriform
in outline, measuring on an average 1*5 mm. in length, by
1 mm. in width. Its longer diameter corresponds to the axis
of the blastodermic vesicle, and, as a rule, to that of the uterus
as well. From their relations to the embryo at a later stao-e.

THE GERMINAL LAYERS. 861

the broader end of the embryonal area may be called the

anterior end, and the narrower one the posterior end.

As regards the structure of the embryonal area. Rauber's
layer lias disappeared almost completely; a few individual cells
may still be recognised here and there, but there is no longer a
continuous stratum of cells. In consequence of the disap-
pearance of Rauber's layer (cf. Fig. 141), the embryonal area
now consists of only two layers of cells : — (i) the epiblast, or
former middle layer, which now becomes the superficial layer,
consists, as before, of a single layer of short columnar cells ; it
thins towards the margin of the embryonal area, and at its
margin is said to become continuous with the outer layer of
cells, or epiblast cells of the rest of the blastodermic vesicle :
(ii) the hypoblast, in the embryonal area, has the same characters
as before : beyond the embryonal area, it has now extended about
half way round the inner surface of the blastodermic vesicle.

The blastodermic vesicle at the end of the seventh day is.
therefore, an ellipsoidal sac filled with fluid. Its wall consists,
in the upper half of the vesicle, of two layers of cells, epiblast
and hypoblast : in the lower half, of a single layer, the epiblast
alone. In the middle of the upper half of the vesicle is the
embryonal area, which is also two-layered, but in which the
epiblast differs from that of the rest of the vesicle in consisting
of columnar instead of pavement cells.

1. The Primitive Streak and the Mesoblast.

Towards the close of the seventh day, the primitive streak
appears. This structure, which in the mode of its formation,
and in its relations to other parts, agrees closely with that of the
chick, is at first an axial thickening of the epiblast at the
posterior, or narrower, end of the embryonal area. It rapidly
lengthens, and by the end of the seventh day (Fig. 143. PS)
it extends, as a linear opacity, along about two-thirds of the
length of the area, having a faint longitudinal groove, the primi-
tive groove, along its dorsal surface.

Transverse sections at this stage (Fig. 142. rs) show that
the primitive streak is formed by proliferation of cells from the
der surface of the epiblast, in the median plane.

The mesoblast, The cells of the primitive streak spread
at. bevond the margins of the thickened streak itself, as two

3G2 tiik RABBIT,

thin lateral sheets of cells (Fig. 142, m), which lie between the
epiblast and hypoblast, and which give rise to the middle
germinal layer or mesoblast. In the primitive streak itself the
cells are spherical and closely compacted ; but in the lateral
mesoblast sheets the cells are more loosely arranged, and are
stellate in shape.

The layer of mesoblast spreads rapidly, both laterally and
posteriorly ; at the end of the seventh day, its limits are indicated
by the shaded area bounded by the dotted line, M, in Fig. 143,
a figure that may with advantage be compared with Fig. 107,
which shows the corresponding stage in the development of the
chick.

While it is certain that the mesoblast in the posterior part
of the embryonal area of the rabbit, i.e. in the region of the
primitive streak, arises in the manner just described, by pro-
liferation of cells of epiblastic origin, it is by no means clear
that the whole of the mesoblast is formed in this way ; and,
although further observations are wanted on the point, it seems
probable that in front of the primitive streak, in the part
of the embryonal area in which the embryo will appear, the
mesoblast arises, as in the chick, by budding off of cells from the
hypoblast.

GENERAL HISTORY OF THE DEVELOPMENT OF
THE RABBIT EMBRYO.

In the preceding section the development of the rabbit's
ovum has been followed up to the end of the seventh day, that
is, up to a point corresponding to that reached by a hen's egg
about the sixteenth hour of incubation. At this stage all three
germinal layers, epiblast, mesoblast, and hypoblast, are estab-
lished ; a primitive streak and primitive groove are present ; but
there is as yet no trace of the embryo itself.

It will be convenient to give, in the present section, a brief
summary of the mode of formation, and of the general course
of development of the embryo, before considering in detail the
history of the several systems and organs.

1 . The Formation of the Embryo.

The embryonal area of the blastodermic vesicle of the rabbit
at the end of the seventh day (Fig. 143, ad) corresponds very

GENEEAL HISTORY OF THE EMBRYO.

363

closely, as just noticed, with the area pellucida of a hen's egg
about the sixteenth hour of incubation.

The formation of the rabbit embryo is also effected in very
similar fashion to the chick. The embryonal area increases in
size, especially by growth at its anterior end. Immediately in
front of the primitive streak a neural groove (Fig. 141, xg) is

Fie 145. — A Kabbit Embryo at the end of the ninth day. The entire blasto-
dermic vesicle is represented, with the embryo in siiv, as seen from the
dorsal surface. (Cf. Fig. 146, which represents an embryo of the same age
in sagittal section.) x 10.

AW, proamnion. BM, mid-brain. E'. horse-shoe shaped patch of thickened epiblast,
by which the blastodermic vesicle is attached to the wall of the uterus (cf. Fig. 169).
MS, mesoblastic somite or protovertebra. It, right half of heart. SI, sinus terminalis.

TA, allantois.

formed, bordered by neural folds, NF, which speedily unite, con-
verting the groove into a tube. This tube becomes the central

$e\

eral

nervous system, and in its anterior or cerebral part the
brain vesicles are early established (Fig. 115).

By means of head, tail, and side folds the embryo is con

364

THE RABBIT.

stricted off from the rest of the blastodermic vesicle, in a manner
practically identical with that in which the embryo chick is con-
stricted off from the yolk-sac (cf. Figs. 145, 146, and 110, 112).

By the end of the ninth day the rabbit embryo (Figs. 145, 146)
has acquired shape, structure, and proportions agreeing very
closely with those of a chick embryo of about the twenty-sixth
hour, with which it also corresponds almost exactly in size.

Up to this stage the embryo has been practically straight

Fig. 146. — A median longitudinal, or sagittal, section through a Babbit Embryo
and blastodermic vesicle at the end of the ninth day. {Cf. Fig. 145.)
(In part after Van Beneden and Julin.) x 10.

AN. tail fold of amnion. AN', proamnion. BM, mid-brain. C. extra-embiyonio part
of tin- ccelomor body-cavity. CP, pericardial cavity. E, epiblast. E', thickened epiblast
by which the blastodermic vesicle is attached to the uterus {cf. Pig. 16D). EK, epiblastio
villi. GF. fore-gut. 6H, hind-gut. GT, mid-gut. H. hypoblast. M, mesoblasfc

SI, sinus terniinalis. TA, allantoic YS. cavity of yolk-sac, or blastodermic vesicle

(Fig. 140), lying with its dorsal surface upwards, towards the
wall of the uterus, and its ventral surface downwards towards
the yolk-sac. From this time, however, the dorsal surface of
the embryo grows more rapidly than the ventral surface, and the
whole embryo in consequence becomes strongly Hexed. Ity the
end of the tenth day (Fig. 147), while the middle portion of the

GENERAL HISTORY OF THE EMBRYO.

365

body, to which the yolk-stalk is attached, remains in the same
position as before, the head and neck, which have greatly in-
creased in size, are bent downwards at right angles to the trunk,
and, pushing down the wall of the yolk-sac before them, appear
to project into this latter : the head and neck are, however,
really separated from the cavity of the yolk-sac, as shown in
Fig. 147, by the wall of the sac itself. The hinder or tail end
of the embryo, the basal part of which is alone shown in

FlG. 147. — A Rabbit Embryo and blastodermic vesicle at the end of the tenth
day. The embryo is represented in surface view from the right side, the
course of the alimentary canal being indicated by the broad dotted line ;
the blastodermic vesicle is shown in median longitudinal, or sagittal section.
The greater part of the tail, which in the natural condition is twisted
spirally, has been removed. (In part after Van Beneden and Julin.) x 10.

AW, proamnion. AX, amnionic cavity, between the inner or true amnion and the
embryo. C, extra-embryonic part of the coelom or body-cavity. E, epiblast. E', thickened
epiblast, by which the blastodermic vesicle is attached to the uterus, and from which the
foetal part of the placenta is formed. EI, auditory vesicle. EK, cpiblastic villi. GF, fore-
gut. GH, hind-gut. G-T, mid-gut. H, hypoblast. OL, lens of eye. R, heart. SI, sinus
terminalis. TA, cavity of allantois. YS, cavity of yolk-sac or blastodermic vesicle.

Fig. 147, has also grown considerably, and is wrapped spirally
round the stalk of the allantois.

By the twelfth day the embryo has acquired the form shown

366

TI1K RABBIT.

in Fig. 148. The several divisions of the brain are clearly re-
cognisable, as are also the nose, and the eyes and ears. On the

Fig. 148. — A Rabbit Embryo and foetal appendages at the end of the twelfth
day. The embryo is represented in surface view from the right side ; the
yolk-sac and foetal membranes are shown in median longitudinal, or sagittal
section. The hind-limb and part of the tail have been removed to allow
the yolk-stalk and allantoic stalk to be fully seen. In part after Van
iieneden and Julin. x 8.

AX, amnionic cavity, between the inner or true amnion and the embryo. C, CX.

extra-embryonic part of the coelom or body-cavity. E, epiblast. E', ect'oplacenta, or
thickened part of the epiblast, from which the placenta is formed. EK, epiblastic
villi. H, hypoblast. M, mesoblast. SI, sinus terminalis. TA, cavity of allantois. YS,
cavitv of yolk-sac or blastodermic vesicle.

sides of the head and neck the visceral arches and clefts are
well seen ; and both fore and hind limbs have attained consider-
able size, and show indications of division into their several
segments.

The twelfth-day rabbit embryo corresponds closely in form
and in structure to a chick embryo of the middle of the sixth
day, and is of very nearly the same actual size. The chief points
of difference between the two are the much smaller size of the

GENERAL HISTORY OF THE EMBRYO.

367

brain and of the sense organs, more especially the eye, in the
rabbit.

The first trace of the neural groove appears in the chick
embryo about the eighteenth hour of incubation, and in the
rabbit embryo early on the eighth day. Starting from this
stage, the rate of development is approximately the same in the
two embryos ; the twelfth day rabbit embryo corresponding to
the chick embryo about the middle of
the sixth day.

By the twentieth day the rabbit
embryo has attained the shape and size
shown in Fig. 149 ; in grade of develop-
ment, and also in actual dimensions,
it corresponds very closely to a chick
embryo of the twelfth day .

The young rabbit is born on the
thirtieth day, i.e. about twenty-two
days from the time of first appearance
of the neural groove, the earliest formed
organ in the body. The chick is
hatched on the twenty-first day of
incubation, or rather more than twenty
days from the same starting-point.
The young rabbit at birth is of con-
siderably greater bulk than the chick
on hatching, but is in a far more helpless condition ; the eyelids
are still united together, and the young animal is quite incapable
of looking after itself, and would perish but for the supply of
milk afforded it by the mother.

Fig. 149.— A Rabbit Em-
bryo of the twentieth-
day, seen from the right
side. The rows of spots,
round the nose and
above the eye, and the
single large spot below
the eye, represent hair
follicles, the last-men-
tioned one being of
especial size, x 1.

2. The Yolk-sac.

The yolk-sac is the extra-embryonic portion of the blasto-
dermic vesicle ; i.e. the part which is left after the embryo is
constricted off by the head, tail, and side folds.

The yolk-sac (Figs. 146, 147, 148, ys), though corresponding
in its mode of formation, and in its relations to the embryo, with
the yolk-sac of the chick embryo (c/. Fig. 100), differs from this
latter in one very important respect. The yolk-sac of the bird
is filled with food matter for the nutrition of the embryo, and
affords the supply of nourishment at the expense of which the

1368 . THE RABBIT.

whole development is effected. The yolk-sac of tin- Mammal,
on the other hand, is a thin- walled vesicle, containing fluid, but
no food matter.

Hence the causes that led to the formation of a yolk-sac
in the bird, i.e. the necessity of constricting off the active
from the inactive part of the vgg in order to avoid undue
distortion of the embryo, will not come into play in the case of
the Mammal ; and the formation of a yolk-sac by the rabbit
embryo must be explained as due to an inherited tendency,
and compels us to infer that Mammals are descended from
-ancestors which produced large eggs, provided with much food-
yolk. Further evidence in support of this view has already been
given in the earlier portions of this chapter.

With regard to the structure of the yolk-sac of the rabbit
embryo, it will be seen in Fig. 146 that the wall of the upper
portion, rather less than half the entire surface, consists of all
three embryonic layers — epiblast, mesoblast, and hypoblast —
excepting only a small patch (Figs. 145, 146, an'), immediately
in front of the head of the embryo, which will be referred to
shortly. The wall of the lower half of the yolk-sac contains
no mesoblast, but is formed of epiblast and hypoblast alone.

In the mesoblast of the upper half of the yolk-sac, blood-
vessels are present, forming the vitelline circulation, or circula-
tion of the vascular area (Fig. 145). The margin of this vascular
area, or, what is the same thing, the margin of the mesoblast,
is indicated by a circular vessel, the sinus terminalis (Figs. 145,
146, 147, 148, si), into which the vitelline artery opens, and
from which the blood is distributed over the vascular area
before it is returned to the heart by the vitelline veins.

By the downward projection of the head of the embryo on
the tenth day (Fig. 147), the upper wall of the yolk-sac becomes
driven ventralwards, and during the succeeding days, as the
embryo gets bigger (Fig. 148), this doubling up of the yolk-sac
becomes more and more marked. By the thirteenth day the two
layers, vascular and non-vascular, are almost in contact with each
other, and the cavity of the yolk-sac is practically obliterated.

The outer or non-vascular wall, which is in contact with the
wall of the uterus (cf. Fig. 170, yl), now breaks down and be-
comes absorbed. Portions of it persist for a time ; but by
about the sixteenth day it has practically disappeared, and the

be

THE YOLK-SAC AND AMNION. 369

vascular, or original upper wall of the yolk-sac, comes into con-
tact with the wall of the uterus, the hypoblast of the yolk-sac
lying in contact with the uterine epithelium.

About the eighth day, irregular epiblastic buds (Figs. 146,
117. EK) arise from the surface of the lower, or non-vascular, half
of the yolk-sac. These acquire close attachment to the mucous
membrane of the uterus, and aid in fixing the blastodermic
vesicle in position, while it is possible that they have also some
nutritive function. They begin to degenerate about the ninth
day, and by the fourteenth or fifteenth day have disappeared.

3. The Amnion.

The amnion of the rabbit, while agreeing in most respects
with that of the chick, differs from this in the prominent share
taken by the tail-fold, which, as was first pointed out by Van
Beneden and Julin, practically forms the whole amnionic
covering of the embryo.

On the ninth day, as already mentioned, there is, immediately
in front of the head of the embryo, a patch of the blastoderm,
roughly circular in outline (Figs. 145, 146, an'), into which the
mesoblast does not yet extend, and which therefore consists of
epiblast and hypoblast alone. This patch is termed the pro-
amnion, and corresponds to the similarly named structure in the
chick.

The rapid growth of the head of the embryo forwards, and
then downwards, depresses the pro-amnion so as to form a deep
pocket, projecting into the yolk-sac. This is a well-marked
feature on the tenth and eleventh days (Fig. 147, an'), but
from the twelfth day onwards it becomes less obvious (Fig. 148),
owing to the general depression of the upper wall of the yolk-
sac which is then occurring.

The pro-amnion, as a special part of the wall of the yolk-
sac, has only a temporary existence. The mesoblast gradually
invades it from the sides, spreading inwards between the
epiblast and hypoblast, and, on the three-layered condition
ing definitely attained, the pro-amnion as such ceases to
xist.

The amnion itself is formed almost entirely by the tail-fold,
aided to a slight extent by the side-folds. The tail-fold of
the amnion (Fig. 146, an) is formed immediately behind the

B B

:-V70 THK BABBIT.

tail end of the embryo, partly by depression of the embryo Into
the yolk-sac, and partly by the actual uprising of a fold of the
somatopleure, or body wall.

After it is once started, the amnion grows rapidly, and by
the end of the tenth day has spread forwards so as to roof
over the whole body of the embryo. In front of the embryo
it meets and fuses with the somatopleure, at the anterior border
of the pro-amnionic pit (Fig. 147).

Apart from the prominent share taken by the tail-fold, the
formation and relations of the amnion are practically the same
in the rabbit as in the chick. As the amnion is a fold of
somatopleure (Figs. 146, 147), the space between its inner and
outer layers is necessarily continuous, as in the chick, with the
ccelom or body cavity of the embryo.

4. The Allan tois.

The allantois arises, on the ninth day (Fig. 146, ta), as a
hollow diverticulum from the ventral surface of the hinder end
of the alimentary canal, appearing almost like a posterior pro-
longation of the embryo itself. It consists, at first, of a thick
wall of mesoblast, in which the allantoic vessels develop very
early, and a lining of hypoblastic epithelium ; and from its first
appearance it is in very close relation with the amnion, or
actually continuous with this (Fig. 146).

As the amnion extends forwards, the allantois grows with
it, spreading rapidly between its two layers, and over the back
of the embryo (Fig. 147, 148, ta). Owing to its early fusion
with the outer layer of the amnion, the upper surface of the
allantois lies practically in contact with the wall of the uterus.

The cavity of the allantois is at first small, but from the
tenth to the twelfth days it enlarges very greatly (Figs. 117.
148, ta).

5. The Placenta.

The placenta is the organ through which the embryo receives,
from the blood of the mother, the nutriment by which it is enabled
to develop. It is a structure of great importance and great
complexity ; the mode of its formation will be dealt with fully
at the end of this chapter, but a brief outline may be given here,
in order to render its relations to the blood-vessels and other
organs of the embryo more intelligible.

THE ALLANTOIS AND PLACENTA. 371

Up to the seventh day the blastodermic vesicle lies quite
freely in the uterus (Fig. 168), but towards the end of the
seventh day it begins to acquire adhesions to the uterine wall.
These are effected partly by the small epiblastic villi of the lower
pole of the vesicle (Fig. 116, ek), but principally by the epiblast
cells of the vascular area : these latter proliferating freely over a
horse-shoe-shaped patch (Fig. 145, e'), which surrounds the sides
and hinder end of the embryonic region ; and growing out into
irregular tags and processes, which adhere firmly to the wall of
the uterus. By the ninth day (Figs. 1 15, 1 16, and 169), this
adhesion has become so firm, that, if the blastodermic vesicle is
pulled away from the uterus, the thickened epithelium over this
horse-shoe area is torn from the vesicle and remains attached to
the uterine wall.

By the tenth day (Fig. 1 17), the allantois has come into
extensive contact with the wall of the blastodermic vesicle
beneath this area of attachment ; and the blood-vessels of the
allantois are thus brought very close to the uterine vessels of
the mother. By a further series of changes, which will be
fully described later on in this chapter, the surface of contact
between the maternal and foatal blood-vessels is greatly in-
creased, and the highly complicated structure of the placenta is
gradually elaborated (cf. Fig. 170).

DEVELOPMENT OF THE NERVOUS SYSTEM.

1. General Account.

In the rabbit, as in the chick and other Vertebrates, the
nervous system is established very early. The neural groove
(Fig. 144, ng) appears, about half way through the eighth day,
as a shallow longitudinal depression, in front of the primitive
streak, and bordered laterally by the neural folds.

The neural groove rapidly increases, both in length and in
depth. By the end of the ninth day (Fig. 145) the lips of the
groove have met and fused along the greater part of their length,
though still remaining separate at both the anterior and pos-
terior ends. The distinction between the wider anterior part, or
brain, and the narrower posterior portion, or spinal cord, is very
evident ; and the vesicles of the fore-brain, mid-brain, BM, and
hind-brain are already well established.

372 THE RABBIT.

The general history of the development of the nervous system,
both central and peripheral, of the rabbit is closely similar to
that of the chick, and it will be only necessary to describe in the
present section the points of more special interest, and in part i-
cular those characteristic of Mammals as contrasted with Birds.

2. The Brain.

Cranial flexure commences towards the end of the ninth day
(Fig. 146), before closure of the neural canal is completed ; and
proceeds rapidly. By the tenth day (Fig. 147) the brain and
spinal cord are closed along their whole length; cranial flexure
is strongly pronounced ; and the head of the embryo, mainly
owing to the rapid growth of the brain, has acquired a shape, and
proportions, similar to those of a chick embryo towards the close
of the fourth day of incubation.

By the twelfth day (Fig. 161) the head has increased con-
siderably in size, and, when the embryo is viewed from the side,
appears to be bent twice at right angles ; the first bend being near
the junction of the brain and spinal cord, opposite the reference
line hy in Fig. 161 ; and the second bend being marked by the
mid-brain, BM, which forms the most prominent part of the brain
at this stao'e.

o

Sagittal sections of twelve-day embryos (Fig. 150) show
that flexure has really taken place to a far greater extent than
is apparent from the surface. Following the floor of the brain
from behind forwards, there is first, at the junction of spinal
cord and mid-brain, between the reference lines TH and CH in
Fig. 150, a rather sharp bend, ventralwards. of nearly 90° ; this is
corrected a little further forwards by a second and more gradual
bend dorsalwards, at the level of the reference line pt. At the
base of the mid-brain, bm, there is a very sharp and sudden
bend of about 180°, by which the infundibulum, in, and the
floor of the hind-brain are brought almost into contact with each
other. In front of the infundibulum the floor of the brain
again bends dorsalwards, and nearly at right angles.

These flexures are even more strongly marked in the later
stages of development, the angles formed by them becoming
sharper and more pronounced. This is well shown in Fig. 151,
which represents the condition of the brain on the eighteenth
day, as seen in sagittal section. The extreme sharpness of the

THE BRAIN.

373

Fig. 1 50. — A median longitudinal, or sagittal section through a Rabbit Embryo
at the end of the twelfth day. The section is a strictly median one except
in two respects : the cerebral hemisphere of the left side has been intro-
duced in order to render the figure more complete; and the Wolffian body
and ureter of the right side. The terminal portion of the tail has been
removed. (Compare Fig. 161 for a surface view of an embryo of the
same age.) x 14.

BF, cavity of fore-brain or thalamencephaloni BH, cavity of hind-brain, or fourth
ventricle. BIi, cerebellum. BM, cavity of mid-brain. BS, cavity of cerebral hemi-
sphere, or lateral ventricle. CH, notoohord. GP, post-anal gut. IN, Anger-like
process of mfundibulum. KC, Wolffian duct. KD, ureter. KM, Wolffian body.
LE, epiglottis. LG, lung. LR, trachea. PN, pineal body. PT, pituitary body.
PS, sinus venosua. RT, truneus arteriosus. RV, ventricle of heart. T, glottis.
TA. stalk of aUantois, cut short. TC, cloaca. TH, thyroid body. TO, oesophagus.
TP, pharynx. W, liver. WD, bile duct. YK. yolk-stalk, cut short.

874 THE RABBIT.

bend at the junction of the spinal cord and brain, between the
reference lines kp and 01 in Fig. 151, is very characteristic of
mammalian embryos at this stage; while the sharp bend at the
base of the mid-brain is quite as conspicuous as in the earlier-
stage.

The general relations of the brain to the head are much the
same in the rabbit embryo as in the chick. In the early stages
(Figs. 147, 148, and 150) the brain forms practically the whole of
the head, and determines its shape almost exclusively; but in
the later stages (Figs. 149, 151), as the parts of the face assume
definite shape, and grow forwards to form the snout, the brain
becomes thrown more and more on to the dorsal surface, and
ultimately to the posterior part of the head, and takes a much
less prominent share in determining the general contour.

In dealing with the several parts of the brain it will be con-
venient to commence with the thalamencephalon, and the struc-
tures developed from, or in connection with it.

The thalamencephalon (Fig. 150, ef) is the anterior cerebral
vesicle, or fore-brain, of the early stages (cf. Figs. 145, 116).
From it the optic vesicles arise as lateral outgrowths ; these
appear very early, and attain some size before the roof of the
fore-brain is closed (Fig. 1 15) ; their further development, and
their conversion into the essential parts of the eyes, will be
described in the next section of this chapter, p. 387.

The side walls of the thalamencephalon thicken very quickly,
to form the optic thalami (Fig. 155, bu) ; and, owing to this thick-
ening of its sides, the central cavity, or third ventricle, becomes
reduced to a vertical cleft, very narrow from side to side. The
anterior wall of the thalamencephalon remains thin, as the lamina
terminalis (Fig. 151, bt), lying between the roots of the two
cerebral hemispheres.

The roof of the thalamencephalon remains thin, consisting in
the greater part of its extent of a single layer of epithelial cells,
devoid of nervous elements of any kind. From this roof, rather
behind the middle of its length, the pineal body arises about the
twelfth day, as a hollow median papilla (Fig. 150, pn). This
rapidly lengthens, forming a tubular and backwardly directed
diverticulum of the brain. Its distal end dilates (Fig. 151, PN),
to form a slightly expanded vesicle, from the sides of which

THE BKAIN.

375

irregular hollow outgrowths arise : at a later stage these out-
growths become solid, and separate completely from the stalk.
In front of the pineal body the roof of the third ventricle,

Fig. 151.— A median longitudinal, or sagittal section through the head of a

Rabbit Embryo of the eighteenth day. (Compare Fig. 149 for a surface
view of an embryo at a slightly later stage.) x 10.

BF, cavity <>t" tftalamencephalon, or third ventricle. BH, cavity of medulla
oblongata, or fourth ventricle. BL. cerebellum. BH, cavity of mid-brain. BS. cavity
of cerebral hemisphere, or lateral ventricle. BT, lamina terminalia. BY, cavity of
olfactory lobe. OT, thyroid cartilage. ET, mesethmoid cartilage. P.l, first cervical
or atlas vertebra. F.2. second cervical or axis vertebra. IN, infundibulum. LB,
trachea. LT. glottis. MC. Meckel's cartilage. NS, central canal of spinal cord.
OB, buccal cavity. OI, odontoid process of axis vertebra. OX, supraoccipital
cartilage. PP, posterior nan'al chamber. PL, palate. PN, pineal body. PT,
pituitary body. BP. basilar plate. TINT, tongue. TO, oesophagus. TP, pharynx.
XA. choroid plexus of thinl ventricle. XB. choroid plexus of fourth ventricle.

which is here excessively thin, becomes thrown into folds, which
hang down into the cavity of the ventricle (Fig. 151, xa). Be-
tween these folds blood-vessels pass in, from the vascular meso-

376 THE BABBIT.

blast outside the brain, and give rise to the choroid plexus of
the third ventricle.

The floor of the thalainencephalon also remains thin, though
not so thin as the roof. The anterior part of the floor is crossed
by a shallow transverse groove, which is prolonged outwards
into the optic stalks (Figs. 150, 151, and 155). The posterior
part of the floor gives rise to the infundibuhim. This is a
median, thin- walled depression, from the hinder end of which a
hollow finger-like diverticulum arises on the tenth day (cf. Fig.
150, in); this diverticulum lies, from the first, in very close
relation with the anterior end of the notochord, and with the
pocket-like outgrowth from the stomatodamm (Fig. 150, it),
which gives rise to the pituitary body. This anterior end of the
notochord is ultimately absorbed and obliterated ; but the infun-
dibular diverticulum and the pituitary body remain in intimate
relation throughout life ; the diverticulum forming what is spoken
of, in the adult rabbit, as the posterior lobe of the pituitary body.

The further development of the pituitary body itself, which may
conveniently be dealt with here, is as follows. The stomatodreal
diverticulum (Fig. 150, pt) dilates at its blind end, and gives
off from this terminal dilatation hollow outgrowths ; these branch
freely (Fig. 151, pt), but in the later stages of development
become solid. The central dilated cavity of the pituitary body
persists; it retains its communication with the buccal cavity,
through the tubular stalk, for some time. The formation of the
palate (Fig. 151, pl) leaves the pituitary stalk in communica-
tion with the narial passage, but cuts it off from direct com-
munication with the buccal cavity. In the later stages, the
pituitary stalk loses its connection with the narial passage, and
becomes obliterated.

The cerebral hemispheres arise, as in the chick, in the first in-
stance as a median anterior prolongation of the thalamencephalon,
which may be termed the vesicle of the hemispheres. This soon
becomes divided by an inwardly projecting fold of its anterior
wall into right and left lobes, which by further growth become
the cerebral hemispheres ; the median anterior wall of the vesicle,
between the bases of the hemispheres, persisting as the lamina
terminalis.

From their mode of formation the hemispheres are necessarily

THE BRAIN,

377

hollow ; and their cavities, the lateral ventricles, retain through-
out life their communications with the third ventricle through
the foramina of Monro, a pair of apertures which are at first wide,
but which gradually become reduced, by thickening of their lips,
to narrow slits.

The cerebral hemispheres first become prominent about the
twelfth day (Fig. 150, BS) ; from this date they grow actively,

PlG. 152. — The brain dissected from above. Enlarged. (From Marshall

and Hurst.)

C, lateral lobe of cerebellum. CA, pillars of cerebellum. CP». cut edge of velum
medullas posterius. CC, corpus callosum : the right half is removed. CD, cut edge of
corpus callosum. CIP, floccular lobe of cerebellum. CS, corpus striatum. F, anterior
limit of body of fornix. H, hippocampus major. M, medulla oblongata. O, olfactory
Lobe. OP, anterior optic lobe. P, pineal body. "V, fourth ventricle.

extending forwards, and still more rapidly backwards, so as to
overlie and cover the roof of the thalamencephalon, and at a later
stage the mid-brain as well (cf. Figs. 151 and 152).

The wall of each hemisphere is at first thin on all sides, and
the cavity, or lateral ventricle, is consequently large (Fig. 151).
The inner wall remains thin, but the outer wall (Fig. 152)
thickens considerably in the later stages of development ; while

378

THE RABBIT.

the binder ends of the hemispheres thicken still more, to form
the corpora striata (Pig. 155, Bi), a pair of prominent swellings,
lying in front and to the outer sides of the optic thalami, B0,
and separated from these by well-marked grooves.

The hippocampus major (Fig. 152, h) is a prominent curved
ridge, projecting into the lateral ventricle, and extending round
into its descending cornu ; it is really an inwardly projecting

Fig. 163. — A longitudinal and vertical section of the brain of an adult Rabbit,
taken in the median plane. (From Marshall and Hurst.)

A* pituitary body. AC, anterior commissure. AF, anterior pillar of the fornix,
seen in the wall of the third ventricle. C, cerebellum. CA, corpus albicans. CC,
corpus callosum. CH, inner surface of left cerebral hemisphere. F, body of the fornix.
FM, foramen of Monro. &. velum interpositum. I, infundibulum.* MC, middle
commissure. N, anterior lobe of corpora quadrigemina, or 'nates.' O, olfactory lobe.
OC, optic chiasma. ON, left optic nerve. P, pineal body. PC, posterior commissure.
PV, pons Varolii. T, posterior lobe of corpora quadrigemina, or ' testis.' VA, velum
mednllffi antcrius, or valve of Vieussens. VP, velum inedullae posterius.

Ill, third ventricle. IV, fourth ventricle. V, fifth ventricle.

fold of the wall of the hemisphere, formed by a deep groove on
the surface of its inner wall.

The choroid plexus of the lateral ventricle (Fig. 155, x) is
a somewhat similar, but much thinner fold of the inner wall of
each hemisphere, between the two layers of which blood-vessels,
xd, pass in freely. It lies immediately below the hippocampal fold.

The commissural bands between the two hemispheres are
very characteristic structures in Mammals, in which they attain
a much higher development than in other Vertebrates. The
most important of these are the corpus callosum, the fornix, and

:

THE ERAIff. 879

the anterior, middle, and posterior commissures of the third
ventricle. Their development is complicated, and difficult to
follow.

In front of the lamina terminalis, the two hemispheres extend
forwards side by side, very close to each other (Fig. 152), but
separated by a median cleft in which lies the connective tissue
lamina from which the falx cerebri is formed. At the hinder
end of this cleft, just in front of the lamina terminalis, the walls
of the two hemispheres come in contact and fuse ; and from this
fused patch, which is somewhat triangular in shape as seen in
sagittal section, the commissural bands are formed. The corpus
callosum (Fig. 15-3. CC), the most important of them, is formed
from the dorsal part of the fused patch ; it develops from before
backwards, the anterior end being formed first. The fornix, in
which the fibres are mainly longitudinal in direction, is formed
from the ventral part of the patch; and the anterior commissure
from its hinder end, just in front of the lamina terminalis. It
is not quite clear whether the fifth ventricle (Fig. 153, \ ) is
formed by the breaking down of the central part of the fused
patch, or is merely a persistent part of the original cleft between
the two hemispheres.

The surfaces of the hemispheres are at first smooth, and even
in the adult rabbit are only slightly convoluted. The convolu-
tions arise as foldings or grooves of the surface, extending to a
greater or less depth, and are classed as primary or secondary
according to whether they are folds involving the whole thickness
of the wall of the hemisphere, or mere grooves in its substance.

The olfactory lobes appear, about the fourteenth day, as a
pair of hollow outgrowths from the ventral surface of the anterior
ends of the cerebral hemispheres ; by the eighteenth day (Fig.
151 , BY) they have become prominent structures.

The mid-Drain. In the early stages, up to about the twelfth
day (Fig. 150), the mid-brain is very imperfectly marked oft
from the fore-brain ; later on (Fig. 151), the boundary between
the two divisions becomes well defined.

As compared with the chick, the mid-1 train of the rabbit is
f rather smaller size, and less prominent : it is further dis-
tinguished by its tendency to grow backwards over the hind-
brain, a tendency already present on the twelfth day (Fig. 150),

380 THE BABBIT.

but much more pronounced in the later stages, the posterior
lobes of the mid-brain on the eighteenth day (Fig. 151 ) com-
pletely overlapping the cerebellum, BL.

The roof of the mid-brain gives rise to the corpora quadri-
gemina. A transverse furrow first appears, dividing it into a
larger anterior, and a smaller posterior portion ; the anterior
portion soon becomes divided, by a median longitudinal groove,
into the anterior lobes of the corpora quadrigemina, or nates ;
the posterior portion, overhanging the cerebellum, is not divided
until a much later stage.

The floor of the mid-brain forms the very sharp bend at the
base of the brain which has already been noticed (Figs. 150 and
151) ; as in the chick, it becomes greatly thickened on the
formation of the longitudinal pillars of nerve fibres known as
the crura cerebri (Fig. 154, cc), which connect the optic
thalami and corpora striata with the hind brain.

The cerebellum is developed from the roof of the anterior part
of the hind-brain, in much the same way as in the chick. On
the tenth day (Fig. 147) a slight thickening is formed across
the roof of the anterior end of the hind-brain ; by the twelfth
day this has become more conspicuous (Fig. 150, bl), but is
still only a slightly thickened transverse band. By the eigh-
teenth day (Fig. 151, bl) it has increased considerably in
thickness ; and, shortly after this stage, it becomes folded trans-
versely on itself, as in the chick (cf. Fig. 116, bl). Secondary
foldings appear on its surface, and the complicated structure of
the adult cerebellum is gradually acquired.

Of the several parts of the adult cerebellum (cf. Fig. 152),
the median lobe, or vermis, is the first to be formed ; the lateral
lobes and floccular lobes appearing at a much later date.

Immediately in front of the cerebellum, between it and the
mid-brain, the roof of the brain becomes extremely thin, forming
the velum medullaa anterius (Fig. 153, va), which is ultimately
reduced to a single layer of epithelial cells devoid of nervous
elements.

The medulla oblongata is formed from the floor of the
hind-brain, and from the part posterior to the cerebellum. The
floor of the medulla oblongata remains thin in the actual mid-

THE BKAIN AND SPINAL CORD. 381

ventral plane (Pig. 158) ; the lateral parts of the floor, and the
sides as well, thicken very considerably. The roof of the fourth
ventricle, as in the chick, widens out considerably (Fig. 158),
and at an early stage becomes exceedingly thin. Immediately
behind the cerebellum, the roof of the medulla remains com-
paratively smooth, as the velum medullae posterius (Fig. 153,
VP) ; but a short way further back it becomes thrown into a
coi i) plicated series of folds (Fig. 151, xb), which hang down into
the fourth ventricle, and between the layers of which the blood-
vessels penetrate in large numbers to form the choroid plexus
of the ventricle.

The walls of the medulla oblongata consist mainly of longi-
tudinal fibres. The pons Varolii (Fig. 153 and 154, PV), the
great transverse band of nerve fibres which connects the two
halves of the cerebellum together, develops very late ; its posi-
tion is indicated, about the eighteenth day, by the sharp rect-
angular bend in the floor of the medulla, opposite the cerebellum
(Fig. 151).

Before leaving the brain, it should be noted that, in spite of the
complicated foldings which various parts of it undergo, and the
extreme thinness to which its walls are reduced in places, notably
in the roof of the third, and in that of the fourth ventricle, the
cavity of the brain remains a closed one, and its walls are not
actually perforated at any place.

The membranes of the brain, i.e. the pia mater and dura
mater, are connective-tissue structures, of mesoblastic origin.

3. The Spinal Cord.

The development of the spinal cord of the rabbit need not
be described in detail, as in all essential respects it agrees with
that of the chick.

I. The Histological Development of the Brain and the Spinal
Cord.

This will be more fully dealt with in the chapter on Human
Embryology.

In the spinal cord, the changes undergone are essentially
similar to those in the chick. The original walls of the neural
canal give rise mainly to the grey nervous matter, the innermost

•382 THE BABBIT.

layer of cells forming the epithelial Lining of the central canal.
The white matter develops later ; in the spinal cord it appears
on the eleventh day, the ventral and lateral bands of white
matter being formed practically simultaneously. The central
canal of the spinal cord remains a narrow vertical cleft until
about the sixteenth day, when its dorsal part becomes obliterated,
as in the chick, preparatory to the formation of the dorsal
fissure.

In the medulla oblongata, the arrangement of white and
grey matter is essentially the same as in the spinal cord, the
white matter forming, about the eleventh day, on the outer surface
of the grey matter ; in the later stages the relations between
white and grey matter become much more complicated.

In the fore-brain the conditions are somewhat different.
The walls of the hemispheres, which at first are very thin,
become early differentiated into an outer layer of rounded
elements, which later on give rise to grey matter, and an inner
epithelial layer, which becomes the epithelial lining of the
ventricle. About the sixteenth or seventeenth day, bands of
white fibres grow upwards from the crura cerebri, through the
optic thalami and corpora striata, and make their way between
the two layers of the wall of the hemisphere ; while a little
later a very thin superficial layer of white matter forms on the
surface of the brain, outside the grey matter. In this way
the characteristic distribution of white and grey matter in the
hemispheres of the adult is brought about.

5. The Peripheral Nervous System.

The general history of the peripheral nervous system is the
same in the rabbit as in the chick ; but the earliest stages of
development have not yet been worked out in such detail. By
the ninth day both spinal and cranial nerves are established,
and by the eleventh day (Fig. 165) all the principal branches of
distribution are present.

The Cranial Nerves.

I. The olfactory or first cranial nerve. The time of first
appearance of the olfactory nerve in the rabbit has not been
definitely determined. The nerve is, however, clearly recog-
nisable, as a short stem, connecting the cerebral hemisphere
with the olfactory pit, before the olfactory lobe is formed.

THE CRANIAL NERVES.

383

II. The optic or second cranial nerve will be considered in
the section dealing with the development of the eye, p. 387.

Fig. 154. — The brain of an adult Kabbit from the ventral surface. The
greater part of the left temporal lobe has been sliced off horizontally.
The planes of the three semicircular canals of the left side are indicated
by the thick lines surrounding the rloccular lobe of the cerebellum.
(From Marshall and Hurst.) x 2.

CC, eras cerebri. CGr, corpus geniculatum. D, descending cornu of left lateral
ventricle. H, hippocampus major. LF, rloccular lobe of cerebellum. LL, lateral lobe
of cerebellum. OC, optic cluasma. OT, optic tract. P, pituitary body. PV. pons
Varolii. SA. anterior vertical semicircular canal. SH, horizontal" semicircular canal.
SP, posterior vertical semicircular canal. T, temporal lobe of cerebral hemisphere.

I, olfactory lobe, with roots of olfactory nerves. II, optic nerve. Ill, third nerve or
motor oculi. IV, fourth nerve. V, trigeminal nerve. VI, sixth nerve or abducens.
VII, facial nerve. VIII, auditory nerve. IX, glosso-pharyngeal nerve. X, pneunio-
gastric nerve. XI, spinal accessory nerve. XII, hypoglossal nerve.

Ill, IV, and VI. The third, fourth, and sixth cranial nerves.
There are no exact observations recorded on the development of

384 THE RABBIT.

these three nerves in the rabbit. From the time when they can
be clearly recognised, about the eleventh day, their course and
relations are the same as in the adult.

V. The trigeminal or fifth cranial nerve. This nerve can
be recognised on the ninth day, at a stage immediately after
closure of the brain has been effected. The nerve appears at
this stage as a pyriform ganglionic mass, the narrow end of
which is in close contact with the dorsal surface of the brain,
while the rest of the nerve lies close alongside the brain,
extending about half way down its side. By the tenth day the
roof of the hind-brain has widened very greatly, and the root of
the fifth nerve is now attached to the junction of the thin roof
and thickened side of the brain. Before the end of the tenth
day, the nerve branches distally ; and by the twelfth day the
ophthalmic, maxillary, and mandibular branches are of con-
siderable length, and have courses and relations very similar to
those of the same nerves in a five-day chick embryo (cf.
Fig. 115).

VII and VIII. The facial or seventh, and auditory or eighth
cranial nerves are, as in the chick, very intimately connected, or
actually continuous with each other, from their first appearance.
They can be recognised before the end of the ninth day ; and by
the tenth day the facial nerve has acquired its definite relation
to the hyoicl arch, as in a five-day chick embryo (Fig. 115).
The auditory nerve appears to be continuous with the epi-
thelium of the auditory vesicle from its earliest appearance.

IX. The glossopharyngeal, or ninth cranial nerve, as in the
chick, arises by multiple roots from the side of the medulla
oblongata, immediately behind the auditory vesicle. Its main
stem lies in the first branchial arch.

X. The pneumogastric, or vagus, or tenth cranial nerve has,
in the rabbit embryo of the twelfth day, almost exactly the
same course and relations as in a chick embryo of the fifth day
(Fig. 115). It arises by a considerable number of roots from
the side of the medulla oblongata, behind the roots of the glosso-
pharyngeal nerve ; the most anterior of the roots of the pneumo-
gastric nerve is, as in the chick, directly continuous with the
posterior root of the glosso-pharyngeal nerve ; while the hind-
most root of the pneumogastric is of great length, and runs
backwards along the side of the medulla oblongata to the gan-

THE CKANIAL AND SPINAL NERVES. 385

glion of the first spinal nerve, with which it is closely connected
(cf. Fig. 115). The roots of origin of the pneumogastric nerve
converge to form a single large trunk, which lies immediately
behind the trunk of the glosso-pharyngeal nerve ; it expands to
form a large oval ganglion, beyond which it divides into a small
branch to the second branchial arch, and a much larger visceral
branch which runs backwards to the heart, lungs, and stomach.

XI. The spinal accessory or eleventh cranial nerve. The
development of this nerve has not yet been determined in the
rabbit.

XII. The hypoglossal or twelfth cranial nerve. Exactly
opposite the roots of the pneumogastric nerve in rabbit embryos
of the twelfth day, but arising from the medulla oblongata at a
more ventral level, and nearer the median plane, is a second
set of nerve roots. These are quite as numerous as the more
dorsally placed pneumogastric roots, but are more slender ; they
converge, and unite to form the hypoglossal nerve. Anatomically,
the roots of the pneumogastric and hypoglossal nerves, at this
stage, have relations closely comparable to those between the
dorsal and ventral roots of a spinal nerve ; but it is not yet
clear whether this comparison has any morphological value. The
roots of the hypoglossal nerve are probably correctly regarded
as belonging to the same category as the ventral spinal roots,
but their relations to the roots of the pneumogastric must be
considered at present as much more doubtful.

The Spinal Nerves.

The earliest stages in the development of the spinal nerves
have not yet been described in the rabbit. The dorsal roots
and the ganglia are clearly established by the end of the ninth
day ; the ventral roots develop later, apparently during the
tenth day.

The ganglia of the dorsal roots are at first of very consider-
able width ; almost as wide, in fact, as the mesoblastic somites ;
so that on the twelfth day there are hardly any intervals between
the spinal nerves, the successive ganglia being practically in
contact with one another along the greater part of the length of
the spinal cord.

Beyond the ganglia the nerves narrow rapidly, and have the
normal proportions. The main divisions of the spinal nerves are

C C

386 THE RABBIT.

early established, and are all present by the eleventh day (Fig.
165). The nerve trunk, beyond the ganglion, divides at once into
a smaller dorsal branch, nn' ; and a larger ventral branch, NN ;
each branch containing nerve fibres from both the dorsal and
ventral roots of the nerve. The dorsal branch, nn', runs out-
wards and upwards, to the muscles and skin of the back ; while
the ventral or larger branch, NN, runs downwards in the body
wall, and in the somites opposite the limbs sends branches into
these latter.

6. The Sympathetic Nervous System.

This has received much attention, but several points con-
cerning its early origin still remain in doubt. Before the end
of the eleventh day it is already well established (Fig. 165, Ny),
consisting of a main ganglionated cord running along each side
of the body, close to the dorsal surface of the aorta, and receiving
branches from the ventral branches of the several spinal nerves
as it passes these. Dr. Paterson, from observations chiefly on
rat embryos, but partly on rabbits, concludes that the longi-
tudinal cord is the first part of the sympathetic system to be
developed, that it arises in the mesoblast entirely indepen-
dently of the spinal nerves, and is at first devoid of ganglia ; he
believes that the connection of the longitudinal cord with the
spinal nerves is a secondary one, and is effected by outgrowths
from the ventral branches of the spinal nerves after the longi-
tudinal cords are established. These observations are, however,
so entirely at variance with what is known as to the develop-
ment of the sympathetic nervous system in other Vertebrates, in
which the sympathetic develops merely as a specialised portion
of the spinal nervous system, that it seems preferable to suspend
judgment on the matter, pending renewed investigations.

The connections of the spinal nerves with the longitudinal
sympathetic cords are limited to the thoracic and lumbar
regions, but the cords themselves extend forwards along the
neck to the head, where they acquire connections with the
hinder cranial nerves.

7. The Supra-renal Bodies.

The supra-renal or adrenal bodies, which in the adult rabbit
form a pair of small, round, yellow bodies, a little way in front
of the kidneys, are developed from two distinct sources.

THE SENSE OKGANS. 387

The outer or cortical portion of the supra-renal body is
developed from a mass of mesoblast cells which appears about the
twelfth day, lying in front of the kidney, and between the aorta
and the root of the mesentery.

The medullary portion of the supra-renal body is formed from
a column of cells, which grows out from the longitudinal sym-
pathetic cord about the fifteenth day, and makes its way into the
mesoblastic mass which gives rise to the cortical layer. This
mode of development agrees with what is known as to the for-
mation of the supra- renal bodies in other Vertebrates, but leaves
the real morphological meaning of these curious structures still
undecided.

THE DEVELOPMENT OF THE SENSE ORGANS.

1 . The Nose.

There is very little of special interest about the olfactory
organ of the rabbit, which resembles in most features of its
development that of the chick. A special diverticulum arises
at an early stage from each olfactory sac, which acquires a
separate opening into the mouth, through the naso-palatine
canal, and becomes the organ of Jacobson.

2. The Eye.

The general history of development of the eye of the rabbit
is very similar to that of the chick ; the formation of the optic
vesicles as outgrowths of the fore-brain, the doubling up of the
vesicles to form the optic cups, the pitting-in of the surface
epiblast to form the lens, and the subsequent fate of the several
parts being essentially the same in the two cases. One of the
most marked points of difference is the much smaller size of the
eye in the rabbit (cf. Figs. 115 and 148).

The optic vesicles arise as lateral outgrowths of the fore-
brain, at a very early stage. Before the end of the ninth day,
i.e. before the fore-brain is closed dorsally, the optic vesicles
(Fig. 145) are already conspicuous structures. The vesicles
soon become constricted at their origins from the fore-brain, the
constricted portions giving rise to the optic stalks. As the con-
strictions proceed from above downwards, the optic stalks remain

c c 2

388

THE RAJilMT.

connected with tlie ventral surface or floor of the fore-brain
(Fig. 155).

The optic cup. On the tenth day, the doubling up of the
optic vesicle to form the optic cup commences, and by the
fourteenth day (Fig. 155) it is completed. The process of
doubling up takes place in very much the same way as in the

Fig. 155. — A transverse section across the head of a Kabbit Embryo of the
fourteenth day, the section passing through the eyes, the fore-brain and
the cerebral hemispheres, x 14.

AO, central artery of retina, arising from internal carotid artery. BD, fold of the
inner wall of the cerebral hemisphere, which forms the hippocampus major. BI, corpus
striatum. BS, lateral ventricle, or cavity of the cerebral hemisphere. BIT, optic
thalamus. FM, foramen of Monro, leading from the third ventricle to the lateral
ventricle. OC, inner or retinal wall of optic cup. OD, outer or pigment wall of optic
cup. OL, lens. OM, upper eyelid. OO, lower eyelid. VJ, jugular vein. X, fold
of the inner wall of the cerebral hemisphere, which forms the choroid plexus of the lateral
ventricle. XD, blood-vessels of the choroid plexus.

chick ; and, although occurring simultaneously with the forma-
tion of the lens, it is to be ascribed rather to a complicated mode
of growth on the part of the walls of the optic vesicle itself,
than to mechanical inpushing by the developing lens.

A choroidal fissure is formed along the ventral surface of the
optic cup, as in the chick ; the only important point of difference
being that, in the rabbit embryo, the process of folding or

THE EYE. 389

doubling up is not confined to the optic cup itself, but extends
a certain distance along the optic stalk towards the brain. The
consequence is that the part of the optic stalk near to the
eye (Fig. 155) is not a simple tube with thick walls, but a
tube deeply grooved along its under surface by folding of its
walls.

This groove, being continuous with the choroidal fissure,
leads into the cavity of the optic cup, i.e. of the globe of
the eye ; and it is by running along this groove that the
central artery of the retina, a branch of the internal carotid
artery (Fig. 155, ao), gains admittance to the interior of the
eye. This artery supplies the retina throughout life, and during
the development of the eye supplies the vitreous body and the
capsule of the lens as well.

Of the two walls of the optic cup, the distal or inner wall
(Fig. 155. oc) is, from the first, much thicker than the proximal
or outer wall, OD ; the difference being a very pronounced one by
the fourteenth day. From the inner layer, oc, the entire thick-
ness of the retina proper is developed, the rods and cones being
processes from its outer surface, which do not appear until shortly
before birth.

The outer and thinner wall of the optic cup, OD, becomes con-
verted, as in Vertebrates generally, into the pigment layer of
the retina ; a stratum of hexagonal cells, closely fitted together,
with which the retinal rods ultimately acquire very close rela-
tions. In the cells of this layer, granules of pigment are deposited
at an early stage ; and up to a late period of development the
black colour of the eye is due to this layer, the choroid coat of
the eye not developing, or acquiring pigment, until very near the
time of birth (c/. Fig. 156).

Near the free edge of the optic cup, the two layers are ot
approximately equal thickness (Fig. 156), and grow forwards
in front of the lens to form the pigmented epithelium of the
posterior surface of the iris.

The optic nerve. It is at present uncertain whether the
fibres of the optic nerve of the rabbit are developed in situ,
from the walls of the tubular optic stalk, or whether, as seems
far more probable, they arise in the retina and grow inwards
along the optic stalk to the brain. The nerve fibres from the

31)0 THE RABBIT.

two eyes cross one another in the floor of the third ventricle,
to form the optic chiasma (Fig. 154, ot), and then continue
their growth upwards and backwards, round the walls of the
fore-brain, to the mid-brain.

The lens. During the tenth day the deeper part of the
surface epiblast, opposite the optic vesicle, becomes thickened,
forming the first rudiment of the lens. On the eleventh day
this thickened patch becomes invaginated, forming the vesicle
of the lens ; the mouth of the pit narrows, and by the end of the
twelfth day or early on the thirteenth day it closes, completing
the vesicle, which soon separates from the surface epiblast.

From the first, the inner or deeper wall of the lens vesicle is
much thicker than the outer wall. The inner wall continues to
increase in thickness, through elongation of the cells composing
it, until by the end of the fourteenth day (Fig. 155, ol), the
cavity of the vesicle is almost completely obliterated.

The lens continues to grow rapidly, and throughout the
later stages of development is of large proportionate size. Its
structure and relations on the twenty-first day are shown in Fig.
156, OL, where its inner surface is seen to be very strongly
convex, and the outer surface less markedly so. The axial cells
of the lens remain straight or nearly so, while the more marginal
one3 are curved in the direction indicated by the lines crossing
the lens in the figure. The first formed part of the lens acts as
a nucleus, round which successive layers of cells are added on in
the later stages of development. These arise at the equator of
the lens, and, increasing rapidly in length, spread on to the faces
of the lens, over the ends of the first formed cells.

The lens, during the time of its formation, is invested by a
sheath of mesoblast. According to Kolliker, this is present
from the first as a thin layer, between the epiblastic thickening,
which gives rise to the lens, and the optic vesicle ; and is carried
in by the lens as this becomes invaginated. Most other in-
vestigators maintain that it arises from mesoblast which gains
admittance into the globe of the eye through the choroidal
fissure. This mesoblastic investment of the lens is very vascular,
the blood being brought to it by a branch of the central artery
of the retina (Fig. 155, ao). This divides into very numerous
branches on the inner or deeper .surface of the lens, which

THE EYE.

391

extend round its margin and cover the outer surface as well.
There are no veins which correspond exactly to this artery, the

Fig. 156. — A transverse section across the head of a Babbit Embryo of the
twenty-first day, the section passing through the centres of the eyes {of.
Figs. 149 and 151). x 8.

BE, orbit o-sphenoidal cartilage. BY, olfactory lobe. CC, cornea. DA, suborbital
gland. DB, submaxillary gland. DC, duct of submaxillary gland. DE, hair follicles.
ET, pre-sphenoidal cartilage. PA, frontal bone. HB, basihyal cartilage. MB,
mandible. MC, Meckel's cartilage. OB, buccal cavity. OC, inner or retinal layer of
optic cup. OD, outer or pigment layer of optic cup. OG, iris. OL, lens. OM,
upper eyelid. OO, lower eyelid. PP, posterior narial passage. PI, palatine bone.
TE, decidiious grinding tooth of upper jaw. TP, permanent grinding tooth of upper
jaw. TG, deciduous grinding tooth of lower jaw. TK, permanent grinding tooth of
lower jaw. TM. tooth papilla. TW, tongue. TT, enamel organ. ZO, zygoma. V,
mandibular branch of trigeminal nerve.

blood being returned by veins in connection with the iris. This
vascular investment to the lens is merely a provisional structure,

392 THE RABBIT.

serving for its nutrition during growth ; it disappears completely
when the lens has reached its full size.

The vitreous body is derived from the mesoblast which grows
into the cavity of the optic cup, through the choroidal fissure.
It is extremely vascular in the early stages of its formation,
receiving its blood from the central artery of the retina.
There is no structure in the rabbit corresponding to the pecten
of the bird.

The cornea (Fig. 156, cc) is formed, as in the chick, from
mesoblast, which spreads in between the surface epiblast and
the lens ; and the anterior chamber of the eye is a space which
appears at a rather late stage, between the cornea and the lens.

The choroid and sclerotic are formed, as in the chick, from
the mesoblast surrounding the optic cup. They are developed
very late; and on the twenty-first day (Fig. 156), when the
cornea is well developed, the choroid and sclerotic are merely
represented by a thin layer of connective tissue, devoid of pig-
ment. The sclerotic of the rabbit is not cartilaginous at any
stage.

The eyelids are folds of skin, above and below the eyeball.
They appear early, and by the fourteenth day have attained
some size (Fig. 155, om, oo). By the nineteenth or twentieth
day they have grown completely over the eye, and meet each
other along their free edges (cf. Fig. 149) ; a little later (Fig.
156, om, oo) the edges of the two eyelids fuse together, the
epidermal layers becoming continuous with each other ; this
fusion persists throughout the remaining period of development,
and is the cause of the blindness characteristic of the young at
birth.

The third eyelid, or nictitating membrane, is a similar fold of
skin, arising at the inner angle of the eye, and lying between
the other two eyelids and the eyeball.

The lacrymal glands arise as solid ingrowths of epiblast into
the underlying connective tissue, which subsequently become
hollowed out to form the cavities of the glands and ducts.

THE EYE AND EAR, 393

3. The Ear.

The ear of the rabbit, like that of the chick, is derived from
a pitting-in of the epiblast at the side of the hind-brain. By
closure of its mouth, the pit becomes a vesicle or sac, imbedded
in the mesoblast of the side of the head ; and from the walls of
this sac, which are of epiblastic origin, the epithelial lining of the
vestibule and of its various prolongations is derived ; the semi-
circular canals, cochlea, and other parts being formed by out-
growths or constrictions of the originally simple sac.

The mesoblast immediately surrounding the sac gives rise
to the connective tissue wall of the auditory labyrinth, while
the mesoblast a little distance off gives origin to the cartilagi-
nous auditory capsule (cf. Fig. 159). Between the labyrinth and
the cartilaginous capsule a series of lymphatic spaces appear,
filled with fluid, in which the labyrinth hangs suspended.
Finally, important series of accessory organs, characteristic of
air-breathing Vertebrates — the tympanic membrane, Eustachian
tube, auditory meatus, auditory ossicles, and external ear — are
formed, and acquire definite relations with the essential organ
of hearing, i.e. the auditory vesicle itself.

The auditory vesicles arise in the rabbit, towards the end of the
ninth day, as a pair of shallow depressions of the epiblast at the
sides of the hind-brain. During the tenth day each pit deepens
rapidly, and by the end of the day the mouth of the pit narrows
and closes, converting the pit into the closed auditory sac or
vesicle (Fig. 147, Ei), which lies imbedded in the side wall of
the head, opposite the first branchial arch.

The auditory vesicle is at first spherical, but soon becomes
triangular in outline as seen in transverse sections. The dorsal
angle of the triangle, which marks the place where the vesicle
separates from the external epiblast, grows upwards as a long
tubular process, the recessus vestibuli (Fig. 158, er), which
follows the curvature of the brain wall, and ends blindly at its
dorsal extremity.

From the outer side of the vestibule, a wide lateral diverti-
culum arises, from which the semicircular canals are developed
at a slightly later stage (Fig. 158, ED, eh). The ventral angle
of the vestibule is prolonged downwards and inwards as a curved
finger-like process, the cochlear canal (Fig. 158, el).

394

THE RABBIT.

The auditory nerve (Fig. 158, viii) develops very early, as
already noticed, and by the tenth day, if not indeed from its

Fig. 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. The section is drawn as seen from the front, and is
taken along a line joining the reference letters so and mn in Fig. 1G2
(p. 405). The external ears are cut short, close to their bases, and the
floccular lobes of the cerebellum, which lie between the three semicircular
canals of each side, are omitted entirely. (From Marshall and Hurst.)

B, buccal cavity. BO, basi-occipital. C, cochlea. CA, right external carotid
artery. CE, cerebellum. E, external ear or pinna. EM, external auditory meatus.
ET, Eustachian tube. BE, body of the hyoid. LA, right lingual artery. M, nialleus ; to
its inner side are seen the incus and stapes. MN", mandible. MO, medulla oblongata.
!N", posterior nasal chamber. B, soft palate. BO, periotic. S, post-tympanic process
of squamosal. SC. anterior vertical seinicircular canal. SO, supra-occipital. T,
tympanic bone. TC. tympanic cavity. TM, tympanic membrane.

earliest appearance, is intimately connected with the inner wall
of the vestibular sac.

THE EAR.

395

The condition on the fifteenth day is shown in Fig. 159, the
section, on the left side, being taken at a level slightly anterior
to that of the right side. The recessus vestibuli, rat, is still
large, and is dilated at its upper end in a club-shaped manner.
The three semicircular canals are well established : they are
formed, as in the chick and frog, from flattened saccular out-

CH TP

EiG. 158. — A transverse section across the head of a Rabbit Embrj^o at the
end of the eleventh day, the section passing- through the medulla oblongata,
the ears, and the pharynx. The plane of section of the right half of the
figure is slightly anterior to that of the left half. (Compare Fig. 147.)
x30.

CH, notochord. EB, membrane closing hyomandibular cleft. ED, common stem
of the two vertical semicircular canals. EEL rudiment of the external or horizontal
semicircular canal. EL, cochlear canal. ER, recessus vestibuli. EV, auditory
vesicle. HM. hyomandibular pouch. TP, pharynx. VP, fourth ventricle. VJ,
jugular vein. VIII, auditory nerve.

growths of the auditory vesicle, the two walls of each outgrowth
coming in contact and fusing, so as to form a curved tube open-
ing into the vestibule at both ends. The section (Fig. 159)
passes through the stem common to the two vertical semicircular
canals, ED, and also through the horizontal canal, eh. Each
semicircular canal has already acquired an ampulla at one end.

396

THE RABBIT.

The body of the vestibule is partially divided by a constric-
tion into a larger division, the utriculus, with which the semi-
circular canals are connected ; and a smaller division, the sac-
culus, which opens through a narrow neck, the canalis reuniens,
into the cochlear canal, el. This latter is a tube of fairly

Fig. 159.— A transverse section across the Lead of a Kabbit Embryo of the
fifteenth day, passing through the medulla oblongata, the ears, and the
pharynx. The plane of section of the left side of the figure is slightly
anterior to that of the right side. x 16.

AC, carotid artery, giving off a small branch which runs through the arch of the
stapes. CH, notochord. EB, tympanic membrane. EC, cartilaginous auditory
capsule. ED, common stem of the two vertical semicircular canals. EH, external or
horizontal semicircular canal. EL, cochlear canal. EO, external auditory meatus.
EP, posterior vertical semicircular canal. EE, recessus vestibuli. ES, Eustachian
passage. EX, external ear. MA, malleus. SA, stapes. TP, pharynx. VF, fourth
ventricle. VJ, jugular vein. VII, facial or seventh cranial nerve.

uniform diameter, curved as shown in the figure, and with its
wall markedly thicker along the inner than the outer side of
the curve. A cartilaginous periotic capsule, EC, is present, sur-
rounding the ear, but at some little distance from it ; the re-
cessus vestibuli alone projecting beyond the capsule.

THE EAR. 397

At a stage a little later than that shown in Fig. 159, the
cochlear canal, which up to this point has been only slightly
curved, begins to form the spiral turns, so characteristic of the
adult (Fig. 157, c), the twisting being brought about by growth
in a spiral manner of the blind end of the canal.

The cochlear canal becomes the scala media of the cochlea in
the adult. Immediately outside it the mesoblast becomes ex-
cavated to form a couple of tubular passages, the scala vestibuli
and scala tympani, which lie respectively above and below the
scala media or cochlear canal. The scala vestibuli and scala
tympani commence at the basal end of the cochlea, and gradually
extend along it towards its apex, following the turns of the
spiral ; and ultimately, on reaching the apex, they open into each
other, though not until a very late stage of development. From
the epithelium of the floor of the cochlear canal, or basilar mem-
brane, the organ of Corti is developed ; while the roof of the
cochlear canal, separating it from the scala vestibuli, is spoken
of in the adult as the membrane of Reissner.

At the base of the cochlea, the scala vestibuli opens into the
peri-lymphatic space surrounding the central part of the
vestibule, while the scala tympani is closed at its base by the
membrane of the fenestra rotunda.

Similar peri-lymphatic passages are formed, by excavation of
the mesoblast, around the semicircular canals.

As in the frog, there is at first a single patch of the
epithelium of the auditory vesicle with which the auditory nerve
is continuous. This single, large patch becomes ultimately
broken up into several smaller ones, which by growth of the
intervening strips of epithelium are separated further and further
from one another until they reach their adult positions.

The accessory auditory apparatus of the rabbit is, in a general
way, similar to that of the frog or the chick, but is more com-
plicated.

The Eustachian tube (Fig. 157, et) is formed from the
hyomandibular gill-pouch. This reaches very close to the surface
in the early stages of development, but does not open to the
exterior at any period. On the eleventh day (Fig. 158, Hm)
the hyomandibular pouch reaches almost to the surface, the
hypoblast of the pouch meeting the epiblast at the bottom of

398 THE RABBIT.

the external groove, so that the cleft is at this stage closed only
by a very thin branchial membrane, EB, formed of epiblastic
and hypoblastic layers without any intervening mesoblast. As
shown in the figure, the hyomandibular pouch lies at this stage
some distance ventral to the ear, and the two structures are
completely independent of each other.

By the fifteenth day (Fig. 159) the conditions have changed
materially. A thick layer of mesoblast has grown in between
the epiblast and hypoblast of the branchial membrane, so that
the hyomandibular pouch is now separated from the surface of
the head by a thick plate, eb, which becomes later on the
tympanic membrane. Further, by growth upwards of its lips,
and through the general thickening of the side walls of the
head, the shallow hyomandibular groove of the earlier stage is
converted into a deep pit, eo, the external auditory meatus, the
margin of which is already commencing to grow out as the
rudiment of the pinna or external ear (cf. Figs. 149 and 157).

In the later stages (cf. Fig. 157), the external meatus, em,
becomes much longer, and the pinna attains enormous dimen-
sions ; the tympanic membrane, tm, becomes relatively much
thinner than at the fifteenth day ; while the Eustachian passage
becomes more distinctly tubular, and, owing to the formation of
the palate, P, now opens into the posterior narial chamber
instead of directly into the buccal cavity.

With regard to the auditory ossicles of the rabbit, it is
difficult to speak with certainty. The stapes (Fig. 159, sa)
forms, about the fifteenth day, as a ring of cartilage, which
from its first appearance is in close connection with the outer
wall of the periotic capsule, and apparently continuous with
this, at the place where the fenestra ovalis is formed a little
later. The ring-like form of the stapes is apparently due, as
shown in Fig. 159, to its being formed around a small branch
of the carotid artery, AC.

Concerning the origin of the other two auditory ossicles of
the mammal, the malleus and incus (Figs. 157, M,and 159, ma),
there has been much discussion. While it appears very probable
that they are formed in connection with the cartilaginous bars
of one or more of the visceral arches, investigators differ widely
■as to whether both are developed from the mandibular bar,

THE EAR, AND THE DIGESTIVE SYSTEM.

which is perhaps the most generally accepted view, or both
from the hyoidean bar, or one from each of these bars. There
are, at present, no recorded observations which determine the
matter satisfactorily in the case of the rabbit.

The malleus can be recognised on the fifteenth day (Fig. 159,
MA) ; it is, from its first appearance, imbedded in the substance
of the tympanic membrane, EB, and is for some time continuous
with the posterior end of the mandibular bar, or Meckel's
cartilage.

THE DEVELOPMENT OF THE DIGESTIVE SYSTEM.
A. The Alimentary Canal.
1 . General Account.

The general history of the development of the alimentary
canal of the rabbit is closely similar to that of the chick. The
greater part of the length of the canal is formed from the
mesenteron, which, as in the chick, is a tubular cavity included
within the embryo by the process of constriction, through which
the embryo becomes separated from the yolk-sac (cf. Figs. 146
and 147). Owing to this mode of formation of the mesenteron,
it necessarily communicates with the cavity of the yolk-sac in
the early stages, and so long as the yolk-stalk remains tubular.
The mesenteron may, therefore, as in the chick, be divided into
three lengths : fore-gut, mid-gut, and hind-gut ; the fore-gut
(Figs. 146 and 147, Gf) being the anterior portion, in which
roof, sides, and floor are all alike present ; the hind-gut, GH,
being the similar portion at the hinder end of the body ; and
the mid-gut, gt, being the median portion, which opens through
the yolk-stalk into the cavity of the yolk-sac, and which con-
sequently has no floor. Fore-gut and hind-gut increase in
length, at the expense of the mid-gut, as the embryo becomes
more and more sharply constricted from the yolk-sac ; and
ultimately, when the yolk-stalk becomes solid, about the thir-
teenth day, the mid-gut as a separate division of the alimentary
canal ceases to exist (cf. Figs. 146, 147, and 150).

The mouth and anal openings are formed, as in other Ver-
tebrates, by stomatodseal and proctodaeal invaginations of the
epiblast at the anterior and posterior ends of the embryo

400 TILE BABBIT.

respectively, which meet and open into the mesenteron, and
so place it in communication with the exterior.

The alimentary canal is at first straight, or merely follows
the curvature of the body, and is situated immediately ventral
to the notochord. It remains in this condition, throughout life,
in the pharyngeal and oesophageal regions, and also at its ex-
treme hinder end ; but along the rest of its extent it shifts
ventral wards, remaining connected with the dorsal wall of the
body cavity by a mesentery (Fig. 165, mh). In the region of
the small intestine the alimentary canal increases in length far
more rapidly than the body of the embryo, and becomes in
consequence thrown into folds, in order that it may be accom-
modated within the body cavity.

2. The Stomatodseum.

The relations of the stoniatodaeal pit are practically the same
as in the chick. Perforation of the stoniatodaeal membrane is
effected at an early stage, before the end of the tenth day. The
pituitary body arises, still earlier, as a diverticulum from the
posterior and dorsal angle of the stoniatodaeal pit ; its further
development has already been described in the section dealing
with the formation of the brain (p. 376).

3. The Buccal Cavity and Pharynx.

The pharyngeal region of the mesenteron is, from the first,
distinguished by its great width (Figs. 158 and 159, tp). Early
on the tenth day, the branchial pouches arise as paired diverticula
from the sides of the pharynx ; and, opposite to the outer ends
of the branchial pouches, branchial grooves are formed on the
surface of the neck, marking out the boundaries of the several
visceral arches. The walls of the branchial pouches and of the
corresponding branchial grooves come into close contact, a thin
branchial membrane (Fig. 158, eb), consisting of epiblast and
hypoblast, without any intervening mesoblast, alone separating
the two. This membrane, however, remains intact ; and in the
rabbit none of the gill-clefts are ever completely formed, or
open to the exterior at any stage of development. There are
also no traces of gills, either external or internal, at any period
in the rabbit.

The visceral arches are well developed, and on the twelfth

THE PHARYNX.

401

day (Fig. 161) the maxillary, MX, mandibular, mn, hyoid, hy,
and first branchial arches form conspicuous ridge-like projections

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in the side walls of the pharyngeal region. Of the branchial
grooves, or external depressions separating the successive arches,

D D

402

THE RABBIT.

the hyomandibular groove, iim, is by far the most conspicuous.
The further development of the hyomandibular groove, and the
mode in which it gives rise to the external auditory meatus, have
already been described in the section dealing with the ear (p. 397).
The tongue is developed as a swelling in the floor of the
buccal cavity ; it commences to form on the twelfth or thirteenth

Fig. 161. — A Rabbit Embryo at the end of the twelfth day, seen from the
right side. The yolk- stalk and allantoic stalk are cut short, close to the
body of the embryo, x 9.

BL, cerebellum. BM, mid-brain. BE.1, first branchial arch. EI, auditory
vesicle. EM, hyomandibular groove. HY, hyoid arch. LA, fore limb. LP, hind
limb. MN, mandibular arch. MS, mesoblastic somite or protovertebra. MX,
maxillary arch. OC, eye. OF, olfactory pit. TA, allantoic stalk, cut short. TL,
tail. VF, fourth ventricle of brain. YS, j'olk-stalk, cut short.

day, and by the eighteenth day (Fig. 151, tn) has attained the
form characteristic of the adult.

The boundary line between stomatodseum and mesenteron is
impossible to fix absolutely, in the later stages of development ;
but its position may be approximately determined, if it be re-
membered that the stalk of the pituitary body (Fig. 151, pt)
marks the posterior boundary of the stomatodasum in the mid-

THE PHARYNX AND MESENTERON. 403

dorsal line ; while, on the floor of the buccal cavity, the boundary
line lies in front of the root of the tongue ; the whole of the tongue
being formed from the mesenteron, and being therefore covered
with hypoblastic epithelium.

The palate is formed, about the fifteenth day, by a pair of
horizontal ridges which grow inwards from the sides of the
buccal cavity, and, meeting each other in the median plane, fuse
to form a horizontal shelf (Fig. 151, pl), which separates the
nasal chamber above from the buccal cavity below. The fusion
■of the two halves of the palate proceeds from before backwards ;
and the palate ends with a free posterior edge, behind which
the nasal and buccal chambers are continuous with each other
(Fig. 151, tp).

4. The (Esophagus.

The hinder end of the pharynx narrows very rapidly, and
passes abruptly into the straight tubular oesophagus (Figs. 150,
and 151, to). It has not yet been determined whether the
oesophagus of the rabbit, like that of the chick and tadpole,
passes through a stage in which it is solid for a time.

5. The Stomach and Intestine.

The stomach becomes evident, as a distinct dilatation of the
alimentary canal, about the thirteenth day; its long axis at first
corresponds with that of the body, but later on it shifts its
position, and becomes placed at first obliquely, and then almost
directly across the body.

The intestine undergoes changes corresponding fairly closely
with those already described for the chick. The lengthening
of the intestine is effected almost entirely in two situations,
giving rise to two ventrally directed loops. Of these, the
proximal or duodenal loop is formed immediately behind the
stomach, and in the rabbit attains a considerable length (Fig.
160, e). The distal or vitelline loop is formed by elongation of
the > -shaped loop of the intestine already present on the
twelfth day, and from the apex of which the yolk-stalk, yk,
; arises ; the vitelline loop attains an enormous length in the
I rabbit.

A short length of the intestine, between the duodenal and
i. "vitelline loops, remains in the rabbit, as in the chick, stationary

D D 2

I

404 THE RABBIT.

throughout the whole period of development ; it is attached to
the dorsal wall of the body cavity by a very short mesenterial
fold, and is easily recognised in an adult rabbit.

A well-developed post-anal gut, or prolongation of the hinder
end of the intestine into the tail, is present on the tenth and
eleventh days. By the twelfth day (Fig. 150), the greater part
of this has already disappeared ; a small diverticulum of the
cloacal cavity, GP, marks its basal portion, and detached frag-
ments of it may persist for a time at intervals along the tail.

This post-anal gut is probably a secondary feature, and due,
as in the frog, to the drawing out of the alimentary canal into
the tail as this latter lengthens.

6. The Proctodeum.

The proctodeum in the rabbit is little more than the actual
anal opening ; it develops late, and is usually not formed until
about the sixteenth day.

B. Organs Developed in connection with the Alimentary Canal.

1. The Teeth.

Teeth are cutaneous structures, developed from the mucous
membrane covering the jaws. They appear in rabbit embryos
during the third week, and are at first independent of the bones
of the jaws ; indeed, the upper teeth develop before the
maxillary bones are formed (cf. Fig. 156). The jawbones, how-
ever, soon acquire close relations with the teeth, growing round
them, and inclosing them in sockets.

In the rabbit, as in Mammals generally, there are two sets of
teeth, known as milk or deciduous, and adult or permanent,
respectively. The deciduous dentition of the rabbit is repre-

2 0 3

sented by the formula : — di. - ; dc. ; dm. - ; the correspond-
ing formula for the permanent dentition being,
.20 3 3

L P C> 6' Pm- 2' m' 3-
The milk, or deciduous, teeth in the rabbit are lost very early.
The deciduous incisors, corresponding to the large chisel-shaped
incisors of the permanent set, are very small, and are shed before
the birth of the young rabbit. The second pair of deciduous

THE TEETH.

405

incisors of the upper jaw are much larger, and persist as
functional teeth for about three weeks after birth, lying wedged
in between the large and small permanent incisors. For the
first three weeks after birth there are therefore three upper
incisors on each side in the rabbit ; the first and third being the
permanent incisors, and the middle one being the deciduous
second incisor, which has not yet been shed. The deciduous
molars (Fig. 156, tf, tg) are of considerable size, and persist

Z.S. F.A. O.F. F 0

riG. 162. — The skull of the Rabbit, from the right side. The middle portion
of the zygomatic arch and the right half of the mandible have been
removed. (From Marshall and Hurst.)

A, external pterygoid process of all-sphenoid. AS, ali-sphenoid. B, internal orbital
foramen. BO, basi-oocipital. BS, basi-sphenoid. C, occipital condyle. D, mandibular
symphysis. EO, ex-occipital. P, frontal. PA, foramen lacerum anterius. FM,
foramen lacerum medium. G, orbital groove, for ophthalmia division of trigeminal
nerve. I, anterior upper incisor. IP, infra-orbital foramen. IP, inter-parietal. J,lower
incisor. L, lacrymal hone. LP, laerymal foramen. M, maxilla. MW, mandible.
N, nasal hone. OP, optic foramen. OS, orbito-sphenoid. _P, parietal. PE, par-
occipital process of ex-occipital. PL, palatine bone. PM. pre-maxilla. PO, periotic.
PT. pterygoid. S, squamosal. SP. stylo-mastoid foramen. SO, supra-occipitaL T,
tympanic bone. ZM. zygomatic process of maxilla, cut short. ZS, zygomatic process
of squamosal, cut short. '

until three or four weeks after birth, when they are pushed out
by the permanent premolars developed beneath them.

A full}7 formed tooth consists chiefly of dentine, covered on
its crown, or grinding surface, with a cap of a very hard and
densely calcified substance, the enamel ; and invested, especially
round its deeper part or root, by a layer of bony substance, the
cement. The dentine is hollowed out by the pulp cavity, in
which are lodged the blood-vessels and nerves of the tooth.

406 THE RABBIT.

These gain admittance through a larger or smaller hole in the
root, or fang, of the tooth ; the hole remaining widely open
throughout life in the rabbit, and other animals in which the teeth
grow continuously throughout life, but becoming reduced to
one or more very small apertures in the majority of Mammals, in
which the teeth cease growing after they have reached their full
size.

Teeth, as already noticed, are cutaneous structures; and of
the substances of which the tooth consists, the enamel is formed
from the epithelium, and the dentine and cement from the under-
lying connective tissue layer or dermis.

The first step in the formation of a tooth consists in an in-
growth from the deeper layer of the epithelium into the con-
nective tissue of the gum (cf. Fig. 156). This ingrowth
soon becomes hollow and flask-shaped, its deeper end dilating*
into a sac (Fig. 156, tt), while its superficial part forms a narrow
solid neck or stalk, which remains in connection with the surface
epithelium. Opposite the deeper end of the flask, the connective
tissue of the gum becomes condensed to form the dental papilla
(Fig. 156, tm). The deeper end of the epithelial flask, or enamel
organ as it is called (Fig. 156, tt), now becomes closely applied
to the dental papilla, which gradually acquires the definite shape
of the crown of the tooth to which it is going to give rise.

The enamel organ (Fig. 156, tt), at this stage, is a flattened
sac, consisting of outer and inner epithelial layers, and having
its cavity occupied by a reticulum of stellate cells ; the outer
epithelial layer is still connected with the surface epithelium
by a narrow stalk or string of cells, while the inner layer
forms a cap, closely embracing the top and sides of the dental
papilla.

This cap consists of a single layer of very regularly arranged,
six-sided, columnar epithelial cells ; and it is by calcification of
the substance of these cells that the enamel layer of the tooth
is produced. Calcification commences at the surface of the
enamel organ next to the dental papilla, and gradually spreads
outwards through the cells of the enamel organ.

The dentine is formed by calcification of the dental papilla,
and is therefore of mesoblastic origin. Calcification appears
first at the surface of the papilla next to the enamel organ, so
that the crown of the tooth is the first part to be formed ; and,

THE TEETH. 407

when once completed, no further change in the shape of the crown
can occur.

The mode of formation of the dentine is as follows. The
cells at the surface of the dental papilla form a single layer
of finely granular nucleated cells, closely arranged side by side,
and spoken of as odontoblasts. The most superficial parts of
the odontoblasts become converted into, or else form by ex-
cretion, a gelatinous matrix in which calcification soon occurs,
forming the dentine. The deeper parts of the odontoblasts,
containing the nuclei, remain soft and unaltered ; they give off
fine processes towards the surface of the tooth, which lie in
channels in the dentine, these channels being the dentinal
tubules of the adult. By a continuance of this process the
dentine increases in thickness ; the odontoblasts, which are the
active agents in the process, forming a layer on the inner
surface of the dentine, and sending out fine radial prolongations
into the dentinal matrix.

The follicle, or tooth-sac, is formed by a condensation of the
vascular mesoblast around the tooth. The cement is a thin
layer of bone formed round the tooth by the wall of the follicle,
which acts as the periosteal membrane.

The permanent teeth are formed in the same way as the
deciduous teeth ; their enamel organs arising as outgrowths from
the necks of those of the deciduous teeth (Fig. 156).

2. The Thyroid Body.

The thyroid body of the rabbit arises early in the tenth day,
as a median thickening of the epithelium of the floor of the
pharynx, which grows downwards into the connective tissue
immediately in front of the pericardial cavity. The stalk of
connection with the pharyngeal floor narrows, and during the
eleventh day disappears, leaving the thyroid as a solid epithelial
body (Fig. 150, th) embedded in the mesoblast of the floor
of the pharynx, immediately in front of the truncus arteriosus,
and between the roots of the carotid arches.

In the later stages the thyroid body widens transversely,
giving off two lateral lobes which rapidly increase in size. A
cavity appears in the median portion, and soon extends into the
lateral lobes, from which outgrowths, some hollow and some
solid, soon arise. As the heart shifts backwards into the thorax,

408 UN1: KAI313IT.

the thyroid body also moves its position, coming into close
relation with the upper rings of the trachea.

3. The Thymus.

The thymus of the rabbit is formed by bud-like outgrowths
from the epithelium of one of the hinder branchial pouches.
These buds first become conspicuous about the fourteenth day ;
they soon separate from the walls of the pharynx, and gradually
shift backwards, increasing greatly in size as they do so, until
they reach their final position at the anterior end of the thorax.

4. The Lungs.

The lungs arise in the rabbit, much as in the chick or frog,
from the ventral wall of the mesenteron, at the place where
it narrows, immediately behind the pharyngeal region, to form
the oesophagus.

On the tenth day the cavity of the oesophagus, which is
elsewhere circular in transverse section, becomes laterally com-
pressed at its anterior end, immediately behind the pharyngeal
region. By the outgrowth of two horizontal ridges from its
side walls, which meet and unite in the median plane, a short
length of the oesophagus becomes divided into two tubes : of
these, the dorsal tube is the oesophagus itself; while the ventral
one, or laryngeal chamber, is a short tube, ending blindly behind ,
but opening in front into the oesophagus through the orifice
which afterwards becomes the glottis (cf. Fig. 150).

From the laryngeal chamber the lungs arise, on the eleventh
day, as a pair of lateral diverticula, which grow backwards along
the dorsal part of the body cavity and the sides of the oesophagus
(Fig. 150, lg).

The lungs, being thus formed as outgrowths from the ali-
mentary canal, will, like the canal itself, have mesoblastic walls,
lined by a hypoblastic epithelium.

On the twelfth day secondary outgrowths arise from the main
tube or bronchus of each lung, and these in the later stages
branch freely to form the smaller bronchi, from the terminal
branches of which the air cells are formed about the time of birth.

The branchings of the bronchi occur almost entirely towards
the dorsal and outer surfaces of the lungs (Fig. 163), so that the
original or main bronchial tubes, lb, lie close to the inner surfaces

THE LUNGS.

409

of the lungs. The smaller bronchi divide, for the most part, in
a regular, dichotomous manner, as shown on the right-hand
side of Fig. 1G3. The branching at first affects the hypo-
plastic lining alone, but about the thirteenth or fourteenth dav
(Fig. 1G3) the mesoblastic wall becomes divided by external

FlG. 1G3. — A transverse section across the thorax of a Rabbit Embryo of the
sixteenth day. x 15.

A, dorsal aorta. CH, notochord. CP, pericardial cavity. CB, pleural cavity.
FN, neural arch of vertebra. LB, bronchus. LG, lung. ITS, spinal cord. NY,
sympathetic nerve cord. RA,right auricle. BB. left auricle. ED, inter-auricular
septum. EI, rib. BN, capitulum of rib. RO, tubercle of rib. RV, right ventricle.
BY. left ventricle. ST, ventral end of rib, from which the sternum is formed. TO,
oesophagus. VD, left anterior vena cava. VI, posterior vena cava.

grooves or clefts, which mark out the boundaries of the main
lobes of the lungs. The trachea, or median part of the air
passage, is at first very short ; but, as the neck elongates, and
the lungs get carried back into the thorax, the trachea rapidly
increases in length, and by the eighteenth day (cf, Fig. 151)

410 THE RABBIT.

the proportions are not very unlike, those of the adult. The
glottis, LT, at this stage is a longitudinal slit-like opening in the
ventral wall of the oesophagus ; it is overhung by the epiglottis
(cf Fig. 150, lk), a fold at the back of the pharynx, marked off
from the tongue by a well-marked transverse groove. The glottis
leads into a dilated laryngeal chamber (Fig. 151), which is
continued down the neck as the trachea, LR. The thyroid, CT,
cricoid, and tracheal cartilaginous rings are already present, and
have the same relations as in the adult.

5. The Liver.

The liver of the rabbit, like so many other important organs,
commences on the tenth day, arising as a diverticulum from the
ventral surface of the mesenteron, about the junction of the
stomach and duodenum. This diverticulum, which becomes the
left bile duct of the adult, is directed ventralwards, and its blind
end is in close relation with a thickened mass of condensed
mesoblast which forms part of the ventral body-wall of the
embryo, behind the heart, and in front of the yolk-stalk.

On the eleventh day, the right bile duct arises as an out-
growth from the left duct, close to its opening into the
duodenum. From the lining epithelium of both right and
left ducts, solid rods of hypoblast cells, the hepatic cylinders,
grow out into the mass of condensed mesoblast around them.
The hepatic cylinders branch and anastomose freely, forming a
reticulum, the meshes of which are occupied by the connective
tissue, in which blood-vessels early appear in large numbers.

By a series of further changes, which have not yet been
accurately determined in the rabbit, the adult liver is formed.
The cells of the reticulum become the hepatic cells, which are
thus of hypoblastic origin ; while some at least of the cylindrical
rods become hollow, and form the bile passages, which com-
municate directly or indirectly with the bile ducts (cf Fig.
150, w).

• The gall bladder arises on the eleventh day, as a diverticulum
-fbim the right bile duct.

The relations of the blood-vessels to the liver are much the
same as in the chick, and will be described more fully in the
section dealing with the development of the vascular system.
It may be noticed here that in its early stages the liver has, as

THE LIVER AND PANCREAS. 411

in the chick, especially close relations with the vitelline veins,
through which the blood is returned to the embryo from the
yolk-sac.

From the twelfth day onwards, the common part of the bile
duct, where the right and left bile ducts join to enter the
duodenum, lengthens rapidly, thus giving rise to the single bile
duct of the adult rabbit (cf. Fig. 160, 11).

C. The Pancreas.

The pancreas arises, on the twelfth day, as a swelling or
bulging of the dorsal wall of the intestine, slightly further back
than the bile duct, and opposite the yolk-stalk. On the eleventh
day it becomes much more sharply defined, and constricted off
from the gut as a somewhat pyriform, hollow sac, which opens
into the dorsal wall of the intestine, and gives off small buds from
its surface. In the succeeding days these buds enlarge, and
give off other similar buds from their sides, and so form a com-
pound gland of the racemose type. The pancreas, therefore, in
its mode of development agrees closely with the salivary or other
ordinary glands, and differs markedly from the liver.

As the alimentary canal is at first straight, and lies but a
little distance ventral to the notochord (Fig. 150), the pancreas,
in its early stages, is embedded in the dorsal wall of the body
cavity, and wedged in between the intestine and the dorsal aorta.
As the intestine lengthens, to form the duodenal loop, the
pancreas is drawn down- with the mesentery, between the
two limbs of the loop, and so attains its adult position (Fig.
160, f).

7. The Cloaca.

On the twelfth clay (Fig. 150), the rectum opens into a
dilated cloacal chamber, tc, from which the stalk of the allantois,
ta, opens, and into which the Wolffian ducts, KC, and ureters,
kd, discharge. This cloacal dilatation lies in a rounded cloacal
papilla, which forms a well-marked projection on the ventral
surface of the embryo, behind the allantoic and yolk stalks,- and
in front of the root of the tail (Fig. 150).

Just before entering the cloacal dilatation, the intestine makes
a rather sharp bend ventralwards, the distinctness of which is
slightly exaggerated in Fig. 150. Between the intestine and the

412 THE RABBIT.

cloacal dilatation, and separating the two structures from each
other behind the point of opening of the Wolffian duct, is a
septum of connective tissue, which is well seen in the figure,
where it is crossed by the reference line, tc.

During the next three days this septum extends backwards,
its growth being effected by the union, in the median plane, of
two lateral ridges to form a median partition. This partition
divides the cloacal chamber into two separate portions, a dorsal
or rectal chamber, and a ventral or urino-genital chamber. The
proctoda3al opening is established by this time, and the partition,
on reaching the surface, divides this opening into separate
anal and urino-genital apertures, the partition itself forming the
perinasum, or transverse septum between the two apertures.

The anal opening lies on the posterior surface of the cloacal
papilla, almost in the angle between the papilla and the tail, so
that the cloacal papilla is from this time concerned with the
urino-genital organs alone. The cloacal, or genital papilla as it
may now be termed, elongates considerably, and the urino-genital
aperture is prolonged as a median groove along its dorsal or pos-
terior surface. From this stage, development differs in the two
sexes : in the male the two lips of the groove unite to form the
penial urethra, the papilla itself becoming the corpus spongiosum
of the penis. In the female the groove remains open, its borders
forming the lips of the vulva.

DEVELOPMENT OF THE HEART AND BLOOD-
VESSELS.

The general relations of the heart and its various cavities,
and of the great arterial and venous trunks, and the changes
which they undergo during development, are much the same in
the rabbit as in the chick, and it will not be necessary to describe
them in detail in this chapter. The changes in the heart itself,
and especially the mode of formation of the septa, by which the
several cavities are shut off one another, will require closer con-
sideration.

Besides the vessels of the embryo itself, there are two extra-
embryonic vascular systems : — (i) the vitelline, or yolk-sac cir-
culation, which is comparatively unimportant in the rabbit ;
and (ii) the allantoic or placental circulation, which is of the

THE CLOACA, AND THE BLOOD-VESSELS. 413

utmost importance, as it affords the means through which the <
embryo receives its supply of nutriment from the mother, and I
is enabled to effect the necessary respiratory and excretory
interchanges.

The vitelline circulation will be dealt with in the present
section ; the relations of the allantoic vessels in the placenta
will })v treated separately, in the concluding section of this
chapter.

1. The Heart.

The heart of the rabbit, like that of the chick, is formed by
the union of the two vitelline veins, which return to the embryo
the blood from the vascular area.

The vitelline veins are formed in the mesoblast of the
splanchnopleure, and appear at an early stage of development,
when the folding-off of the embryo from the yolk-sac, by the side
folds, has only just commenced. The right and left vitelline
veins, and consequently the two halves of the heart as well, are
therefore at first a considerable distance apart ; and in rabbit
embryos of the ninth day (Fig. 145, r) they appear as a pair of
tubes, lying along the sides of the head, opposite the hind-brain.

As the side-folds deepen, constricting off the embryo from
the yolk-sac, the two tubes get carried round to the ventral
surface of the embryo, where they lie close together, side by
side. About the middle of the tenth day they fuse together to
form a single tubular heart, lying in the floor of the pharyngeal
region of the mesenteron, and having relations very similar to
those of the heart in a chick embryo of about the thirtieth
hour.

In the latter part of the tenth day, the heart, while it remains
attached to the floor of the pharynx at both its ends, becomes
free in the middle portion of its length ; and, growing rapidly,
hangs down into the body cavity as a loop, which soon becomes
twisted on itself like a letter S, and partially divided by con-
strictions into chambers (Fig. 147, r). The hinder or proximal
limb of the heart, which receives the great veins, is the sinus
venosus ; the first loop of the S is the auricular portion ; the
second loop is the ventricular portion ; and the distal or anterior
limb is the truncus arteriosus, from which the aortic arches arise
as right and left branches.

414 THE BABBIT.

The later changes undergone by the heart are of very con-
• siderable interest, and have been described with great care by
Born. It will be convenient to deal with the several cavities
in order, beginning at the hinder or venous end of the heart.

The sinus venosus, on the tenth day, is a vessel running
transversely across the body, and slightly enlarged at its two
ends to form the right and left cornua or horns. Each horn
receives three veins : — (i) the Cuvierian vein, which is formed by
the junction of the anterior and posterior cardinal veins, return-
ing venous blood from the body of the embryo generally ; (ii) the
vitelline vein, returning blood from the yolk-sac ; (iii) the allan-
toic vein, returning blood from the allantois. The sinus venosus
is at this stage nearly symmetrical, and opens into the auricular
cavity by a wide median aperture.

By the eleventh or twelfth day, the right horn of the sinus
venosus has become much larger than the left horn. The allan-
toic and vitelline veins, in place of opening separately into the
sinus venosus, now unite before reaching the heart, and discharge
into the sinus through a single vein, the posterior vena cava.
The posterior vena cava and the right Cuvierian vein, or right
anterior vena cava as it is now termed, open into the larger or
right horn of the sinus venosus ; while the smaller left horn
receives only the left Cuvierian vein, or left anterior vena cava.
The opening from the sinus venosus into the auricle has now
become more slit-like, and leads distinctly into the right half
of the auricular chamber ; the slit-like opening is bounded by
two valve-like folds of the endocardial lining of the heart, which
may be termed the right and left venous valves respectively.

At a later stage the sinus venosus becomes absorbed into
the right auricle, of which it now forms part ; the three venas cavse
opening separately into the auricular cavity. Of the two venous
valves, the left one disappears, while the right one becomes the
Eustachian valve, by which the blood from the posterior vena
cava, and for a time that from the right anterior vena cava as
well, is directed into the left auricle.

The auricular portion of the heart. The originally single
auricular chamber becomes divided into right and left auricles
by a septum, which arises during the twelfth day from the dorsal

THE HEART. 415

wall of the auricular chamber, and grows down into its cavity
(cf. Fig. 163, RD). For a time the lower and posterior edge of
the auricular septum is free, but during the fourteenth day it
meets and fuses with a cushion-like thickening of the margin
of the auriculo-ventricular aperture.

Before this fusion is completed, however, a new aperture, the
foramen ovale, is formed in the dorsal and anterior part of the
auricular septum, through which free communication between
the two auricles is maintained up to the time of birth of the
young rabbit.

The pulmonary veins develop rather late, and are of small
size until near the time of birth ; the two veins, from the right
and left lungs respectively, unite to form a single vessel, which
opens into the dorsal wall of the left auricle, very close to the
auricular septum.

The ventricular portion of the heart. The ventricular cavity
is at first single, and receives the blood from the auricular cavity
through a transverse slit in its dorsal wall.

The division of the ventricular cavity into right and left
ventricles is effected by a septum, which grows upwards from
the apex of the ventricle towards the auriculo-ventricular aper-
ture. This ventricular septum (cf. Fig. 163) appears about the
twelfth day, and its position is indicated from an early period by
a groove on the surface of the heart. The septum remains in-
complete for some time, the two ventricles communicating above
its free edge. About the fifteenth day the septum meets, and
unites with, the cushion-like thickenings of the margin of the
auriculo-ventricular aperture, and so completes the separation
between the two ventricles.

The thickening of the wall of the ventricle is effected in the
first instance, just as in the frog, by the ingrowth of muscular
trabecules into the cavity, which unite to form a reticulum (cf.
Fig. 163), the proper wall of the ventricle remaining thin.
In the later stages, however, the outer walls of the ventricles
thicken considerably throughout their entire substance. Up to
the time of birth there is practically no difference in thickness
between the walls of the right and left ventricles, the resistance
to be overcome being approximately the same in the two
cases.

-il(> THE RABBIT.

The truncus arteriosus becomes divided, much as in the chick
or in the frog, by an internal longitudinal septum ; which, arising
between the roots of the systemic and pulmonary arches, grows
backwards in a somewhat spiral course, dividing the truncus
arteriosus into right or pulmonary, and left or aortic tubes.
The septum continues its growth backwards until it meets the
upper free edge of the ventricular septum, with which it fuses.

After the truncus arteriosus is thus divided internally, an
external groove appears on its surface, opposite the internal
septum ; and this groove deepens until it splits the truncus
arteriosus into two completely separate and independent ves-
sels, of which the right one, or pulmonary trunk, arises from the
right ventricle, and the left one, or aortic trunk, from the left
ventricle.

The semilunar valves are formed by projections of the
thickened endocardium at the roots of the pulmonary and aortic
trunks : the valves are at first thick and soft, but later on
become membranous.

2. The Arteries.

In the rabbit, as in the chick, five pairs of aortic arches are
developed, which appear in order from before backwards. By
the middle of the tenth day the first two pairs are present, in
the mandibular and hyoidean arches respectively. By the end
of the tenth day a third pair of aortic arches is present, in the
first branchial arches ; and before the end of the eleventh day the
remaining two pairs are established, in the second and third
branchial arches respectively.

Of these five pairs of aortic arches, the first two pairs, in the
mandibular and hyoidean arches respectively, lose their connec-
tions with the dorsal aortas during the eleventh day, and become
reduced to the arteries of the floor of the mouth and of the
tongue.

The aortic arches of the third pair, in the first branchial arches,
persist as the carotid arteries. They retain for a time their
connections at their dorsal ends with the fourth pair of arches,
but ultimately lose these, and from this time send blood to the
head alone ; each divides into external and internal carotid
arteries, supplying the parts outside and inside the skull
respectively.

THE ARTEEIKS. 417

The aortic arches of the fourth pair, in the second branchial
arches, are the systemic arches, which by their union form
the dorsal aorta. At first the vessels of the two sides, right
and left, are of equal size, but from a very early period the left
one becomes the larger, and ultimately forms the arch of the
aorta in the adult. The right systemic arch persists for some
time, but ultimately disappears, with the exception of its
proximal part, which is said to give origin to the right sub-
clavian artery.

The aortic arches of the fifth pair, in the third branchial
arches, are the pulmonary arches ; from them the pulmonary
arteries arise as posteriorly directed branches. The pulmonary
arches retain their connections with the dorsal aorta? throughout
the whole period of intra-uterine life, up to the time of birth ; these
connections having, as in the chick embryo, a most important
influence on the course of the circulation. At the time of birth,
the part of each pulmouary arch between the origin of the
pulmonary artery and the aorta (cf. Fig. 128), a part known as
the ductus arteriosus or ductus Botalli, becomes obliterated;
and from this time the blood driven into the pulmonary
arches by the right ventricle can no longer pass directly to
the aorta, but is all sent through the pulmonary arteries to the
lungs.

Zimmermann has found traces, in rabbit embryos of the
eleventh day, of a pair of aortic arches between the systemic and
pulmonary arches. This observation, if confirmed by future
investigation, will be of considerable interest, as showing that
the pulmonary arches of the rabbit are the sixth and not the
fifth pair, and that the pulmonary arteries therefore arise in the
rabbit from the same pair of arches as in the frog ; in other
words, that the pulmonary arteries are strictly corresponding
structures in these two types.

As regards the arteries of the trunk, the two dorsal aorta?
are at first distinct along their whole length, and the allantoic
arteries appear as though they were direct posterior continuations
of the aorta?. Later on, the two aortas unite to form the definite
dorsal aorta, which is continued as a narrow median caudal
artery to the hinder end of the embryo ; the allantoic arteries
from this time appearing as branches of the aorta.

E E

418 THE BABBIT.

3. The Veins.

The general relations of the veins in the rabbit, and the
changes which they undergo during development, are very
similar to those described in the next chapter as seen in the
human embryo, and will not be dealt with further in this
section (c/. pp. 578 to 583).

4. The Course of the Circulation.

It will be convenient to give here a brief account of the
course of the circulation during the latter half of intra-uterinc
life, when the placental circulation is in full swing ; and also
a summary of the changes which occur at the time of birth.

As regards the heart, the ventricular septum is complete, as is
also the septum of the truncus arteriosus. The auricular septum
is, however, incomplete, the foramen ovale allowing blood to
pass across directly from the right to the left auricle.

The blood is brought to the right auricle by the three vense
cavaa. Of these, the right and left anterior venas cavse — the
Cuvierian veins of the earlier stages — return to the heart venous
blood from the head and from part of the trunk of the embryo.
This is received into the right auricle and driven by it into the
right ventricle.

The blood in the posterior vena cava is derived from many
sources. The main factors are the allantoic veins, which return
to the heart the blood from the placenta, blood which is arte-
rial both as regards nutritive matter and as regards its contained
gases.

The other factors of the posterior vena cava are, the vitelline
veins from the yolk-sac, which are small and comparatively un-
important ; the mesenteric veins, which return venous blood
from the alimentary canal of the embryo, and which are of small
size ; and the posterior vena cava itself, which returns blood
from the kidneys and the hinder part of the body. Of these
factors, the allantoic veins are so much the largest that the blood
returned by the posterior vena cava to the heart may be rightly
spoken of as arterial. This arterial blood is discharged into the
right auricle, but never really enters the cavity of the auricle,
since it is directed at once, by the Eustachian valve, through the
foramen ovale into the left auricle, and driven thence into the
left ventricle.

s

THE COURSE OF THE CIRCULATION. 419

The right ventricle is thus filled with venous blood, and the
left ventricle with arterial blood. On the ventricular systole,
the arterial blood from the left ventricle is driven through the
aortic trunk and the carotid arteries to the head ; while the
venous blood from the right ventricle is driven through the
pulmonary trunk and pulmonary arches into the dorsal aorta,
and then backwards along the body, the greater part passing
along the large allantoic arteries to the placenta.

The changes that occur at birth are practically the same as
those which are effected in the chick on hatching (cf. p. 314).

(i) The vitelline and allantoic circulations are stopped. The
result of this is that the blood in the posterior vena cava is
from this time venous, since the arterial supply previously
brought by the allantoic veins is now cut off.

(ii) The ductus venosus, or direct passage through the liver,
is closed. The effect of this change is that all the blood brought
to the liver must now pass through its capillaries in order to get
to the heart, whereas formerly the ductus venosus afforded a short
cut by which the liver capillaries could be avoided.

(iii) The ductus arteriosus, or ductus Botalli, closes on both
sides of the body. This renders it impossible for blood from
the right ventricle to get directly into the aorta. All the blood
from the right ventricle has now to pass along the pulmonary
arteries to the lungs, and the pulmonary vessels consequently
dilate very considerably, to accommodate this increased quantity
of blood. A further effect is that the dorsal aorta now receives
its blood supply from the left ventricle instead of, as formerly,
from the right ventricle ; i.e. it now contains arterial, instead of
venous blood.

(iv) The foramen ovale closes. This is effected at a rather
later stage than the other changes. When it is completed, the
blood from all three vena3 cavas enters the right auricle, and is
driven from this into the right ventricle ; while the only blood
ntering the left auricle is now the blood returned from the
ngs by the pulmonary veins, vessels which up to the time of
birth are comparatively small and insignificant, but which dilate
very greatly as soon as lung breathing is established.

The circulation, by these changes, becomes that of the adult.
A complete double circulation is established ; the right and left

E E 2

420 THE RABBIT.

sides of the heart are perfectly distinct from each other; and to
get from one side to the other the blood must pass through
either the pulmonary or the systemic circulation.

5. The Circulation in the Yolk-sac.

The circulation in the yolk-sac is definitely established by
the tenth day, and presents some points of interest.

In the rabbit, as in other Vertebrates, the vitelline vessels
are developed in the inner, or splanchnopleuric layer of the
mesoblast, beyond the embryonal area (Figs. 146 and 147). The
mesoblast, as already noticed, only extends over the upper half
of the blastodermic vesicle ; the lower half, or hemisphere, having
a wall composed of epiblast and hypoblast alone. The boundary
between these two halves is a sharp one, and is indicated by an
annular vessel, the sinus terminalis (Figs. 14G and 147, Si),
which runs round the margin of the mesoblast, and marks the
outer limit of the vascular area.

The course of the vitelline vessels in the rabbit differs in
some important respects from that of the chick. In the chick
the vitelline arteries and veins lie in two layers, the veins being
dorsal or superficial to the arteries ; and the sinus terminalis is
a vein, which collects the blood from the marginal part of the
vascular area and returns it, by branches which form main
factors of the vitelline veins, to the heart (cf. Fig. 99).

In the rabbit, on the other hand, all the vessels of the vascular
area lie in one plane. The vitelline arteries run straight back-
wards from the embryo, and open at once into the sinus terminalis,
which is therefore an artery, and not, as in the chick, a vein. From
the vitelline arteries themselves, and from the sinus terminalis,
smaller arteries arise, which branch freely and end in capillaries ;
the capillaries unite to form veins which open finally into the
vitelline veins themselves, a pair of large vessels which run in
the vascular area, concentrically with the sinus terminalis, but
about midway between this and the embryo. Opposite the
anterior or head end of the embryo, the vitelline veins turn
sharply backwards, and, entering the embryo along the yolk-
stalk, run forwards to the heart.

There are at first two vitelline arteries, and two vitelline
veins. Of the two arteries, the left one soon becomes much the
larger, the right one appearing as a mere branch of it. Both

THE EXCRETORY SYSTEM. 421

vitelline veins may persist, but more usually the right one becomes
much reduced in size, or else atrophies completely.

In the rabbit, the vitelline circulation is of much less impor-
tance than in the chick, inasmuch as the nutrition of the rabbit
embryo is effected, not by the yolk-sac, but by the placenta.

DEVELOPMENT OF THE EXCRETORY SYSTEM.

The general history of the development of the excretory
organs and their ducts in the rabbit is much like that of the
chick.

Xo trace of a head kidney, or pronephros, has yet been de-
scribed, and it may be assumed that this structure is either
altogether absent, or else very small and rudimentary. A seg-
mental, or Wolffian duct is early formed along each side of the
body ; and in connection with each duct a Wolffian body is
developed, which is large in the embryo, but which becomes
replaced functionally by the metanephros or permanent kidney
in the later stages and in the adult animal. The Miillerian
duct develops rather later than the Wolffian duct and Wolffian
body ; it lies very close to the Wolffian duct, but is apparently
independent of this.

1. The Wolffian Duct.

The mode of development of the Wolffian duct in the rabbit
has been much debated ; the point in dispute being whether it
is formed from mesoblast, or directly from the external epiblast.

According to the observations of Hensen, supported by
Flemming, the Wolffian duct arises, early in the ninth day, as a
solid ridge-like thickening of the epiblast (Fig. 164, kc), at the
level of the fourth and fifth mesoblastic somites, and close to their
outer borders. It soon separates from the epiblast, and then lies
as a solid rod of cells between the epiblast and mesoblast ; this
rod grows rapidly backwards, becomes tubular by the formation
of an axial cavity or lumen, and on the eleventh day reaches
the hinder end of the body, and opens into the dorsal surface of
the allantois, just in front of the union of the rectum and the
allantois to form the cloaca (cf. Fig. 150, KC, tc).

There is no doubt that the Wolffian duct, in the early stages
of its development, lies very close indeed to the epiblast, espe-

422

THE RABBIT.

cially at its hinder end ; but the more recent and very careful
observations of Martin show conclusively that it is merely a
case of very close apposition, and that the duct is really of
mesoblastic origin along its entire length : its mode of formation
being practically the same as that already described in the case
of the chick, p. 315.

2. The Wolffian Body.

The Wolffian body commences to form, in the latter part of
the ninth or early part of the tenth day, as a series of solid strings

AN

Fig. 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.) x 75.

A, dorsal aorta. AN, amnion. C. ccelorn, or body cavity. CH, notocbord. H,
hypoblast. KC, Wolffian duct. ME, somatopleuric layer of mesoblast. MH,
splanchnopleuric layer of mesoblast. MS, mesoblastic somite or protovertebra. NS,
central canal of spinal cord.

of cells, which lie to the inner side of the Wolffian duct along
almost its entire length.

These strings of cells are stated to arise as ingrowths from
the peritoneal epithelium ; but the point is not definitely esta-
blished, and from a very early stage the strings lie embedded in
the mesoblast, and quite independent of the peritoneum. The
strings soon become tubular, and are then spoken of as the
Wolffian tubules. Each tubule opens at one end into the Wolffian
duct ; while its opposite, or blind end, becomes expanded into a
vesicle, and then doubled up on itself to form a Malpighian body,
into which a branch of the aorta quickly penetrates to form the
glomerulus (Pig. 165, gm). The Wolffian tubules are not seg-

THE WOLFFIAN BODY

423

mentally arranged ; two or three corresponding to each somite
in the region of the body in which they occur.

The relations of the Wolffian body to the blood-vessels are
well seen in Fig. 165. The dorsal aorta, A, lies between the
two Wolffian bodies, and gives off branches which supply the

ME

Fig. 165. — A transverse section across the body of a Kabbit Embryo at the
end of the eleventh da}7. x 45.

A, dorsal aorta. A"W, lumbar artery. O»coelom, or body cavity. CH, notochord.
Q-M, glomerulus of Malpighian body. GR. genital ridge. KC. Wolffian duct, KM,
tubule of Wolffian body. ME, somatqpleure. MH, mesentery. ND, dorsal root of
spinal nerve. WE, spinal ganglion. NN, ventral division of spinal nerve. NI',
dorsal division of spinal nerve. NS, spinal cord. NV, ventral root of spinal nerve.
NY, sympathetic ganglion. TI, intestine. V, vein. VC, posterior cardinal vein.

glomeruli ; while the posterior cardinal veins, VC, lie along their
dorsal surfaces, and give off numerous branches, which lie in
very close relation with the tubules, and from which the epi-
thelial cells of the tubules withdraw the excretory products.

The Wolffian body increases rapidly in size, and soon becomes
more compact and of more definite shape. Its position and rela-

424

THE RABBIT.

tions on the twelfth day are well seen in Fig. 1 50, where the
transversely running Wolffian tubules, KM, opening into the
longitudinal Wolffian duct, KC, give the whole organ a some-
what comb-like appearance.

By the fourteenth day, the Wolffian body (Fig. 1 66, km) has
increased still further in size, especially at its hinder end; and

Fig. 166. — A transverse section across the hinder part of the body of a Rabbit
Embryo of the fourteenth day, the section passing through the hind
limbs, x 12.

A, dorsal aorta. AA, allantoic artery. CH, notochord. 1*0, centrum of vertebra.
KC, Wolffian duct. KD, ureter, or metanephric duct. KM, Wolffian body. KT,
kidney or metanephros. LP, hind limb. ND, dorsal root of spinal nerve. NE. spinal
ganglion. "NN, ventral division of spinal nerve. NS, spinal cord. NV ventral root
of spinal nerve. NY, longitudinal sympathetic cord. TA, stalk of allantois. TC,
cloacal aperture on cloacal papilla. TI, intestine. VC, posterior cardinal vein.

it remains of large size until within a short time of birth. It
is the excretory organ of the embryo, and its great size and
abundant vascular supply indicate that it is of considerable
functional importance.

3. The Kidney and Ureter.

The adult kidney, or metanephros, develops in the rabbit
in much the same manner as in the chick.

THE KIDNEY AND URETER. 425

On the eleventh day a diverticulum arises from each Wolf-
fian duct, just before this reaches the cloaca, and grows forwards
dorsal to the Wolffian duct. During the twelfth day this
diverticulum (Fig. 150, kd) increases in length ; its blind anterior
extremity dilates, and gives rise to branching tubular processes ;
while the mesoblast surrounding the processes becomes more
compact than elsewhere, giving definite shape to the organ.

The structure formed in this way becomes the kidney, the
original diverticulum from the Wolffian duct forming the ureter.
The kidney extends forwards, dorsal to the Wolffian body, and
overlapping this, so that both the Wolffian body and the kidney
may be cut in the same transverse section (Fig. 1GG, km and kt).

In the later stages, the lateral branches from the ureter sub-
divide, and elongate considerably to form the kidney tubules.
There are for some time no Malpighian bodies in the kidney,
but these are ultimately formed in connection with the blind
ends of the tubules, in the same manner as in the Wolffian
body.

The kidneys are, from the first, compact bodies ; they early
acquire their characteristic shape (Fig. 1(J0, k), and also their
asymmetry in position, the right kidney moving some distance
further forwards than the left.

The Wolffian duct and the ureter of each side open, on the
twelfth day (Fig. 150, kc, kd), by a common duct into the
urino-genital sinus. In the later stages, by unequal growth in
different directions, and by absorption of the common duct into
the urino-genital sinus, the relations become altered ; the Wolffian
ducts still opening into the urino-genital sinus, but the ureters
now opening directly into the bladder.

4. The Mullerian Duct.

The mode of the development of the Mullerian duct in the
rabbit has not been very clearly ascertained. About the twelfth
Of thirteenth day it is present as a peritoneal funnel, lying to
the inner side of the Wolffian body, close to its anterior end ;
from the funnel a duct arises which crosses over, dorsal to the
Wolffian body, and then runs backwards a short distance, lying-
very close to the outer side of the Wolffian duct, and ending
blindly behind. During the succeeding days the Mullerian

426 THE RABBIT.

duct grows slowly backwards, but does not reach the level of
the urino-genital sinus until about the twentieth day.

5. The Genital Ducts and Accessory Genital Organs.

In the male, or buck rabbit, outgrowths from the tubules of
the Wolffian body penetrate into the testis at a very early stage
of development, forming the tubuliferous tissue already described,
and giving rise ultimately to the vasa efferentia. The Wolffian
body becomes greatly reduced in size, and is converted into the
head of the epididymis ; the proximal part of the Wolffian duct,
which is greatly convoluted, gives rise to the body and tail of
the epididymis (Fig. 160, w) ; and the distal part of the duct
forms the vas deferens, x.

The testes originally lie opposite the anterior ends of the
Wolffian bodies, and attached to the dorsal wall of the abdomen ;
ultimately they shift their position from the dorsal to the ventral
wall of the abdomen, and, passing through the inguinal rings,
become lodged in a pair of pouch-like folds of the skin, the
scrotal sacs (Fig. 160).

The Miillerian ducts, in the male rabbit, disappear completely.
The uterus masculinus (Fig. 160, s)has been stated to be formed
from their hinder or distal ends, but according to Kolliker it
is derived from the Wolffian, and not from the Miillerian ducts ;
these latter in the male never opening into, or even reaching, the
urino-genital passage.

In the female, or doe rabbit, the Miillerian ducts become
greatly enlarged, and form the oviducts. Their abdominal
openings persist as the open fimbriated mouths of the Fallopian
tubes ; the proximal portions of the ducts become the Fallopian
tubes themselves ; the middle portions become the uteri ; and
the terminal, or distal segments unite to form the vagina.

The Wolffian bodies, in the female, undergo degenerative
changes ; they become greatly reduced in size, and are ultimately
converted into the parovaria. The Wolffian ducts either
disappear completely, or else small portions of them persist as
rudimentary or vestigial structures.

nc
pr

THE CCELOM. 427

DEVELOPMENT OF THE CCELOM.

The ccelom, or body-cavity, of the rabbit appears, as in most
Vertebrates, as a cleft in the mesoblast, formed by splitting, or
rather by rearrangement, of its cells into two layers, somatic and
splanchnic.

The ccelom appears first on the eighth day, and by the ninth
day (Fig. 1 46, c) has become a cavity of considerable size. It
is not confined to the embryo, but stretches out beyond this,
and in all directions, reaching almost to the margin of the
mesoblast, indicated by the sinus terminalis, Si.

Immediately in front of the embryo, in the pro-amnion
(Fig. 145, a?nt1), there is at first no mesoblast, and consequently
no ccelom ; but in the later stages, as the mesoblast invades the
proamnion from its sides, the ccelomic cavity extends into this
gion also.

Within the embryo itself, the ccelom is confined to the body
egion, and does not extend forwards into the head. The
abdominal portion of the ccelom presents no further changes of
special interest, but in the thorax the development of the
pericardial and pleural cavities, and also the formation of the
diaphragm, require notice.

1 . The Pericardial Cavity.

Early on the ninth day, the heart consists of two tubes,
lying along the sides of the head, and widely separate from each
other (Fig. 145, R). The parts of the ccelom into which these
tubes project become later on the pericardial cavhty, so that this
cavity, like the heart itself, consists at first of two separate
halves, right and left respectively.

As the side-folds deepen, the two halves of the heart are
brought together beneath the pharynx ; and, early on the tenth
day, the right and left halves of the pericardial cavity meet
beneath the throat, and become continuous with each other.

The pericardial cavity (Fig. 147) is thus merely the anterior
part of the general body-cavity or ccelom, and there is at first no
boundary between the two, except the very imperfect partitions
formed by- the right and left vitelline veins, where they diverge
behind the heart.

Towards the close of the tenth day, and during the early

428 THE RABBIT.

part of the eleventh day, the pericardial cavity becomes slml off
from the body cavity by a couple of septa. One of these, which
is ventral in position, is formed by a thick transverse fold of the
splanchnopleuric mesoblast, immediately behind the heart, and
between this and the liver. The second, or dorsal septum is
much thinner, and grows forwards from the walls of the
Cuvierian veins to the anterior end of the body cavity. These
two septa, between them, shut off the ventral and anterior portion
of the ccelom as a pericardial cavity, distinct from the general
body cavity (Figs. 150 and 163, cr).

2. The Pleural Cavities.

After the boxing-in of the pericardial cavity, by the dorsal
and ventral septa, is completed, the general body cavity still
extends forwards as a pair of pocket-like diverticula, dorsal to
the pericardial cavity, and along the sides of the oesophagus.
Into these pocket-like cavities the lungs hang freely, and the
pockets themselves become the pleural cavities.

As the lungs enlarge, the pleural cavities, which at first lie
entirely dorsal to the pericardial cavity, gradually extend down-
wards so as to embrace its sides (Fig. 163, cr), and ultimately
reaching almost to the mid-ventral wall of the chest.

3. The Diaphragm.

The diaphragm is formed from a couple of septa, dorsal and
ventral respectively, which arise independently, and are for some
time quite distinct from each other.

Of these, the ventral septum is the thick transverse par-
tition already described as forming the ventral part of the hinder
wall of the pericardial cavity.

The dorsal septum of the diaphragm arises, on the thirteenth
day, as a transverse fold of mesoblast, which grows downwards
from the dorsal wall of the body cavity, just behind the Cu-
vierian veins. It has for a time a free ventral edge, crescentic in
shape ; but it ultimately meets, and fuses with, the ventral septum,
or posterior wall of the pericardial cavity, thereby completing
the diaphragm, and shutting off the pleural cavities from com-
munication with the body cavity.

THE CCELOM AND THE SKELETON. 429

DEVELOPMENT OF THE MUSCULAR SYSTEM.

The majority of the body muscles are developed, as in the
chick, from the muscle plates of the protovertebras, or meso-
blastic somites.

The great dorsal muscles of the neck and trunk, and the
muscles of the thoracic and abdominal walls, are derived directly
from the muscle plates, but the origin of many of the other
muscles is not determined with certainty. The muscles of the
head arise independently of the muscle plates ; and the muscles
of the limbs also arise independently, and in situ. It is pro-
bable, however, that in both these cases the mode of development
has undergone secondary modifications and abbreviations.

DEVELOPMENT OF THE SKELETON

There are in the rabbit, as in the chick or frog, three stages
in the development of the skeleton. The first, or earliest, stage
is that in which the notochord is the only specially skeletal
structure present ; the second stage is that in which a carti-
laginous skeleton is developed, not only in relation with the
notochord, but in the head and limbs as well ; while the third
or final stage is characterised by the development of bone,
which gradually becomes the dominant and essential constituent
of the skeleton. Bones arise either as cartilage-bones, in direct
connection with the cartilaginous skeleton ; or else independent^
of this, as membrane-bones.

It is important to remember that each of these stages is not
a further development of the preceding stage, but an indepen-
dently arising one, which displaces its predecessor. Thus the
cartilaginous skeleton does not arise from the notochord, but
outside this and independently of it, and gradually displaces
and obliterates it ; to be displaced in its turn by the bony
skeleton. So too the lower jaw is not formed from Meckel's
cartilage, but around it ; and the formation of the bone leads
ultimately to the obliteration of its cartilaginous predecessor.

The development of the skeleton of the rabbit has not yet
been studied in detail, and there are many points on which our
knowledge is still very incomplete.

430 THE RABBIT.

1. The Vertebral Column.

On the tenth day, the note-chord, which up to this time has
been the sole skeletal structure present, becomes surrounded
by a membranous sheath ; and, during the eleventh and twelfth
days, a cartilaginous tube begins to form around this sheath.

By the fourteenth day, the cartilaginous tube is definitely
established ; and in it a distinction, as regards histological cha-
racters, is apparent, from the first, between the vertebral and
the intervertebral regions. The tube thickens on its inner
surface, and so begins to encroach upon the notochord. Opposite
the centra of the vertebrae the notochord becomes constricted,
and finally completely obliterated.

Between the successive vertebras, in the intervertebral re-
gions, the notochord remains of full width for a long time ; and,
according to Kolliker and others, it even persists throughout life,
as part of the nucleus pulposus in the axes of the intervertebral
ligaments.

From the vertebral centra the neural arches (Fig. 163, FN)
grow up at the sides of the spinal cord, during the fifteenth and
sixteenth days ; but the completion of the neural canal dorsally
does not occur until a late stage.

The first two vertebras undergo modifications similar to those
already described in the bird ; the centrum of the first vertebra
or atlas (Fig. 167) separating from the rest of the vertebra, and
fusing. with the centrum of the second, or axis, vertebra to
form its odontoid process.

The transverse and other processes of the vertebras arise as
outgrowths from the cartilaginous centra or from the neural
arches.

2. The Ribs and Sternum.

The ribs (Fig. 163, m) arise as bars of cartilage, in the con-
nective tissue septa between the several muscle-segments or
myotomes of the thorax. In the rabbit, the two or three most
anterior ribs are at first continuous with the vertebras, and
appear as elongated transverse processes. At a later stage a
joint is formed between the rib and the vertebra, and in this way
the tubercular articulation is acquired (cf. Figs. 163 and 167).
The head or capitulum of the rib develops as an outgrowth

Fig. 167. — Selected vertebra from the Eabbit. (From Marshall and

Hurst.)

I. First cervical vertebra, or atlas, from the dorsal surface. II. Second
cervical vertebra, or axis, from the right side. III. Fifth cervical vertebra;
anterior surface. IV. Fourth thoracic vertebra, from the right side.

V. Fourth thoracic vertebra, and fourth pair of ribs ; anterior surface.

VI. Second lumbar vertebra, from the right side. VII. Second lumbar vertebra ;
anterior surface.

AP, anapophysis. AZ, anterior, or pre-zygapophysis. C. centrum. CE, epiphysis
of centrum. CJEt, cervical rib. PH, facet for capitulum or head of the fourth rib.
PH', facet for capitulum of the fifth rib. FO, facet for odontoid process. FT.
for tubercle of fourth rib. HP, hypapophysis. MP, metapophysis. NS, neural
spine, or spinous process. OP, odontoid process. PZ. post-zygapophysis. PC,
capitulum or head of rib. RP, process of rib for attachment of ligaments. RS, sternal
Portion of rib. RT, tubercle of rib. RV, vertebral portion of rib. S, sternum. TP,
transverse process. VA, vertebrarterial canal. Z, articular surface for axis vertebra.

432 THE RABBIT.

from the proximal end of the rib, and does not articulate with
the vertebra until a considerably later period. The posterior ribs,
behind the first two or three, develop from the start indepen-
dently of the vertebrae.

The sternum is formed in two halves, and from the ventral
ends of the ribs. Each rib is at first slightly dilated at its
ventral end (Fig. 163, st), and these enlarged ends of successive
ribs, growing both anteriorly and posteriorly, meet and fuse, so
as to form along either side a longitudinal cartilaginous bar,
connecting the ventral ends of the ribs of its side of the body.
The two bars, right and left, approach each other, meet in the
median plane, and fuse to form the sternum.

3. The Skull (cf. Figs. 160 and 162).

The cartilaginous skull of the rabbit is formed from the same
essential elements — parachordals, trabecule, sense capsules, and
visceral bars — as in the chick or frog; and the general relations
of these parts to one another are very similar in the three animals.
Cartilage appears in the head of the rabbit embryo about the
fourteenth day, and by the sixteenth or seventeenth day the
cartilaginous skull is practically completed.

The two parachordal cartilages fuse together very early to
form the basilar plate (Fig. 151, rp), which underlies the medulla
oblongata, and forms the floor of the hinder part of the skull.
The edges of the basilar plate grow up at the sides of the brain,
and fuse with the independently arising periotic capsules
(Fig. 159, ec) ; and then, growing in towards each other, meet
above the cerebellum to complete the occipital ring (Fig. 151.
ox). In front of the supra-occipital cartilage, the roof of the
skull remains membranous until the formation of the bones.

The trabecule are a pair of rods of cartilage, which are con-
tinuous at their hinder ends with the basilar plate : further
forwards they lie at the sides of the pituitary body, and in
front of this unite to form the ethmoidal plate (Fig. 151, et).
This latter is at first small, and never becomes so large as in the
bird, but as the nose grows forwards, and the face assumes its
definite form, the ethmoidal plate extends forwards with it, giving
off from its upper surface a median vertical septum between the
two olfactory organs. The cartilaginous olfactory capsules arise
independently, but very early fuse with the ethmoidal septum. ,

THE SKULL. 433

With regard to the cartilaginous bars developed in the visceral
arches, the maxillary or palatopterygoid bar forms the basis of
the upper jaw, but it is not clear whether it arises independently,
or as an outgrowth from the mandibular bar.

The mandibular bar is a rod of cartilage, which along the
greater part of its length is known as Meckel's cartilage (Figs.
151 and 156, mc) ; it forms the basis of the lower jaw, the
bones of the mandible being formed around it, though not in
direct connection with it, except at the chin.

The hyoid bar is at first cartilaginous along its whole length,
but subsequently disappears in great part. Its lower or ventral
end forms the basis of the anterior, or lesser cornu of the hyoid
bone of the adult.

Of the first branchial bar, the only part that persists is the
ventral end, which forms the basis of the posterior, or greater
cornu of the hyoid bone. The body of the hyoid bone is
formed from the median elements of the hyoid and first
branchial arches.

The development of the auditory ossicles, and their relations
to the mandibular and hyoid bars have already been considered
in the section dealing with the development of the ear (p. 398).

Concerning the appendicular skeleton there is nothing special
to note, except that Kolliker has shown that the clavicle in
rabbit embryos of about the seventeenth day is cartilaginous ;
and that the clavicle, though presenting some peculiarities in
the details of its mode of ossification, ought to be viewed as a
cartilage bone, and not, as is commonly stated, as a membrane
bone.

DEVELOPMENT OF THE SKIN.
1. The Hairs.

Hairs are epidermal structures, and are as characteristic of
Mammals as are feathers of Birds. The first stage, in the for-
mation of a hair consists in the growth of a small solid process
from the deeper or mucous> layer of the epidermis, into the
underlying connective tissue (Fig. 15G, de). A small papilla
of vascular connective tissue grows into the deeper end of the
epidermal process, and serves for its nutrition. The hair itself

F F

434 THE RABBIT.

is formed by cornification of the axial or central cells of the
process, while the outer or peripheral cells form the hair-sheath ,
or follicle. The hair grows upwards from its base, and the free
tip soon projects above the surface of the skin.

From the first, the hairs of the eyebrows, and of the upper
lip and nose are of exceptionally large size (Fig. 149) ; and one
particularly large hair, arising from the cheek immediately
below the eye, forms a prominent feature in rabbit embryos from
about the nineteenth day onwards, and is also very large in the
adult rabbit.

2. The Claws.

The claws are formed by cornification of the epidermis at
the ends of the fingers and toes. The layer of epidermis that
becomes converted into the claw is not, in the first instance, the
most superficial one, but is a special stratum, developed between
the superficial and the deeper or Malpighian layers of the epi-
dermis ; the Malpighian layer, with the underlying dermis, being
modified to form the bed of the claw. The distal border of the
claw soon projects freely at the end of the digit, and its further
growth is effected by additions at its hinder or attached border,
and to its under surface.

3. The Mammary Glands.

The mammary glands, like the other cutaneous glands, are
formed by ingrowths of the epidermis into the underlying con-
nective tissue. These ingrowths give off secondary branches,
which are at first solid, but soon become hollow, and form the
gland cavities ; the ducts being derived from the original epi-
dermal ingrowths.

DEVELOPMENT OF THE PLACENTA.

The placenta is the organ by which the nutrition of the
embryo is effected during the period of its stay in the uterus ;
and it is through the placenta that the mammalian embryo is
enabled to attain so large a size, and so high a grade of develop-
ment at the time of birth, although formed from an ovum of
extremely small size and almost devoid of food-yolk.

The placenta (Fig. 170) is formed partly from the mother,

THE PLACENTA. 435

and partly from the embryo or foetus ; the foetal element being
supplied by the wall of the blastodermic vesicle, and by the
allantois ; and the maternal element by the part of the wall of
the uterus to which the blastodermic vesicle becomes attached.

In the chick, the allantois (Fig. 101) attains a great size, and
forms the respiratory organ of the embryo during the later stages
of its development.

In the rabbit the allantois becomes still larger and more
important, subserving nutrition as well as respiration. It be-
comes firmly attached to the wall of the uterus (Figs. 148 and
170), and then gives off, from its outer surface, vascular tufts or
villi into the substance of the uterine wall. The vessels of these
villi, which are derived from the allantoic arteries and veins, and
are therefore continuous with the blood-vessels of the embryo,
lie in close contact with the dilated maternal capillaries of the
uterus. The intervening walls between the two sets of blood-
vessels, foetal or allantoic, and maternal or uterine, become so
greatly reduced in thickness that diffusion readily takes place
between the two blood streams, through these very thin parti-
tions. In this way the foetal blood derives nutrient matter
from the maternal blood, and gives up to it the gaseous and
other excretory matters that are formed in the embryo, as a
necessary consequence of the chemical changes associated with
its growth and development.

The actual details of development of the rabbit's placenta
are extremely complicated, and the accounts given by different
investigators are at variance with one another, even in points of
primary importance. The most complete and consistent account
is that given by Duval ; it is supported in many important
respects by Minot's investigations, and has afforded the basis on
which the following description has been founded.

The mucous membrane of the uterus, in the unimpregnated
condition, is thrown into six longitudinal folds, which project
into the uterine cavity, and give it a stellate appearance in
transverse section. Of these folds, two (Fig. 168, pk) lie on the
side of the uterus next to the mesometrium, or mesenterial fold,
mm, which attaches the uterus to the abdominal wall ; these are
termed by Minot the placental folds or placental lobes. The
second pair, or periplacental folds, pm, lie at the sides of the

F F 2

436

THE RABBIT.

uterus; and the third pair, or obplacental folds, lie opposite
the placental folds, along the free or unattached border of the
uterus.

It is from the mesometrial, or placental, folds alone that the
maternal part of the placenta is derived : the periplacental and

Fig. 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.) x 12.

E, epiblast of blastodermic vesicle. EK, thickened epiblast of embryonal area.
GU, uterine glands. H, hypoblast of blastodermic vesicle. MI, outer or longitudinal
muscles of the wall of the uterus. MK, inner or circular muscles of the wall of the
uterus. MM, mesometrium, or mesenterial fold connecting the uterus with the dorsal
wall of the abdomen. PK, placental fold of uterus. PM, periplacental fold of uterus.
PR, median cleft between the two placental folds. TJC, dilated capillaries of submucous
layer of uterus. YS, yolk-sac.

obplacental folds undergo considerable changes, but do not take
any direct part in the formation of the placenta.

On the seventh day the blastodermic vesicles are spaced out
along the uterus, and the swellings or loculi of the uterus, indi-
cating their position, are well marked externally.

The blastodermic vesicle, the structure of which at this stage

THE PLACENTA. 437

lias already been described (p. 860), lies quite freely within the
uterus, and the structure of the uterine walls is as follows
(cf. Fig. 168).

The muscular walls of the uterus are well marked, consisting
of outer longitudinal, mi, and inner circular layers, mk. "Within
the layer of circular muscles come the submucous and glandular
layers. Of the six longitudinal folds of the uterus, the two
placental folds, PK, form large and prominent ridges, separated
by a deep median cleft, PR. The periplacental folds, pm, are
similar, but much smaller ; while the obplacental folds are no
longer recognisable, having become flattened out and obliterated
by the stretching, which this part of the wall of the uterus has
undergone, to make room for the embryo.

The submucous layer, which is very thick in the placental
folds, pk, but comparatively scanty elsewhere, consists of loose
connective tissue, with very numerous, branched connective-
tissue cells, and is very vascular. The blood-vessels, which are
derived from the mesometrium, perforate the muscular walls of
the uterus as small arteries and veins, and then dilate, within
the submucous layer, into large but very thin-walled capillaries
(Fig. 168, uc), which are especially numerous in the subglandular
layer of connective tissue, immediately below the surface epi-
thelium.

The epithelium lining the uterus is pitted to form the
uterine glands, GU, which are very deep and freely branched
in the placental and periplacental folds ; while in the obplacental
area, owing to the stretching which this part of the uterus has
undergone, the mouths of the glands are greatly dilated, and
the glands themselves widened out.

Early on the eighth day the attachment of the blastodermic
vesicle to the wall of the uterus commences, and by the ninth
day it is completed. The attachment is effected, as already
noticed, by thickening and proliferation of the epiblast cells of
the blastodermic vesicle over a horse-shoe shaped patch, the
placental area, which surrounds the sides and hinder end of the
embryo (Fig. 145, e'). The epiblast cells of this placental area
become more numerous, by repeated divisions, and grow out into
irregular processes which fuse firmly with the surface of the
placental lobes of the uterus (Fig. 169, e). By this time, accord-

438 THE RABBIT.

ing to Duval, the uterine epithelium of the placental lobes has
entirely disappeared, by absorption, though it remains unaltered
in the deeper parts of the glands for some time longer : the
embryonic epiblast of the placental area is, therefore, in direct
contact with the connective tissue of the uterine wall.

This thickened epithelium of the placental area of the blasto-
dermic vesicle is a structure of very great importance, and has
been named by Duval the ectoplacenta. It serves in the first
instance, as just noticed, to attach the embryo to the uterine
wall, and in the later stages it plays a very prominent part in
the formation of the placenta. It must be borne in mind
throughout the following description that, if Duval's account
is correct, the ectoplacenta is entirely of foetal origin, and is not
derived, even in part, from the uterine epithelium. This is a
point, however, on which difference of opinion obtains ; Strahl,
for instance, maintaining that the ectoplacenta is formed by
proliferation of the uterine epithelium, and not from the em-
bryonic epiblast.

The embryo normally lies with its long axis coinciding with
that of the blastodermic vesicle, and therefore with that of the
uterus, so that a transverse section of the uterus cuts the
embryo transversely (Fig. 169, NG). The embryo is usually in
the middle of the upper surface of the blastodermic vesicle, and
lies opposite the deep cleft, PR, between the two placental lobes,
The position of the embryo is, however, variable, especially in
the earlier stages : it may lie obliquely across the vesicle ; or
may, more rarely, lie opposite one or other of the placental lobes,
instead of opposite the cleft between them.

In the submucous layer of the placental lobes important
changes occur during the ninth day. The capillaries dilate very
considerably (Fig. 169, uc), becoming much larger than the
arteries and veins in connection with them. They retain their
simple epithelial walls, but thick adventitious perivascular walls
are formed outside these by the surrounding connective-tissue
cells. These perivascular cells are at first ordinary connective-
tissue cells, which increase in number, draw in their processes,
and become arranged in layers, two or three cells thick, around
the capillaries. This perivascular thickening of the walls of the
capillaries occurs throughout the greater part of the submucous
layer, but does not affect the capillaries immediately beneath the

THE PLACENTA.

439

surface of the uterus, nor those of the outermost layer, next to
the circular muscles of the uterine wall.

Fig. 169. — A transverse section across the uterus and the contained blasto-
dermic vesicle of a Eabbit at the end of the ninth day. Cf. Figs. 145 and
146, in which the blastodermic vesicle and embryo of this age are shown
in surface view and in sagittal section. (In part after Duval.) x 8.

AW, side fold of the amnion. C, mesoblast of the upper wall of the blastodermic
vesicle, beyond the embryonal area. E, epiblast of the blastodermic vesicle ; the upper
reference is t) the thickened epiblast of the placental area. GU, uterine glands. G"W,
modified uterine glands of placental region. H, hypoblast of blastodermic vesicle. MI,
outer or longitudinal muscles of the wall of the uterus. MR inner or circular muscles of
the wall of the uterus. MM, mesometrinm. NG, neural groove of embryo, in trans-
verse section. PR, median cleft between the placental lobes of the uterus. SI, sinus
terminalis. TJC, capillaries of placental region, with thickened perivascular walls.
UL, giant cells. UV, blood-vessels of uterus. YS, yolk-sac, or cavity of blastodermic
vesicle.

During the tenth day, the ectoplacental epithelium increases
greatly in thickness, and becomes excavated by irregular chan-
nels or lacunse, which according to Duval open into the maternal

440 THE RABBIT.

or uterine capillaries. At the same time the inner or deeper
surface of the ectoplacenta is becoming folded, and the meso-
blast of the somatopleure grows in between these folds (cf. Fig.
147, e'). The uterine glands of the placental lobes have by this
time almost disappeared (cf. Fig. 169, gw); and a little later they
are completely absorbed. In the submucous layer the perivas-
cular thickening of the walls of the capillaries has proceeded still
further, while the capillaries themselves are even larger than
before; and ultimately nearly the whole of the connective tissue
of the submucous layer becomes converted into perivascular, or,
as they are now termed, decidual cells.

In the periplacental lobes somewhat similar changes occur.
The superficial epithelium of the uterus, and the epithelium
lining the mouths of the glands degenerate and disappear ; and
perivascular thickenings of the capillary walls occur, although
to a less marked extent than in the placental lobes.

In the obplacental region also, the uterine epithelium de-
generates and becomes absorbed ; but the epithelium of the
glands themselves remains, and at a later stage, by spreading
outwards from the mouths of the glands, reconstitutes an epi-
thelial lining to this part of the uterus.

Towards the end of the tenth day, and during the eleventh
day, the allantois is growing rapidly. As shown in Figs. 146
and 147, the outer or mesoblastic wall of the allantois very early
coalesces with the mesoblast of the outer layer of the amnion,
opposite the placental area ; and in this way the blood-vessels
of the allantois are brought immediately beneath the ectopla-
cental epithelium, and consequently into close proximity with
the dilated capillaries of the uterus : and the placenta is thus
established.

The further development of the placenta consists mainly
in a gradually increasing complication and elaboration, by
which folds of the mesoblast, containing the allantoic vessels, are
carried deeply into the ectoplacenta from its inner surface ;
while from the outer surface the maternal vessels extend in
further, and in larger numbers, than before. This interdigita-
tion of the fcetal and maternal blood-vessels is accompanied by
progressive thinning of the layer of ectoplacental epithelium

THE PLACENTA. 441

intervening between them, until ultimately the two sets of
blood-vessels are separated by exceedingly thin partitions.

The successive steps in this process are as follows ; Duval's
descriptions being mainly followed in the account here given.

From the tenth day onwards the growth of the vascular
septa, or villi, from the allantois into the ectoplacenta proceeds
very rapidly, so that the latter becomes cut up into a series
of radially arranged columns, or lobules, within which lie the
lacunas opening into the maternal capillaries. At this stage
the foetal blood is separated from the maternal blood by three
structures : — (i) the endothelial wall of the foetal or allantoic
capillaries ; (ii) a layer, several cells in thickness, of the ecto-
placental epithelium ; (iii) the endothelial wall of the maternal
capillaries. There is some doubt, however, with regard to the
third layer ; according to Duval, this has already disappeared,
and the maternal vessels of the placenta are merely lacunar
spaces hollowed out in the ectoplacental epithelium, and devoid
•of true walls.

During the twelfth to the fourteenth days, each of the ecto-
placental columns or lobules becomes subdivided, by longitudinal
folding of its walls, and ingrowth of septa, into a set of closely
placed parallel tubules, the general direction of which is radial,
i.e. vertical to the inner surface of the uterus.

These lobules, in the later stages, become larger and more
minutely subdivided, and by the nineteenth or twentieth day the
relations are as shown in Fig. 170. Each of the two placental
lobes now consists of a number of lobules, th, which are some-
what fusiform in shape, radially arranged, and packed closely
together side by side. Each lobule is further subdivided into
a complicated system of branching tubular passages, which at
each end of the lobule open into larger chambers, UP. Through
these passages, which, according to Duval, are simply lacuna?
excavated in the ectoplacental epithelium, the maternal blood
circulates. Large afferent channels, derived from the uterine
arteries, convey the maternal blood directly to the dilated
chambers at the inner ends of the lobules, next to the surface of
the uterus. From these chambers it flows back, through the
-complicated system of tubules of which the lobule consists, to
•the chambers at the outer ends of the lobules, from which it is

442 THE RABBIT.

carried away by efferent vessels which open into the uterine
veins.

The foetal or allantoic vessels, aa, pass between the several
lobules to their outer ends, and then return as thin-walled
capillaries, which pass through the lobules, lying between the
tubules into which these are divided ; on reaching the inner
surface of the uterus the capillaries open into the allantoic veins,
VA, which return the blood from the placenta to the embryo.

In the later stages, from the twenty-fifth to the thirtieth day,
the chief changes consist in the gradual thinning of the parti-
tions separating the foetal and maternal streams in the lobules.
The ectoplacental wall becomes gradually absorbed, more and
more completely, until ultimately, according to Duval, a single
layer of epithelial cells, the endothelial wall of the foetal capil-
lary, is alone left between the two streams of blood.

The changes that occur in the deeper, or submucous part
of the placenta require further notice. In the early stages
(Fig. 169), the submucous layer is very thick, and the ecto-
placenta very thin. By the nineteenth day (Fig. 170) the two
have become of about equal thickness. From this time the
submucous layer is the thinner of the two, and towards the
close of gestation it becomes comparatively insignificant.

The chief changes that occur in the submucous layer during
these later stages are : — a still further dilatation of the capillary
vessels ; an increase in the decidual cells surrounding the capil-
laries ; and the appearance in the subglandular region of a layer
of special cells, spoken of as glycogenous cells. These latter
are large, roundish, or ovoid vesicular bodies, each consisting of
an outer capsule and a central multinucleate protoplasmic body,,
from which strands of protoplasm radiate outwards to the cap-
sule. In the meshes between the strands lie faintly glistening
glycogen masses. Each of these glycogenous cells is said to be
formed by the fusion of a number, from three to six, of origi-
nally separate cells.

The perivascular and glycogenous cells are probably to be-
regarded as having some function in connection with the ela-
boration, or preparation, of the maternal blood, before it is sent
to the placenta for the nourishment of the embryo. The blood

THE PLACENTA.

443-

Fig. 170. — A transverse section across the uterus and the contained embryo
of a Rabbit at the end of the nineteenth day. The embryo is cut trans-
versely, about the middle of the body, the section passing through the
yolk-stalk and allantoic stalk, x 3^.

A, dorsal aorta of the embryo. AA, allantoic artery. AX, amnionic cavity, be-
tween the inner or true amnion and the embryo. CX, apace between the inner
and outer layers of the amnion {cf. Fig. 148). GrU, uterine glands of obplacental
region. H, hypoblast of upper or vascnlar wall of yolk-sac. MI, outer or longitudinal
muscles of the wall of the uterus. MK inner or circular muscles of the wall of the
uterus. MM, mesometrium. NS, spinal cord of the embryo. NY, sympathetic nerve
cord. PH, lobule of placenta. PO, region along which the separation of the placenta
occurs at birth. PR, interplacental groove. P"W\ subplacental cavity. SI, sinus
terminalis. TA, cavity of the allantois (cf. Fig. 148). TS, stomach of einbryo. UC,
dilated uterine capillary, with thick perivascular wall. UP, titerine or maternal sinuses
i >f placenta. UV, blood-vessels of uterus. VA, allantoic vein. "W, liver of embryo.
YK, yolk-stalk. YL, dotted line representing the lower or non-vascular wall of the yolk-
sac, now completely absorbed. YS, cavity of yolk-sac, continuous with the uterine
cavity owing to absorption of the lower wall of the yolk-sac.

444 THE BABBIT.

in the maternal capillaries of the placenta is specially charac-
terised by the relatively enormous number of leucocytes which
it contains, at all stages from about the eleventh or twelfth day
onwards.

Curious modifications occur in the lining epithelium of the
maternal capillaries of the submucous layer during the forma-
tion of the placenta. The ordinary endothelial walls, which
these vessels at first have, become replaced by a layer of
irregular, thickened, and often columnar cells. According to
Duval, this layer is formed by extension outwards of cells from
the ectoplacenta along the interior of the maternal vessels ;
while Minot regards it as formed by degenerative changes in
the proper endothelial wall of the capillaries.

In the obplacental region, and to a less extent in the peri-
placental region, certain peculiar cells, characterised chiefly by
their enormous size, and hence spoken of as colossal or giant
cells, appear in the submucous layer at an early stage (Fig. 169,
ul). These are stated to be derived from the uterine epithelium
of these regions ; they form marked features from the ninth
day onwards, but their function is entirely unknown. Cells of
exceptional size are commonly associated with absorptive, rather
than with formative changes, but the actual absorption occur-
ring in this region of the uterus is comparatively slight in
amount.

Parturition. The outer layer of the submucous connective
tissue, next to the circular muscle layer of the uterus, is charac-
terised by the small size of its blood-vessels (Fig. 170, po) ; and
it is along this line that separation takes place at the time of
birth, the entire placenta, both foetal and maternal, coming away
with the young animal.

The actual separation is effected by strong contractions of
the muscles of the uterus. The haemorrhage at parturition is
but slight, partly because the blood-vessels along the plane of
separation are small, and partly because of the rapidity with
which complete contraction of the uterus is effected.

Long before the birth of the young animal, the mucous
membrane of the obplacental region of the uterus has been
completely re-established ; this mucous membrane is attached
to the muscular walls of the uterus by very loose connective

THE PLACENTA. 445

tissue. Owing to the strong contraction of the muscles of the
uterus at the time of birth of the young, the bare patch, from
which the placenta has been separated, is at once greatly re-
duced in size, while the loosely attached mucous membrane
of the obplacental region slips over it, and closes the wound
almost instantly. The complete regeneration of the uterine
epithelium after parturition is effected with astonishing rapidity,
and the doe is ready to receive the buck almost immediately
after she has given birth to the young.

The placenta is commonly regarded as essentially an allantoic
structure. But the facts, that the attachment to the uterus is
first effected, not by the allantois, but by the epiblast of the
blastodermic vesicle ; and that the allantois merely utilises this
attachment as a means of getting access to the uterus, suggest
that the participation of the allantois in the formation of the
placenta is probably a secondary and not a primitive character.

The further fact that changes occur in the mucous membrane
of the obplacental and periplacental regions, similar to the earlier
changes seen in the placental region, suggests that the area of
attachment of the blastodermic vesicle to the uterus was origin-
ally a more extensive one. Minot has contended, from these
and similar considerations, that the mammalian placenta was
originally formed from the chorion, i.e. from the extra- embryonic
part of the blastodermic vesicle, and not from the allantois ;
and the history of the formation of the placenta in the lower
groups of Mammals strongly supports this view.

List of the more important Publications dealing with the
Development of the Rabbit.

Barry, M. : ' Eesearches in Embryology.' Philosophical Transactions of the
Royal Society. 1840.

Beneden, E, van: * Recherches sur la Composition et la Signification de
l'CEuf.' Bruxelles. 1870.

'Recherches sur l'Embryologie des Mammiferes. La Formation
des Feuillets chez le Lapin.' Archives de Biologie, i. 1880.

* Observations sur la Maturation, la Fecondation et la Segmenta-
tion de l'CEuf des Cheiropteres.' Archives de Biologie, i. 1880.

Beneden, E. van, and Julin, C. : ' Recherches sur la Formation des Annexes
Fcetales chez les Mammiferes.' Archives de Biologie, v. 1881.

Bischoff, T. L. W. : ' Entwicklungsgeschichte des Kaninchen-Eies.' Braun-
schweig. 1842.

440 THE RABBIT.

Born, G. : 4 Beitriige zur Entwickelungsgeschichte des Siiugethicrherzens.

Archiv fur niikroskopische Anatomie, xxxiii. 1889.
Coste, M. : ' Histoire gen6rale et particuliere du Developpement des Corps

organises.' Paris. 1847-18;")*.).
Duval, M. : ' Le Placenta des Rongeurs.' Journal de 1' Anatomie et de la

Physiologie, xxv. and xxvi. 1889-00.
Flemming, W. : ' Ueber die Bildung von Richtungsfiguren in Siiugethiereiern

beim Untergang Graaf'scher Follikel.' Archiv fur Anatomie und Ent-

wickeluogsgeschichte. 1885.

' Die ektoblastische Anlage des Urogenitalsystems beim Kanin-

chen.' Archiv fiir Anatomie und Entwickelungsgeschichte. 1886.
Foster, M., and Balfour, F. M. : « The Elements of Embryology; ' second edition

by .Sedgwick and Heape. 1883.
Fraser, A. : 'On the Development of the Ossicula Auditus in the Higher Mam-
malia.' Philosophical Transactions of the Royal Society. 1882.
Goette, A. : ' Beitriige zur vergleichenden Morphologie des Skeletssystems der

Wirbelthiere.' Archiv fiir mikroskopische Anatomie, xiv. 1877.
Haddon, A. C. : ' Suggestions respecting the Epiblastic Origin of the Segmental

Duct.' Scientific Proceedings of the Royal Dublin Society. 1887.
Hensen, V. : ' Beobachtungen iiber die Befruchtung und Entwicklung des

Kaninchens und Meerschweinchens.' Archiv fiir Anatomie und Ent-
wickelungsgeschichte. 1875.
Hochstetter, F. : ' Ueber die urspriingliche Hauptschlagader der hinteren

Gliedmasse des Menschen und der Siiugethiere.' Morphologisches

Jahrbuch, xvi. 1890.
Hubrecht, A. A. W. : ' Studies in Mammalian Embryology : i. The Placenta-

tion of Erinaceus Europtcus, with Remarks on the Phylogeny of the

Placenta.' Quarterly Journal of Microscopical Science, xxx. 1889.
Huxley, T. H. : 'Evolution and the Arrangement of the Vertebrata.' Pro-
ceedings of the Zoological Society. 1880.
Kastschenko, N. : * Das Schicksal der embryonalen Schlundspalten bei Sauge-

thieren.' Archiv fiir mikroskopische Anatomie, xxx. 1887.
Kolliker, A. : ' Entwicklungsgeschichte des Menschen und der hoheren Thiere.'

Leipzig. 1879.

' Ueber die Chordahohle und die Bildung der Chorda beim Kaninchen.'

Sitzungsberichte der phys.-med. Gesellschaft in Wiirzburg. 1883.

< Die Entwicklung der Keimbliitter des Kanincheos.' Leipzig.

1882.
Krause, W. : ' Die Anatomie des Kaninchens.' Leipzig. 1868.
Lockwood, C. B. : • The Early Development of the Pericardium, Diaphragm,

and Great Veins.' Philosophical Transactions of the Royal Society.

1888.
Lowe, L. : ■ Beitriige zur Anatomie und zur Entwickelungsgeschichte des

Nervensystems der Siiugethiere und des Menschen.' Berlin. 1880.
Martin, E. : ' Ueber die Anlage der Urniere beim Kaninchen.' Archiv fiir

Anatomie und Entwickelungsgeschichte. 1888.
Masius, J. : * De la Genese du Placenta chez le Lapin.' Archives de Biologie,

ix. 1889.
Masquelin, H., and Swaen, A.: 'Premieres phases du Developpement du

Placenta maternel chez le Lapin.' Archives de Biologie, i. 1880.

BIBLIOGRAPHY. 447

Minot, C. S. : ' Uterus and Embryo. I. Rabbit. II. Man.' Journal of Mor-
phology, ii. 1889.

1 Die Placenta cles Kaninchens.' Biologisches Centralblatt, x. 1890.
' Zur Morphologie der Blutkorperchen.' Anatomischer Anzeiger, v.
1890.

• A Theory of the Structure of the Placenta.' Anatomischer Anzeiger,
vi. 1891.
Owen, R. : ' Comparative Anatomy and Physiology of Vertebrates,' iii. 1868.
Parker, W. K. : ' Mammalian Descent.' London, 1885.
Paterson, A. M. : ' Development of the Sympathetic Nervous System in

Mammals.' Philosophical Transactions of the Royal Society. 1890.
Retterer, E. : 'Sur l'Origine et l'Evolution de la Region ano-genitale des
Mammiferes.' Journal de l'Anatomie et de la Physiologie, xxvi. 1890.
Robinson, A. : • Observations upon the Development of the Segmentation
Cavity, the Archenteron, the Germinal Layers, and the Amnion in
Mammals.' Quarterly Journal of Microscopical Science, xxxiii. 1892.
mafer, E. A. : • Quain's Anatomy.' Tenth edition, vol. i., part 1. 1890.
brahl, H. : ' Zur Bildung der Cloake des Kaninchenembryo.' Archiv fur
Anatomie und Entwickelungsgeschichte. 1880.

' Untersuchungen iiber den Bau der Placenta.' Archiv fur Anatomie
und Entwickelungsgeschichte. 1889.
mrneux, F. : ■ Sur les Modifications que subit l'CEuf de la Lapine pendant
sa Migration dans l'Oviducte, et sur la duree de cette Migration.'
Comptes Renclus de la Societe de Biologic Serie ix., tome 1. Paris.
1889.
Vassaux, G. : ' Recherches sur les premieres phases du Developpement de

l'CEil chez le Lapin.' Archives d'Ophthalmologie, viii. 1888.
Woodward, M. F.: 'On the Milk-dentition of Hyrax and of the Rabbit.'

Proceedings of the Zoological Society. 1892.
Zimmermann, W. : ' Ueber einen zwischen Aorten- und Pulmonalbogen
gelegenen Kiemenarterienbogen beim Kaninchen.' Anatomischer
Anzeiger, iv. 1889.

lis

Chapter VI.

THE DEVELOPMENT OF THE HUMAN
EMBRYO.

PRELIMINARY ACCOUNT.

The human embryo is developed from an egg, which, like that
of other animals, is a single nucleated cell, derived from the
peritoneal cells forming the outermost layer of the ovary. The
human egg, or ovum, measures 0*2 mm. in diameter, i.e. is
rather less than double the diameter of the ovum of the rabbit.

The ovum, when ripe, is discharged from the ovary, and is
taken up by the open mouth of the Fallopian tube or oviduct,
down which it travels to the uterus, where it remains during
the rest of the period of development. Prior to the arrival of the
ovum, the mucous membrane lining the uterus undergoes impor-
tant changes, and gives rise to a special layer, the decidua, to
which the ovum is attached and in which it becomes embedded.
During gestation the cavity of the uterus gradually becomes
filled up by the growth of the embryo and of its inclosing
membranes.

As the ovum is of very small size, the nutriment at the
expense of which development takes place must be obtained
from without. This is effected, as in the rabbit, by means of
the placenta, an organ in which the blood-vessels of the embryo
and tfrose-'of the wall of the uterus are brought into extensive
contact, so that free interchange of contents can take place
through their walls.

The process of fertilisation of the human ovum, and the early
stages of development have not yet been seen ; and of specimens
showing the first formation of the embryo only a very limited
number have been obtained, and very few of these in fit
condition for microscopical investigation. Of later embryos,
numerous specimens have been examined and described, and

THE HUMAN OVUM. 440

from a stage corresponding to about a forty-eight hour chick
embryo, or to a nine-day rabbit embryo, the history of human
development has been determined in considerable detail.

For this satisfactory condition of our knowledge we are very
largely indebted to the labours of Professor His, whose detailed
and careful descriptions, and splendid series of figures, form the
basis on which the account in the present chapter has been in
chief part founded.

The total period of human development is usually estimated
at slightly under ten lunar months. The exact period cannot
be ascertained, owing to the impossibility of determining the
time at which fertilisation of the egg is effected, i.e. at which
■development commences.

The details of development of the human embryo are closely
similar to those of the rabbit ; the chief points of difference
being : — (i) the far longer time occupied by the human embryo,
more than nine times as long as the rabbit ; (ii) the extreme
slowness with which the early stages of development are effected
in the human embryo ; and (iii) the early stage at which the
allantois is established in the human embryo, and the peculiar
mode in wliich it is formed.

THE HUMAN OVUM.
1 . Formation of the Ovum.

The earlier stages in the development of the ova are already
completed in the female child before birth ; and after birth the
formation of ova only goes on for a very short time, and to a
very limited extent. According to Bischoff, Waldeyer, Foulis,
and others, the formationof new ova ceases about the age of
two years ; in other words, the ovaries of a female child already
contain, at the end of the second year, all the ova that will ever
be developed in them. As each ovum is morphologically a
single cell, this means that an individual cell may live, and
retain all its characteristic activities, for a period of forty-five
years or more.

The early development of the human ova must, therefore, be
studied, not in the woman or child, but in the embryo. The
several stages of its formation are so closely similar to those

G Cr

450 THE HUMAN EMBRYO.

.already described in the rabbit that a detailed account will be
unnecessary.

The germinal epithelium. In embryos of about the fifth
week, the genital ridges appear, as a pair of longitudinal bands
along the dorsal wall of the abdominal cavity, close to the inner
borders of the Wolffian bodies. The ridges, which at first are
merely caused by the epithelial cells becoming columnar in place
of squamous in shape, rapidly increase in thickness, partly owing
to active division of the cells of the germinal epithelium
covering the ridge, and partly owing to ingrowth of connective
tissue along their axes.

At an early age, an intimate relation is established between
each genital ridge and the corresponding Wolffian body, a
number of rod-like outgrowths arising from the Malpighian
bodies of the Wolffian body, and growing into the substance of the
genital ridge. These rods subsequently become hollow, and form
the so-called tubuliferous tissue of the ovary. This lies, at first,
close beneath the germinal epithelium, but soon withdraws into
the deeper part of the ovary ; it has nothing to do with the
formation of the ova, and merely requires mention on account
of its great prominence during the early stages of develop-
ment.

The germinal epithelium gives rise to the ova in much the
same way as in the rabbit. In its earliest stages it is a single
layer of columnar epithelium cells, with large nuclei, the cells
measuring on an average about 0*014 mm. in length by 0-007
mm. in width.

By division of its cells, the germinal epithelium rapidly
increases m thickness. The surface cells remain columnar, but
the deeper cells, which are spherical or polygonal in shape,
grow down into the connective-tissue stroma as irregular
branching rods of cells, the egg columns (Fig. 171). By further
growth inwards of the egg columns, accompanied by active
growth outwards of the vascular connective tissue, the structure
of the ovary rapidly becomes more complicated. In place of the
original arrangement, of a layer of epithelial cells clothing a
central connective-tissue core, there is now (Fig. 171) a super-
ficial layer of columnar epithelium, a, beneath which is a reti-
cular framework of connective tissue, the meshes of which are

THE OVUM.

451

filled with irregular columns or rods of epithelial cells, arranged
for the most part vertical to the surface.

The primitive ova. The columnar epithelial cells of the
surface layer are at first all much the same size, but they do not
long remain so. At an early period, about the sixth or seventh
week, certain of the cells become conspicuous by their larger
size and more spherical shape ; these are the primitive ova
(Fig. 171, c, c), each of which is capable of developing into a
definitive or permanent ovum, and then, if fertilised, of giving

Fig. 171.

-Part of a vertical section of the ovary of a new-born Infant.
(From Strieker's ' Histology.') x 150.

a, superficial layer of columnar epithelium, b, plate of epithelial cells, formed by
irregular growth of the ovary, c c, primitive ova. de, nests of various shapes, containing
ova and commencing follicles. /, isolated follicle with its contained ovum, g, blood-
vessel.

rise to an embryo. Each of these enlarged epithelial cells is in
fact a potential human being.

On the formation of the egg columns, by proliferation of the
deeper surface of the germinal epithelium, the primitive ova are
carried down into them in large numbers. As the egg columns
penetrate deeper and deeper into the substance of the ovary,
they become broken up, by further growth of the connective
tissue, into groups or nests of cells (Fig. 171, d, e), each nest
containing one or more primitive ova as well as a number of
indifferent epithelial cells. In these nests a tendency soon

G o 2

452 THE HUMAN EMBRYO.

manifests itself for the smaller or indifferent cells to arrange
themselves round the primitive ova, so as to inclose these in
follicles (Fig. 171, d, e,f). At first there may be in a single
nest several of these follicles, each containing a single ovum, but
the continued growth of the connective tissue stroma gradually
breaks up the nests, and tends to isolate the several follicles from
one another, forming around each of them a separate connective
tissue investment.

At the time of birth of the infant, the structure of the ovary
is as shown in Fig. 171. The germinal epithelium, a, or super-
ficial layer of columnar epithelial cells, is separated from the
deeper layers of the ovary, at almost all parts, by a thin layer
of connective tissue, the tunica albuginea. A little deeper down
are seen large nests of epithelial cells, formed by proliferation
from the deeper surface of the germinal epithelium, but cut off
and isolated by growth of the connective tissue stroma. In
these nests certain of the cells, the primitive ova, are dis-
tinguished by their larger size, and round these the smaller cells
tend to arrange themselves so as to form capsules or follicles.
In the deeper parts of the ovary the vascular connective tissue
has, by its further growth, broken up the nests, and separated
the follicles more or less completely from one another.

In passing from the exterior towards the deeper parts of
the ovary, successive stages in the development of the ova are
met with. In the superficial layer of columnar epithelial cells
the earliest stages are seen ; certain of these cells, the primi-
tive ova, being of rather larger size than their neighbours.

Beneath this surface layer are large nests, composed of
epithelial cells, which, except in the larger size of the primitive
ova, differ but little from one another, and present no regularity
of arrangement. In the more deeply placed nests, the cells
immediately adjacent to the ova have arranged themselves round
these latter so as to form follicles ; but there are, in such nests,
many cells of indifferent character, whose ultimate fate is still
uncertain. Deeper still, the number of these indifferent cells is
greatly diminished; and the follicles are larger, more clearly
defined, and separated from one another by connective tissue
trabecule.

In such a section, therefore, as in Fig. 171, the most deeply
situated ova are the oldest and most mature, and have, in

THE OVUM. 453

attaining their present position, passed in succession through the
several stages which are met with in passing from the surface
to the deeper parts of the ovary.

The primitive ova are spherical cells, from 0-05 to 0*07 mm.
in diameter, with granular and rather ill-defined nuclei, and
devoid of nuclear membranes. Each primitive ovum is inclosed
in a follicle, consisting of a single layer of small cubical or
flattened epithelial cells.

The permanent ova. About the time the egg follicles or
capsules commence to form around the primitive ova, these
latter undergo changes by which they become converted into
the permanent ova. Primitive ova occur in both sexes, and the
early stages in the development of the genital organs are the
same in both ; but the change to permanent ova occurs in the
female only, and marks the establishment of sexuality.

The change, as in other animals, chiefly concerns the nucleus.
In the primitive ovum this is uniformly granular, with a rather
ill-defined outline ; in the permanent ovum it becomes converted
into a spherical vesicular body, of much larger size than before,
with a sharply defined double-contoured wall, fluid contents,
and a nuclear reticulum with one or more nucleolar enlarge-
ments at the nodes.

Besides the changes in the nucleus, the whole egg increases
in size ; its protoplasm, previously clear, becomes granular ; and
around the egg, between it and the follicle, a thin elastic invest-
ing membrane, the zona radiata, is formed.

The Graafian follicle. Each ovum is surrounded at first by
a single layer of cells, derived, like the ovum itself, from the
germinal epithelium. These cells are at first flattened, but very
shortly become cubical or columnar in shape. Since they lie
between the ovum and the blood-vessels of the ovary, the nutrient
matter must pass through the follicular cells in order to reach
the ovum ; and it is probable that these cells do not merely
transmit the food, but play some part in elaborating it.

A second layer of cells soon appears in each follicle, formed,
as in the rabbit, between the original layer and the ovum, and
probably by division of this originally single layer of cells
into two. Shortly afterwards, by further division, the follicle

454 THE HUMAN EMBRYO.

becomes several cells thick. By splitting apart of these cells,
accompanied by rapid growth of the outer layer, a cavity is
formed in the thickness of the wall of the follicle; and this
cavity, which is filled with fluid, rapidly increases in size, dividing
the follicle into an outer wall, the tunica granulosa, and an
inner one, or discus proligerus, which immediately invests the
ovum (cf. p. 349).

The fully formed Graafian follicle (cf Fig. 133, gk, p. 347)
is ovoid or ellipsoidal in shape: its walls consist of: — (i) an
outer investment of vascular connective tissue, derived from the
stroma of the ovary, and divisible into a rather ill-defined outer
layer, the tunica fibrosa folliculi ; and an inner well-marked layer
of fine connective tissue, abundantly supplied with capillary
blood-vessels, the tunica propria folliculi. (ii) Within this
latter is the tunica granulosa (Fig. 133, gb), a thick layer
of granular, spherical or polygonal cells. At one part, the
tunica granulosa is much thickened, forming a roundish mass
projecting into the cavity of the follicle ; and embedded in the
middle of this roundish mass, or discus proligerus, is the ovum,
OW. The cells immediately surrounding the ovum are distinctly
columnar in shape, while the remaining cells of the follicle
are spherical or polygonal. The cavity of the follicle is filled by
the watery liquor folliculi.

In the early stages of their formation (Fig. 133) the more
mature Graafian follicles lie in the deepest parts of the ovary ;
but, as they increase in size, their growth takes place in all
directions ; and ultimately the outer walls of the follicles approach
very close to the surface of the ovary, or actually push the super-
ficial layer of epithelium and connective tissue of the ovary
before them, and so form rounded external projections on its
surface.

At the most prominent part of the ripe Graafian follicle is a
small spot, the hilum folliculi, distinguished from the rest of the
follicle by being devoid of blood-vessels : at this place, shortly
after the follicle has reached its full dimensions, i.e. a diameter
of from 1*25 to 4 mm., rupture of the follicular wall occurs, and
the ovum, together with the liquor folliculi, is discharged on the
surface of the ovary.

This rupture of the wall of the Graafian follicle is due in part
to fatty degeneration of the cells composing the wall ; and in

THE GRAAFIAN FOLLICLE AND CORPUS LUTEUM. 455

part to increased pressure on the follicle, caused by a sudden
accession of blood to the ovary.

The ripe ovarian ovum. The ripe human ovum is a spherical
cell, about 0'2 mm. in diameter. It consists of a granular mass
of protoplasm ; within which is a nucleus, or germinal vesicle,
about 0*045 mm. in diameter, containing a nuclear reticulum
and a conspicuous nucleolus or germinal spot. The ovum is
invested by a transparent elastic membrane, the zona pellucida,
which is about 0*01 mm. thick.

Each Graafian follicle, as a rule, contains only a single ovum ;
in exceptional cases two ova, and in a few instances three, have
been seen in the same follicle.

2. The Corpus Luteum.

After the discharge of the ovum, important changes occur in
the Graafian follicle, leading to the formation of the body known
as the corpus luteum, which occupies and fills up the cavity of
the follicle.

The corpus luteum is formed by rapid growth of the wall of
the empty follicle, which becomes thrown into radial folds, pro-
jecting into the cavity of the follicle, and blocking this up almost
completely. The folding involves both the follicular epithelium
and the connective-tissue wall of the follicle, but the latter
takes the most active share in the process. The characteristic
yellow colour of the folded wall, which has given rise to the
name corpus luteum, is due to large numbers of yellowish cells,
derived apparently from the connective tissue stroma of the
ovary. Between the two layers of each of the folds, blood-
vessels pass in freely ; and the central cavity of the follicle,
which, by ingrowth of the radial folds, is reduced to an irre-
gularly stellate space, becomes occupied by a cicatricial fibrous
tissue, which is red in the early stages, but in the later ones
becomes grey.

The subsequent changes in the corpus luteum differ con-
siderably according to whether the ovum, which has been dis-
charged from the follicle, (i) is fertilised and develops into an
embryo ; or (ii) is not fertilised, but dies without undergoing
any further development.

In the latter case, i.e. if the ovum is not fertilised, the corpus

45G THE HUMAN EMBRYO.

luteum spurium, as it is then called, increases slightly in size for
a few days ; but ten or twelve days after the discharge of the
ovum, commences to shrink, and disappears completely in a few
weeks' time.

If, however, the ovum that has escaped from the follicle is
fertilised, and gives rise to an embryo, the corpus luteum, now
spoken of as corpus luteum verum, or corpus luteum of pregnancy,
does not reach its full development until two or three months
after the discharge of the ovum. It persists throughout the greater
part, or the whole, of pregnancy, contracting towards the close of
the period to a small white stellate cicatrix, the corpus albicans,
which may persist for some months after delivery.

The fully developed corpus luteum verum, or corpus luteum
of pregnancy, is a firm body, larger than the original follicle,
and attaining one-fourth, or even one-third, the size of the
entire ovary (Fig. 255).

The presence of a corpus luteum verum in one of the ovaries
is a matter of considerable medico-legal importance, inasmuch
as it has been appealed to as positive evidence of pregnancy
having occurred ; but the best authorities now agree that there
is no infallible sign by which the corpus luteum of pregnancy
can be distinguished from that of the non-fertilised ovum. The
differences between the two are chiefly those of size, and length
of duration, and cannot always be relied on in determining
disputed cases. The terms ' true ' and l false,' as applied to the
two kinds of corpora lutea, appear, indeed, to be erroneous ; as
the two structures are essentially similar, and in many cases
indistinguishable from each other.

3. Ovulation.

From the time of puberty, and throughout the whole of the
child-bearing period of life, i.e. from about the fifteenth to about
the forty-fifth year, the gradual maturation of the Graafian
follicles, ending in rupture of the follicles and discharge of the
ova, is continually going on ; and in the healthy condition this
discharge of ova occurs, not in an indefinite manner, but at
regular, and usually monthly intervals, one or more ova being
set free at each period.

This periodical maturation and discharge of ova is spoken of
as ovulation. It goes on independently of sexual intercourse,

OVULATION AND MENSTRUATION. 457

or of any kind of influence from the male ; but it is possible
that, as held by many authorities, the discharge of ova, though
in no way dependent on sexual intercourse, may yet be hastened
by this.

4. Menstruation.

Menstruation is the periodical discharge from the uterus of
a certain amount of blood, mixed with mucus from the uterine
glands, and with epithelial and connective-tissue cells, derived
from disintegration of the mucous membrane of the uterus
itself.

There is a close connection between menstruation and ovu-
lation. Both processes commence at puberty, and last through-
out the child-bearing period. They both recur periodically ;
and, further than this, the intervals are the same, and the two
processes occur, as a rule, simultaneously. The true nature
and extent of the connection between the two will be discussed
after the nature of the menstrual process has been considered
more fully.

During the period of pregnancy, that is, during the whole
time that an ovum or embryo is developing within the uterus,
menstruation ceases, recommencing six or seven weeks after the
birth of the child. The normal occurrence of the menstrual
periods may also be affected by a variety of accidental or
pathological conditions, for the consideration of which reference
must be made to works dealing with obstetrics.

Menstruation, i.e. the actual discharge from the uterus of
blood and other matters, is not an isolated process, but is the
terminal act of a series of changes, which occur at regular
intervals in the walls of the uterus, and of which the sequence is
as follows.

In the quiescent condition the uterus is lined by a smooth
mucous membrane, of a soft, spongy consistence, and pale red
colour. It consists of a single layer of ciliated epithelial cells,
resting on a very delicate basement membrane, beneath which
is the connective-tissue layer of the mucous membrane. This
latter is about 1*5 mm. in thickness, and consists of connective
tissue, with very numerous connective-tissue cells, and traversed
by irregularly arranged muscle fibres. It is attached by its
outer surface to the muscular wall of the uterus.

458 THE HUMAN EMBRYO.

The epithelium lining the uterus is pitted to form the uterine
glands. These (Fig. 175) are tubular glands, embedded in large
numbers in the connective-tissue layer of the mucous membrane,
vertically to the inner surface of the uterus ; they are straight, or
slightly convoluted ; their blind or outer ends are usually slightly
dilated ; and they secrete a transparent, glutinous, alkaline fluid.

Changes in the mucous membrane accompanying menstruation.

These changes commence with congestion and tumefaction of the
mucous membrane lining the entire uterus. This swells up
considerably, becoming softer and more vascular than before,
and forming ridge-like folds which project into the cavity of the
uterus. The connective-tissue cells increase considerably in
number, and the uterine glands become longer, wider, and more
convoluted. The whole layer of mucous membrane increases
in thickness from 1'5 mm. to from 3 to 5 mm. ; while the glands
increase in diameter from 0*08 to 012 mm. This swollen and
hypertrophied mucous membrane forms what is called the
menstrual decidua.

At the menstrual period, the superficial layer of the mucous
membrane, about a fourth of the entire thickness, breaks down
and is thrown off, usually in detached fragments, but sometimes,
in cases of dysmenorrhea membranacea, as a single piece,
forming a complete cast of the interior of the uterus. Fatty
degeneration has been noticed in these cast-off cells, but only in
the later stages, after the menstrual discharge has actually
commenced.

This disintegration, and casting off, involves the loss of
the epithelial lining of the uterine cavity, of the mouths of the
uterine glands, and also of about one-fourth of the entire thick-
ness of the swollen mucous membrane. It of necessity causes
rupture of the blood-vessels of the detached portions, and so occa-
sions more or less free haemorrhage ; and the blood so discharged,
together with the broken-down mucous membrane of the uterus,
and with a certain amount of mucus from the uterine glands,
forms the menstrual or catamenial flow.

The menstrual flow lasts, as a rule, from three to five days,
but may be protracted for a week or more. It is accompanied
by nervous and other disturbances, which are fully described in
works on obstetrics.

MENSTRUATION. 459

At the commencement of a period, the menstrual discharge
is viscous, consisting largely of mucus from the uterine glands,
slightly tinged with blood ; in the middle of the period the flow
becomes almost pure blood ; while towards the end it becomes
paler, the mucus again preponderating. Owing to mixture with
the uterine mucus, the blood of the menstrual flow does not coagu-
late. The total amount of the menstrual discharge is usually
from four to six ounces ; but this may be widely departed from
in individual cases, either in the way of diminution or of excess.

On the cessation of the menstrual flow, the uterine epithelium
is very quickly regenerated, spreading over the surface from the
necks of the uterine glands. It is completely reformed within
three or four days of the end of the menstrual period. After
this re-establishment of the uterine mucous membrane, the
uterus remains in a quiescent condition for from ten days to a
fortnight ; at the end of this time it begins to swell again, and
the menstrual process is repeated. This repetition occurs, as
already noticed, at intervals, usually of four weeks, throughout
the whole child-bearing period ; the only normal disturbing
element being gestation, during which menstruation is in
abeyance, recommencing a short time after the birth of the child.

5. Explanation of the Menstrual Process.

The complete menstrual cycle, occupying in typical cases
twenty-eight days, may be divided into four stages, which
follow one another in regular sequence.

(i) The first or constructive stage is characterised by swelling
of the mucous membrane, enlargement of the uterine glands,
and increase in the connective-tissue cells of the mucous mem-
brane ; it results in the formation of a menstrual decidua, lining
the entire uterus.

(ii) The second or destructive stage includes what is ordinarily
known as the menstrual or catamenial period. It is marked by
abundant discharge of mucus from the enlarged glands, and by
the disintegration and discharge from the uterus of the inner
layer of the mucous membrane. It involves loss of the epithelial
lining of the uterus and of the necks of the glands, and is accom-
panied by haemorrhage.

(iii) The stage of repair comes next, during which the
uterus is recovering from the destructive changes. The uterine

460 THE HUMAN EMBRYO.

epithelium is restored, by growth from the lips of the deeper
parts of the uterine glands ; and the swelling of the mucous
membrane subsides.

(iv) The fourth stage is the period of quiescence, during which
the uterus, having regained its normal structure, remains with-
out further change until the commencement of the next suc-
ceeding constructive stage.

The actual and relative durations of the several stages
enumerated above are not determined with certainty, and are
subject to individual variations. It will, perhaps, be right to
assign about a week to the constructive stage ; rather less than
a week (five days on an average) to the destructive stage ; three
or four days to the stage of repair ; and twelve or fourteen days to
the quiescent period ; the four stages together occupying the
twenty-eight days which make up the normal menstrual cycle.

Of the above four stages, the first and second require further
attention ; the fourth stage is the normal condition ; and the
third stage is merely the return of the uterus to the normal con-
dition after a period of disturbance.

Concerning the first or constructive period, there is hardly
any room for doubt that it is to be regarded as a preparation on
the part of the uterus for the reception of an ovum.

The several stages of the process correspond closely, in
essential respects, with those that occur in the placental lobes ot
the rabbit's uterus from about the fourth to the eighth day. In
the rabbit, as in the human uterus, there occur swelling of the
mucous and submucous tissues, increased vascularity, a large
increase in the number of the connective-tissue cells, and a great
enlargement of the uterine glands, which become larger, wider,
and more freely branched. These changes, in the rabbit's uterus,
are clearly related to the nutrition of the embryo, for it is to this
hypertrophied and modified area of the uterine mucous membrane
that the embryo becomes attached on the eighth day ; and it is
from this area that the maternal part of the placenta is formed.

The most important difference between the rabbit's and the
human uterus, as regards these stages, is that in the rabbit the
ovum, or rather the blastodermic vesicle, is present within the
uterus during the whole of the series of changes, although it lies
quite freely and does not acquire attachment until the eighth
day ; while in the human uterus, on the other hand, the men-

MENSTRUATION. 461

strual constructive process goes on without the stimulus afforded
by the presence of an ovum.

As regards the actual changes in the uterus itself, the
resemblance between the two cases is so great that it seems neces-
sary to suppose that their significance is the same ; and it must,
therefore, be concluded that the human uterus periodically pre-
pares itself, by the formation of a decidual lining, for the reception
of an ovum ; the process occurring at monthly intervals through-
out the child-bearing period, and quite irrespectively of the
presence or arrival of a fertilised ovum.

The second or destructive stage, constituting the act of
menstruation in the ordinary sense of the term, is much more
difficult to explain. At first sight it appears to consist simply
in a rapid, and somewhat violent, undoing of the work accom-
plished in the preceding stage.

If, however, it is compared with the changes that take place
in the rabbit's uterus during gestation, it is found that the
human uterus at the end of the constructive period of menstrua-
tion has reached a stage corresponding to that of a rabbit's
uterus at the end of the seventh or beginning of the eighth day
of pregnancy, when the blastodermic vesicle is still lying freely
within the uterus, but is just about to acquire its attachment.

In the rabbit this attachment is effected, early on the eighth
day, by fusion of the wall of the blastodermic vesicle with the
epithelium of the modified and hypertrophied placental lobes of
the uterus (Fig. 169). This fusion is immediately followed, or
rather is accompanied, by degenerative changes in the uterine
mucous membrane opposite the area of attachment, which
rapidly lead to absorption of the uterine epithelium, and of the
mouths and necks of the uterine glands.

Similar changes occur during the formation of the human
placenta, and will be described in the concluding section of this
chapter ; and inasmuch as the portion of the wall of the uterus
which is concerned in the changes is the same in menstruation
and in pregnancy, the menstrual discharge may be viewed, not
merely as a destructive process, but as corresponding in a modi-
fied form to the rapid absorption of the same parts which occurs
normally during pregnancy.

The constructive stage of menstruation, and, as just seen,
the destructive stage as well, may be regarded as phases in the

462 THE HUMAN EMBRYo.

preparation of the uterus for the formation of a placenta ; stages
which can be carried up to a certain point without needing the
stimulus of the presence of an ovum or embryo, but which,
] laving reached a point at which further development is impos-
sible without an embryo, stop abruptly. The constructive stage
has been shown to be an active preparation of the uterus for the
reception of a fertilised ovum; the succeeding or destructive
stage is not to be regarded as a simple undoing of this prepa-
ration, but as a further continuance, in a modified form, of the
act of preparation, which leaves the uterus in a condition in
which, for further elaboration to occur, the presence of an embryo
is indispensable.

6. The Connection between Ovulation and Menstruation.

Ovulation and menstruation, or the discharge of ova from the
ovary, and of the disintegrated decidua from the uterus, are
processes which occur periodically, and as a rule simultaneously ;
and it becomes a matter of interest to inquire into the nature of
the connection between them.

The ovaries swell up, and become tender, at monthly intervals.
The enlargement commences, as a rule, a few days before the
menstrual period, attains its maximum about the time of the
period, and gradually subsides after the period is over.

As the ovary is known to become congested just before the
rupture of a Graafian follicle and the discharge of an ovum, it
appears a fair inference that this discharge occurs about the
same time as the menstrual flow, i.e. that ovulation and men-
struation are practically simultaneous. However, although
this may be, and probably is, the rule, yet it is far from being
an invariable one. Thus Kolliker, on examining the ovaries of
seven women who had died directly after menstruation, found
that in two of the cases there was no fresh corpus luteum
in either ovary ; that is, that no ovum had been discharged
at the time of menstruation ; and Coste has cited similar
instances.

Ovulation and menstruation may be assumed to occur as a
rule about the same time, but it is by no means clear what is
the precise nature of the connection between the two processes.
Authorities differ as to the stage in the menstrual period at
which ovulation occurs, the majority holding that it takes place

CONNECTION BETWEEN OVULATION AND MENSTRUATION. 463

two or three days before the commencement of the period,
while others maintain that it happens at the middle, or even
towards the end, of the period. It is very possible that there is
no constancy in this particular respect.

A still more difficult point remains to be considered. The
menstrual decidua is to be viewed as a preparation on the part
of the uterus for the reception of an ovum ; but it has still to
be determined whether the decidua which is broken up and
discharged at a given menstrual period is the one prepared for
the ovum which is set free from the ovary at the same period, or
for an ovum liberated at some previous or subsequent period.
The question is one of great importance, as the means of deter-
mining the age of human embryos are very materially affected
by the answer given to it.

The menstrual cycle has been seen to consist essentially in
a periodically recurring preparation of the uterus for the recep-
tion of an ovum. It is important to determine, if possible, at
what particular phase of the cycle, the uterus is in the condition
most favourable for the reception of an ovum. Very different
views have been expressed on this point, and two of these call
for special notice.

(i) That the end of the constructive period is the natural
and most favourable moment for the ovum to enter the uterus.

(ii) That the period of quiescence is the most favourable
time.

In support of the former view, it is urged that the formation
of the decidua is unintelligible except on the supposition that it
is a preparation for the reception of the ovum ; and that the
analogy of the rabbit's uterus, in which the sequence of changes
is strikingly similar, is in favour of the end of the constructive
period, or perhaps the commencement of the destructive period,
being the one specially concerned with the fixation of the ovum
to the wall of the uterus.

It must be noticed, however, that if the normal time of
attachment to the uterus is, in the human ovum, the end of the
mstructive period, i.e. the commencement of the menstrual
Jriod, then it is clear that the ovum which is to be attached
tnnot be the one discharged from the ovary at the same period.
>r the discharge of the ovum is practically coincident with the
lset of the menstrual period ; and the ovum, after leaving the

4G4 THE HUMAN EMBRYO.

ovary, has still, in order to reach the uterus, to travel along the
entire length of the Fallopian tube, a passage which is known
to take three days in the rabbit, and eight to ten days in the
dog, and which in all probability takes at least a week in the
human species. It follows that the decidua which is discharged
at a given menstrual period cannot have been prepared for
the ovum discharged at the same period, but must be the pre-
paration for the ovum which was discharged at the preceding
menstrual period.

The second view, that the period of quiescence in the
menstrual cycle is the most favourable time for the entrance of
the ovum into the uterus, leads to the same conclusion, inas-
much as the only ovum which could reach the uterus during
the quiescent stage is the one discharged at the previous
menstrual period.

In favour of this second view, that the quiescent period in
the menstrual cycle is the most favourable time for the ovum to
enter the uterus, the following considerations may be urged.

(a) A much greater range of time is given, within which
the uterus is ready for the reception of an ovum. The quiescent
period is the longest of the four stages which compose the men-
strual cycle, lasting from twelve to fourteen days ; while, on the
view that the completion of the constructive process marks the
time at which the uterus is best fitted to receive an ovum, the
range of time is limited to two or three days at most ; and the
longer period is more in accordance with what is known of the
range of time within which conception may occur.

(Jj) The stages in the formation of the menstrual decidua
have been compared, above, with the changes which occur in the
uterus of a rabbit, from the fourth to the seventh or eighth day
of pregnancy ; and the close similarity between the two cases has
been insisted on. It should now be noticed that these changes
in the rabbit occur after the entrance of the ovum into the
uterus ; i.e. that in the rabbit the ovum enters the uterus while
this latter is in the quiescent stage.

Neither of the above arguments is at all conclusive, and the
question is still an open one. It must be repeated, however,
that if either of these views is correct, the same conclusion
follows with regard to the relation between ovulation and men-
struation, viz. that the decidua of a particular menstrual period

CONNECTION BETWEEN OVULATION AND MENSTEUATION. 465

is related, not to the ovum discharged at that period, but to the
ovum discharged at the preceding period.

It follows that there is no necessary connection between
ovulation and the occurrence of the menstrual flow ; a point
which helps to explain the cases quoted by Kolliker, Coste, and
others, in which there was no discharge of ova at the time of
menstruation.

The fact that the two processes, ovulation and menstruation,
occur normally at or about the same time, may perhaps be
explained by the consideration that at the time of ovulation
there is very considerable congestion of the ovaries and Fal-
lopian tubes ; and this, owing to the free communication between
the ovarian and uterine arteries, must almost necessarily cause
congestion of the uterus ; and this determination of blood to
the large and thin-walled vessels of the decidua is probably an
important factor in causing the menstrual hemorrhage.

7. The Duration of Pregnancy.

Much has been written on this point, and many elaborate
tables have been compiled from which it appears :

(i) That there is no absolutely fixed period of gestation.

(ii) That there is no means of determining with certainty
the commencement of gestation, as the precise time of fertilisa-
tion of the ovum cannot be ascertained.

It is customary to calculate the duration of pregnancy from
the last occurring menstrual period ; and this, if the argument
given above is correct, will correspond with the discharge, from
the ovary, of the ovum from which the child is developed. The
most reliable estimates indicate a normal duration of pregnancy,
dating from the last occurring menstrual period, of 270 to 280
days. This is, however, estimated by some authorities from the
first day of the period ; by others, and more usually, from the
last day.

It is possible that the actual limits, in normal pregnancy, are
not so wide as indicated above. Apart from the difficulty of
determining the date of fertilisation, the chief causes of uncer-
tainty arise from our ignorance of the length of time during
which the ova and spermatozoa retain their vital activity, after
leaving the ovary and testis respectively.

H H

4GG THE HUMAN EMBRYO.

Concerning the spermatozoa, we have very little precise
knowledge. It is known that spermatozoa, introduced into the
vagina, may retain their vitality, and presumably their fertilising
power as well, for a week ; and the fact that successful impreg-
nation may occur at any time in the menstrual cycle, strength-
ened by the analogy of other animals, suggests that human
spermatozoa may retain their power for considerably longer
periods. It is stated that ripe spermatozoa may remain for
months in the testis before being discharged, without losing
their fertilising power.

The time taken by the spermatozoa to travel along the
vagina, uterus, and Fallopian tube to the ovary, is not known,
but is probably very short ; in the rabbit it does not occupy
more than a quarter of an hour to two hours.

If the ovum is not fertilised it soon dies. How long an
ovum may retain its vitality, and capacity for fertilisation, is not
known; indeed, no unfertilised human ovum has yet been seen,
outside the ovary. Some experiments of Bischoff, on lower
Mammals, point to the conclusion that, in these, the ovum, if not
fertilised, dies in the lower part of the Fallopian tube, before
reaching the uterus. Assuming that the human ovum also dies
shortly before reaching the uterus ; and assuming further, as is
done by most authorities, that the human ovum takes at least
eight days to travel down the Fallopian tube, it may be stated
that the human ovum probably retains its vitality, and power
of being fertilised, for sometime, perhaps a week, after discharge
from the ovary ; but ultimately loses it, probably before reaching
the uterus. This is, however, at present little more than specu-
lation.

If the above considerations prove well founded, and if, as
suggested above, the length of time during which an ovum
remains alive and fertilisable, after leaving the ovary, is less
than the interval between two successive periods of ovulation, it
will follow that there must be certain times during which there
are ova ready to be fertilised, and certain times during which
there are none ; i.e. that fertilisation can only be effected at
certain recurring periods, and cannot occur in the intervals
between these periods.

Concerning the respective lengths of these periods we have

THE DURATION OF PREGNANCY. 467

no certain knowledge, but it is commonly held that the intervals
during which there are no ova capable of being fertilised are at
least as long as the periods in which there are such ova. In
other words, assuming that the ova discharged at a given men-
strual period retain their vitality for from ten to fourteen days
— a pure assumption — there would be an interval of about two
weeks before the next menstrual period, i.e. before the next dis-
charge of ova, and during this interval there would be no ferti-
lisable ova in the oviduct, and fertilisation could not take place.
Any spermatozoa received during this interval would have to
wait until its close, at the next period of ovulation, before they
had a chance of meeting with ova capable of being fertilised.

There seems to be a general consensus of opinion that the
first day or days after the cessation of the menstrual period are
the most favourable time for fertilisation to take place. This is
in complete accordance with what has been said above, both
with regard to the ovum and the decidua, for the ovum will be
lying within the Fallopian tube in a healthy fertilisable condi-
tion and easily accessible to the spermatozoa ; while if the ovum
takes another week or so to travel down the tube to the uterus it
will enter this latter while it is in the quiescent state, which, it has
been shown above, there is reason for regarding as the most
favourable one for the reception of the ovum.

8. Estimation of the Age of Human Embryos.

It follows from what has been said above, that there is no
means of determining with certainty the age of a human
embryo prematurely discharged from the uterus; for develop-
ment dates, not from the discharge of the ovum from the ovary,
but from the moment of fertilisation ; and this latter cannot be
determined.

Ovulation is a process easily overlooked, but the fact that it
occurs simultaneously with the menstrual periods renders its
date readily determinable, but within certain limits only. The
connection between the two processes is a loose one, and it is
probable that ovulation may occur either from two to three days
before a menstrual period, or during the period ; giving a pos-
sible error of about a week in estimating the age of an embryo
from the date of menstruation.

468 THE HUMAN EMBRYO.

Professor His, in the first part of his monograph on the
development of the human embryo, laid down the following
rule : —

' The age of an embryo is the time that has elapsed since
the first day of the first omitted period.'

Thus, supposing the commencement of a menstrual period
to be due on January 5, and that when this time comes, the period
is omitted ; but that at some subsequent time, say February 9,
an embryo is aborted ; then, according to Professor His' rule,
the age of the embryo would be the interval between January 5
and February 9, i.e. thirty-five days.

In arriving at this result, Professor His argues in the follow-
ing manner : The ovum leaves the ovary either at, or shortly
before, the menstrual period ; if it is fertilised, presumably by
spermatozoa previously introduced, menstruation does not occur ;
but the changes in the uterine mucous membrane, instead of, as
usual, becoming retrogressive, either remain stationary or else
continue to be progressive ; and so prepare the uterus for the
reception of the ovum. Hence the first omitted menstrual
period corresponds in point of time with the fertilisation of the
ovum ; and hence the age of the embryo may be taken as the
time that has elapsed since the first omitted period.

This method of calculation is, however, open to very grave
objections, the more important of which are as follows : —

(i) There are strong reasons, which have been fully con-
sidered in the previous portion of this chapter, for regarding
the decidua which is broken up and discharged at a menstrual
period to be related, not to the ovum discharged from the
ovary at the same period, but to the ovum discharged at the
preceding period.

(ii) Professor His' rule assumes that the ovum is invariably
fertilised on the first day of the first omitted period. There is
no direct evidence in support of this ; and the loose nature of
the connection between ovulation and menstruation renders it
highly improbable.

(iii) The rule assumes that the act of fertilisation of an ovum ,
which in all probability will not reach the uterus for at least a
week, is able to arrest the degenerative changes already com-
menced in the decidua, to suddenly stop the menstrual flow that
is on the verge of taking place, or has actually commenced, and

ESTIMATION OF AGE OF EMBRYOS. 469

to convert the retrogressive changes of the uterus into pro-
gressive ones.

(iv) The rule is not in accord with the well-established fact
that, in order to insure pregnancy, the most favourable time for
intercourse is shortly, or immediately, after the conclusion of a
menstrual period. This is intelligible enough if the ovum to be
fertilised is the one discharged at that period ; but is hard to
understand if, as the rule requires, these spermatozoa have to
wait for a period of three weeks or more, until the next discharge
of 07a.

These objections are serious ones, and Professor His, in the
second part of his work, recognises that the rule as originally
formulated cannot apply to all cases. He quotes instances in
which the dates were accurately recorded, and in which the
fertilised ovum must have belonged to the last occurring period,
and not to the first omitted one ; he is of opinion, however, that
the rule as stated above will still apply to the majority of
cases.

This more recent view may be expressed graphically, thus.
If I. is the first day of the last actually occurring menstrual
period, and II. is the first day of the first omitted period; then
the possible days of fertilisation are as follows : —

I, 2, 3, 4, 5, 6, 7 26, 27, 28, II.

That is, an ovum discharged during an actually occurring period
remains capable of fertilisation for a certain number of days, ex-
pressed in the formula as a week, commencing with I., and
ending at 7. During this time it may be fertilised, either by
spermatozoa received after the period is over, or received before
the period and retained in the oviduct during it. In the case
of these embryos the age should be calculated from I., the first
day of the last actually occurring period.

On the other hand, Professor His, and others, maintain that
there are possibilities of fertilisation at the other end of the
series ; and that an ovum, discharged from the ovary a day or two
before the next period, II., is due, may, if fertilised, stop that
period from occurring; and in such cases, if they really happen,
the age of the embryo should be calculated from the first omitted
period, and not from the last occurring one. It is not yet
certain which of these two possibilities is the normal mode of

470 THE HUMAN EMBRYO.

occurrence, but such evidence as we have is in favour of the
former.

It is customary, however, to adopt His' original rule, and to
estimate the age of human embryos from the first day of the first
omitted period, and this method will be followed in this chapter.
It must be repeated, however, that this is done merely from
convenience, and from the absence of any other precisely formu-
lated system. Viewed on its own merits, His' rule will certainly
not apply generally.

THE GENERAL HISTORY OF DEVELOPMENT OF
THE HUMAN EMBRYO.

In this section the earliest stages in the formation of the
human embryo will be described, so far as they are at present
known ; and an account will be given of the external characters
of the embryo at the several stages up to the time of birth.

These descriptions are in the great majority of cases taken
from Professor His' monograph, and, as explained in the pre-
ceding section, the ages given are those assigned by him to the
several stages. In the following sections the development of
the nervous, digestive, and other systems will be considered in
detail ; and in the concluding section the placenta, the foetal
membranes, and the relations of the embryo to the uterus will
be described.

The actual length of an embryo is not always easy to deter-
mine, owing to the varying amount of flexure of the head and
body at different stages. By the length of an embryo, in the
following descriptions, is always meant the longest straight line
that can be drawn through it in the sagittal plane. In the
earliest stages of development this coincides fairly well with the
longitudinal axis of the embryo (Figs. 176 to 179); from the
beginning of the fourth week to the end of the fifth week (Figs.
200, 203, and 205) it is a line drawn from the prominent hump
at the junction of the head and body, to the pelvic region ; and
from the end of the fifth week onwards, as the head is gradually
lifted up by straightening of the neck (Figs. 211, 212), the
line once more approximates to the longitudinal axis of the
foetus.

With regard to the general course of development, the first

GENERAL HISTORY OF DEVELOPMENT. 471

fortnight is occupied in preliminary processes, no trace of the
embryo appearing until the twelfth or thirteenth day. From
the end of the second to the end of the fourth week, the embryo
is acquiring definite form, and the various organs and systems
are being established. From the fourth to the sixth or seventh
week there is a gradual change from the embryonic to the foetal
form ; the head becoming uplifted, the nose, ears, and lips esta-
blished, the limbs divided by joints, and the fingers and toes
formed. By the end of the second month, the general form is as
shown in Fig. 212, and from this time onwards the further
changes consist chiefly in increase of size, and in proportionately
greater development of the limbs.

The changes that occur in the shape and size of the embryo
up to the end of the second month are well shown in the series
of outlines given in Figs. 176 to 178, 189 to 195, 199 to 203,
205, 211, and 212. These figures, which are borrowed from
Professor His, are in each case five times the linear dimensions
of the embryos themselves.

1. The First Week.

The fertilisation of the human ovum has not been studied.
A single observation, by Nagel, of a ripe ovarian ovum, removed
by operation and examined in a fresh condition, showed that
two polar bodies were present, lying on the surface of the ovum
within the zona pellucida.

There is no reason for supposing that fertilisation is effected
in other than the normal manner ; and it is probable that it
takes place at or about the time the ovum leaves the ovary and
enters the oviduct.

The segmentation of the human ovum has not been seen. It is
highly probable, from analogy of other Mammals, that it occurs
during the passage of the ovum along the Fallopian tube towards
the uterus.

The ovum of the dog, which is slightly smaller than the
human ovum, travels quickly along the first part of the oviduct
but stays some days in the distal or lower part, where it under-
goes segmentation, entering the uterus eight or ten days after
leaving the ovary. Bischoff and others believe, though there is
no direct evidence on the point, that the human ovum agrees in
this respect fairly closely with that of the dog ; undergoing

472

THE HUMAN EMBRYO.

segmentation in the lower part of the oviduct, and not entering
the uterus until from eight to ten days, or perhaps longer, after
the time of discharge from the ovary.

2. The Second Week.

Of ova or embryos which are believed to belong to the end
of the second week a few examples have been described. These
are of great interest, although there is room for doubt in some
of these cases whether the specimens can be regarded as perfectly
normal.

Reichert's ovum. The best known instance is an ovum de-
scribed by Reichert, and believed to be of about the twelfth or
thirteenth day. This ovum, which is represented four times the

Fig. 172. Fig. 173.

Figs. 172 and 173.— Front and side views of Reichert's Ovum. (FromKolliker,
after Reichert.) x 4.

natural size in Figs. 172 and 173, was found, in situ, in the
uterus of a woman who had committed suicide, and gave every
indication of being perfectly normal.

The ovum was a vesicular body, lenticular in shape, and
measuring 5*5 mm. across its greater diameter, and 3*3 mm.
from side to side. Of the two surfaces, the one turned towards
the wall of the uterus, the upper one in Fig. 173, was more
convex than the opposite surface, which faced towards the cavity
of the uterus. The margin of the vesicle was thickly fringed
with villi, the largest of which were 0*2 mm. long, and slightly
branched; the middle portions of both surfaces were smooth,
and devoid of villi ; and in the centre of the more convex or
uterine surface was a small circular spot (Fig. 172), 1*6 mm.
in diameter, and of a darker colour than the rest of the vesicle.

The relations of the ovum to the uterus were as follows. The

THE SECOND WEEK. 473

entire uterus was lined by a decidua, described as not differing
in any special manner from an ordinary menstrual decidua, and
forming the usual ridge-like projections into the cavity of the
uterus. To one of these ridges, on the dorsal surface of the
fundus of the uterus, the ovum was attached, the decidua
spreading over it as a thin layer so as to completely encapsule it
(cf. Fig. 175). The marginal villi were
described and figured by Reichert as
penetrating a little distance into the
enlarged uterine glands.

In the ovum itself there was no
indication of primitive or neural grooves,
nor of any other part of the embryo. FlG- 174. — Diagram-
The wall of the vesicle was described by Reichert's Ovum.
Reichert as consisting of a single layer (From His.) x 5.
of flattened epithelial cells, prolonged a' eembryo areftl
outwards to form the hollow villi. In the circular patch on
the uterine surface, spoken of as the germinal or embryonal
area, a second or inner layer of finely granular nucleated cells
was present. The cavity of the vesicle was occupied by a
gelatinous fluid, traversed by a network of fibres, and con-
taining within it a rounded body attached to the germinal area.
Lining the whole vesicle was a second, fairly coherent
membrane, with which the fibres were continuous. By Reichert
this second membrane, the network of fibres, and the central
rounded body were all alike considered to be artificial products,
due to coagulation of the fluid contents of the vesicle by the
alcohol in which the specimen was preserved.

Ova of similar appearance, and of apparently about the same
age, have been described by Wharton Jones, Breuss, Kollmann,
and others ; and in none of these cases was any trace of an
embryo present.

The position held by the ovum in relation to the uterus, in
the case recorded by Kollmann, is shown in Fig. 175. The
whole uterus was lined by a decidual membrane, dv ; this was
greatly thickened about the middle of the ventral wall, forming
the decidua serotina, DW, to which the ovum, cv, was attached ;
the decidua extending over the ovum so as to completely en-
capsule it. The ovum itself was in the form of a hollow, thin-
walled vesicle, with short branched villi projecting from its

474

THE HUMAN EMBRYO.

surface, apparently on all sides. The villi were stated not to
penetrate into the uterine glands. The minute structure of
the ovum, or blastodermic vesicle as it may more properly be
called, was not ascertained.

The chief additional points learnt from these further

Fig. 175. — A longitudinal section of the Uterus with an Ovum in si
mated as about the thirteenth day. (After Kollmann.) x 1.

c, esti-

CV, cavity of ovum or blastodermic vesicle. C"W\ wall of blastodermic vesicle.
CZ, villi projecting from wall of blastodermic vesicle. DV, decidua vera. D"W\
decidua serotina. DX, decidua refiexa. GrV, vagina. IT A, cavity of uterus. TJB,
dorsal wall of uterus. TJC, ventral wall of uterus. TJO, os uteri. TJK, uterine glands.
TJX, cervix uteri.

specimens are : (i) that the rounded body described by Eeichert
as lying within the vesicle is made up of nucleated cells, and is
apparently solid, and attached to the embryonal or germinal
area ; (ii) that there is strong reason for thinking that the wall
of the vesicle really consists, not of a single layer of cells, but of
two layers, of which the inner one, regarded by Reichert as a

THE SECOND WEEK. 475

coagulation product, is of the nature of connective tissue, and
therefore of mesoblastic origin. This latter point is, however, a
doubtful one.

In the present state of our knowledge it is hardly possible
to make any satisfactory comparison between these early human
ova, and the stages already described as occurring in the rabbit.
The difficulty is much increased by the absence of detailed
histological description, and by the doubt as to whether the ova
are in all respects normal. It must also be borne in mind that
we are absolutely ignorant of the mode in which segmentation
of the human ovum, and the immediately succeeding stages are
effected ; and that great uncertainty still exists with regard to
the details of these processes in the rabbit.

As, however, the several human ova of the stage in question
agree in a number of important points, and as, in the case of
Reichert's ovum, there is every reason for regarding the specimen
as normal, it is advisable to make such comparison as is possible
between these ova and the several stages of development of such
a Mammal as the rabbit.

In the first place, the complete absence of any trace of an
embryo indicates that the stage is a very early one. In ova
which there are strong reasons for regarding as but one or two
days older, an embryo is present ; so that the stage represented
by Reichert's ovum may be described as one shortly before the
first appearance of the embryo, and as corresponding in this
respect with the blastodermic vesicle of a rabbit at about the
fifth or sixth day.

In its vesicular character, the thinness of its walls, and the
presence of a central embryonal area of different constitution to
the rest of the wall, there are additional points of resemblance
between Reichert's ovum and the blastodermic vesicle of a
rabbit of the sixth day, or of a dog in the early part of the
third week. There is also a close correspondence in actual size
between these stages in the three instances.

Reichert was of opinion that this comparison was a true
one ; and the view is supported by His, who gives in illustra-
tion of it the diagrammatic section (Fig. 174). His considers
that the outer wall of the vesicle consists of epiblast only, and
tli at the hypoblast forms the inner circular patch of cells in the
embryonal area ; he also regards the central rounded mass of

470 THE HUMAN EMBKYO.

cells as hypoblastic, and as destined to become hollowed out at
a later stage to form the yolk-sac.

Concerning this comparison, it must be borne in mind that
we have as yet no satisfactory knowledge of the histological
structure of these early human ova, and that the stage is one
about which much doubt exists even in the case of the rabbit.
It must further be noticed that there are some points of impor-
tance which tell directly against the interpretation suggested by
Keichert and His.

In the first place, there is nothing in the blastodermic vesicle
of a rabbit on the sixth day that can be compared with the
central mass of cells in Reichert's ovum.

Secondly, if His is right in interpreting this central mass of
cells as the yolk-sac, then the yolk-sac of the human embryo is
developed in a manner entirely different from that of the rabbit.
In the rabbit (Fig. 146), the yolk-sac is part of the blastodermic
vesicle itself, while in the human embryo it appears to be, from
the first, independent of this. In other words, if the central
mass of cells in Reichert's ovum is the yolk-sac, and the later
stages strongly support this interpretation, then the wall of the
vesicle of Reichert's ovum cannot correspond to the wall of the
rabbit's blastodermic vesicle (cf. Figs. 146 and 188).

Thirdly, there is strong reason for thinking, as already
noticed, that the wall of Reichert's ovum is double ; an inner
mesoblastic lining being already present, as well as the outer
epithelial layer. To this inner layer, if it really exists, there is
nothing corresponding in the rabbit's blastodermic vesicle until
a later stage.

On the whole, then, the evidence, while not excluding a
general correspondence in grade of development between
Reichert's ovum and a rabbit's blastodermic vesicle of about the
sixth day, appears to be against a close or exact agreement
between the two. There are features in Reichert's ovum which
do not fit in with the processes of development as known in
the rabbit, or in other Mammals ; peculiarities which will pro-
bably not be understood until opportunity has occurred for
study of the segmentation of the human ovum, and of the stages
immediately following it. Light will perhaps be thrown on the
question by investigations on Mammals more nearly allied to
man than are rabbits or dogs.

THE THIRTEENTH AND FOURTEENTH DAYS. 477

Embryos of the Thirteenth and Fourteenth Days.

His' embryo, E. One of the youngest human ova containing
a distinct embryo was obtained by Professor His in 1869, and
was carefully described by him under the distinguishing letter
E. This embryo, which is at present deposited in the Anato-
mical Museum at Basle, is estimated to be about thirteen days
old : it is represented from the right side in Fig. 176 ; and in
diagrammatic sagittal section in Fig. 188.

The entire vesicle (Fig. 188) is a thin-walled sac, measur-
ing 8-5 mm. by 5*5 mm. and covered all over with branched
villi. The contained embryo (Fig. 176) is 2*1 mm. long, and
is attached at its hinder end, by a short thick stalk, to the inner
surface of the vesicle. A slight constriction separates the

:

Fig. 17(5. Fig. 177. Fig. 178.

FlGS. 176, 177, 178. — Outline figures, from the right side, of three Human
Embryos, estimated to be of the thirteenth or fourteenth days. (From
His.) x 5.

Fig. 176.— Embryo lettered by Professor His, E (of. Fig. 188).

Fig. 177.— Embryo described by Allen Thomson.

Fig. 178.— Embryo lettered by Professor His, SR (cf. Fig. 179).

embryo ventrally from the yolk-sac, which measures 2#3 by
1*6 mm. Covering the embryo, but at a short distance from it,
is a membranous fold, which is clearly the inner or true amnion.
The embryo itself presents along its dorsal surface a shallow
neural groove, bounded by prominent neural folds ; and the
only other organs visible on the surface are a pair of longi-
tudinal folds, formed by the two halves of the heart, and lying
between the anterior end of the embryo and the yolk-sac.
From the heart, vessels can be traced, running over the surface

f the yolk-sac.

His' embryo, SR. This is a well-preserved embryo of the

hirteenth day, slightly older than the embryo E, but very
similar to it in all important respects.

The entire vesicle measures 8 to 9 mm. in diameter, and

478

TIIK HUMAN EMBRYO.

is covered over its whole surface by branched villi, as in the case
of the embryo E (of. Fig. 1 88). The embryo (Figs. 178 and 1 79)
is 2-2 mm. long : it is attached to the inner surface of the
vesicle by a short thick stalk, tz, and is separated from the yolk-
sac, YS, by a slight constriction.

In the embryo itself, the head end, HD, is more markedly
raised above the yolk-sac than in the embryo E ; and the neural
groove, NG, is widely open along its whole length. The dorsal

Fig. 179. — Human Embryo lettered by Professor His, SR, and estimated as of
the thirteenth day. The wall of the blastodermic vesicle has been removed,
except the part to which the allantoic stalk is attached. (After His.) x 25.

AN, inner or true amnion. HD, head end of the embryo. R, heart. NG, neur
groove. TL, tail. TZ, allantoic stalk, connecting the embryo with the wall of the
blastodermic vesicle. VN, villi. YS, yolk-sac.

surface of the embryo is somewhat sinuous in outline, present
ing alternate convexities and concavities : the most anterior
and largest swelling is formed by the head ; then comes a con-
cavity opposite the middle of the length of the yolk-sac ; and
then another marked convexity further back. The hinder end
of the embryo projects freely as a short blunt tail, TL, beyond
the stalk of attachment to the wall of the vesicle, tz, which now
arises from the ventral surface of the hinder end of the embryo.
The two halves of the heart, R, form prominent swellings between

THE THIRTEENTH AND FOURTEENTH DAYS.

479

the head and the yolk-sac. There is as yet no trace of either
visceral arches or clefts ; and the dorsal surface of the embryo is
enveloped in the thin membranous amnion, an, which now lies
rather closer to it than in the case of the embryo E.

Human embryos of about the same age as the embryos E
and SR have been described by Allen Thomson (Fig. 177),
Keibel, V. Spee (Figs. 180 to 184), Kollmann (Fig. 185), and
others. These all agree in essential respects, and leave no doubt
that the stage must be regarded as a perfectly normal one.

V. Spee's embryo, which was studied by means of sections, is
of considerable importance, as it has shown the internal struc-

Fig. 180. — A Human Embryo of about the thirteenth day, from the left side :
the wall of the blastodermic vesicle has been in chief part removed.
(After V. Spee.) x 8.

Fig. 181. — The same embryo from the dorsal surface. (After V. Spee.) x 14.

AW, inner or true amnion. HD, bead end of embryo. NGr, neural groove. NT.
neurenteric canal. PS, primitive streak. VN, villi of chorion. YS, yolk-sac.

ture, and the relations of the germinal layers, at the stage in
question.

This embryo is represented from the left side in Fig. 180, and
from the dorsal surface in Fig. 181. It is, if anything, slightly
younger than the embryo E, and the constriction separating the
embryo from the yolk-sac has hardly commenced to form. The
head of the embryo (Fig. 181, hd) is wide and flat, and the
neural groove is shallow. At the hinder end, the two neural
folds diverge from each other, and embrace between them the
anterior end of a well-marked primitive streak (Fig. 181, rs) ;
and just in front of the primitive streak is a small but well-

480

THE HUMAN EMBRYO.

defined neurenteric passage, NT, leading from the surface into the
cavity of the yolk-sac.

The sections (Figs. 182-84) will render the relations of the
parts clearer. Fig. 182, which passes across the anterior part of
the head, shows the widely open neural groove, ng ; the fore-gut.

Figs. 182-184. — Sections across the Human Embryo of the thirteenth day,

represented in Figs. 180 and 181. (After v. Spee.) x 45.
Fig. 182. — Transverse section across the head end of the embryo.
Fig. 183. — Transverse section across the middle of the body.
Fig. 184. — Transverse section across the hinder end of the embryo and the
yolk-sac, the section passing through the neurenteric canal.

AN", Inner or true amnion. CH, commencing notochord. E, epiblast of tlie
embryo. GF, fore-gut. G-T, mid-gut. H, hypoblast. M, mesoblast. ME, somato-
pleuric layer of mesoblast. MH, splanclmopleuric layer of mesoblast. NG, neural
groove. NT, neurenteric canal. YS, cavity of yolk-sac.

GF, lined by hypoblast, and just shut off from the yolk-sac, YS ;
and the mesoblast, with its contained blood-vessels. Fig. 183
passes through the body region, and shows the commencing*
notochord, CH, as a thickened plate of hypoblast cells in the
mid-dorsal wall of the gut.

Fig. 184 passes through the neurenteric canal, NT, which

THE THIRTEENTH AND FOURTEENTH DAYS. 481

places the mid-gut and yolk-sac in direct communication with
the exterior. It will be noticed that the wall of the yolk-sac
consists of an inner lining of hypoblast, H, and an outer wall
of mesoblast, mh, in which are very numerous blood-vessels.
On the right side the mesoblast was torn in the section, and is
indicated by a dotted line in the figure.

The amnion, ax, in all three figures is seen to consist of both
epiblastic and mesoblastic layers.

Kollmann's embryo (Fig. 185) is rather older than the
others described in this section, and may be estimated as of the
fourteenth day. It affords an important transitional stage

AN AK1 MT NG

Fig. 185. — Human Embryo of about the fourteenth day, from the right side.
The yolk-sac and the wall of the blastodermic vesicle have been removed.
(After Kollmarm.) x 27.

AW, outer or true amnion. BF, fore-brain. DS, stomatodaamn. MT, mesoblastic
somite or protovertebra. WGr, neural groove. NGr', point behind which the neural
groove is closed to form the neural tube. R, heart. TL, tail. TZ, allantoic stalk.
W, vitelline vein. YS, yolk-stalk, cut short.

/

between the embryos E and SR on the one hand, and on the
other the embryos of the third week, which will be described in
the next section.

Kollmann's embryo (Fig. 185) measures 2*5 mm. in length.
As compared with the earlier embryos, the head is larger and
more prominent ; and the embryo is much more distinctly
constricted off from the yolk-sac. The neural folds have met
and fused, to complete the neural canal, in the hinder part of
the body, but the neural groove is still widely open in the head
and anterior part of the body. The brain vesicles are becoming
evident, and the flexure of the anterior end of the head is already

i I

482 THE HUMAN EMERYO.

commencing. A distinct stomatodaoal depression, DS, is present
on the under surface of the head. The two halves of the heart,
R, have united ; and the heart, now a single tube, is already
twisted in a characteristic S shape. Fourteen or fifteen pairs of
mesoblastic somites, or protovertebrse, mt, are clearly visible from
the surface.

There are as yet no traces of visceral arches or clefts, nor of
eyes, ears, or limbs.

On comparing the embryos E and SR (Fig. 179), with the
corresponding stages in the development of the rabbit, i.e. with
rabbit embryos towards the end of the eighth day, before closure
of the neural canal has occurred at any point, there are seen to
be several points of difference.

The rabbit embryo at this stage is still on the surface of the
vesicle, while the human embryo is already covered by a well-
formed amnion. In connection with this separation from the
surface there is a further point of difference ; the human embryo
is connected at its hinder end by a short thick stalk (Fig.
179, tz) with the wall of the vesicle; while in the rabbit the
tail fold is only just commencing, and the hinder end of the
embryo is still directly continuous with the wall of the blasto-
dermic vesicle.

This stalk of connection (Fig. 179, tz), between the embryo
and the wall of the vesicle, arises from the under surface of the
hinder end of the embryo, and its relations are practically iden-
tical with those of the allantois of a tenth-day rabbit embryo.
As this stalk contains a tubular diverticulum of the hind-gut,
and transmits the allantoic arteries and veins (cf. Fig. 198),
it clearly corresponds, at any rate in great part, to the rabbit's
allantois, and may consequently be spoken of as the allantoic
stalk.

As regards amnion and allantois, the main difference between
the human embryo and the rabbit may be briefly expressed by
saying, that both amnion and allantois develop in the human
embryo at an earlier stage, relatively to the embryo itself, than is
the case in the rabbit. The probable explanation of this preco-
cious development of amnion and allantois, and comparatively
late appearance of the embryo itself, in the human species, as
compared with the rabbit, will be considered further on.

COMPARISON WITH RABBIT EMBRYOS. 483

Another difference of even greater importance between the
two embryos is found in the relations of the yolk-sac in the
two cases respectively, to which attention has already been
directed. In the rabbit, the yolk-sac (Fig. 146, ys) is part of
the blastodermic vesicle itself; while in the human embryo (cf.
Fig. 188) it lies freely within this. This difference is explained
by Keibel as due to a relatively early extension of the splitting
of the mesoblast, in the human embryo, right round the lower
half of the blastodermic vesicle. In the rabbit (Fig. 147) the
mesoblast, and consequently the cavity, C, between its layers, only
extends halfway round the blastodermic vesicle, stopping at
the sinus terminalis, ST. If, in the rabbit, the mesoblast, and
the split between its somatic and splanchnic layers, were to
extend round to the lower pole of the blastodermic vesicle, then
the yolk-sac would be completely split away from the wall of the
vesicle, and a condition similar to that of the human embryo
would be attained.

No stages intermediate between Eeichert's ovum and His'
embryo E, or V. Spee's embryo, have yet been described. The
gap, though probably only a slight one in actual time, is of
great importance ; for, while Eeichert's ovum has no trace of
an embryo, His' embryo E possesses neural groove and folds,
heart and yolk-sac, and has both amnion and allantois well
developed.

His has attempted to bridge over the interval, and has
given a series of diagrams, reproduced in Figs. 186-188, show-
ing hypothetical intermediate stages.

The figures represent diagrammatic longitudinal sections
through embryos at successive stages of development, and should
be compared with Fig. 174, which represents a similar section
through Eeichert's ovum.

In Fig. 186, which is a hypothetical stage, the commence-
ment of the formation of the embryo is indicated. The
embryonal or germinal area has become somewhat depressed,
but at its anterior end, to the right in the figure, is lifted up
by the commencing head fold.

In Fig. 187, also a hypothetical stage, the general depression
of the embryonal area has increased, the embryo being pushed
down into the blastodermic vesicle. The head fold has deepened,

i i 2

484

THE HUMAN K.MlSIiYO.

and the head end of the embryo is now more prominent, and is
raised distinctly above the yolk-sac. At the hinder end of the
embryo, to the left in the figure, the embryonal area still pre-
serves its primitive connection with the chorion or wall of the
vesicle. The head end of the embryo is covered over by the
commencing head fold of the amnion.

Fig. 1

is a somewhat diagrammatic section,

Jiut

at a stage

Fig. 187. Fig. 188.

FlGS. 186-188. — Diagrammatic longitudinal sections through Human Embryos,
representing hypothetical stages intermediate between Reichert's ovum
and His' Embryos, E or SR. (From His.) x 5.

FlG. 186. — Shows the commencement of the head fold of the embryo, and of the

amnion.
Fig. 187. — A rather later stage, in which the embryo is depressed into the

blastodermic vesicle, but still remains in connection with the wall of the

vesicle through the allantoic stalk. The dotted lines indicate, l^potheti-

cally, the further growth of the amnion. ■
FlG. 188. — A later stage, equivalent to that of His' embryos E or SR (Fig.

179). The amnion is complete, and the villi extend the whole way round

the vesicle.

Am, inner layer of the amnion, or true amnion. S.Ch, dotted line representing the
future extension of the outer layer of the amnion. Ys, yolk-sac.

corresponding to that of the embryos E or SR (Fig. 179). The
changes necessary to derive it from the stage shown in Fig. 187
are very slight. The hinder end of the embryonal area now
forms the thick allantoic stalk, connecting the embryo with the
chorion ; and a tubular diverticulum of the ventral wall of the
hind gut, the allantois proper, now extends some way along the
stalk. The amnion extends over the whole back of the embryo ;

COMPARISON WITH BABBIT EMBBYOS. 485

a change due possibly, as Professor His suggests, to backward
growth of the head fold of the amnion of the earlier stages, as
indicated by the dotted lines in Fig. 187 ; but more probably
caused, at any rate in part, by approximation and meeting of
the side folds of the amnion along the mid-dorsal line.

His' diagrams (Figs. 186 to 188) undoubtedly give an intel-
ligible and consistent theory concerning the development of
the human embryo from the stage indicated by Reichert's ovum
to that of the embryos E and SR, and it is greatly to be hoped
that opportunity may occur for testing their correctness by
actual observation.

If the transition occurs in the way suggested by Professor
His, then both the amnion and the allantois of the human
embryo present features differing widely from those of the rabbit.
The amnion has no tail fold, which is almost the only part
developed in the rabbit ; while the allantois is, from the first,
continuous with the chorion.

Even in the rabbit, however, an approach is made towards
the mode of development of the allantois supposed to occur in
the human embryo ; the mesoblast of the allantois in the rabbit
being, from the first, continuous with the mesoblast of the tail
fold of the amnion (Fig. 146), and very early fusing with the
chorion as well (Fig. 147).

The precocious development of the allantois, which is one
of the most striking points about the human embryo, may be
connected with the precocious appearance of the vascular layer
of mesoblast lining the blastodermic vesicle ; and both features,
in so far as they are exceptional , may be regarded as examples of
the tendency to shortening or abbreviation of the processes of
development, which is so constantly encountered by the student
of embryology.

The establishment of a vascular connection between the
embryo and the chorion, and so indirectly with the mother, is
the characteristic feature of mammalian development ; and it is
not surprising to find, in the most highly developed of all
Mammals, this feature thrown back to an earlier stage than that
at which it originally appeared ; and hurried on prematurely,
even at the expense, as it would seem, of the embryo itself, whose
development is unusually retarded.

486 THE HUMAN EMBRYO.

The germinal layers of the human embryo. JSut little can
be said on this point at present. Concerning the mode of
establishment and of differentiation of the germinal layers, we
know nothing. In Reichert's ovum the outer wall of the vesicle
consists of a single layer of epithelial cells, which from Reichert's
figures appear to be flattened, or pavement cells, and which must
almost certainly be epiblastic. The central mass of cells, form-
ing the yolk-sac, is certainly hypoplastic ; and so also, in all
probability, is the thick disc of granular cells forming the deeper
layer of the embryonal area.

Whether mesoblast is, or is not, present in Reichert's ovum
is a disputed point; but at a stage not much later, in the
embryos E and SR, layers of vascular mesoblast are present, not
only in the embryo itself, but covering the outer surface of the
yolk-sac, and lining the wall of the blastodermic vesicle as well
(cf. Figs. 182, 183, and 184). The mode of origin, and the time
of appearance, of the mesoblast are unknown ; but from the
stage represented by the embryos E and SR, the history of the
layer is practically the same as in the rabbit or chick.

3. The Third Week.

During the third week, the embryo assumes more definite
form. The neural canal is closed along its whole length ; the
brain vesicles, optic vesicles, and auditory sacs are formed ; the
visceral arches and clefts develop ; and the head and neck
acquire their characteristic embryonic shape. The embryo in-
creases considerably in size ; the constriction between embryo
and yolk-sac becomes much more marked ; and towards the end
of the week the first rudiments of the limbs appear.

During this time, development proceeds rather slowly, the
changes passed through in the course of the week corresponding
roughly to those effected during the second and third days in a
chick embryo.

Only a limited number of embryos of the third week have
been described, not much more than a dozen in all, and only a
few of these were in satisfactory condition for detailed exami-
nation. Outlines of some of the more important specimens are
given in Figs. 189 to 195, and more detailed drawings on a
larger scale in Figs. 196, 197, and 198.

THE THIRD WEEK.

487

Coste's embryo (Fig. 196). An embryo described and figured by
Coste, about the precise age of which there is some doubt, appears
to belong to the commencement of the third week. The whole
vesicle measures 16'2 mm. along its greater diameter, and is
covered externally with short, slightly branched villi. The em-
bryo is attached to the inner surface of the vesicle by a short

Fig. 189.

Fig. 190.

Fig. 191.

Fig. 192.

Fig. 193.

Fig. 194.

Fig. 195.

Figs. 189 to 195. — Outline figures of seven Human Embryos of the third week.
(From His.) x 5.

Fig. 189. — Embryo lettered by Professor His, Lg, and estimated as fifteen

days old. {Cf. Fig. 197, p. 489.)
Fig. 190. — Embryo lettered by Professor His, Sch, and estimated as fifteen

days old.
Fig. 191.— Embryo lettered by Professor His, M, and estimated as eighteen

days old.
Fig. 192. — Embryo figured by Professor Allen Thomson, and probably about

eighteen days old.
Fig. 193. — Embryo lettered by Professor His, BB, and estimated as about

eighteen days old.
Fig. 194. — Embryo lettered by Professor His, Kin, and estimated as about

twenty days old.
Fig. 195. — Embryo lettered by Professor His, Lr, and estimated as twenty or

twenty-one days old. (Cf. Fig. 198, p. 491.)

thick allantoic stalk, a.s. The head end of the embryo is well
developed, and raised freely above the yolk-sac ; but the body is
still so closely connected with the yolk-sac that a distinct yolk-
stalk can hardly be said to be present. The body of the embryo
is concave upwards, and the tail has a well-marked upward direc-
tion. In the neck, three visceral arches are visible as thicken-
ings, but the grooves between them are only faintly indicated-

488

THE HUMAN EMBRYO.

Below the neck, in the angle between the embryo and the
yolk-sac, is the heart, H, a large tube sharply twisted on itself.
Blood-vessels are present in the wall of the yolk-sac, ys, and
also in the allantoic stalk, from which latter they pass into the
wall of the blastodermic vesicle, the inner layer of which is vas-

Fig. 196.— Human Embryo at the commencement of the third week. (From
His, after Coste.) x 15.

A, inner or true amnion. ' As, allantoic stalk. H, heart. V, blood-vessel of yolk-sac.
Ys, yolk-sac.

cular throughout its whole extent, although the blood-vessels do
not penetrate into the villi.

The middle portion of the embryo is clearly divided into
proto vertebras, but there are no traces of limbs.

Embryo lettered by Professor His, Lg (Figs. 189 and 197), and
estimated as fifteen days old. The entire blastodermic vesicle
in this case measures 17 by 11 mm., and is covered with villi,
except at two patches at opposite poles of the vesicle. The embryo
(Fig. 197) is rather closely invested by the amnion, an, and is
connected with the wall of the vesicle by a short thick allantoic
stalk, tz. The yolk-sac, ys, is nearly spherical, and about 2 mm.
in diameter, and is still widely continuous with the ventral
surface of the embryo.

The most marked feature in the general form of the embryo
is the very sharp bend in the middle of the back, opposite the
yolk-sac. A similar and equally sharp bend has been noticed
in other embryos of about the same age (cf. Figs. 190, 193), but

THE THIRD WEEK,

489

it is not yet certain whether it is to be regarded as a normal
feature. His has suggested that it may possibly be caused by
the embryo tending to increase in length more rapidly than the
closely fitting amnion will allow it to ; the embryo consequently
becoming bent at the place where its ventral wall is weakest, i.e.
opposite the yolk-sac.

In the head, cranial flexure is well marked, the anterior part
of the head being bent down at right angles to the hinder part,

BM

Fig. 197. — Human Embryo lettered by Professor His, Lg, and estimated as
fifteen days old. The wall of the blastodermic vesicle has been removed,
except the portion with which the allantoic stalk is continuous. (From
His.) x 30.

AN, inner or true amnion. BM, mid-brain. EI, auditory pit. HC.l, first
branchial cleft. EM, hyomanrtibular cleft. M!N\ mandibular arch. E, heart. TL,
tail. TZ, allantoic stalk. V!N", villi of chorion. YS, yolk-sac.

and the fore-brain being in consequence carried round to the under
surface of the head. The prominent angle of the brain is formed
by the mid-brain, bm, behind which comes the nearly straight
hind-brain.

At the sides of the fore-brain are lateral swellings, caused by
the outgrowing optic vesicles, but there is as yet no trace of the
lens. The auditory pits, EI, are a pair of shallow depressions,
with widely open mouths, at the sides of the hind-brain.

490 THE HUMAN EMBRYO.

At each side of the neck there are a couple of slit-like de-
pressions, hm, and HC.i, transverse to the axis of the embryo.
These are the external grooves, which lie opposite the hyoman-
dibular and first branchial gill-pouches, or diverticula from the
pharynx. The epiblastic grooves and the corresponding hypo-
blasts pharyngeal pouches lie iu close contact with one another,
but do not communicate, so that there are at this stage no com-
plete gill-clefts, or actual perforations in the wall of the neck.

The hyoid arch is the ridge or strip of the neck lying
between the first branchial and hyomandibular grooves, HC.i and
hm. The mandibular arch, mn, forms a much more prominent
ridge, in front of the hyomandibular groove ; and, wedged in
between the dorsal end of the mandibular arch and the under
surface of the head, is the comparatively small maxillary arch.

The stomatodaeum, or mouth depression, is a shallow pit on
the under surface of the head, bounded in front by the head
itself, at the sides by the maxillary arches, and behind by the
mandibular arches. It does not yet open into the fore-gut
(cf. Pig. 232).

The heart, R, is large, and lies immediately below the bran-
chial region of the neck, between this and the yolk-sac ; it is a
single tube, twisted so as to form a strongly curved loop, with
its convexity towards the right side of the embryo.

In the body region, the outlines of the somites or proto-
vertebras can be seen through the skin ; about thirty-five pairs
being already present. The tail, TL, forms a prominent rounded
stump ; and from the under surface of its base the short thick
allantoic stalk, tz, arises, which attaches the embryo to the
chorion. There is as yet no trace of the limbs.

An embryo lettered by Professor His, Lr (Figs. 195 and 198),
and estimated as twenty or twenty-one days old, may be taken
as typical of the condition attained by the end of the third
week.

Apart from the great increase in size, best seen by comparing
Figs. 189 and 195, p. 487, the chief points in which the embryo
Lr differs from the embryo Lg, are : — The narrowing of the yolk-
stalk, by which the embryo is separated more markedly from
the yolk-sac ; and the almost complete disappearance of the
sharp bend in the back, opposite the yolk-sac, which is so

THE THIRD WEEK.

491

curiously characteristic of the embryo Lg, and of others of the
same age.

The greatest length of the embryo Lr, measured in a straight
line, from the swelling caused by the mid-brain to the rounded
hinder end of the body, is 4'2 mm.

In the head, the outlines of the several brain vesicles can be

Fig. 198. — Human Embryo lettered by Professor His, Lr, and estimated as
twenty or twenty-one days old. The right wall of the pericardial cavity
has been removed, to expose the heart ; and the arteries, the veins, and
the hinder part of the alimentary canal are represented as though the
embryo were transparent. (From His.) x 23.

A, dorsal aorta. A.l, first aortic arch, in the mandibular arch. A. 2, second aortic
arch, in the hyoid arch. A. 3, third aortic arch, in the first branchial arch. A.4, fourth
aortic arch, in the second branchial arch. A.5, fifth aortic arch, in the third branchial
arch. AA, allantoic artery. EI, auditory vesicle. GH, hind-gut. G-T, mid-gut,
opening into the yolk-stalk. HM, hyoinandibular cleft or groove. OF, olfactory pit.
RA11 right auricle. ET, truncus arteriosus. RV, ventricle. TA, diverticulum of
hind-gut into allantoic stalk. TL, tail. TR, cloaca. TZ, allantoic stalk. "VA,
allantoic vein. VB, anterior cardinal vein. VC, posterior cardinal vein. VD, Cu-
vierian vein. VV, vitelline vein. YK, yolk-stalk.

recognised fairly accurately through the skin ; and the propor-
tions have not altered very greatly from those of the earlier
embryo, Lg. The axes of the fore-brain and of the hind-brain
are still at right angles to each other, the mid-brain forming a
prominent rounded swelling at their junction. The fore-brain
is wider from side to side than before, owing to the optic vesicles,
which project outwards from its sides.

492 THE HUMAN EMBRYO.

The olfactory pits, of, are a pair of shallow depressions, on
the under surface of the extreme anterior end of the head,
above the mouth. The lens of the eye has not yet commenced
to form ; and the auditory vesicles, EI, are now a pair of closed
sacs, embedded in the mesoblast at the sides of the hind-brain.

The gill-cleft region is a triangular patch on each side of
the neck, with the apex directed backwards, and is bounded both
dorsally and ventrally by shallow grooves. The hyomandibular
cleft, hm, and the first, second, and third branchial clefts are all
represented by pharyngeal pouches, and by external grooves
corresponding to them ; but none of the clefts are actually
perforated.

The stomatodaeal pit is much deeper, and more clearly defined
than before, owing to uprising of its lips : it now opens into
the fore-gut.

The heart, ea, rv, rt, is larger than before, and its several
divisions are more clearly marked off from one another by
constrictions. The whole heart has shifted backwards to a
certain extent, the greater part of it now lying behind the gill-
cleft region.

The somites, or protovertebrae, are more distinct than before.
A pair of longitudinal ridges which run along the sides of the
body, ventral to the somites, are spoken of as the Wolffian
ridges. Each of these ridges is more prominent at two places,
opposite the posterior end of the heart, and opposite the allantoic
stalk respectively. These more prominent parts of the continu-
ous Wolffian ridges are the rudiments of the arms and legs.

4. The Fourth Week.

The fourth week is marked by a great increase in the size of
the embryo, growth being relatively more active at this than at
any other period.

In the early part of the fourth week (Figs. 199-201), the
flexure of both head and body is very strongly marked, the
embryo being rolled up on itself so that the head and tail touch
or even overlap, and the outline of the entire embryo being
approximately circular. The several parts of the head are more
conspicuous than before ; the visceral clefts and arches are more
clearly defined ; the nose and ear are more prominent ; the heart
is very large; the Wolffian ridges are still present as con-

THE FOUKTH WEEK.

493

tinuous structures, but the enlargements of the ridges, which
become later the arms and legs, are rapidly increasing in size.
Towards the end of the fourth week (Figs. 202, 203, 204),

Fig. 199.

Fig. 200.

Fig. 201,

Fig. 199-201.

-Outline figures of three Human Embryos, estimated as about
twenty-three days old. (From His.) x 5.

FlG. 199. — Embryo figured and described by Coste. For a figure of the uterus

and blastodermic vesicle from which this embryo was obtained, see Fig.

255, p. G08.
Fig. 200. — Embryo lettered by Professor His a.
Fig. 201. — Embryo figured and described by Allen Thomson. By a mistake

of the engraver's this figure has been reversed ; and it is really the right

side of the embryo, and not the left, that is shown.

the embryo acquires a very characteristic form, corresponding
closely in shape, in size, and in internal structure with a chick

Fig. 202.

Fig. 203.

Figs. 202 and 203. — Outline figures of two Human Embryos, estimated as
twenty-seven days old. (From His.) x 5.

Fig. 202.— Embryo lettered by Professor His, B.
Fig. 203. — Embryo lettered by Professor His, A.

embryo at the end of the fourth day, or a rabbit embryo of the
eleventh day.

The embryo (Fig. 204), which measures 7*5 mm. along its
longest diameter, is still strongly flexed. The back is rather
straighter than before, but, owing to the very sharp bend in

494

THE HUMAN EMBRYO.

the cervical region, at the junction of the head and trunk, the

under surface of the head is still almost in contact with the tail.

The several parts of the brain are readily distinguished

BR. I

Fig. 204. — Human Embryo lettered by Professor His, A, and estimated as
twenty-seven days old. (From His.) x 13.

BR.1, first branchial arch. EI, auditory vesicle. HC.l, first branchial cleft.
HM, byornandibular cleft. HY, hyoid arch. LA, fore-limb or arm. LP, hind-limb
or leg. MN, mandibular arch. MX, maxillary arch. OC, lens. OF, olfactory pit.
OJ, Jacobson's organ. STJ, sinus prsecervicalis. VN, villi of chorion.

through the skin, the mid-brain being especially prominent.
The nerve ganglia, both cranial and spinal, are well developed,
and form swellings that are clearly visible from the surface.
The olfactory pits, of, on the under surface of the fore part

THE FOUKTH WEEK. 495

of the head, are larger and deeper than before, and are bordered
by prominent lips, with somewhat irregular outlines. At the
inner and ventral corner of each olfactory pit there is a small
but deep notch, oj, with a sharply defined border : from this
notch the organ of Jacobson is developed.

The eyes are very much smaller than in a chick embryo at
a corresponding stage of development. The lens is present as
a small circular pit, with an open mouth, oc.

The auditory vesicles, El, appear from the surface as a pair
of rounded swellings, just above the dorsal ends of the hyoid
arches, hy.

The visceral arches have undergone considerable modifica-
tion. The maxillary arch, MX, lies immediately behind the
eye ; it is larger than before, but is still much smaller than
the arches next behind it. The mandibular arch, mn, is the
largest of the series, and is partially divided by a constriction,
about the middle of its length, into dorsal and ventral portions.
The hyoid arch, hy, is nearly as large as the mandibular arch,
and is also constricted across its middle. The first branchial
arch, BR.i, lies behind the hyoid arch, and is of much smaller size
than this. A still smaller second branchial arch is present, but
is not visible from the surface, being overlapped and concealed
by the first (cf. Fig. 239, p. 552).

With regard to the visceral clefts, it is probable that none
are open at this, or indeed at any period in development ; but
the point has not been determined with absolute certainty.
Behind the first branchial arch there is, on each side of the neck,
a deep pit, the sinus prascervicalis, su. This is a blind pocket,
(cf. Fig. 239), formed by a process of telescoping, through
which the hinder pairs of branchial arches are carried forwards,
so as to lie between the anterior arches, instead of behind these.
The sinus prsecervicalis does not open into the pharynx or into
any other cavity, and at a later stage it is obliterated by fusion of
its anterior and posterior walls with each other (cf Fig. 240, su).

The mouth (cf. Fig. 206) is much wider from side to side than
in the earlier stages ; it is bounded in front by the fronto-nasal
process, at the sides by the maxillary arches, and behind by
the mandibular arches.

In the body of the embryo, thirty-five pairs of somites, or
protovertebrge, are clearly visible ; of these, eight are cervical,

496 Tl [E HUMAN EMBRYO.

twelve thoracic, five lumbar, five sacral, and five caudal. Tin-
tail projects freely as a short conical process.

The fore and hind limbs, LA, LP, are flattened buds, with
rounded margins ; they are about as long as they are wide, and
show as yet no trace of a division into segments, or into fingers
and toes. The outer surface of each limb is its extensor surface ;
and the inner, facing the body of the embryo, is the flexor
surface. The root of attachment of the fore-limb, or arm,
extends from the fifth cervical to the second thoracic somite ;
and the attachment of the hind-limb, from the fourth or fifth
lumbar somite to the third or fourth sacral. The Wolffian rids?e
connecting the arm and leg of each side is still present, but is
inconspicuous.

The heart is of great size, and its several divisions can be
easily recognised through the thin wall of the pericardial cavity.
The liver, which is also large, forms a prominent swelling-
bet ween the heart and the fore-limbs.

The yolk-sac is about the size of the head and neck of the
embryo ; and the yolk-stalk is now long and slender. The
inner, or true, amnion is a thin transparent membrane which
invests the embryo rather closely ; and the allantoic stalk,
which lies to the right of the tail, and to the left of the yolk-
stalk, is about 2 mm. long, and rather more than 1 mm. in
diameter.

5. The Fifth Week.

The fifth week is marked by great increase in size of the
whole embryo, and especially of the head ; by further straight-
ening of the back, and uplifting of the head ; by the more definite
formation of the face ; and by rapid growth of the limbs.

The cervical flexure, at the junction of the head and body, is
still very conspicuous, and throughout the greater part of the fifth
week the greatest length of the embryo is, as in the fourth week,
a line drawn from this cervical prominence to the sacral curva-
ture. At the close of the fifth week, the head becomes lifted up
more markedly, and the length of the embryo, about 15 mm., is
now represented by a line drawn from the top of the mid-
brain to the sacral curve (cf. Fig. 211).

Throughout the fifth week, the head of the embryo grows
rapidly, and by the end of the week it forms, with the neck, about

THE FIFTH WEEK.

497

half of the entire embryo. The shape of the head is still deter-
mined almost entirely by the brain, of which the several divisions
are clearly recognisable from the surface. All parts of the brain
increase considerably in
size, and more especially
the cerebral hemispheres.

During the fifth week
the face is gradually ac-
quiring definite form, and
the features are becoming
established.

The olfactory pits deepen
considerably ; and their
inner and outer borders
become raised into pro-
minent lips. The inner
borders are formed by the
lateral margins of the
fronto-nasal process, which
grow out as two rounded
wings, the processus globu-
lares (Figs. 206, 207, fo). The outer borders are formed by
the lateral frontal processes, which separate the olfactory pits
from the eyes.

The lower margin of each olfactory pit is incomplete, and
between the processus globularis and the lateral frontal process
there is a deep nasal groove (Fig. 206), leading from the olfactory
pit to the stomatodaBum. Towards the end of the fifth week,
the maxillary arches (Fig. 207, mx) become more prominent, and
growing inwards meet the processus globulares, FO, and fuse with
these ; thus bridging over the nasal grooves, and converting
them into short tubes, the posterior narial passages, which lead
from the olfactory pits to the mouth. At the same time the
apertures of the olfactory pits become narrowed, to form the
external nostrils.

The bridge of the nose is formed from the median part
the fronto-nasal process (Figs. 206, 207, fp). At the com-
mencement of the fifth week this is a triangular area, slightly
depressed below the level of the surrounding parts ; but towards
the close of the week, a blunt process appears in the centre of

K K

Fig. 205.— Human Embryo lettered by
Professor His, Kg, and estimated as
thirty-two or thirty-three days old.
(From His.) x 5. "

498

THE HUMAN EMBItYO.

the area, formed by a sagittal fold of its surface, and gradually
grows forwards to form the bridge of the nose. For some time
the nose is very short and inconspicuous, and the nostrils very
far apart; but towards the end of the second month (c/. Figs,
213 and 214), the nose grows forwards more prominently, and
the nostrils are brought closer together. The ala3 nasi, forming
the outer borders of the nostrils, are developed from the lateral
nasal processes.

The mouth changes its shape very markedly during the fifth
week. At the beginning of the week (Fig. 206, os) it is a

Fig. 206.

Fig. 207.

Fig. 206. — The under surface of the head of a Human Embryo lettered by-
Professor His, Hn, and estimated as about twenty-nine days old. (From
His.) x 7|.

Fig. 207.— The under surface of the head of a Human Embryo lettered by
Professor His, C.II, and estimated as about thirty- four days old. (From
His.) x 5.

BS, cerebral hemisphere. DS, stornatodseum. FO, processus globularis, or lateral
portion of fronto-nasal process. FP, median portion of fronto-nasal process. HM,
hyomandibular cleft. MN, mandibular arch. MX, maxillary arch. OC, eye. OK,
olfactory pit.

wide opening, extending transversely across the under surface
of the head ; but before the end of the week (Fig. 207) it has
become greatly reduced in size, by convergence of the maxillary
arches and the processus globulares, and is now a narrow trans-
verse slit. Between the maxillary arch and the lateral nasal
process of each side is a depression, the lacrymal groove, which
at first (Fig. 206) leads into the stomatodasum, but which on the
completion of the narial passage opens into this latter (Fig. 207).
In the region of the visceral arches and clefts, important
changes occur. The tendency of the anterior arches to grow
backwards over the hinder arches, or, as it may be better

THE FIFTH WEEK. 499

expressed, the telescoping of the hinder arches within the
anterior ones (cf. Fig. 239), has been already referred to. At
the end of the fourth week (Fig. 201), the second branchial
arch is overlapped by the first, br.i, and is completely concealed
by this in surface views. Early in the fifth week, the second
branchial arch is in its turn overlapped by the hyoid arch ; and
from about the thirtieth day onwards (Fig. 205) the only
arches visible on the surface of the neck are the maxillary,
mandibular, and hyoidean. Behind the hyoidean arch is the
deep fissure caused by the sinus prascervicalis (cf. Fig. 210, su),
which must not be mistaken for a visceral cleft.

During the fifth week the borders of the hyomandibular cleft
become more prominent, and gradually give rise to the folds
from which the external ear is developed, in the following marrher.

At the end of the fourth week (Fig. 204), the hyomandi-
bular cleft, HM, is a deep groove between the mandibular and
hyoid arches, and running about halfway across the head.
The mandibular arch is divided by a slight constriction, about
the middle of its length, into dorsal and ventral portions : of
these, the ventral portion bears at its upper and posterior border
a small rounded process, well shown in the figure, and named
the tuberculum tragicum ; while the dorsal portion of the arch,
to which the reference line, MN, runs, is the tuberculum anterius
helicis. Opposite the dorsal end of the hyomandibular cleft is a
longitudinal ridge, the tuberculum intermedium helicis.

Tbe hyoid arch is divided, by two transverse constrictions,
into dorsal, middle, and ventral lobes : of these, the dorsal lobe is
named the tuberculum anthelicis; the middle lobe, to which the
reference line, hy, in Fig. 204 runs, is the tuberculum anti-
tragicum ; and the ventral lobe, which is the smallest of the
three, is the tuberculum lobular e.

In the course of the fifth week, these swellings assume more
definite form, and gradually give rise to the several parts of the
external ear or pinna. The tuberculum anterius helicis (Fig.
208, 2), and tuberculum intermedium (3) unite together, and
with a vertical ridge, the cauda (3c), which arises along the
posterior border of the hyoid arch, to form the horse-shoe shaped
marginal rim, or helix, of the ear. The ventral ends of the hyoid
and mandibular arches fuse, and so give more definite shape to the
hyomandibular cleft, which latter becomes the external auditory

500

THE HUMAN EMBRYO.

meatus. The tuberculum lobulare (g) fuses with the lower cud
of the cauda helicis (Fig. 209), and at a later stage grow s
ventral wards to give rise to the lobule of the ear. The tubercula
anthelicis, tragicum, and antitragicum, give rise to the antihelix,
tragus, and antitragus respectively of the adult ear.

The body of the embryo presents no external characters of
special interest during the fifth week. Owing to the increasing
thickness of the muscular and connective tissue walls, the out-
lines of the internal organs are less distinctly seen from the
surface than in the earlier stages.

The limbs undergo important changes during the week, and

Fig. 208.

Fig, 209.

Fig. 208. — The left ear of a Human Embryo lettered by Professor His, Br.2,
and estimated as thirty-five days old. (From His.) x 20.

Fig. 209. — The left ear of a Human Embryo, lettered by Professor His, Dr,
and estimated as thirty-eight days old. (From His.) x 20.

1, tuberculum tragicum. 2, tuberculum anterius lielicis. 3, tuberculum inter-
medium helicis. 3c and c, cauda helicis. 4, tuberculum anthelicis. 5, tuberculum anti-
tragicum. 6, tuberculum lobulare.

afford ready means of determining the age of the embryo. In
the early part of the fifth week they become divided, first into two,
then into three segments. By the middle of the week this
division is well marked, the terminal segments, i.e. the hands
and feet, forming broad flattened terminal plates, with free
rounded margins. A day or two later (Fig. 205), a distinction
appears in the hand, between a more swollen basal part, and a
thin flattened marginal part ; and towards the close of the week
the first traces of fingers appear, as small lobes at the boundary
between the basal and marginal portions, which soon extend to
the free edge, but do not project beyond this until the sixth
week.

THE FIFTH WEEK.

501

The hind-limb is slightly behind the fore-limb in its develop-
ment, and at the end of the fifth week the toes are only just
commencing to appear.

The fore- and hind-limbs of each side are still connected by
a low and inconspicuous Wolffian ridge. During the fifth week
the tail (Fig. 205) is more conspicuous than at any other stao-e

Fig. 210.— A Pregnant Uterus of about the fortieth day. The uterus has been
opened from the ventral surface, and the decidua reflexa and chorion cut
through by a crucial incision, and the flaps turned aside to expose the
embryo. The embryo is still inclosed in the amnion, and the small yolk-
sac, with its long stalk, are seen lying between the amnion and the
chorion. At the upper part of the figure the apertures of the Fallopian
tubes are seen. (From Kolliker, after Coste.) x ?.

in development ; it is a thin pointed projection, usually bent
either laterally, or backwards, by the pressure of adjacent parts.

6. The Sixth Week.

During the sixth week the embryo increases in size, though
not so rapidly as in the earlier stages. At the commencement
of the week it is about 15 mm. long, and at the close about 19
or 20 mm., but the actual measurements depend rather on the

502 THE nr.MAN EMBRYO.

degree to which the head is lifted up, by straightening of the
cervical flexure, than on any real increase in the dimensions.

The position of the embryo within the uterus, about the
fortieth day, is shown in Fig. 210. The embryo is connected
with the placenta by a thick allantoic stalk. The yolk-stalk is
long and thin : its proximal part is bound up with the allantoic
stalk in a sheath formed round both by the inner or true am-
nion ; while its distal portion, ending in the small yolk-sac, lies
between the amnion and the chorion. The amnion is a trans-
parent sac some distance from the embiyo.

The embryo itself is rapidly assuming more definite shape,
and by the end of the week is distinctly human in appearance.
Owing to the thickening of the muscles and of the subcutaneous
connective tissue, and the formation of skeletal elements, the
shape of the embryo as a whole, and especially of the head, is
much less dependent on the internal organs than in the earlier
stages.

The head is still of great size. The face has made consider-
able progress, and the features are now well established. The
nose is larger than before, but is still very broad and flat. The
eyelids are commencing to form, as folds of skin, above and below
the eyes. The lips appear as folds at the margins of the jaws,
but only reach a small development during the sixth week : the
red ridge of each lip arises independently, and not until a much
later period ; about the middle of the third month.

Up to the end of the fifth week there is a distinct notch in
the median plane where the two mandibular arches meet:
during the sixth week this notch is gradually filled up, and the
chin formed as a median projection.

The external ear makes considerable progress during the
week (Figs. 209, 211), and by its close the relations and propor-
tions of the several parts are readily comparable with those of
the adult.

Apart from the external auditory meatus and the external
ear, the visceral clefts and arches are no longer recognisable.
The sinus pragcervicalis has closed, and the neck is becoming
established as a constricted region between the head and
trunk.

The limbs have increased considerably in size, the upper
arm and thigh in particular being much longer than before.

THE SIXTH WEEK.

503

The fingers project beyond the margin of the hand by the
middle of the sixth week ; and the toes become clearly esta-
blished, although they do not reach the margin of the foot until
the early part of the seventh week. The elbow and knee project
outwards at first, but towards the end of the sixth week the limbs

Fig. 211. -Human Embryo about the middle of the sixth week.
(From His.) x 5.

become rotated so as to lie alongside the body, the elbow being
directed backwards, and the knee forwards.

The tail is less conspicuous than before, and owing to the
growth of adjacent parts is gradually becoming incorporated in
the body.

7. The Second Month.

At the end of the second month the embryo measures from
25 to 30 mm. in length, and weighs from 12 to 20 grammes.

The cervical flexure has almost disappeared : the head is
well lifted up, and is still of very large size, forming nearly half
the entire embryo. The eyelids, nose, lips, and external ear
have all made considerable advance ; the nose is still broad
and flat, and the nostrils wide apart, though much closer than

504

TUE HUMAN K.MIiliYO.

before. The median part of the upper lip is formed by th
two processus globulares, which meet and fuse shortly before th
end of the second month. The cheeks are now well formed.

Fig. 212. — Human Embryo at the end of the second month.
(From His.) x 5.

The limbs project some distance beyond the body ; and the

THE SECOND MONTH.

505

fore-limb, which is still the larger of the two, has the charac-
teristic shape of the human arm. The thumb is clearly marked
off from the fingers, and the deltoid swelling at the shoulder is
already prominent. The leg is smaller than the arm, and is so
directed that the soles of the feet are apposed.

The neck is well marked, though short. The ventral wall
of the body is completely formed. The umbilical cord, which
attaches the embryo to the placenta, is about 8 or 10 mm. long :
it is as a rule straight, but may be slightly twisted on itself.
It is formed by the allantoic stalk and yolk-stalk, bound together

HM-

Fig. 213.

Fig. 214.

Fig. 213. — Head of a Human Embryo at the end of the seventh week. HM,

external auditory meatus. (From His.) x 5.
Fig. 214.— Head of a Human Embryo at the end of the second month. (From

His.) x 3.

by the amnion, and it still contains at its base a loop of the
intestine.

From the end of the second month, when the definite human
form is established, up to the time of birth, it is customary to use
the term foetus in place of embryo.

8. The Third Month.

At the end of the third month the foetus measures about
7 cm. in length, or 9 to 10 cm. if the legs be included, and weighs
from 100 to 125 grammes.

The head is still very large relatively to the rest of the body,

506 Till: HUMAN EMBRYO.

but not nearly so much so as in the earlier stages: the lipa
and eyelids are closed, and the helix of the ear is folded down
so as to almost close the meatus. The neck is longer than
before. The limbs, though small, have acquired their definite
shape and proportions ; and nails are present, as thin plates, on
both fingers and toes. The integument is slightly firmer than
before, but is still very thin, transparent, and rose-coloured.

Up to this stage a loop of the intestine has been situated in the
allantoic stalk, and therefore outside the embryo; but by the end
of the third month this loop is withdrawn, and the whole alimen-
tary canal, which has increased greatly in length, is from this
time situated within the abdominal cavity.

During the third month, the external genital organs become
established. The history of their development will be given in
the section dealing with the organs of reproduction.

9. The Fourth Month.

At the end of the fourth month the foetus measures 12 to 13
cm. in length, from the vertex of the head to the coccyx ; or from
16 to 20 cm. if the legs be included. The weight is usually from
230 to 260 grammes.

The skin is of a rosy colour, and is much firmer than before .
Short whitish hairs appear on the head, and a slight down on
other parts of the body. The eyelids, nostrils, and lips are all
closed. The chin, which has hitherto been inconspicuous, begins
to become prominent. The legs and arms are of about equal
length : and the external sexual characters are iisually well-
marked.

The anus is open, and the duodenum contains meconium of
a light greyish-white colour. The umbilicus, or point of origin
of the umbilical cord, is low down, close to the pubes. In the
skull, the bones are still far from meeting one another, so that the
sutures and fontanelles are very wide. The muscles are more
fully developed than before, and may give rise to distinct move-
ments of the foetus. In abortions at this period the foetus may
live for some hours.

10. The Fifth Month.

By the end of the fifth month the foetus measures about
20 cm. in length; or, if the legs be included, 25 to 27 cm.
The average weight is about half a kilogramme.

THE FOURTH, FIFTH, AND SIXTH MONTHS. 507

The skin is more consistent than before, and presents on its
surface at certain places small patches of sebaceous matter.
Hairs are more extensively developed than before, but are still
devoid of any distinct colour. The legs are now longer than
the arms, and the nails are well formed. The umbilicus is
further forward than in the preceding month, and is now some
distance in front of the pubes.

The head is still very large in proportion to the other parts.
The heart, liver, and kidneys are also disproportionately large.
The small intestine contains meconium, which, owing to the
secretion of bile, is now of a pale greenish-yellow colour. The
gall-bladder is of some size. Ossification has commenced in the
pubes, and in the os calcis.

11. The Sixth Month.

The total length of the foetus at the end of the sixth month,
measured from the vertex to the heels, is from 30 to 32 cm.
The weight is very variable ; its average amount is about a
kilogramme.

The skin is of a dirty reddish colour, and much wrinkled ;
it is covered, at any rate in the axilla) and groins, with a seba-
ceous deposit. The hairs are more strongly developed, and of a
darker colour than before. Both eyelashes and eyebrows have
commenced to appear.

The umbilicus is still further forward than before, and the
meconium in the intestine is darker and more viscous. The
testes of the male have not yet descended into the scrotum,
but are situated within the abdominal cavity, lying on the psoas
muscles, immediately behind the kidneys.

The sternum is well developed, and has commenced to ossify.
The nails reach to the ends of the fingers, and extend about a
quarter of the way round them.

12. The Seventh Month.

The total length of the foetus at the end of the seventh
month, measured from the vertex to the heels, is about 35 or
36 cm., and the weight averages about 1^ kilogramme.

The skin is still of a dirty reddish colour, but not so
dark as before. There is an increased deposit of fat in the
cellular tissue, causing the body to appear more plump and

508 THE HI' MAN EMBRYO.

round. The hairs are plentiful, and about 5 or 6 mm. in
length.

The several bones forming the roof of the skull become
strongly convex, the central portion of each, from which ossifica-
tion starts, forming a very evident prominence. The eyelids,
which have been closed since reaching their fall size in the fourth
month, now open.

The whole of the large intestine is filled with a dark olive-
green viscous meconium. The liver is still very large relatively
to the whole body, and is of a deep brownish red colour.

The testes have, as a rule, descended as far as the inguinal
rings, and may even have entered the inguinal canals.

The end of the seventh month is of interest, as being perhaps
the earliest period at which the foetus can be born with any
reasonable chance of surviving.

13. The Eighth Month.

During the eighth month the increase in bulk is more
marked than that in length. At the end of the month the
total length of the foetus, from the head to the coccyx, is about
28 cm. ; and from the head to the heels about 40 cm. The
weight varies from 2 to 2 J kilogrammes.

The skin is of a brighter flesh colour than before, and is
covered all over with the sebaceous deposit known as ' vernix
caseosa.' This substance, which usually makes its appearance
about the middle of gestation, was formerly considered to be a
deposit formed from the liquor amnii, but appears rather to
consist of matter formed by the cutaneous glands of the foetus,
mixed with dead epithelial cells. It varies much in quantity
in different cases, and is always more abundant in certain situa-
tions, notably the head, axilla), and groins.

The chin is now far more prominent than before, the lower
jaw equalling the upper in length. One of the testes, usually
the left one, has passed through the inguinal canal into the
scrotum, while the other is, as a rule, still in the canal. There is
no ossification in the lower epiphysis of the femur.

14. The Ninth Month.

At the full time the foetus measures about 35 cm. from the
head to the coccyx, and 50 cm. from the head to the heels.
The weight is, on the average, from 3 to 3J kilogrammes.

THE EIGHTH AND NINTH MONTHS. 509

The skin is paler than before. The subcutaneous connective
tissue is filled with fat, giving roundness and firmness to the
body and limbs. The hair is thick, long, and fairly abundant
on the head, while the down has begun to disappear from the
body.

The umbilicus is almost exactly in the middle of the body,
or slightly behind this point. Both testes are, as a rule, in the
scrotum, which has now a corrugated surface.

Ossification has commenced in the centre of the cartilage at
the lower end of the femur. This is the first epiphysial ossifica-
tion to appear in the body, and is often the only one present at full
time. Ossification has sometimes commenced in the proximal
epiphyses of the tibia and humerus ; but while the presence of
these centres is a sure sign of full time having been reached,
their absence does not, without further evidence, indicate pre-
mature delivery.

DEVELOPMENT OF THE NERVOUS SYSTEM.

The general history of development of the human nervous
system is the same as in other Vertebrates. Certain points,
especially in connection with the brain, will require detailed
notice ; and, with regard to the histological development of the
nervous elements, recent researches by His, and others, have
shown that human embryos are well suited for the most minute
investigations.

1. The Brain.

a. General account. It will be convenient to give first a
general account of the development of the brain, and of its con-
dition at successive stages, and then a more detailed descrip-
tion of parts, such as the cerebral hemispheres, which are of
special interest.

The second week. In the youngest human embryos, such as
His' embryos E and SR (Figs. 176, 178, and 179), estimated as
about thirteen days old, the neural groove is widely open along
its whole length, but by comparison with later embryos it is
possible to determine, even at this stage, the several regions of
the brain.

Thus, in Fig. 179, the dorsal concavity, opposite the refer-

510 THE lir.MAN EMBRYO.

ence Line an, marks the junction of the brain and spinal cord ;
the highest point of the cephalic convexity, close to the refer-
ence line hd, is the region of the mid-brain ; and the part in
front of this is the fore-brain, which is already flexed ventral-
wards.

The third week. By the fifteenth day (Figs. 197, p. 489,
and 232, p. 545) the neural canal is closed along its whole
length, except at the extreme hinder end ; the several divisions
of the brain — fore-brain, mid-brain, and hind-brain — are well
established; and cranial flexure is strongly marked, a sharp
bend of about 90 degrees taking place opposite the mid-brain,
by which the fore-brain is brought down to the under surface of
the head.

The fore-brain is of considerable length ; its most anterior
part is the vesicle of the hemispheres, a short, rounded, and com-
paratively inconspicuous dilatation, which as yet shows no trace
of division into right and left hemispheres. The thalamen-
cephalon, or fore-brain proper (Fig. 232, bf), is long, and com-
pressed laterally ; from its sides arise the optic vesicles, bo,
which project outwards and slightly backwards, and are already
constricted at their bases to form the optic stalks. The floor of
the thalamencephalon is produced downwards behind the optic
stalks into a shallow pit, the infundibulum.

The mid-brain, bm, is small and rounded ; it is separated by
a constriction from the fore-brain in front, and by a much
sharper one, the isthmus, from the hind-brain.

The hind-brain is the widest as well as the longest part of
the brain ; it is widest in front, and gradually tapers posteriorly
as it passes into the spinal cord. The roof of the hind-brain
is very thin, except at its anterior end, where a slightly
thickened transverse band, BL, marks the commencement of the
cerebellum.

During the third week the brain rapidly increases in size,
and by the end of the week has attained the proportions shown
in Fig. 215. The several divisions of the brain are more dis-
tinctly marked off from one another, and the vesicle of the hemi-
spheres, bs, and the cerebellum, BL, are more conspicuous than
before.

The cervical flexure, by which the entire head is bent ventral-

THE BEAIN.

511

wards on the body, is commencing to appear at the junction of
brain and spinal cord ; it is shown in Fig. 215, at a level between
the reference lines lie. 3 and en.

The fourth week. By the end of the fourth week the shape
of the brain is as shown in Fig. 21G. The flexure at the level
of the mid-brain, or mesencephalic flexure as it may be termed,
has increased greatly in extent, and now amounts to about 180°,

HC.1

HC.3

Fig. 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.
(Cf. Fig. 198.) The brain is exposed from the left side ; the rest of the
embryo is represented in sagittal section. (From His.) x 28.

BP, thalamencephalon. BH, hind-brain, or medulla oblongata. BL, cerebellum.
BM, mid-brain. BO, optic vesicle. BS, vesicle of the cerebral hemispheres. CH,
notochord. EI, auditory vesicle. HC.l, first branchial pouch. HC.3, third bran-
chial pouch. HM, hyomandibular pouch. MIST, mandibular arch. BT, truncus
arteriosus. TO, oesophagus. "W, liver.

the infundibulum and the hind-brain almost touching each other.
The cervical flexure, marking the junction of the brain and spinal
cord, at the level of the reference line a. 5, is also much more
pronounced than before, and forms an angle of about 90°. A
third, or metencephalic flexure, with the concavity directed
dorsal wards, is commencing to form opposite the cerebellum ,
at the level of the reference line pt ; at a slightly later stage
this flexure becomes very strongly marked.

As regards the individual parts of the brain, the vesicle of

12

THE HUMAN EMBRYO.

the hemispheres has greatly increased, and is now divided by a
median fold into right and left hemispheres, BS, which are

Fig. 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. (From His.) x 9.

A, dorsal aorta. A.3, third aortic arch, or carotid arch. A.4, fourth aortic arch, or
systemic arch. A. 5, fifth aortic arch, or pulmonary arch. BL, cerebellum. BM,
mid-brain. BS, cerebral hemisphere. IN, infundibulum. KD, ureter. LG, lung
LT, laryngeal chamber. OC. optic cup. PT, pituitary diverticulum from stomato-
daeuui. EB, left auricle. BV, ventricle. TA, cavity of allantoic stalk. TC, cloaca.
TN, tongue. TB, intestine. TS, stomach. TZ, umbilical stalk. VA, allantoic
vein. VB, anterior cardinal vein. VC, posterior cardinal vein. VE, meatus veiio-
sus. VI, posterior vena cava. VO, vitelline vein. "W, liver. WD, bile duct.
TK, yolk-stalk.

already commencing to grow backwards over the thalamen-
cephalon. This latter is very deep dorso-ventrally, but com-
pressed laterally ; the infundibulum, in, is of considerable size ;

JV v<*>

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Fig. 227.— Diagrammatic figure of a Human Embryo, lettered by Professor
His, Ko, and estimated as thirty-one days old. The brain and spinal
cord, and the cranial and spinal nerves are shown, and certain of the other
organs are represented in outline. The bases of the fore and hind limbs
are indicated by the dotted outlines. In all cases in which the full length
of the nerve is not shown, the end is represented as though cut across.
(After His.) x 10.

BF, thalainencephalon. BM, mid-brain. BS, cerebral hemisphere. BY, olfactory
lobe. EI, auditory vesicle. FGr, Froriep's ganglion. GC, ciliary ganglion. HM,
hvomandibular cleft, or external auditory meatus. M.l, ganglion of first cervical nerve.
M\9, ganglion of first thoracic nerve. M.21, ganglion of first lumbar nerve. M.26,
ganglion of first sacral nerve. M.31, ganglion of first coccygeal nerve. MH, phrenic
nerve. OC, optic cup. OF, olfactory pit. EB, left auricle. EV, ventricle. SU.
sinus praecervicalis. TI, vitelline loop of intestine. TL, tail. W, liver. Ill, third.
cranial nerve. IV, fourth cranial nerve. V, Gasserian ganglion. V«, Ophthalmic
branch of fifth, or trigeminal nerve. ~Vb, maxillary branch of fifth, or trigeminal nerve.
Vc, mandibular branch of fifth, or trigeminal nerve. VII, ganglion of the seventh, or
facial nerve. VIII, ganglion of the eighth, or auditory nerve. IX, ninth, or glosso-
pharyngeal nerve. X, ganglion of the root of the tenth, or pneumogastric nerve. XI,
roots of the eleventh, or spinal accessory nerve. XII, roots of the twelfth, or hypoglossal
nerve.

M M

530 THE HUMAN EMBRYO.

The nerves of the first set, i.e. the motor nerves, havd
localised centres of origin in the brain : the nerves of the second
set, or sensory nerves, are not definitely localised in the brain,
except by the points at which the fibres enter the brain. The
two groups of nerves arise independently, as in the case of the
spinal nerves. They may retain their independence, forming
purely motor, or purely sensory nerves ; or they may become more
or less closely associated with one another to form nerves of
mixed, motor and sensory, function.

The course of the cranial nerves in the early stages of their
development is curiously straight (Fig. 227) ; their main direc-
tion, like that of the spinal nerves, being at right angles to the
axis of the head, or brain, at their points of origin. This initial
course is liable to disturbance through shifting relations of the
parts with which the nerves are in connection, or through
growth of the skeletal or other neighbouring parts. Thus the
facial nerve is at first straight, but, owing to the telescoping of
the hinder visceral arches within the anterior ones, its course
becomes much modified (Fig. 227, vn).

In many instances some further explanation is required :
thus the glossopharyngeal nerve extends forwards in front of its
proper territory, in order to reach the circumvallate papilla? of
the tongue ; while the facial nerve extends forwards to the fore-
head. An interesting case is the extension of the pneumogastric
nerve to the heart, lungs, and stomach. The posterior limit of
the head may be taken as indicated by the hinder border of the
second branchial arch ; or, in the adult, by the boundary line
between the thyroid and cricoid cartilages, if Callender and His
are right in regarding the thyroid cartilage as developed from
the cartilage of the second branchial arch. In any case, the
heart, lungs, and stomach are, in the adult, far behind the head
region. It must be remembered, however, that the heart origi-
nally lies between the ventral ends of the visceral arches, and that
the lungs arise from the floor of the pharynx, so that both heart
and lungs really lie within the proper area of distribution of the
pneumogastric nerve. The stomach, however, does not do so,
and in order to reach it the pneumogastric nerve must pass
beyond the limits of its own territory.

In describing the cranial nerves individually it will be con-

THE CRANIAL NERVES. 531

venient to arrange them in two groups, in accordance with the
distinction laid down above, and to describe the nerves of each
group in order, from behind forwards.

Group A. Nerves arising from groups of neuroblasts in the
substance of the brain, in the same way as the motor or ventral
roots of the spinal nerves.

To this group belong the third, fourth, and sixth nerves ;
the motor root of the trigeminal nerve ; the facial nerve ; the
motor roots of the glossopharyngeal and pneumogastric nerves ;
and the spinal accessory and hypoglossal nerves.

Along the spinal cord, the motor roots all leave the cord at
the same horizontal level, the sole exception being at the anterior
end of the cervical region, where the hinder roots of the spinal
accessory nerve arise at a level dorsal to that of the motor
spinal roots. In the brain there are two series of motor
roots, a ventral series and a lateral series; the ventral series
including the hypoglossal, the sixth, and perhaps the fourth and
third nerves as well ; and the lateral series including the
anterior roots of the spinal accessory, and the motor roots of the
pneumogastric, glossopharyngeal, facial, and trigeminal nerves.

The hypoglossal, or twelfth cranial nerve (Fig. 227, xn),
arises by a long series of roots, each formed by a bundle of axis
cylinders which arise as outgrowths from a group of neuroblasts
in the ventro-lateral wall of the medulla oblongata (Fig. 228, xn).
The roots commence just in front of the motor root of the first
spinal nerve, and in line with this, and extend forwards to the
level of the glossopharyngeal nerve and the posterior border of
the auditory vesicle.

The mode of origin, and the position and relations of these
roots, strongly suggest a comparison with the ventral or motor
spinal roots.

In sheep embryos Froriep describes a dorsal ganglionic root
of the hypoglossal nerve, in addition to the ventral roots, so the
comparison with a spinal nerve or nerves seems quite legitimate.
In human embryos at the end of the fourth week, and beginning
of the fifth week, His has described a small ganglion, which he
names Froriep's ganglion (Fig. 227, fg), lying immediately in
front of the first cervical ganglion, x.i, and in line with this.
Froriep's ganglion is small and gives off no nerves at all, and at
a slightly later stage it disappears altogether ; it appears, how-

532

THE III 'MAN E.MHRYO.

ever, to correspond to the ganglion described by Froriep in
sheep embryos as forming a true dorsal root to the hypoglossal
nerve.

It is probable, therefore, that the hypoglossal nerve is to hi
regarded as formed by the ventral roots of one or more nerves
equivalent to spinal nerves; and of which the dorsal root, or

x.s

X.k

Fig. 228. — Transverse section across the medulla oblongata of a Human
Embryo, lettered by Professor His, Ko, and estimated as thirty-one days
old. The embryo is the same one as that represented in Fig. 227, and
the section passes through one of the roots of the hypoglossal nerve, and
through both the motor and sensory roots of the pneumogastric nerve.
(From His.) x 40.

X.M, motor root of pneumogastric nerve. X.S, sensory root of pneumogastric nerve.
XII, root of hypoglossal nerve.

roots, are represented in man by the rudimentary Froriep's
ganglion alone.

The spinal accessory, or eleventh cranial nerve (Fig. 227, xj),
arises by a number of roots, formed by outgrowths from groups
of neuroblasts in the side of the medulla oblongata ; the roots
lying at a level dorsal to that of the hypoglossal roots, and at the
junction of the ventro-lateral and dorso-lateral regions of the
medulla (cf. Fig. 228).

The roots of the spinal accessory nerve are very numerous.
At the beginning of the fifth week (Fig. 227) the most posterior

THE CRANIAL NERVES. 538

root lies in close relation with Froriep's ganglion, fg, and a very
little distance in front of the first cervical nerve ; while the
most anterior root lies just behind the pnenmogastric nerve.
The cervical roots of the spinal accessory nerve do not appear
until a later stage, a probable indication that the spinal accessory
is to be regarded as a cranial rather than as a spinal nerve.

The motor roots of the pneumogastric, or tenth cranial nerve
(Figs. 227 and 228, XM). These lie immediately in front of the
anterior roots of the spinal accessory nerve, and in line with
them. They arise from groups of neuroblasts in the walls of the
medulla oblongata (Fig. 228), in a manner precisely similar to
that in which the ventral spinal roots are formed. The nerve
fibres converge to form small bundles which leave the medulla
immediately ventral to the much larger and more conspicuous
sensory root (Fig. 228, x.s), by which they are covered and
more or less completely concealed.

The motor roots of the glossopharyngeal, or ninth cranial nerve.
are exactly similar to those of the pneumogastric ; they lie im-
mediately in front of these, and in line with them, and in trans-
verse sections present an appearance practically identical with
that shown for the pneumogastric nerve in Fig. 228, X.m.

The facial, or seventh cranial nerve (Fig. 227, vn), arises
from a group of neuroblasts in the side wall of the medulla
oblongata, opposite the auditory vesicle. The bundle of axis
cylinders, formed as outgrowths of the neuroblasts, does not at
once pass out from the medulla, but runs forwards a short
distance in its substance, and emerges immediately below the
auditory nerve and in very close relation with this. The root of
the facial nerve lies in line with the motor roots of the glosso-
pharyngeal and pneumogastric nerves, i.e. it belongs to the
lateral series of motor roots.

The chorda tympani is present early in the fifth week as an
.ulterior branch of the facial nerve, which runs in the tympanic
membrane, but does not yet reach the trigeminal nerve.

The sixth cranial nerve belongs to the ventral series of motor
roots. It arises from several groups of neuroblasts which lie in
the ventro-lateral area of the medulla oblongata, in line with
the hypoglossal roots, and vertically below the root of the
auditory nerve, i.e. a short distance anterior to the root of the
facial nerve. The sixth nerve, after emerging from the brain.

534 THE HUMAN EMBEYO.

runs almost directly forwards, lying to the inner side of the
Gasserian ganglion, and reaches the external rectus muscle
early in the fifth week. The nerve is drawn, though not named,
in Fig. 227, as a thin band emerging from the ventral surface of
the brain immediately below the ganglion of the facial and
auditory nerves, and running horizontally forwards towards the
hinder border of the eye.

The motor root of the trigeminal, or fifth cranial nerve, lies at
a level slightly ventral to the motor roots of the facial, glosso-
pharyngeal, and pneumogastric nerves, but clearly belongs to the
lateral rather than to the ventral series of roots. It lies to the
inner side of the Gasserian ganglion, and in close relation with
this, and is, in its early stages, slightly anterior to this in position.

The fourth cranial nerve (Fig. 227, iv), though leaving the
brain on the mid-dorsal surface, is stated by His to arise from a
group of neuroblasts on the ventral surface of the isthmus, or
constricted neck between the hind- and mid-brains. These roots
lie close to the mid-ventral plane, and clearly belong to the
ventral series. From this origin the fibres of the fourth nerve
run up, in the sides of the brain, to its dorsal surface, cross those
of the opposite side in the mid-dorsal plane, and finally emerge
from the brain as the definite nerves.

Though very slender, the fourth nerves are of considerable
length by the early part of the fifth week (Fig. 227), already
reaching to the level of the eye.

The fourth nerve has long been a source of trouble to
morphologists. Professor His' observations on its development
in human embryos will, if confirmed and extended to other
Vertebrates, throw a very welcome light on the problem, show-
ing that, in spite of the peculiar position at which it leaves the
brain, the fourth nerve really belongs to the category of ventral
or motor roots.

The third cranial nerve (Fig. 227, in) arises from a group
of neuroblasts in the floor of the mid-brain, rather further apart
than the other ventral roots, but belonging to the same series.

Group B. The nerves included in this series arise from
groups of neuroblasts, not in the brain, but in the ganglia, i.e.
they are developed in the same manner as the dorsal or sensory
roots of the spinal nerves.

THE CRANIAL NERVES. 535

To this group belong the sensory roots of the pneumogastric
and glossopharyngeal nerves ; the auditory nerve ; the sensory
root of the trigeminal nerve ; and probably the olfactory nerve
as well.

In the head there are four primary ganglion masses, those
of the fifth, eighth, ninth, and tenth cranial nerves. Whether
these are connected in the early stages, to form a continuous
neural ridge along each side, has not yet been ascertained ;
neither has the precise mode in which the permanent connection
of these ganglia with the brain is acquired been determined.

The four ganglionic masses are clearly visible at the end of
the third week. During the fourth week they become gradually
divided up, each giving rise to two or more ganglia, which, by
further elongation of the connecting nerve strands, move apart
to a greater or less distance from one another.

The sensory root of the pneumogastric, or tenth cranial nerve,
is from the first in close relation with the motor root, being at-
tached to the brain immediately dorsal to this latter (Figs. 227
and 228, x.s). The ganglion is at first single, but by the end of
the fourth week it becomes divided into a proximal and smaller
part, the ganglion of the root ; and a distal, larger, and fusiform
part, the ganglion of the trunk (Fig. 227). In the later stages
these two ganglia move some distance apart, owing to lengthening
of the nerve trunk between them. The ganglion of the root is
connected with the distal, or petrous, ganglion of the glosso-
pharyngeal nerve by an oblique commissural band, well seen in
Fig. 227 : whether this is a persistent remnant of an originally
continuous neural ridge has not been determined.

By the end of the fourth week, the superior and inferior
laryngeal nerves are present, and also a large branch extending
down the oesophagus towards the stomach.

The sensory root of the glossopharyngeal, or ninth cranial
nerve, is very similar to that of the pneumogastric, but of smaller
size. The ganglion early divides into a proximal 'jugular'
portion, and a distal ' petrous ' portion (Fig. 227). The nerve
itself is straight in the early stages, but becomes curved forwards
at its ventral end (Fig. 227) as the first branchial arch, with
which it is specially associated, is carried forwards along the
inner side of the hyoid arch (cf. Fig. 240).

The auditory nerve. In the case of the auditory ganglion of

536 THE 1 1 (MAN EMBRYO.

the human embryo, nerve fibres have been traced growing out
from the nerve cells of the ganglion into the brain, the attach-
ment taking place immediately dorsal to the point of emergence
of the facial nerve. Beyond its root of attachment, the gang-
lion of the auditory nerve divides into two main portions, the
cochlear and vestibular ganglia : these diverge from each other,
and between them the root of the facial nerve is wedged. The
auditory ganglia very early acquire connection with the wall
of the auditory vesicle, and the several ganglia of the adult
ear are formed by further division of the two ganglia of the
embryo.

It is stated that the geniculate ganglion of the facial nerve
is derived from the same ganglionic mass from which the auditory
ganglion is formed.

The sensory root of the trigeminal, or fifth cranial nerve.
The ganglion of the trigeminal nerve is, from the first, of great
size (Fig. 227). The three principal branches of the nerve —
ophthalmic, maxillary, and mandibular (Fig. 227, v, a h c) — are
already present, and of large size, before the end of the fourth
week ; as these nerves lengthen, the originally single ganglion
gradually breaks up, small portions becoming detached, and
moving out along the growing nerve stems. In this way, early
in the fifth week, the ciliary, sphenopalatine, and otic ganglia
are established ; the submaxillary ganglion is not separated
until a rather later stage. The main ganglion persists as the
Gasserian ganglion of the adult, and the motor root of the
trigeminal nerve lies along its inner side, and in close contact
with it (Fig. 227, v).

The optic nerve. The optic vesicle and optic stalk are parts
of the brain, and cannot be compared with nerves, either sen-
sory or motor. There is, however, strong reason to think that
the actual optic nerve fibres do not arise in the optic stalk, but
are formed independently, as outgrowths from the retinal cells
which grow inwards to the brain, following the line of the optic
stalk, but being fundamentally independent of this. Even then,
however, inasmuch as the retina is developmentally part of the
brain, the optic nerve would rather resemble the intra-cerebral
fibres of the brain than the ordinary sensory nerves ; and for
the present the relations of the optic nerves to the other nerves
must be left undecided.

THE CRANIAL NERVES. 537

The olfactory nerve. According to the observations of
Professor His, the mode of development of the olfactory nerve in
the human embryo is as follows. The olfactory lobe is formed
as an outgrowth of the cerebral hemisphere towards the end of
the fourth week, and very early becomes divided by a transverse
constriction into anterior or distal, and posterior or proximal
portions.

At this stage, in embryos of from twenty-seven to twenty-
eight days, although the olfactory pit is well developed (Figs.
204 and 227), there is, according to His, no trace of either the
olfactory ganglion or olfactory nerve. A day or two later, the
olfactory epithelium begins to undergo changes similar to those
which occur in the wall of the brain, or spinal cord, preparatory
to the appearance of nerves. Neuroblasts are formed near its
inner or deeper surface : these soon become pyriform, and give off
processes which grow into the mesoblast, and towards the brain.
There is thus, early in the fifth week, a mass of neuroblasts
forming a ganglion in direct connection with the olfactory epi-
thelium : from the ganglion, nerve fibres grow out towards the
brain, but do not yet reach this. By the end of the fifth week
the nerve fibres reach the olfactory lobe, meeting it at the
constriction separating the proximal and distal portions, and
thus placing the olfactory epithelium in connection with the
brain.

During the second month the distal portion, or bulb of the
olfactory lobe, which at first lies entirely in front of the nerve,
becomes bent down so as to lie in contact with this ; and by
the end of the second month the olfactory nerve arises by a
number of fibres from the olfactory bulb, instead of by a
single stem from the olfactory lobe behind the bulb, as in the
earlier stages.

The roots of the adult olfactory nerve are formed by bundles
of ascending or centripetal nerve fibres, which grow from the
ganglion into the brain ; they are already present at, or shortly
after, the end of the second month.

At first sight this account appears to differ widely from the
descriptions given above of the development of the other sensory
nerves; but the differences are not really so great as they seem.
In the case of all sensory nerves the connection with the brain,
or spinal cord, is acquired by growth of nerve processes centri-

588 mi: human embryo.

petally, from bhe ganglia Into the brain or cord: the ganglia
themselves, though developed in close relation with the brain and
cord, are not really parts of these, but are independent structure-.
The formation of neuroblasts in the olfactory epithelium presents
no difficulty, when it is remembered that the wall of the brain
or spinal cord is itself merely a specialised portion of the surface
epithelium ; while, finally, it has been shown in earlier parts
of this book that in other Vertebrates, such as the frog and
chick for example, the surface epithelium may, in the hinder
cranial nerves, take a direct share in the formation of the
nerve ganglia.

It is probable indeed that the mode of development of the
olfactory nerve described above, as observed in human embryos,
represents a more primitive type of nerve development, from
which that of the other sensory nerves has been derived.

4. The Sympathetic Nervous System.

The mode of development of the sympathetic nervous system
has not been accurately determined in human embryos. From
the close resemblance in the mode of development of other parts
of the nervous system, it is probable that it takes place in essen-
tially the same manner as described above in the case of the
chick and rabbit.

THE DEVELOPMENT OF THE SENSE ORGANS.

1. The Nose.

All the essential points in the development of the olfactory
organ have been already described. The formation of the
olfactory pit, the organ of Jacobson, the external nostrils, the
bridge and alas of the nose, and the posterior narial passages,
are dealt with on pp. 494 to 498 ; and the development of the
olfactory lobe and olfactory nerve on pp. 517 and 537.

During the third month the olfactory pits, which are at first
simple, become greatly complicated by folding of their walls ;
and in this way the nasal labyrinth, supported by the turbinal
scrolls, is established. The accessory cavities communicating
with the nose, i.e. the antrum, and the frontal, sphenoidal, and
ethmoidal sinuses, are not established until a later stage.

THE NOSE AND EYE. 539

2. The Eye.

The mode of development of the human eye is so closely
similar to that of the rabbit that it will be needless to describe
it in detail.

The optic vesicles appear as lateral outgrowths of the fore-
brain as early as the fifteenth day (Fig. 232, bo). They soon
become constricted at their bases, and then doubled up to form
the optic cups, in the same manner as in other Vertebrates.
Owing to the mode in which this doubling up is effected, a
choroidal fissure is left, leading into the cavity of the cup ; and,
as in the rabbit, the choroidal fissure extends a little way along
the optic stalk, towards the brain. Throughout all the earlier
stages of development (Fig. 204), the eye is very small, as in
Mammals generally, standing in this respect in marked contrast
to the eye of the chick embryo at a corresponding stage of
development (cf. Fig. 115).

The inner wall of the optic cup (cf Fig. 155) is from the
first the thicker of the two ; and by the end of the fourth week
is at least four times as thick as the outer wall.

The lens develops late. In embryos three weeks old it is
still an open pit : at four weeks the mouth of the pit has closed
(Fig. 204), and from this time the cavity of the lens vesicle
becomes rapidly filled up by elongation of the cells forming its
inner or deeper wall. During the whole period of its develop-
ment, the lens is enveloped in a vascular capsule which serves
for its nourishment. New cells are constantly added on round
the equator of the lens, and growth continues until the time of
birth, when the vascular capsule atrophies and disappears.

The vitreous body is formed of mesoblast, which makes its
way into the cavity of the optic cup through the choroidal
fissure : it is very vascular during the earlier stages of its
formation.

The cornea is formed from a layer of mesoblast which grows
across the front of the eye, between the outer conjunctival
epithelium and the lens, towards the end of the second month.
In the deeper part of this layer a cavity appears, which becomes
the anterior chamber of the eye. The thick layer of mesoblast
in front of this chamber becomes the cornea, while the much
thinner layer between the chamber and the lens forms ' the

540 THE HUMAN EMBRYO.

anterior wall of the lens capsule. The cornea becomes trans-
parent during the fourth month, at which time it is strongly
convex, more so than in the adult. The cornea is very thick
from the first, and much thicker than the sclerotic; and at the
time of birth it is said to be absolutely thicker than in the
adult.

The choroid is a very vascular layer, in which pigment
begins to appear towards the end of the second month. The
occasional occurrence in the adult human eye of a non-pig-
mented streak along the under surface of the eyeball, or even
of complete fissure of the iris, or coloboma iridis, is commonly
attributed to incomplete closure of the choroidal fissure. Inas-
much, however, as the choroidal fissure concerns primarily the
optic cup alone, and not the choroid coat, it is probable that
coloboma iridis should not be regarded as a simple case of
arrested development, but as involving some further pathological
process as well. The choroidal fissure normally closes about the
seventh week.

The retina is formed, as in other Vertebrates, from the inner
and thicker layer of the optic cup. After its first establishment,
it grows for a time more rapidly than the outer coats of the eye-
ball, and consequently becomes thrown into folds which during
the second month project freely into the cavity of the eye. In
the later stages it becomes flattened out again. The rods and
cones are formed as outgrowths from the outer surface of the
thickened inner layer of the optic cup : they do not appear
until very late ; shortly before the time of birth.

The mode of origin of the fibres of the optic nerve of the
human eye has not been determined with exactness. It appears
certain, however, that the nerve fibres are not formed out of
the walls of the optic stalk, and it is probable that they arise
as outgrowths from neuroblasts in the retina itself, which grow
inwards towards the brain along the path afforded by the optic-
stalk ; and on reaching the base of the thalamencephalon pass
across to the opposite side of the brain, thus forming the optic
chiasma, and continue their course up the sides of the brain as
the optic tracts, until they finally reach the corpora quadri-
gemina.

The eyelids appear, towards the close of the second month,
as folds of skin above and below the eyeball (Fig. .213); they

THE EYE AND EAE. 541

unite with each other, thus closing the eye, about the third or
fourth month, and separate again shortly before birth.

The lacrymal duct is formed along the line of the lacrymal
groove, as a linear depression running from the eye to the nose,
along the line of meeting of the external nasal process and the
maxillary arch (Fig. 207). The duct itself arises as a solid rod
of epithelial cells split off from the floor of the groove : it
becomes sinuous at an early stage, and from its sides the lacry-
mal glands arise as solid branched outgrowths, with dilated,
hollow, bulb-like ends. At a later stage the solid cords become
hollow along their axes, and converted into the lacrymal
ducts. At the inner canthus of each eye the duct bifurcates,
while still a solid rod, to form the rudiments of the upper and
lower lacrymal canals.

The third eyelid, or plica semilunaris, which is rudimentary
in man, arises as a small fold of the conjunctiva at the inner
canthus of the eye, within the upper and lower eyelids.

3. The Ear.

The ears appear as a pair of open pits at the sides of the
hind-brain on the fifteenth day (Fig. 197, El). Almost directly
afterwards the mouths of the pits close, and the vesicles, thus
formed, separate from the skin. The original mouth of each pit
lengthens out into an elongated neck, the recessus labyrinthi
(Fig. 229, er), while the vesicle itself forms a flattened sac, ev,
somewhat oval in outline, and lying embedded in the connective
tissue at the side of the hind-brain.

At the commencement of the fifth week the auditory vesicle
becomes more irregular in shape. Its ventral and anterior end
(Fig. 227, El) grows forwards as a short blunt process, which
forms the rudiment of the cochlea ; while, near its dorsal end,
three flattened projections appear on its outer surface, which
are the first stages in the formation of the three semicircular
canals.

By the end of the fifth week (Fig. 230), the auditory vesicle
has increased considerably in size, and its main divisions are
well established. The body of the vesicle is divided by a fold
into two main portions : a dorsal division, or utriculus, UT ; and
a ventral division, or sacculus, S. From the utriculus the three
semicircular canals arise, the two vertical canals, EA, EP, being

42

THE lir.MAN EMBBYO,

already separated from the vesicle, except at their ends, while
the horizontal canal, EH, is still a wide, flattened, pouch-like
outgrowth from the vesicle. From the sacculus, the cochlea,
EL, arises as a short, blunt, anteriorly directed outgrowth.
The recessus labyrinthi, ER, is much larger than before, and is
divided at its lower part by a partition into two tubes, of whicli
one opens into the sacculus and the other into the utriculus ;
these tubes affording the sole communication between the two
chambers, sacculus and utriculus, of the auditory vesicle.

Fig. 229.

Fig. 230.

Fig. 229. — The left auditory vesicle of a Human Embryo four weeks old, seen
from the outer surface (cf. Fig. 227). From W. His, jun. x 3.").

Fig. 230. — The left auditory vesicle of a Human Embryo five weeks old, seen
from the outer surface. From W. His, jun. x 35.

EA, anterior vertical semicircular canal. ED, common stem of the two vertical
semicircular canals. EH, horizontal or external semicircular canal. EL, cochlea.
EP, posterior vertical semicircular canal. ER, recessus labyrinthi. EV, auditory
vesicle. S, sacculus. UT, utriculus.

By the eighth week, the shape and proportions of the internal
ear are as shown in Fig. 231. The sacculus, s, and utriculus,
UT, are now comparatively small parts of the internal ear.
The semicircular canals have greatly increased in length ; and
the cochlea, el, has grown enormously, and is rolled up spirally
on itself.

The auditory nerve, as already noticed (p. 536), very early
becomes continuous with the auditory epithelium : it soon

THE EAR. 543

divides into two main portions, the vestibular and cochlear
ganglia, and from these, by further division, the several nerve
endings of the adult ear are derived.

The epithelial cells of the auditory vesicle, which, it will be
remembered, are derived directly from the surface epidermis of
the head, become variously modified in different parts of the
vesicle. Over the greater part of its surface they remain flat
pavement cells, while opposite the nerve endings they become
altered into the hair-cells, rods of Oorti, sense cells of the am-
pullae, and other specialised structures.

The mesoblast of the side of the head, in which the auditorv

Fig. 231.— The left auditory vesicle, or internal ear, of a Human Embryo of
the eighth week, seen from the left side. From W. His, jun. x 17.

EA, anterior vertical semicircular canal. EA', ampulla of anterior vertical semi-
circular canal. ED, common stem of the two vertical semicircular canals. EH, horizontal

semicircular canal. EH', ampulla of horizontal semicircular canal. EL, cochlea. EP,
posterior vertical semicircular canal. EP', ampulla of posterior vertical semicircular
canal. ER, recessus labyrinthi. S, sacculus. UT, utriculus.

vesicle is embedded, undergoes important changes. The layer
in immediate contact with the epithelial vesicle becomes closely
connected with this, and forms the connective tissue wall of the
labyrinth ; at a little distance from the labyrinth the mesoblast
becomes converted into cartilage, which forms the periotic capsule.
Between the cartilaginous capsule and the labyrinth itself, the
intervening mesoblast breaks down to form the perilymphatic
spaces surrounding the vestibule and the semicircular canals, and
the two lymphatic canals — seal a tympani and scala vestibuli —
which lie above and below the scala media, or cochlear outgrowth
from the labyrinth.

5 II THE III MAN EMBRYO.

In the later stages the cartilaginous periotic capsule becomes
replaced by bone. This is in chief part spongy bone, but is
lined on the surface towards the labyrinth by layers of compact
bone, formed by the periosteal membrane. The modiolus and
septa of the cochlea, as well as the osseous spiral lamina, are
formed wholly in connective tissue, without any preformation in
cartilage.

The Accessory Auditory Organs.

The Eustachian tube and tympanic cavity are formed from the
hyomandibular pouch, or diverticulum from the pharynx. The
pouch does not quite reach the surface at any stage of develop-
ment : the membrane closing it at its outer end becomes the
tympanic membrane ; and the groove or depression on the
surface of the head, opposite the hyomandibular pouch, becomes,
as already noticed, the external auditory meatus ; the external
ear, or pinna, being formed from a series of processes deve-
loped round its margin (vide pp. 499, 500).

Up to the time of birth, the Eustachian tube, and the tym-
panic cavity itself in great part, are practically obliterated ; their
walls being brought into contact with each other by the deve-
lopment of very abundant, gelatinous, connective tissue in their
substance. This is absorbed about the time of birth, and the
tympanic cavity and Eustachian tube opened out.

DEVELOPMENT OF THE DIGESTIVE SYSTEM.

1. General Account.

The alimentary canal of the human embryo, like that of the
chick, is at first merely the part of the cavity of the yolk-sac
which is included within the embryo, as this is constricted off by
the head-, side-, and tail-folds (cf. Fig. 188).

As the constriction deepens, to form the yolk-stalk, the part
of the cavity within the embryo, or mesenteron, gradually be-
comes more and more sharply separated from the cavity of the
yolk-sac proper ; the two cavities still, however, communicating
freely through the yolk-stalk.

The mesenteron soon acquires a definite tubular form, and
on the fifteenth day has the shape and relations shown in Fig. 232.

THE ALIMENTARY CANAL.

545

It consists of three parts, fore-gut, mid-gut, and hind-gut, which

are approximately equal in length.

The fore-gut is widened transversely at its anterior end to

form the pharynx, tp, which is separated in front by a thin,
, obliquely placed septum, du, from the bottom of the stomatodasal,
I or mouth invagination, ds. Behind the pharynx, the fore-gut

narrows to form a short tubular portion, the oesophagus, which
i lies immediately above the heart. Behind the oesophagus is a
I fusiform dilatation, the stomach, ts, beyond which the fore-gut

TR W TS TP

BM

Fig. 232. — Human Embryo, lettered by Professor His, Lg, and estimated as
fifteen days old (cf. Fig. 197). The brain and heart are exposed from the
right side ; the alimentary canal and the yolk-stalk are represented in
median sagittal section. (From His.) x 30.

AA, allantoic artery. B3P, thalamencephalop. BL, cerebellum. BM, mid-brain.
BO, optic vesicle. DS, stoma todasuin. DTJ, septum between stomatodasum and pharynx.
EI, auditory pit. GH, hind-gut. GT, mid-gut and yolk-stalk. ET, truncus
arteriosus. B,V, ventricular portion of heart. TA, allantoic diverticulum. TP,
pharyngeal region of fore-gut. TR, cloacal dilatation of hind-gut. TS, stomach. TZ,
allantoic stalk. VA, allantoic vein. "W, liver.

passes into the mid-gut, GT, which latter opens through the wide
yolk-stalk into the yolk-sac. The hind-gut, GH, is at first narrow
and tubular ; but at its hinder end it dilates to form the large
cloacal chamber, TR, from the ventral surface of which the
allantois, ta, arises as a narrow tubular diverticulum. There is
as yet no trace of a proctodaeal, or anal invagination.

In embryos about a day older than the one represented
in Fig. 232, i.e. of about the sixteenth day, the stomatodaeal
septum is perforated, and the mouth opening established

N N

i4G

THE HUMAN EMBRYO.

(cf. Fig. 215). The proctodaeal opening is not formed until a
much later stage, about the end of the fifth week, and i
perforation of the integument rather than a distinct pit.

During the fourth week the alimentary canal rapidly assumes
more definite form. The pharynx (Figs. 216 and 233) remains of
great width from side to side, and in connection with it the gill-
pouches, lungs, and other important structures are formed. The
oesophagus rapidly increases in length as the neck elongates ; and

Fig. 233. — Outline figure of the alimentary canal of a Human Embryo,
lettered by Professor His, Pr, and estimated as twenty- eight days old (cf.
Fig. 216). The figure is drawn from the right side, and the cavity of the
alimentary canal is alone represented, not the thickness of its walls. The
curved line bounding the figure on the left is the notochord. (From His.)
x 15.

All. allantoic. B, cloaca. Ds, yolk-stalk. Ep, epiglottis. Kk, laryngeal chamber!.
Lbg, bile-duct. Lg, lung. Mg, stomach. N, meter. J0, pancreas. RT, pituitary body.

Uk, mandibular arch. W, Wolffian duct. Zg, tongue.

the stomach becomes a more conspicuous dilatation. The intestine
is long, narrow, and tubular ; it forms a prominent, ventrally
directed vitelline loop, from the apex of which the narrow yolk-
stalk arises, connecting the intestine with the yolk-sac.

The alimentary canal is at first (Fig. 232) closely attached

THE ALIMENTAKY CANAL.

547

to the dorsal wall of the body along its whole length, lying*
immediately ventral to the notochord ; and is hence equal in
length to the part of the body in which it lies. During the
fourth week, the intestine grows much more rapidly than the
body of the embryo, and becomes thrown into loops which pro-
ject ventralwards (Fig. 233). A small duodenal loop is formed
immediately beyond the stomach, and opposite the bile-duct,

Fig. 234. — Outline figure of the alimentary canal of a Human Embryo,
lettered by Professor His, Sch, and estimated as thirty- five days old.
The figure is drawn from the right side, and the cavity of the alimentary
canal is alone represented, not the thickness of its walls. The curved
line bounding the figure on the left is the notochord. (From His.) x 10.
An, position at which the anus will be formed. Cc, caecum. Ch, notochord. CI, rectum.

Ep, epiglottis. Hb, basal portion of alia ntois, which becomes the bladder. Kl; larynx.

Lb;/, bile-duct. Lg, lung. Mg, stomach. N, rudiment of permanent kidney ormetanephros.

1\ pancreas. RT, pituitary bod y. Sgf clitoro-penis. St, tail. Tr, trachea. TO, mandible,

or lower jaw. Zg, tongue.

Lbg ; and a much larger vitelline loop is formed lower down, from
the apex of which the yolk-stalk, Ds, arises. As the intestine
lengthens, its attachment to the dorsal wall of the body becomes
drawn out into a thin vertical sheet of mesoblast, the mesentery,
between the layers of which the blood-vessels of the alimentary
canal run.

148

THE HUMAN EMBRYO.

During the fifth week (Figs. 234, 235, and 23G), the oeso-
phagus lengthens very greatly; the stomach in consequent-
shifts backwards, and at the same time acquires its characteris-
tic shape (Fig. 236, Mg), and becomes placed across the body
instead of along it. The vitelline loop of the intestine (Fig.
234) passes out some distance beyond the body ; it lengthens

Fig. 235.

Fig. 236.

Fig. 235. — Outline figure of the alimentary canal of a Human Embryo,
estimated as thirty-two days old. The figure is drawn from the ventral
surface, and the cavity of the alimentary canal is alone represented, not
the thickness of its walls. (From His.) x 12.

Fig. 236. — Outline figure of the alimentary canal of a Human Embryo,
estimated as thirty-five days old. The figure is drawn from the ventral
surface, and the cavity of the alimentary canal is alone represented, not
the thickness of its walls. (From His.) x 10.

Cc, caecum. Cl, rectum. Dd, duodenum. Ds, yolk-stalk, gb, gall bladder. EbjK
bile-duct. Lg, lung. Mg, stomach. P, pancreas.

considerably, and becomes at the same time twisted on itself.
Before the end of the week, the tubular yolk-stalk (Fig. 235, Ds)
separates from the intestine, although detached portions of the
tube may persist along the yolk-stalk for some time longer.
The csecum, Cc, arises during the fifth week as a diverticulum
from the distal limb of the vitelline loop, not far from the point
of attachment of the yolk-stalk.

THE ALIMENTARY CANAL. 549

During the fifth week the cloaca, which up to this time has
been a single dilated chamber (Fig. 233), becomes divided, by
the growth backwards of a septum from the angle between the
allantoic stalk and the intestine, into two separate tubes ; of
these, the dorsal one (Fig. 234, CI) is continuous with the
intestine and forms the rectum ; while the ventral one, Hb,
receives the allantoic stalk, and the Wolffian ducts and ureters,
and forms the urino-genital passage.

The septum which thus divides the cloaca into rectal and
urino-genital chambers is formed by the union in the median
plane of two lateral folds or ridges, which arise from its sides ;
it reaches the surface of the body just below the root of the tail,
about the end of the fifth week (Fig. 234). The proctodseal
opening is formed about the same time, but it is not certain
whether this takes place before or after the completion of the
septum : in the former case there would be for a short time a
single cloacal aperture ; in the latter case the rectal and urino-
genital apertures would be distinct from the first.

The later stages in the development of the part of the
alimentary canal from the oesophagus to the rectum present
few features of special interest. The epithelium lining the
oesophagus is ciliated during the fifth and sixth months, and
perhaps for a longer period.

The mucous membrane of the stomach is smooth up to the
end of the second month ; during the third month it becomes
much folded, especially at the pyloric end, and in the course of
the fourth month the glands commence to develop. In the
intestine the villi appear towards the end of the second month,
and the glands of Lieberkulm about the beginning of the fourth
month. The large intestine is at first closely similar to the
small intestine, and contains numerous villi, which about the
fourth or fifth month become united by folds of the mucous mem-
brane to form a honeycomb pattern. Peyer's patches appear
about the sixth month.

2. The Pharynx.

The pharynx requires special notice on account of the im-
portance of the structures developed in connection with it.

From the first the pharynx is distinguished from the rest of
the length of the alimentary canal by its great width.

550

THE HUMAN EMJJKYO.

At its first formation (Fig. 237) the pharynx is of approxi-
mately uniform width along its whole length ; but at an early
stage the anterior part widens very greatly and the whole
pharynx becomes funnel-shaped, with the apex directed back-
wards (Figs. 238 and 239).

The condition of the pharynx on the fifteenth day is shown
in horizontal section in Fig. 237, which should be compared

with Figs. 197 and 232,
which represent the same
embryo in surface view,
and in sagittal section.
The visceral arches are
seen to form prominent
ridges projecting into the
pharynx, and separated
from one another by
grooves, the visceral
pouches. Of the visceral
arches, the mandibular,
MN, and hyoidean, hy, are
well developed; and be-
hind these the first and
second branchial arches,
BR1 and BR2
able, thoug
defined.

The hyomandibular and first branchial pouches are well
formed ; and corresponding to them on the outer surface of the
pharynx are well-marked external visceral grooves, clearly seen
in surface views of the embryo (Fig. 197, hm, hc1). The corre-
sponding visceral pouches and grooves, on the inner and outer
surfaces of the pharynx respectively, do not quite meet, but are
separated by thin membranous partitions, of which the most
anterior one, eb, between the mandibular and hyoidean arches,
becomes ultimately the tympanic membrane.

Further back, there are less strongly marked second bran-
chial, and third branchial pouches or grooves on the inner surface
of the pharynx, with slight indications of corresponding visceral
grooves on the outer surface.

Towards the end of the third week, and in the early part of

Fig. 237.— The floor, of the pharynx of a
Human Embryo fifteen days old, seen
fy from above. (Cf. Figs. 197 and 232.)
~ | (From His.) x 50.

Al, first aortic arch, in the mandibular
arch. A2, second aortic arch, in the hyoid
arch. BR1, first branchial arch. BR2,
second branchial arch. C, body cavity, or
coelom. EB, membrane closing the hyoman-
dibular cleft, which becomes afterwards the
tympanic membrane. FL, furcnla. HY,
hyoid arch. MN, mandibular arch. TU,
tuberculum impar.

, are recognis-
;h less clearly

THE PHARYNX.

551

the fourth week, the hinder visceral arches, and the pouches
separating them from one another, become much more clearly
defined : the pharynx also changes its shape, becoming much
wider in front, and narrowing posteriorly towards the oesophagus
(Fig. 238).

The mandibular, hyoid, and first and second branchial arches
are well defined (Fig. 238, MX, hy, BR1, and BR2), the hyoid
arch being especially large. Both the internal, visceral pouches,
and the external visceral grooves between the successive arches
are well marked. There is some doubt as to whether any of the
gill-clefts are actually open in the human embryo ; such evidence

Fig. 238.— The floor of the pharynx of a Human Embryo, twenty-three days
old, seen from above. Cf. Fig. ?43, which represents the same embryo.
(From His.) x 30.

A2, second aortic arch, in the hyoidcan arch. A3, third aortic arch, in the first
branchial arch. A4, fourth aortic arch, in the second branchial arch. A5, fifth aortic
arch in the third branchial arch. BB1, first branchial arch. BB2, second branchial
arch'. BBS, third branchial arch. EB, membrane closing the hyomandibular cleft,
which afterwards becomes the tympanic membrane. FL, furcula. HY, hyoid arch.
LG-, lung. MM", mandibular arch. TIT, tuberenhim impar.

as has been obtained points to the conclusion that none of the
clefts are really completed either at this or any other stage in
development; the visceral pouches and the corresponding vis-
ceral grooves being always separated by thin partitions, as at
eb in Fig. 238.

The second branchial arch, BR2, is bounded posteriorly by
the conspicuous and deep third branchial pouch ; immediately
behind this is a ridge, BR3, projecting into the cavity of the
pharynx, and bounding laterally the entrance to the oesophagus.
Although there is no external ridge on the surface of the embryo
corresponding to this internal ridge, yet its relations to other

552

TIN-: HUMAN EMBRYO.

organs, and more especially the fact that in it, as in the anterior
arches, an aortic arch, or branch of the truncus arteriosus, A5, is
present, show that the ridge in question, BR3, is really a third
branchial arch.

In Fig. 238 it is seen that the second branchial arches,
BR2, not only lie nearer the middle line than the first branchial
arches, BR1, but are also in part overlapped by these. During
the latter part of the fourth week, this overlapping becomes
much more marked, the posterior visceral arches shifting for-
wards, and being telescoped within the arches in front of them.

In Fig. 239 the condition at the end of the fourth week
is shown, at which time the first branchial arches have com-

FlG. 239. — The floor of the pharynx of a Human Embryo twenty-eight clays
old, seen from above. Cf. Fig. 216, which represents the same embryo
(From His.) x 30.

A.3, third aortic arch, in the first branchial arch. A.4, fourth aortic arch, in the second
branchial arch. A.5, fifth aortic arch, in thethird branchial arch. BR.1, first branchial
arch. BB.2, second branchial arch. EB, membrane closing the hyomandibular cleft
which afterwards becomes the tympanic membrane. FK, foramen caecum. HY,'
byoid arch. MN, mandibular arch. STJ, sinus prascervicalis. TH, median thyroid'
rudiment. TTJ, tuberculum irnpar. V.3- mandibular branch of trigeminal nerve.
VII, hyoidean branch of facial nerve. IX, glosso-pharyngeal nerve. X, branchial
branches of pneumogastric nerve.

pletely overlapped the second branchial arches, br2, so as to
conceal them in surface views of the embryo.

During the fifth week the first branchial arches are in their
turn overlapped and concealed by the hyoid arches (Fig. 210),
so that in surface views of embryos of this age none of the arches
behind the hyoid can be seen (cf. Fig. 205).

THE PHARYNX.

553

By this telescoping of the visceral arches a deep cleft is
formed at each side of the neck, extending round to its ventral
surface, and dividing the pharyngeal region from the trunk.
This cleft, which presents a certain resemblance to the opercular
cavity of a tadpole, is the sinus prascervicalis (Fig. 240, su) ; it
ultimately becomes obliterated by fusion of its anterior and pos-
terior walls.

3. The Tipper Lip and the Palate.

The fronto-nasal process consists, as already described, of a
median area (Fig. 240, fp), and two lateral lobes, the processus

BR.2.

Fig. 240. — The head and neck of a Human Embryo thirty-two days old, seen
from the ventral surface. The floor of the mouth and pharynx has been
removed. Of. Fig. 205, which is an outline figure of the same embryo.
(From His.) x 12.

BK.1, first branchial arch. BR.2, second branchial arch. EB, membrane closing
the hyornandibular cleft, which afterwards becomes the tympanic membrane. PC pro-
cessus globularis. FP, median part of fronto-nasal process. HM, hyornandibular
pouch. HY, hyoid arch. LG. lung. LE, larynx. MN, mandibular arc]]. MX.
maxillary arch." OD, eye. OK, mouth of olfactory pit, or external nostril. PT,
pituitary body. SU, sinus praecervicalis.

globulares, FC. The processus globulares form the inner lips
of the nasal grooves, which connect the olfactory pits with the
mouth, and of which the outer lips are formed by the inner
edges of the maxillary arches, MX. By fusion of their inner and
outer lips, the nasal grooves become converted into the posterior

554

THE HUMAN KM1JRY0.

nasal passages, a pair of short tubes leading from the'olfactory pits
to the fore part of the roof of the mouth, into which they open
in much the same position as the posterior nares in an adult frog.

At a later stage, after the outgrowth of the median part
or bridge of the nose, the two processus globulares meet each
other in the median plane, and fuse to form the median part of
the upper lip (c/. Figs. 207 and 241).

There are, thus, in the upper lip three sutural lines : a median
one, where the inner borders of the two processus globulares
meet and fuse with each other ; and a pair of lateral ones, where

.MX

MX

FlG. 241.— The roof of the mouth of a Human Embryo about two and a half
months old, showing the mode of formation of the palate. (From His.)
xlO.

PO, processus globularis. PO', palatal process of processus globularis. MX,
maxillary arch MX', palatal process of maxillary arch. OB, mouth cavity. OD,
eye. OBZ, aperture of olfactory pit, or nostril. OL, lens.

the outer borders of the processus globulares meet and fuse with
the inner ends of the maxillary arches.

The median cleft is the one which persists throughout life
in the hare or rabbit, but it is doubtful whether it ever remains
open in man ; what is called hare- lip in man being due to imper-
fect closure of one or other of the lateral clefts.

The palate is formed, as regards its most anterior portion,
by a pair of horizontal shelf-like outgrowths from the pro-
cessus globulares (Fig. 241, fo'), which meet and fuse in the
median plane. The rest of the palate, comprising the greater
part of its length, is formed by two similar outgrowths, MX', from

THE PALATE AND TONGUE. 555

the inner surfaces of the maxillary arches. The palatal pro-
cesses grow rapid]ty, and by the beginning of the third month
the anterior ends of the maxillary processes, MX ', have met and
fused with each other in the median plane, immediately behind
the premaxillary processes, or outgrowths from the processus
globulares, fo'. A small aperture is left in the median plane
between the four palatal processes, and persists as the foramen
incisivum. The completion of the palate is effected by the
extension backwards of the fusion of the inner edges of the
maxillary processes, towards their hinder ends. Occasionally the
union fails to take place properly, and the malformation known as
cleft palate results.

By the formation of the palate, the anterior part of the mouth
cavity becomes divided into dorsal or nasal, and ventral or buccal
portions, and the communication between the posterior nostrils
and the buccal cavity is shifted backwards to the level of the
hinder edge of the palate.

The septum nariuin is formed in the first instance by
upgrowths from the inner edges of the palatal processes, which
fuse together in the median plane, and grow dorsalwards as a
partition, dividing the nasal chamber into right and left halves.

4. The Tongue.

The tongue arises from the floor of the fore-gut, so that its
epithelial covering is entirely of hypoblastic origin. It is formed
from two rudiments, which are at first completely separate from
each other ; an anterior median swelling, the tuberculum impar,
from which the body and tip of the tongue are developed ; and
a posterior V-shaped ridge, which gives rise to the root of the
tongue.

On the fifteenth day (Fig. 237) the ventral ends of the
mandibular arches, mjv, almost meet each other in the median
plane ; the ventral ends of the hyoid arches, hy, are some
little distance from each other ; and the ventral ends of the
first and second branchial arches, BR.i, BK.2, are still further
apart. There is thus left in the floor of the pharynx, between
the ventral ends of the visceral arches, a triangular, meso-
branchial area, the apex of which is directed forwards. From
the dorsal surface of this area the tongue is developed ; while
the heart (Fig. 232) lies immediately beneath it.

r>r>{\ THE HUMAN EMBBYO.

At the anterior end of the mesobranchial area, between the
ventral ends of the mandibular and hyoid arches, is a small
rounded elevation, the tuberculum impar (Fig. 237, tu). Behind
this, and between the ventral ends of the first and second
branchial arches, there is a much larger elevation, with prominent
rounded margins and a median longitudinal furrow. This is
the furcula (Fig. 237, fl) ; and from it the epiglottis will be
developed at a later stage, while the median groove will become
the glottis.

The furcula lies at first immediately behind the tuberculum
impar ; but in the early part of the fourth week (Fig. 238) the
two become separated by a transverse ridge, formed from the
ventral ends of the hyoid and first branchial arches, which unite
together and extend across the floor of the mouth. This ridge

/-Tu

Fig. 242. — The tongue and floor of the mouth of a Human Embryo at the end
of the second month. (From His.)

BE.l, first branchial arch. FK, foramen cascum. HY, hyoid arch. LT, glottis.
MN, mandibular arch. TU, body of the tongue, formed from the tuberculum impar.

soon grows forwards at the sides of the tuberculum impar,
embracing it like a V. At the angle of the V, between the
ridge and the tuberculum, a small backwardly directed pit is
formed, the mouth of which becomes the foramen caecum (Fig.
239, fk), while the pit itself becomes the median portion of the
thyroid body, TH.

The median part of the transverse ridge soon becomes
marked off by lateral grooves, and fusing with the tuberculum
impar gives rise to the root of the tongue (Fig. 242). The
V-shaped groove, marking the boundary between the two
originally separate elements of which the tongue consists, is
very conspicuous throughout development, and is often well

THE TONGUE AND THE THYROID BODY. 557

marked in the adult : it is always indicated, in the median plane,
by the foramen caecum (Fig. 242, fk). The line of circum-
vallate papillae, which appears during the third month, lies
immediately in front of this groove, and therefore in the part
of the tongue formed from the tuberculum impar : immediately
in front of the foramen caecum, and sometimes surrounding it, is
a single, very deeply depressed circumvallate papilla.

The double origin of the tongue is indicated by its nerve
supply; the body and tip of the tongue, developed from the
tuberculum impar, are supplied by the gustatory branch of the
trigeminal nerve ; while the root and sides of the tongue,
developed from the transverse ridge, are supplied by the glosso-
pharyngeal. It must be noted, however, that in order to reach
the circumvallate papillae the branches of the glosso-pharyngeal
nerve have to overstep the boundary between the two parts of
the tongue, and invade the part formed from the tuberculum
impar.

5. The Thyroid Body.

The thyroid body is formed from three independently arising
rudiments, which remain distinct until a rather late stage in deve-
lopment : (i) a middle thyroid rudiment (Fig. 239, th), which is a
deep pit commencing at the foramen caecum, at the junction of
the body and root of the tongue, and extending downwards and
backwards in the floor of the mouth ; and (ii) a pair of lateral
thyroid rudiments, which are outgrowths of epithelium from the
floor of the mouth at the sides of the larynx, in close relation
with the third branchial pouches.

The middle thyroid rudiment, which appears about the middle
of the fourth week, consists at first of a short tubular duct,
which divides at its blind end into right and left lobes (Fig. 239,
th). During the fifth week the median duct, or thyro-glossal
duct, elongates rapidly, growing downwards and backwards until
its bifurcated distal end lies opposite the larynx, or upper end
of the trachea. During this rapid growth the duct usually loses
its lumen, and becomes a solid rod of epithelial cells extending,
in the median plane, from the foramen cscuin to the trachea.

Towards the end of the fifth week, this epithelial cord
usually becomes broken up in the middle part of its course into
a number of detached fragments ; and a little later it becomes

558 THE HUMAN EMBRYO.

still further interrupted by the formation of the cartilaginous
body of the hyoid, which lies exactly in its path.

The paired lateral thyroid rudiments early separate from
the epithelium, and form a pair of lobed masses lying at the sides
of the larynx, and of considerably larger size than the bifurcated
median rudiment. At a later stage they shift still further back,
so as to lie alongside the trachea, and then fuse with the median
rudiment to form the definite thyroid body. The median
rudiment gives rise to the isthmus of the adult thyroid, and
probably to parts of the lateral lobes as well ; the greater part
of the lateral lobes, however, are formed from the much larger
lateral rudiments.

At an early stage the lobes are excavated by a number of
detached cavities, which become the vesicles of the adult thyroid
From the history of their development it follows that the epi-
thelial walls of these vesicles are of hypoblastic origin.

The duct or stalk of the middle thyroid rudiment usually dis-
appears in great part ; detached portions of it not uncommonly
persist as accessory suprahyoid or epihyoid bodies, or as cysts.

Occasionally the upper part of the stalk persists as a tube,
the lingual duct, extending from the foramen caecum, on the
dorsum of the tongue, backwards aud downwards towards the
body of the hyoid, or actually reaching this in some cases.

The lower or posterior part of the stalk may also occasionally
persist, forming the so-called pyramid of the thyroid, a some-
what pyriform body, enlarged and saccular at its lower or
posterior end, and tapering upwards to a fibrous cord which is
attached to the dorsal surface of the hyoid bone. The pyramid is
apparently formed by persistence and enlargement of one of the
two branches into which the stalk bifurcates at its lower end.
When present, it is usually single, but cases have occurred in
which two pyramids were found, due apparently to persistence
of both branches of the bifurcation.

6. The Thymus.

The thymus is a paired organ of epithelial origin, developed
in connection with the second and third branchial clefts, and
perhaps the first branchial cleft as well.

It appears about the middle of the fifth week ; but as to the
precise mode of its formation there is still some doubt. Born

THE THYROID, THYMUS, AND THE TEETH. 559

maintains that the thymus of man, like that of other Vertebrates,
is developed from the hypoblastic lining of the pharynx ; His'
observations, on the other hand, support an epiblastic origin ;
the thymus, according to him, being formed from the epiblastic
walls of the sinus praBcervicalis, the deep fissure at the side of
the neck caused by the overlapping of the hinder visceral arches
by the more anterior ones (cf. Fig. 240, su).

The thymus gradually shifts backwards towards the root of
the neck, extending along the pneumogastric nerve and carotid
artery almost as far as the heart. It attains a great size in later
foetal life, and continues to increase after birth up to about the
end of the second year, when it measures two inches or more in
leugth.

7. The Salivary Glands.

The salivary glands commence to form early in the second
month, and by the end of the month have attained a considerable
size. The ducts arise as grooves of the buccal epithelium,
which by fusion of their lips become tubes ; the glands them-
selves are, at first, solid outgrowths of epithelial cells, which
later become hollowed out by extension of the cavities of the
ducts into their substance. The submaxillary glands appear
first, then the parotid, and lastly the sublingual glands.

8. The Teeth.

The teeth are developed in man in very much the same way
as in the rabbit. In embryos about seven weeks old the epi-
thelium becomes thickened along the border of each jaw, and the
deeper or Malpighian layer of the epithelium grows down into the
substance of the jaw as a continuous keel-like ridge, the common
enamel germ. This soon becomes enlarged at intervals to form
the enamel organs of the milk or deciduous teeth, while between
the enamel organs the ridge becomes less conspicuous, and
ultimately disappears.

Each enamel organ is flask-shaped, consisting of a terminal
enlarged portion, buried deeply in the jaw, and a narrow neck
or stalk which connects the enlarged part with the surface
epithelium of the jaw. Opposite each enamel organ the con-
nective tissue of the jaw becomes more compactly arranged to
form the dental papilla (cf. Fig. 156, tm). The dental papilla
soon becomes moulded into the shape of the future tooth, and

5G0 THE HUMAN EMBRYO.

the enanii-1 organ becomes closely fitted, like a cap, over the
surface of the papilla, which acquires the form of the crown ol
the future tooth.

From the dental papilla the main substance of the tooth, or
dentine, is formed in the following manner. On the surface of
the papilla next the enamel organ a layer of special cells, the
odontoblasts, appear. These form, by excretion on their outer
surfaces, a dense matrix in which fine filamentous processes of
the odontoblasts are embedded ; by calcification of the matrix
the dentinal substance is formed, the dentinal tubules being the
narrow channels in the matrix occupied by the processes of the
odontoblasts. The first formed part of the dentine is the outer-
most layer of the crown of the tooth, and this layer thickens by
further formation of dentine on its inner surface, the odonto-
blasts gradually withdrawing further and further from the
surface, as the dentine increases in thickness.

The enamel is formed from the layer of epithelial cells of the
enamel organ which lies in immediate contact with the dental
papilla. This layer consists of closely set, columnar or prismatic
cells, and it is by direct calcification of these cells that the
enamel is produced. The rest of the enamel organ is merely
nutritive in function, and does not give rise directly to any
part of the tooth.

The crown is thus the first part of the tooth to be formed.
After it is completed, the tooth increases in length by the
further formation of dentine round the lower part of the papilla.
The aperture at the base of the tooth is at first a widely open
one ; but, as the tooth approaches its full size, the aperture
becomes gradually narrowed to form the root or fang of the
tooth. In the case of the grinding teeth, the aperture becomes
■divided by bridges of dentine into two or three separate openings,
and by elongation of the margins of these openings the double or
triple fangs of the adult tooth are produced. At the apex of each
fang a minute hole remains, through which the blood-vessels and
nerves gain admittance to the pulp of the tooth, which latter is the
part of the dental papilla that remains encapsuled in the middle
of the tooth after its completion.

The cement, or outermost layer of the fully formed tooth, is a
bony deposit developed from the connective-tissue sheath which
surrounds it.

THE TEETH.

55

On the first appearance of the bony jaws, the teeth lie in
continuous grooves extending all round their free borders.
Partitions are soon formed, dividing these grooves into separate
compartments or alveoli, which grow round the teeth so as to
closely embrace them.

The order of appearance of the milk teeth takes place in
regular sequence ; but the actual dates at which the several
teeth emerge, or are ' cut,' vary within certain limits. The
cutting of the milk teeth usually commences about seven months
after birth, and is completed by the end of the second year.
The central lower incisors appear first, about the seventh month ;
the upper incisors two or three months later ; a few months later
still the lower lateral incisors, and the first premolars ; four or
five months later the canines ; and about the end of the second
year, the second premolars.

The permanent teeth are developed in the same manner as
the milk teeth. From the stalk or neck of the enamel organ of
each milk tooth, a small outgrowth arises at a very early stage,
about the sixteenth week, which becomes the enamel organ of
the corresponding permanent tooth. A dental papilla is formed
opposite each enamel organ, and the permanent teeth are formed
in the jaw, a little way behind and below the corresponding milk
teeth, and in precisely similar fashion.

The three hinder grinding, or molar teeth, which have no
milk predecessors, are formed by extension backwards of the
original common enamel germ, from which the milk teeth are
developed. The enamel organ for the first permanent molar
appears about the fifteenth week of embryonic life ; that for
the second permanent molar about seven months after birth ;
and that for the third permanent molar, or ' wisdom tooth,' not
until the third year.

The eruption, or cutting, of the permanent teeth of the lower
jaw takes place at the following dates, the teeth of the upper
jaw usually appearing a little later : —

Molars, first
Incisors, central .

„ lateral .
Bicuspids, anterior

„ posterior .

Canines
Molars, second

„ third (or wisdom)

6 years.

7

. 9
. 10

11 to 12

12 „ 13
17 „ 25

0 0

£62 THE HUMAN EMBRYO.

9. The Lungs.

On the fifteenth day (Fig. 237), a swelling is present on the
floor of the pharynx, opposite the first, second, and third branchial
arches ; and along the middle of this swelling, or furcula, fl,
there runs a longitudinal groove.

By the sixteenth day this groove is much more pronounced,
and its posterior end leads into a short blind pocket.

By the end of the third week the pocket has become much
■deeper, extending backwards, ventral to the oesophagus and
independently of this ; and its hinder end is split into right and
left lobes (cf. Fig. 238, lg). These lobes are the rudiments of
the lungs ; the tube leading to them is the trachea ; and the
slit-like opening, or groove, on the floor of the pharynx is the
future glottis.

During the fourth week the lungs grow rapidly, extending
back alongside the oesophagus, and dorsal to the heart (Figs.
216 and 233, Lg) • their distal ends are enlarged and are
commencing to divide into lobes. The right lung has three
terminal buds or lobes, and the left lung two ; these buds form-
ing the rudiments of the five lobes of the adult lungs.

During the fifth week the lungs continue to increase rapidly ;
the main lobes elongate greatly, and give rise to secondary and
tertiary buds, which end in slightly expanded ampullae (Figs.
235 and 236, Lg).

The further development of the lungs consists in a continua-
tion of the process of budding, by which new tubules and
ampullae arise from the older ones, either by dichotomous
division, or, as in the later stages, by lateral branching. The
air cells appear, as closely set pouchings of the walls of the
ampullae, which are not recognisable until the time of birth.

The trachea is at first short, but rapidly elongates during
the fifth and following weeks. The larynx first becomes evident,
as a dilatation of the anterior part of the trachea, towards the
end of the fifth week (Fig. 234). The vocal cords, and the
ventricles of the larynx, are not formed until about the fourth
month.

The anterior, median part of the furcula becomes the epi-
glottis (Fig, 233, Ep) ; while the lateral ridges give rise to the
ary-epiglottic folds and the arytenoid cartilages. The thyroid
•cartilage is said by Callender, and by His, to be formed from the

THE LUNGS AND THE LIVER. 563

cartilages of the second branchial arches, though Kolliker believes
it to arise independently.

The lungs, as they grow backwards, project into the dorsal
part of the body cavity, pushing before them the peritoneal
lining of the cavity, which forms their pleural covering. At a
later stage, the portions of the body cavity into which the lungs
hang are shut off, by the diaphragm and pericardium, from the
rest of the cavity, and become the definite pleural sacs.

10. The Liver.

The liver is present on the fifteenth day (Fig. 232, w) as
a short, hollow diverticulum, with a compact mass of cells at
its blind end, arising from the ventral wall of the fore-gut and
the anterior wall of the yolk-stalk, immediately behind the heart.

By the end of the third week (Fig. 215, w) the liver is of
larger size, and the bile-duct, or wide tubular passage connecting
the liver with the gut, is longer than before, but otherwise the
relations are much the same as in the earlier stage.

During the fourth week the liver enlarges very rapidly
(Figs. 216, 243, w). It consists of a close network of anastomos-
ing epithelial cylinders, the development of which has not been
followed accurately : the meshes of the network are chiefly occu-
pied by blood-vessels, which are present in large numbers, and of
great size. The development and relations of these blood-vessels
of the liver will be described in the next section of this chapter.

The rapid growth of the liver continues during the succeeding
weeks. In the second half of gestation it is rather less marked
in proportion to the other viscera, but even at the end of preg-
nancy the weight of the liver is to that of the whole embryo as 1
to 18, while in the adult it is only 1 to 36. Afterbirth the liver
diminishes rapidly, both in size and weight, owing to the cutting
off of the blood supply previously brought to it by the allantoic
veins.

The bile-duct rapidly lengthens during the fourth week ;
and the gall-bladder appears, as a diverticulum of the bile-duct,
before the end of the fifth week (Fig. 236, g.b).

The large size of the liver during almost the whole period of
gestation, and its abundant vascular supply, indicate that it must
be of great physiological importance. It probably serves to
modify in some way the nutrient material brought from the

o o 2

564 THE HUMAN EMBRYO.

placenta by the allantoic veins ; and it almost certainly ad
an important excretory organ during embryonic and foetal life.

The brown, or greenish-brown, mass known as meconium,
which occurs in the small intestine from the third to the fifth
month, and lower down, in the large intestine and rectum, during
the later months of pregnancy, contains bile in considerable
quantity, as well as mucus, and epithelial and other debris.

1 1 . The Pancreas.

The pancreas arises, towards the close of the fourth week, as a
dorsally directed diverticulum from the duodenum, almost oppo-
site the opening of the bile-duct (Figs. 233 and 234, P), and lying
in the thickness of the mesentery which attaches the duodenum
to the dorsal body-wall. The pancreas grows rapidly, giving oft
lobed offshoots, from which the acini and their ducts are formed.
The original diverticulum from the duodenum persists as the
pancreatic duct ; it at first opens a little distance from the
bile-duct, but ultimately the two ducts lie close alongside each
other, and open into the duodenum by a single orifice.

12. The Mesentery.

The mesentery is the thin vertical sheet of mesoblast which
slings the stomach and intestine to the body- wall. The relations of
the mesentery are at first extremely simple, but as the intestine
lengthens, and especially as it becomes thrown into convolutions,
they become greatly complicated. The attachment of the dorsal
border of the mesentery to the body-wall remains comparatively
unmodified throughout life, though at certain places oblique or
transverse lines of attachment are acquired in addition to, or in
place of, the original simple longitudinal attachment.

The part of the mesentery which attaches the stomach to the
body-wall, commonly spoken of as the mesogaster, undergoes
special modification. The stomach originally lies lengthways
along the body (Fig. 233, Mg, and 243), the mesogaster being at-
tached along the border which will afterwards become the greater
curvature of the stomach, and the pancreas (Fig. 233, P) lying
in the thickness of the mesogaster near its posterior limit.
When the stomach shifts its position, and becomes placed trans-
versely across the body (Fig. 236), its original left side becomes
ventral, and its right side dorsal ; while the mesogaster remains
attached along what is now the posterior border of the stomach.

THE PANCREAS AND THE MESENTERY. 565

This part of the mesogaster, along the posterior border, or greater
curvature of the stomach, becomes produced into a double fold or
sac, the great omentum, which hangs down, like a curtain, over
the coiled mass of the intestine, close to the ventral wall of the
abdomen.

Shortly after birth, the two layers of the omental sac coalesce,
so that the omentum becomes a single membranous layer, in
which fat early tends to accumulate.

The dorsal part of the mesogaster, which is attached to the
•dorsal body-wall, and in the thickness of which the pancreas is
contained, comes into close contact with the layer of mesentery
suspending the transverse colon, and ultimately fuses completely
with this ; a change which causes the pancreas to appear to lie
altogether dorsal to the mesentery, instead of in its substance.

THE DEVELOPMENT OF THE CIRCULATORY
SYSTEM.

The general history of the development of the blood-vessels
in man, their relations at the different periods of embryonic and
of foetal life, and the changes by which at the time of birth the
adult circulation is established, are closely similar to those
already described in the rabbit. Certain differences have been
noticed in the mode of formation of the valves of the heart, and
in the development of the great veins, more especially of those
in relation with the liver ; these are, however, of comparatively
small importance, and are possibly, in some cases, due to the
difficulty of obtaining human embryos in satisfactory histo-
logical condition, and of the particular age desired.

In the following account, which is based mainly on the
descriptions of Professor His, the development of the heart will
be dealt with first, then that of the arteries and the veins, and
finally a brief description will be given of the course of the cir-
culation in the embryo and foetus, and of the changes which
occur at birth.

1 . Development of the Heart.

General Account. The early stages in the development of the
heart in the human embryo are known very imperfectly, and
only as regards the external form of the organ.

56G THE HUMAN EMBliYO.

In the youngest human embryos, as in the corresponding
stages of the rabbit, the heart consists of two symmetrical and
perfectly distinct halves. On the thirteenth day (Fig. 179, r)
the heart is present as a pair of straight tubes lying along the
sides of the anterior end of the embryo, between the neural
folds and the yolk-sac, and in connection at their hinder ends
with the vessels which return the blood from the yolk-sac.

At a slightly later stage (Fig. 185, r), the two halves of the
heart have united to form a single tube, which is already twisted
on itself.

By the fifteenth day (Figs. 197 and 232), the heart has
advanced considerably in development, and forms a prominent
swelling on the under surface of the embryo, between the head
and the yolk-sac. It is a single tube, of considerable size ;
attached, both in front and behind, to the floor of the fore-gut,
but free along the middle portion of its length, which is twisted
into a prominent S-shaped loop. The dorsal and posterior end
of the loop is the auricular portion of the heart ; and is separated
by a slight constriction, the canalis auricularis, from the succeed-
ing or ventricular portion (Fig. 232, Rv). This forms the widest
and most prominent part of the loop ; it runs almost transversely
across the body from the left to the right side, and then turns
forwards rather sharply, and passes into the truncus arteriosus,
RT, or terminal limb of the loop. The anterior end of the
truncus arteriosus (Fig. 232) is attached to the floor of the
fore-gut, very far forwards, opposite the mandibular arches.

The wall of the heart (Fig. 232) is double along its entire
length, consisting of an outer mesoblastic tube, in which muscle-
cells are already present on the fifteenth day, and an inner
endothelial tube, the origin of which has not been determined.
The endothelial tube is very much smaller than the muscular
tube, and the space between the two is occupied by a gelatinous
substance traversed by fine radial fibres, apparently of the
nature of connective tissue (cf. Fig. 215).

During the third week the heart continues to increase rapidly
in size, and its several divisions become more clearly marked off
from one another by constrictions. By the end of the third
week it has reached the condition shown in Figs. 198 and 215.
The auricular portion (Fig. 198, Ra) is much larger than before ;
it is very wide from side to side, and is produced into conspicuous

THE HEART.

567

ear-like appendages. A marked constriction, the canalis auri-

cularis, separates it from the ventricular portion. This latter

rv, is shaped something like the adult stomach, and lies almost

AP A.S RA

Fig. 243. — Human Embryo, lettered by Professor His, Bl, 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. (From His.) x 20.

A, dorsal aorta, A.2, second aortic arch, in the hyoidean arch. A. 3, third aortic, or
carotid arch, in the first branchial arch. A. 4, fourth aortic, or systemic arch, in the
second branchial arch. A. 5, fifth aortic, or pulmonary arch, in the third branchial arch.
AP, pulmonary artery. BL, cerebellum. BM, mid-brain. BS, cerebral hemisphei e.
CH, notochord. EI, ear. IJsT, infundibulum. KC, Wolffian duct, KD, ureter,
LG-, lung. OC, optic cup. PT. pituitary body. BA. right auricle. TI, Intestine
TN, tongue. TB,, rectum. TS, stomach! VD, Cuvierian vein. W, vitelline vein
YK, yolk-stalk.

directly across the body ; its right-hand or distal end bends
sharply forwards, and passes into the truncus arteriosus, RT,
which is attached to the floor of the fore-gut rather further back

568 THE HUMAN EMBRYO.

than before, opposite the hyoidean and first brand lial arches
(Fig. 215). The structure of the heart is the same as in the earlier
stages, except that the muscular elements have increased consider-
ably. The wide space between the muscular and the endothelial
walls is well shown in Fig. 215, as is also the fibrous network
connecting the two walls.

In describing the further development of the heart it will be
convenient to take the several divisions one by one.

The sinus venosus. The blood is returned to the heart by
three main veins on each side : — the Cuvierian vein (Fig. 243,
VD), from the body of the embryo ; the vitelline vein, from the
yolk-sac ; and the allantoic vein, from the placenta.* These three
pairs of veins form by their union a single large vessel, the
sinus venosus, which runs transversely across the body, imme-
diately in front of the liver, and opens through a median ape *f
ture into the auricular portion of the heart.

The sinus venosus is at first situated behind the diaphragm,
but during the fourth week it gradually extends over, and in
front of this, and so comes to lie in the pericardial cavity, imme-
diately behind the auricle (Fig. 243).

Towards the end of the fourth week the sinus venosus
becomes placed somewhat obliquely, in place of transversely,
across the body ; at the same time its right side becomes larger
than the left, and the opening into the auricular cavity, which was
at first median, shifts so as to lead distinctly into the right side
of the auricle (Fig. 244, rs). During the fifth week, the open-
ing from the sinus venosus into the auricle widens out very con-
siderably, so that the sinus becomes part of the auricle itself,
and ceases to exist as a separate cavity. The left horn of the
sinus venosus, which now only receives the left Cuvierian vein,
retains its independence more completely, and persists as the
coronary sinus.

The auricles. The auricular chamber is at first single, but
towards the end of the fourth week it becomes imperfectly divided
into the right and left auricles (Fig. 244). The division is
indicated externally by a slight constriction, and more markedly
by the outgrowth of the auricular appendices, which very early
show characteristic crenations along their margins.

THE HEART.

569

Seen from within, the auricular portion of the heart has, at
the end of the fourth week, the appearance shown in Fig. 244.
Opposite the external constriction, a fold, sk, the septum superius,
projects into the cavity from its anterior end and ventral wall, and
reduces the communication between the two auricles to a rather

Fig. 244. — The dorsal half of the heart of a Human Embryo, twenty-eight
days old, seen from within. The heart has been bisected lengthways, and
the ventral half removed. (From His.) x 32.

EA, right auricle. EB, left auricle. EP, auriculo-ventricular aperture. EK,
ctinalis auriculari s. ES, opening of sinus venosus into right auricle. EV, right
ventricle. EY, left ventricle. SB, septum spurium. SD, septum inferius. SK,
septum superius. SZ, spina vestibuli. VD, right vena cava anterior. VU, Eustachian

valve.

small circular aperture, nearer the dorsal than the ventral
surface.

The conspicuous projection into the dorsal part of the right
auricle, shown in the figure, is caused by the sinus venosus.
The aperture from the sinus venosus into the auricle is an
obliquely placed slit, RS, of which the outer lip is thickened, and
forms the Eustachian valve, VU ; while the opposite, or inner lip,
is a thin fold, which at the lower end of the slit passes into a
triangular thickening of connective tissue, the spina vestibuli,

570 THE HUMAN EMBRYO.

sz, projecting into, and partially blocking up, the aperture be-
tween the right and left auricles. This spina vestibuli, accord-
ing to Professor His, plays an important part in the formation
of both the interauricular and interventricular septa.

An additional fold, the septum spurium, SB, projects into the
cavity of the right auricle, opposite the upper end of the slit-like
opening of the sinus venosus ; it is a transient structure, and
ultimately disappears completely.

Of the two auricles, the right one, RA, is at first (Fig. 244)
much the larger. The walls of the auricles, like the rest of the
heart, consist of two layers, muscular and endothelial ; these
are at first some distance apart, but about the twenty-third day
they come in contact, and unite firmly to form the definite
auricular wall. The connective-tissue elements of the wall are
derived apparently from the gelatinous matter which originally
separates the muscular and endothelial walls from each other.

The interauricular septum is formed, according to His, by
coalescence of the septum superius (Fig. 244, sk) with the spina
vestibuli, sz ; the latter growing downwards towards the ven-
tricle as a thickened plug, which before the end of the fifth
week divides the originally single auriculo-ventricular ajoerture
into separate right and left openings.

It is not quite clear whether the foramen ovale in the human
embryo is merely due to the interauricular septum remaining
incomplete dorsally ; or whether it is a new aperture formed in
the dorsal part of the septum, as described by Born in the case
of the rabbit.

The canalis auricularis, at the beginning of the fourth week
(Fig. 243), is a short, straight, and rather narrow tube, connect-
ing the auricular and ventricular portions of the heart. In the
latter part of the fourth week this portion of the heart shortens
somewhat, the auricular and ventricular portions approach each
other, and the canalis auricularis becomes telescoped within
them (Fig. 244, ek), projecting partly into the auricular and
partly into the ventricular cavity, and being no longer visible
from the surface except as a sharply marked annular constric-
tion.

The lumen of the canalis auricularis becomes at the same
time reduced to a narrow transverse slit, its dorsal and ventral
walls thickening to form a pair of endothelial cushions, which

THE HEAET. 571

fuse with the lower border of the spina vestibuli to complete
the interauricular septum, and from which also the auriculo-
ventricular valves are derived.

The ventricles. The ventricular cavity becomes partially
divided towards the close of the fourth week by a fold, the
septum inferius (Fig. 244, sd), which arises from its dorsal and
posterior wall, and the position of which is indicated externally
by a slight groove on the surface of the heart. The completion
of the interventricular septum is a somewhat complicated pro-
cess, and will be described after the truncus arteriosus has been
dealt with.

The ventricular wall consists at first of an outer muscular
tube, and an inner and much smaller endothelial tube, the two
tubes being separated by a considerable quantity of gelatinous
connective tissue (c/. Fig. 215). The thickening of the ventricular
wall is effected, in the first instance, by the outgrowth of bands
from the muscular tube into the gelatinous tissue : these bands
interlace and unite with one another to form a spongework
of muscular trabecule. The gelatinous tissue now becomes
greatly reduced in amount, so that the endothelial and muscular
walls are brought much closer together, and the endothelium
becomes moulded to the surface of the muscular wall, covering
the trabeculse, and lining the depressions of the spongework.
The wall of the ventricle is now in much the same condition as
it remains in throughout life in the frog. In the later stages of
development, the outer, compact muscular wall thickens very
considerably, and the spongework becomes less conspicuous,
forming ultimately the columnse carneae.

The walls of the two ventricles are of equal thickness through-
out almost the whole of fcetal life, as the resistance to be over-
come by the two is approximately equal until the time of birth.

The truncus arteriosus. In the truncus arteriosus the most
important change is the formation of the aortic septum, by
which the single tube becomes divided into two, lying side by
side, which become the systemic and pulmonary trunks re-
spectively ; or, in the adult, the ascending aorta and the pul-
monary artery.

This division of the truncus arteriosus is effected by two

572 THE HUMAN EMBRYO.

longitudinal ridge-like thickenings of the endothelial lining,
which, arising from opposite sides, encroach on the lumen, re-
ducing it to a slit, dumb-bell shaped in section ; by further
growth, the two ridges meet each other and fuse, so as to divide
the lumen into two completely separate passages.

The endothelial ridges, and consequently the septum itself,
appear first at the distal end of the truncus arteriosus, between the
origins of the systemic and pulmonary aortic arches, and gradu-
ally extend backwards towards the ventricle. The septum first
appears towards the end of the fourth week, and is completed
before the end of the fifth week ; it has a slightly spiral course,
so that the two tubes, into which it divides the truncus arteriosus,
are respectively dorsal and ventral at the proximal end, next to
the ventricle, and right and left at the distal end of the truncus.

Of the two tubes, the one (Fig. 245, ex) which lies dorsally
at its proximal end, and on the right side distally, is the sys-
temic trunk ; the other, rw, which is ventral proximally, and on
the left side distally, is the pulmonary trunk ; and the same
relations are retained throughout life by the ascending aorta
and the root of the pulmonary artery, into which the trunks
develop respectively.

The separation of the systemic and pulmonary trunks at first
concerns their internal cavities alone ; but it is soon followed by
the appearance of external grooves, which deepen until they
completely separate the two trunks from each other.

The interventricular septum. The truncus arteriosus origi-
nally arises from the right-hand corner of the ventricular cavity,
and the two trunks into which it becomes split retain for a
time the same relations. In other words, at a time when the
interventricular septum is already partially formed (Fig. 244,
SD), both the systemic and pulmonary trunks arise from the
right ventricle, and the left ventricle has for a time no outlet,
except through the right ventricle.

The completion of the interventricular septum has to be
effected in such a way that while the pulmonary trunk is left in
connection with the right ventricle, the systemic trunk shall be
cut off from the right ventricle and placed in communication
with the left ventricle.

The formation of the interventricular septum is consequently
somewhat complicated. The greater part of the septum is

THE HEART. 57&

formed from the septum inferius (Fig. 244, sd), but it is com-
pleted above, partly by the lower edge of the interauricular sep-
tum, and partly by a prolongation of the aortic septum, which
divides the truncus arteriosus into systemic and pulmonary
trunks.

The aortic septum grows back beyond the truncus arteriosus,
so as to project a certain distance into the ventricular cavity ;
it then fuses with the free lower edge of the interauricular sep-
tum, in such a way as to cut off the systemic trunk from the
right ventricle, and to place it in communication with the left
ventricle ; while finally the septum inferius extends so as to meet
and fuse with the interauricular septum, and so completes the
separation of the ventricles from each other.

The valves of the heart. The outer flaps of the auriculo-
ventricular valves, both mitral and tricuspid, are formed from
the lower lips of the canalis auricularis, which hang down into
the ventricular cavity (Fig. 244) ; the inner flaps of the valves are
derived from the lower edge of the interauricular septum. The
valves are at first very thick and soft, and only later become
thin and membranous.

The semilunar valves are formed, about the end of the fifth
week, as cushion-like thickenings of the endothelium, which soon
become hollowed out into pockets.

2. The Arteries.

The general plan of arrangement of the arteries in the
human embryo is the same as in other Vertebrates ; and has
already been described, in previous chapters, in the case of the
rabbit, the chick, and the frog.

From the anterior end of the truncus arteriosus a series of
pairs of aortic arches arise, which run round the sides of the
pharynx, lying in the visceral arches (Fig. 243). On reaching
the dorsal surface of the pharynx, the aortic arches of each side
open into a longitudinal vessel, the aorta. The two aortas run
backwards along the body, ventral to the notochord ; they are
at first separate along their whole length, but early fuse together
in the hinder part of their course to form the definite dorsal aorta.
From the dorsal aorta, vitelline arteries are given oft' to the yolk-
sac ; and at the posterior end of the embryo the aorta divides

574 THE HUMAN EMBRYO.

into the two large allantoic arteries, which carry blood from the
embryo to the placenta.

The aortic arches of man, as of other Vertebrates, are de-
veloped in order from before backwards.

At the fifteenth day (Figs. 197 and 232) there are two pairs
of aortic arches present, lying in the mandibular and hyoideanj
arches, and corresponding, therefore, to the most anterior
pairs in rabbit, chick, or frog embryos. By the sixteenth day
three additional pairs have appeared, in the first, second, and
third branchial arches ; and up to the end of the third week all
five pairs are still present (Fig. 198, A.1-A.5).

The point of attachment of the truncus arteriosus to the
floor of the mouth shifts backwards during development, as
already noticed, and at the end of the third week is opposite the
hyoidean and first branchial arches. The truncus arteriosus, at
this stage, immediately on entering the floor of the mouth, divides
into two branches on each side (Fig. 198). Of these, the an-
terior branch runs forwards, and divides into the mandibular, A.i,
and hyoidean, a. 2, aortic arches ; while the posterior branch runs
backwards, and divides into the three hinder aortic arches, A.3,

A.4, A.5.

The aortic arches diminish in size from before backwards
(Fig. 198) ; and, owing to the funnel-like shape of the pharynx
(cf. Fig. 238), the hinder arches lie much nearer the median plane
than do those further forward.

All five pairs of arches are complete, opening at their dorsal
ends into the aortas (Fig. 198). In front of the first, or mandi-
bular arch, each aorta is continued forwards as the internal
carotid artery, which runs along the side of the brain, and gives
off branches supplying this.

During the fourth week important changes occur in the
aortic arches, closely comparable with those already described in
other Vertebrates, and leading to the establishment of the adult
scheme of circulation.

Early in the fourth week (Fig. 243) the middle portion of
the first, or mandibular, aortic arch of each side becomes obli-
terated, and disappears ; and very shortly afterwards the cor-
responding portion of the second, or hyoidean, aortic arch
disappears in the same fashion.

By the end of the fourth week the condition of the aortic

THE ARTERIES. 575

arches is as shown in Fig. 216. The mandibular and hyoidean
aortic arches have lost their connection with the aortas. Their
ventral or proximal ends persist as the external carotid arteries
and their various branches ; the mandibular arch, according to
His, giving rise to the external and internal maxillary arteries,
and the temporal artery ; while from the second, or hyoidean
arch, the lingual and ascending pharyngeal arteries arise, and
perhaps also the occipital and posterior auricular arteries.

The third aortic arch, A. 3, in the first branchial arch, remains
complete. As seen from the side (Fig. 216), it is somewhat
S-shaped, its curvature being such that the direction of flow of
the blood in it is naturally forwards, along the internal carotid
artery, towards the head.

The fourth and fifth aortic arches, A.4 and A.5, are both
complete, opening at their dorsal ends into the aortas. From
the fifth arches, near their ventral ends, the pulmonary arteries
arise, early in the fourth week, as small branches which run
backwards to the lungs (Fig. 243, ap).

During the fifth week further changes of importance occur.
The division of the truncus arteriosus, by formation of the aortic
septum, is completed, and the systemic and pulmonary trunks
are now entirely independent of each other ; the systemic trunk
(Figs. 245, 246, rx) remaining in connection with the fourth
and third aortic arches, and with the persisting remnants of the
second and first arches as well ; while the pulmonary trunk, RW,
communicates with the fifth pair of aortic arches alone.

The portion of the aorta between the dorsal ends of the
third and fourth, or, as we may now call them, the carotid and
systemic arches, disappears (Fig. 245).

The third, or carotid arch, becomes more directly continuous
with the anterior prolongation of the aorta, the two vessels
together forming the internal carotid artery, Ai ; while the
common carotid artery (Figs. 245, 246) is formed by lengthen-
ing of the arch at its origin from the systemic trunk.

Towards the end of the fifth week the heart travels rapidly
backwards, as the neck elongates ; this causes great lengthening
of the common carotid artery (Fig. 246, ae), and straightening
of the course of the internal carotid artery. It further leads,
among other changes, to the pulling out of the laryngeal branch
of the pneumogastric nerve, to form its recurrent loop.

576

THE HUMAN EMBRYO.

In the early part of the fifth week, the left fourth, or systemic
arch, becomes distinctly larger than the corresponding arch of the
right side ; and this difference soon becomes more pronounced.
By the end of the fifth week the fourth right arch is not only
markedly smaller than the left arch, but has lost its connection
with the aorta, and now forms only the vertebral and subclavian
arteries of the right side.

The fifth aortic arch of the right side disappears, beyond
the origin of the right pulmonary artery. The fifth left arch.

Fig. 245. — The aortic arches of a Human Embryo thirty-two days old, from
the left side. (From His.) x 18.

A, dorsal aorta. A.4, fourth, or systemic aortic arch. A.5, fifth, or pulmonary
aortic arch. AE, external carotid artery. AI, internal carotid artery. AP, pulmonary
artery. AV, vertebral artery. A"W, intervertebral or segmental arteries. LGhlung.
LE, trachea. MN, mandible, or lower jaw. PT, pituitary diverticulum from mouth.
RW, pulmonary trunk. KX, systemic trunk. TN, tongue. TO, oesophagus.

however, remains of large size up to the close of fcetal life ; the
portion of the arch between the root of the left pulmouary
artery and the dorsal aorta being known as the ductus arteriosus
(Figs. 245 and 246, a.5).

The dorsal aorta and its branches. The point at which the
two aortae unite, to form the single dorsal aorta, is about the
junction of the cervical and dorsal regions, in embryos at the
end of the fourth week, but the exact position varies considerably
in different specimens. As the union proceeds backwards, the
dorsal aorta increases considerably in size, and its diameter in

THE ARTERIES.

577

the lumbar region is more than double that in the anterior
thoracic region. At the hinder end of the lumbar region the
aorta divides into the right and left allantoic arteries, which run
along the allantoic stalk to the placenta, and which, at any rate
iu the early stages, appear as direct continuations of the aorta
rather than as branches of it.

The proximal ends, or roots, of the allantoic arteries persist
throughout life as the common iliac arteries, from which the
external iliac arteries arise as branches, on the formation of the

Fig. 246.— The aortic arches of a Human Embryo thirty-five days old, from
the left side. (From His.) x 30."

A.4, fourth, or systemic aortic arch. A.5, fifth, or pulmonary aortic arch. AE,
common carotid artery, at its point of division into internal and external carotid arteries.
AI, internal carotid artery. AP, pulmonary artery. AS, subclavian artery. AV,
vertebral artery. CH, notochord. LR, trachea. R"W, pulmonary trunk. RX,.
systemic trunk. TH, thyroid body. T!NT, tongue. TO, oesophagus.

hind limbs. The hypogastric arteries are the abdominal, or
intra-fcetal, portions of the allantoic arteries, beyond the origin of
the internal iliac arteries ; their cavities become obliterated after
birth, but their walls persist as solid cords, crossing the sides of
the bladder obliquely, and running forwards and upwards to the
umbilicus.

The vertebral arteries appear, about the twenty-fourth day,
as a pair of longitudinal trunks running along the sides of the
brain, and extending from the level of the ears to the commence-

P P

578 THE HUMAN EMBRYO.

ment of the cervical region. They have at first no communica-
tion with the other vessels, but towards the end. of the fourth
week their anterior ends unite to form the median basilar artery,
which becomes connected with the internal carotid arteries to
form the circle of Willis. About the same time, a series of
paired segmental or intervertebral arteries arise, as branches
from the dorsal wall of the aorta, along the cervical and thoracic
regions (Fig. 245, aw), and supply the spinal cord. One, or
more, of the anterior pairs of these intervertebral arteries become
continuous with the hinder ends of the vertebral arteries (Fig.
245, av), which thus acquire their connection with the aortee. In
the later stages, some of the intervertebral arteries further back
become connected in similar fashion with one another, and with
the vertebral artery ; and by the acquisition of these posterior
connections, with simultaneous loss of the older and more an-
terior ones, the point of origin of the vertebral artery from
the aorta is gradually shifted backwards to the root of the neck.

The subclavian arteries arise as branches of the vertebral
arteries (Fig. 246, as) ; but, as the fore limbs increase in size, the
relative proportions of the two vessels soon become reversed, and
the vertebral arteries appear as branches of the subclavians.

From the sides of the dorsal aorta a series of pairs of arteries
arise, which supply the Wolffian bodies. The coeliac axis is from
the first a median artery ; it arises from the ventral wall of the
aorta, in the anterior thoracic region, and gradually shifts back-
wards, until its adult point of origin, opposite the last thoracic
vertebra, is attained.

In the development of the aorta, and in that of all the other
arteries as well, the wall of the vessel consists at first of a single
layer of endothelial cells. Outside this, the layer of circular
muscle-fibres is formed from the surrounding mesoblast, early in
the third week. At a later stage a layer of connective tissue is
formed between the muscular and the endothelial layers, but it
is not clear from what source this connective tissue is derived.
His suggests that it is formed directly from the blood in the
blood-vessel itself.

3. The Veins.

The general arrangement, and mode of development, of the
veins in man is the same as in the rabbit. The most important

THE VEINS. 579

differences consist in the disappearance of the left anterior vena
cava, and in certain modifications in connection with the veins
of the liver.

In the latter part of the third week (Fig. 198), the blood is
returned to the heart by three pairs of veins, of approximately
equal size : — the Cuvierian, vitelline, and allantoic veins.

Of these, the Cuvierian veins, vd, return blood from the
embryo itself, and are formed on each side, as in the rabbit and
the chick, by the union of an anterior cardinal or jugular vein,
VB, from the head, with a posterior cardinal vein, vc, from the
trunk.

The vitelline veins, W, return blood from the yolk-sac, and
enter the embryo by the yolk-stalk.

The allantoic veins, va, return blood from the placenta ; they
enter the embryo along the allantoic stalk, and run forwards in
the side walls of the body to the heart.

The veins are at first of equal size on the two sides of the
body, and by the union of the six veins the transversely placed
sinus venosus is formed. In following their further development
it will be convenient to take the several veins separately.

The vitelline veins are comparatively small, as in Mammals
generally, owing to the small size of the yolk-sac. They
lie-in the splanchnopleuric mesoblast, and, after entering the
embryo at the umbilicus, run forwards along the sides of the
alimentary canal to the sinus venosus (Fig. 243, vv). The
vitelline veins are closely associated with the liver, and they
become surrounded by this as it is developed ; furthermore, the
principal changes which they undergo are in connection with the
vascular supply of the liver.

Early in the fourth week, about the twenty-third day (Fig.
243), the vitelline veins become interrupted as they pass through
the liver, breaking up into a set of afferent hepatic vessels
supplying the liver, and a set of efferent hepatic vessels con-
veying the blood from the liver to the heart. The afferent and
efferent hepatic vessels are connected by capillaries only, so that
all the blood entering the liver by the vitelline veins must
traverse the substance of the liver in order to reach the heart.

About the same time, the right and left vitelline veins
become connected together, immediately before they enter the

p p 2

58Q

THE HUMAN EMBRYO.

liver, by three transverse commissural vessels. Two of these
commissural vessels pass ventral to the duodenum, while the
third, or middle one, is dorsal to it ; and the three together form
two vascular rings, or sinus annulares, encircling the duodenum
(Fig. 243). From the anterior ring, afferent vessels arise which
cany blood into the liver.

At a slightly later stage, during the latter part of the fourth

Tl w

Fig. 247. — The liver and the veins in connection with it of a Human Embry<
twenty-four or twenty-five days old, seen from the ventral surface. (Fror
His.)

PA, pancreas. TI, intestine. TS, stomach. VA, left allantoic vein. n
right allantoic vein. VA", anterior detached portions of the allantoic veins,
ductus venosus, or vena Arantii. "VH, efferent hepatic vessel. VL, afferent hepati'
vessel. VO, hepatic portal vein. VV, vitelline vein. W, portions of the sini
anntdares which disappear. W, liver. WD, bile duct.

week, the right and left vitelline veins unite to form a singh
vein, which is joined, before it reaches the liver, by veins re-
turning blood from the intestine, and which may from this time
be spoken of as the hepatic portal vein.

Of the two sinus annulares, the left half of the anterior on(
and the right half of the posterior one, disappear ; the persistent
portions , form a single vessel (Fig. 247, vo), which become
the anterior part of the hepatic portal vein, and which, froi

THE VEINS. 581

the mode of its development, runs round the duodenum with the
spiral course characteristic of the vein in the adult.

The allantoic veins are at first paired, but they soon fuse
together at their hinder ends, within the allantoic stalk, to form
a single vessel ; further forwards, within the embryo itself, they
remain separate, running in the side walls of the body, close to
the base of the amnion folds (Fig. 198).

During the fourth week, both allantoic veins lose their con-
nection with the sinus venosus. The right allantoic vein
(Fig. 247, va'), which is now much the smaller of the two,
breaks up into two sets of vessels : an anterior set, va7', which
run in the body wall, and join the efferent hepatic vessels as
these leave the liver ; and a posterior set, va', which disappear at
a slightly later stage.

The left allantoic vein, va, which is much larger than the
right, also divides into two sets of vessels : an anterior set, va",
which resemble those of the right side ; and a large posterior
vessel, VA, which joins the anterior sinus annularis, or hepatic
portal vein, just as this enters the liver substance.

The ductus venosus. At about the twenty-third day, both the
vitelline and the allantoic vessels have lost their direct connec-
tions with the sinus venosus, and in order to reach the heart the
blood in these vessels must traverse the liver capillaries. A
direct communicating passage is now established between the
portal vein, just before it enters the liver, and the right hepatic
vein just before this reaches the sinus venosus. This communica-
tion (Fig. 247, ve) is the ductus venosus, sometimes called the
vena ascend ens or vena Arantii ; it enlarges very rapidly, and
affords a wide and direct path by which the blood from the
placenta can reach the heart without passing through the liver
capillaries.

In rabbit and chick embryos the ductus venosus is the per-
sistent anterior part of the fused vitelline veins ; in man, accord*
ing to Professor His, whose descriptions have been followed above,
it is, as just described, an entirely new vessel.

The posterior vena cava is a very insignificant vein in the
earlier stages. It is formed by the junction of the iliac veins,

582 THE HUMAN EMBKYO.

and does not appear until the hind-limbs have begun to become
prominent. It joins the ductus venosus as this emerges from
the liver.

The Cuvierian veins. Each Cuvierian vein (Fig. 198, v$) is
formed by the junction of an anterior and a posterior cardinal
vein. The anterior cardinal vein persists as the external jugular
vein, and is joined later on by the internal jugular and sub-
clavian veins.

The posterior cardinal veins disappear, in the middle part
of their course, on the replacement of the Wolffian bodies, with
which they are specially related, by the permanent kidneys.
The hinder ends of the veins become the internal iliac veins, and
acquire connections with the allantoic veins. The anterior
portion of the right posterior cardinal vein gives rise to the
azygos vein.

The Cuvierian veins themselves run at first transversely ; but,
as the heart shifts backwards, their direction becomes at first
oblique, and finally longitudinal.

The right Cuvierian vein persists as the anterior vena cava.
The left Cuvierian vein undergoes important changes : up to
the end of the second month it is as large as the right vein ;
but during the third month a communicating vessel is formed
between the left and right Cuvierian veins, just behind the
junction of the jugular and subclavian veins. Through this com-
municating branch, which is very large and has a somewhat
oblique course, the blood from the left jugular and .subclavian
veins is carried across to the right Cuvierian vein, instead of
returning to the heart as before by the left Cuvierian vein.
The left Cuvierian vein, having no longer any function to
perform, shrinks up and becomes obliterated more or less com-
pletely. Portions may persist, either as fibrous cords, or as
venous channels of greater or less size ; and the posterior end,
where it opens into the sinus venosus, is said to give rise to the
coronary sinus.

The pulmonary veins appear late, about the end of the fifth
week : they open into the left auricle, close to the interauricular
septum. At first there is only a single opening into the auricle,
but at a later stage, about the fourth month, there are two

THE COUESE OF THE CIECULATION. 583

openings, and in slightly older foetuses all four openings are
present ; the change being apparently due to the opening out
of the originally single orifice, and the absorption of the vein, as
far as its first branches, into the wall of the auricle ; much in the
same way as the sinus venosus is opened out, and made part of
the wall of the right auricle.

4. The Course of the Circulation during the first Four Months of
Gestation.

In the early stages, up to the end of the first month, the
blood brought back to the heart — whether from the body of the
embryo itself, from the placenta, or from the yolk-sac — is poured
into the sinus venosus, and thence, through a median slit-like
aperture, into the single auricular cavity. Complete mixture of
the streams from the several sources must necessarily occur, in
both the sinus venosus and the auricle, and the blood driven out
through the truncus arteriosus by the ventricle will be of a
mixed character.

After the sinus venosus is taken into the heart, in the early
part of the second month, there are for a time three separate
openings into the right auricle : those of the right and left
Cuvierian veins, and of the posterior vena cava respectively. The
auricular septum is now partially formed, but there is still free
communication between the two auricles through the foramen
ovale. Of the three veins, the opening of the posterior vena
cava lies nearest to the foramen ovale ; and the Eustachian
valve, a fold of the wall of the auricle along the right-hand side
of the opening, tends to direct the blood from the posterior vena
cava through the foramen ovale into the left auricle. The
foramen ovale is at this stage a mere aperture in the auricular
septum, not guarded by valves, so that a certain amount of direct
mixture of the blood returned to the auricle by the different
veins must of necessity take place.

During the third month, the transverse communication from
the left to the right Cuvierian vein is being established ; and
by the end of the fourth month the left Cuvierian vein has
practically disappeared, the whole of the blood from both sides
of the head, and from both fore limbs, being returned by the
right Cuvierian vein, or anterior vena cava as it may now be
called. Neglecting the coronary sinus, which is comparatively

584 THE HUMAN EMBRYO.

insignificant, there are at this stage only two vessels returning
blood to the right auricle : the anterior vena cava, which returns
venous blood from both sides of the head, and from both fore
limbs ; and the posterior vena cava, which brings back blood,
mainly arterial in character, from the placenta, and also from the
hinder part of the body of the embryo, and from the yolk-sac.

During the fourth month the foramen ovale, which has
hitherto been a free opening, becomes partially blocked up by a
fold, which acts as a valve, allowing blood to pass from the right
to the left auricle, but obstructing its return in the opposite
direction.

The Eustachian valve becomes larger at the same time ; and
partly owing to its increased size, and partly to slight changes in
the position and direction of the opening of the posterior vena
cava, the whole of the blood returned by this latter vessel is now
discharged through the foramen ovale into the left auricle.

5. The Course of the Circulation during the Latter Half of Ges-
tation.

During the latter four months or so of gestation the course
of the circulation is as follows : —

The right auricle receives blood from three sources —

(i) From the anterior vena cava.

(ii) From the coronary sinus.

(iii) From the posterior vena cava.

The anterior vena cava returns venous blood from both sides
of the head, and from both fore-limbs.

The coronary sinus, which is the persistent terminal portion
of the original left anterior vena cava, returns venous blood from
the walls of the heart itself.

The posterior vena cava, which is much the largest of the
three, returns blood : (a) from the hinder part of the body, and
especially the kidneys and the hind limbs ; and (b) from the
placenta, the intestine and the liver. The latter of these two
streams requires further consideration.

Of the two allantoic veins, by which the blood was returned
from the placenta in the earlier stages, the right one has dis-
appeared. The left allantoic vein, which is very large, enters
the body at the umbilicus, and runs forwards to the hinder
border of the liver ; here it is joined by the hepatic portal vein,

THE COUESE OF THE CIRCULATION. 585

which returns blood from the intestine, and is formed in part
from the vitelline veins of the earlier stages.

On reaching the liver, the blood has two alternative routes
open to it, by either of which it can reach the posterior vena
cava. Part of the blood is conveyed by the afferent hepatic
vessels into the substance of the liver, from which it is returned
by the efferent hepatic vessels, or hepatic veins, to the posterior
vena cava ; the greater part, however, continues straight on-
wards through the wide ductus venosus, and so reaches the
Kosterior vena cava without having traversed the liver.
The blood brought back to the heart by the posterior vena
ava is thus derived very largely from the allantoic vein, and
i part from the renal veins ; it is therefore purer as regards
aseous constituents, and freedom from nitrogenous excretory
xxiatters, and is richer in nutrient matters, than the blood
returned by the anterior vena cava ; and the blood in the
anterior and in the posterior venae cavae may consequently be
contrasted as venous and arterial respectively.

The venous blood brought to the right auricle by the ante-
rior vena cava passes, on the auricular contraction, into the
right ventricle. From the ventricle it is driven along the pul-
monary trunk (Fig. 24G, rw) ; a small portion passes along the
pulmonary arteries, AP, to the lungs, but as the lungs are in an
unexpanded condition there is considerable resistance to the
entrance of blood into the pulmonary vessels, and only an
insignificant portion of the stream takes this path. Nearly the
whole of the venous blood in the pulmonary trunk passes along
the ductus arteriosus (Fig. 246, A. 5) to the dorsal aorta, down
which it courses to the bifurcation of the aorta into the two
common iliac arteries ; then down these latter, and partly along
the external iliac arteries to the hind limbs, but maiuly along
the allantoic arteries to the placenta, where it gains nutrient
matter and oxygen, and from which it is returned to the foetus
by the allantoic vein.

The arterial blood brought to the right auricle by the
posterior vena cava does not really enter the cavity of the right
auricle, but is directed at once, by the Eustachian valve, through
the foramen ovale into the left auricle, which also receives the
very small quantity of blood returned from the lungs by the
pulmonary veins. From the left auricle the blood passes into

586 THE HUMAN EMBRYO.

the left ventricle, and is thence driven along the systemic trunk
(Fig. 246, rx), and through the carotid and subclavian arteries
to the head and fore-limbs.

It is probable that very little, if any, blood from the left
ventricle gets into the dorsal aorta, for this is already filled,
through the ductus arteriosus, from the right ventricle ; and as
the two ventricles have at this stage walls of about equal thick-
ness, and presumably of equal strength, there will be as strong a
tendency for the blood of the right ventricle to pass forwards
along the arch of the aorta, as for the blood from the left ven-
tricle to pass backwards along the dorsal aorta.

Theoretically, the aorta might be ligatured just in front of
the point at which the ductus arteriosus joins it, without in any
way disturbing the foetal circulation ; and instances of mal-
formation have occurred, in which the aorta was completely
obliterated at this place, and yet development in other respects-
proceeded normally. Such a malformation, though causing no
disturbance of the circulation so long as the foetus is receiving
nourishment through the placenta, is fatal at the time of birth,
as the arterial supply of the whole body behind the arms is then
cut off.

6. The Changes in the Circulation at the Time of Birth.

At birth, the placental circulation is arrested, and the
allantoic and vitelline vessels are interrupted ; and, as the lungs
become inflated, the pulmonary circulation is fully established.

In connection with this shifting of the seat of respiration,
from the placenta to the lungs, important changes are effected
in the circulation, the principal of which are :

(i) Shrinking and obliteration of the ductus arteriosus, and
of the hypogastric, or allantoic, arteries.

(ii) Obliteration of the ductus venosus, and of the part of
the allantoic vein within the body of the child.

(iii) Closure of the foramen ovale.

By these changes it is brought about that the blood in the
posterior vena cava, which is now entirely venous, is no longer
able to get into the left auricle, owing to closure of the foramen
ovale, but passes, with that of the anterior vena cava, from the
right auricle to the right ventricle. From the right ventricle,
owing to the obliteration of the ductus arteriosus, it can no

THE CHANGES IN THE CIRCULATION AT BIRTH. 587

longer reach the aorta, but passes entirely along the pulmonary
arteries to the lungs. From the lungs it is returned by the
pulmonary veins, which are now greatly enlarged, to the left
auricle, and so to the left ventricle, which drives it not only to
the head and upper limbs, but also along the dorsal aorta to the
hinder part of the body.

By obliteration of the ductus venosus, all the blood in the
hepatic portal vein is compelled to pass through the capillaries
of the liver in order to reach the posterior vena cava. In other
words, by these three changes — obliteration of the ductus arte-
riosus, obliteration of the ductus venosus, and closure of the
foramen ovale — the foetal circulation has been converted into that
of the adult.

These changes do not occur immediately on birth, nor are
they effected simultaneously.

Obliteration of the allantoic or hypogastric arteries occurs
first ; it is effected partly by contraction of the entire vessels,
but chiefly by thickening of their inner coats, and is usually
completed by the third or fourth day after birth.

The allantoic veins and the ductus venosus remain open
rather longer, but are generally obliterated by the sixth or
seventh day.

The ductus arteriosus, according to Allen Thomson, ' is rarely
found open after the eighth or tenth day, and by three weeks it
has, in almost all instances, become completely impervious.'

Closure of the foramen ovale is the last of the changes to be
completed. The closure is at first effected merely by the valve,
which projects into the left auricle, being kept closely applied
to the margin of the aperture by pressure of the increased
quantity of blood now returning by the pulmonary veins. At a
later stage the edge of the valve gradually coalesces with the
margin of the opening, but the union often remains incomplete
for some months ; and it not unfrequently happens that an
oblique valvular aperture, large enough to admit a probe,
persists for the first year of infancy, and may even be perma-
nent throughout life, in which case a direct passage of venous
blood into the left auricle is liable to occur, especially on over-
exertion.

588 THE HUMAN EMBRYO.

DEVELOPMENT OF THE URINARY ORGANS.

The general history of development of the urinary organs in
man is the same as in the rabbit. Paired Wolffian ducts and
Wolffian bodies appear first ; these form the excretory organs of
the early stages, and attain a considerable size during the second
month, after which time they gradually shrink, ultimately
losing their excretory function, and becoming modified to form
accessory parts of the reproductive system.

The permanent or adult kidneys arise, as in the rabbit, as
outgrowths from the hinder ends of the Wolffian ducts : from the
third month onwards they replace the Wolffian bodies as the
functional excretory organs.

A pair of Miillerian ducts is formed, independently of the
Wolffian ducts, and in the female becomes modified to form the
oviducts, uterus, and vagina. The head-kidney, if present at
all, is in a very rudimentary and evanescent condition.

1. The Wolffian Duct and Wolffian Body.

According to Kollmann, the Wolffian ducts appear, about the
fourteenth day, as a pair of longitudinal grooves of the external
epiblast, just below the level of the myotomes (Fig. 248, kc).
By the middle of the third week the ducts are tubular, and lie
embedded in the mesoblast of the intermediate cell mass. It is
not yet certain, however, whether the tubular duct is formed by
closure of the lips of the groove, or by splitting off of a rod of
cells from the thickened floor of the groove, which subsequently
acquires a lumen, and becomes tubular : while the observations
recorded in the case of rabbit embryos render it possible that
the origin of the Wolffian duct from the epiblast may prove
to be apparent rather than real (cf. p. 421).

The Wolffian ducts at first end blindly behind, but about the
end of the third week or beginning of the fourth week they grow
back to the cloaca, and open into its sides (Fig. 243, kc).

The Wolffian bodies appear about the eighteenth day as a
pair of longitudinal ridge-like thickenings of the dorsal wall of
the body cavity, one on each side of the mesentery. These soon
become more prominent, and by the beginning of the fourth
week extend from about the sixth cervical to the last lumbar
somite.

THE WOLFFIAN DUCT AND BODY.

589

Each Wolffian body consists at first of rods of cells, which
appear to arise independently of the Wolffian duct. The rods
soon become S-shaped : early in the fourth week they acquire
axial cavities, and so become tubes ; and by the end of the week
the tubes, or Wolffian tubules as they may now be termed, grow
towards the Wolffian duct and open into it. The opposite, or

Fig. 248. — Transverse section across the body of a Human Embryo, estimated'
as fourteen days old. For a figure of the whole embryo, see Fig. 185,
p. 481. The embryo had thirteen pairs of mesoblastic somites, and the
section figured passes through the tenth pair. (From Kollmann.) x 240.

A, aorta. C,ccelom. CH, notochord. GT, mid-gut. KC, Wolffian duct. ME,
somatopleuric layer of ruesoblast. MH, splanclmopleuric layer of rnesoblast. MT,
myotome or mesoblastic somite. NC, central canal of spinal cord. N S, spinal cord.

closed ends of the tubules become dilated, and then invaginated'
to form Malpighian bodies, the glomeruli being derived from
branches of the aorta which penetrate into the Wolffian body
along its whole length ; while the veins open into the large-
posterior cardinal veins, which are intimately associated with the
Wolffian bodies from their first appearance.

The Malpighian bodies are more abundant along the inner
side of each Wolffian body, while the duct lies along its outer
border, except at the hinder end, where it crosses to the inner side.

During the second month the Wolffian bodies grow rapidly i

590 THE HUMAN EMBRYO.

the Malpighian bodies increase greatly, both in number and in
size ; and new Wolffian'tubules are formed, apparently by budding
from the old ones. In each tubule the part next the Malpighian
body, which is probably the secreting portion, has thicker walls,
formed of larger epithelial cells, than the more distal part which
opens into the Wolffian duct.

The Wolffian body reaches its greatest development about
the eighth week, from which time it slowly diminishes in size.
Degeneration commences, and proceeds more actively at the
anterior end of the Wolffian body, which from the first has lagged
behind the rest of the organ in development. Ultimately the
whole structure becomes affected ; by the fifth month the Mal-
pighian bodies have almost entirely disappeared, and in the end
the Wolffian body becomes reduced to an accessory part of the
reproductive apparatus.

2. The Kidney and Ureter.

The ureter arises on each side as a diverticulum from the
hinder end of the Wolffian duct, in the early part of the fourth
week (Fig. 243, kd). This soon acquires an independent
opening into the cloaca, a little way behind that of the Wolffian
duct (Figs. 216, kd, and 233, n). At its opposite or blind end
the ureter grows forwards, between the hinder end of the Wolffian
body and the vertebrae. It dilates to form a somewhat elongated
sac, which is the pelvis of the future kidney ; and from this
sac branching tubular diverticula grow out (Fig. 234, n), and
become the urinary tubules. These rapidly increase in number
and in length ; Malpighian bodies are formed in connection with
their distal ends, and the kidney structure is definitely acquired
by the end of the second month, at which time the degeneration
of the Wolffian body commences.

The bladder is formed by dilatation of the basal or proximal
part of the allantois. Beyond the bladder the allantoic stalk
loses its cavity and becomes a solid rod, the urachus, leading
from the bladder to the umbilicus. The lumen usually disap-
pears early in the fifth week, but it may persist for a much
longer time, or even be present in the adult.

3. The Mullerian Duct.

About the end of the fourth week, a longitudinal ridge-like
thickening of the peritoneum appears along the outer side of each

THE KIDNEY AND URETER. 591

of the Wolffian bodies. The ridge lies close to the Wolffian
duct, and extends along its whole length, but is quite independent
of this.

Early in the fifth week, the Mullerian duct is formed in this
ridge ; it is a narrow straight tube, lying along the outer side
of the Wolffian duct, but distinct from this. Its anterior end
opens into the body cavity by an elongated slit-like mouth,
situated in a patch of thickened peritoneal epithelium, a little
way in front of the anterior end of the Wolffian body. Poste-
riorly, the Mullerian duct ends blindly.

By the eighth week the Mullerian duct has undergone some
changes. It commences in front with a wide funnel-like mouth,
the margins of which are already slightly fimbriated. Behind
this mouth, the duct runs straight backwards for some distance,
along the outer side of the Wolffian body, then turns sharply
inwards, crosses ventral to the Wolffian duct, and continues back-
wards in close contact with the Mullerian duct of the opposite
side ; it still ends blindly behind.

In the male, the Mullerian ducts begin to atrophy shortly
after reaching this stage. In the female, they undergo further
development, and give rise to the oviducts, uterus, and vagina,
as will be described in the section dealing with the accessory
organs of reproduction.

4. The Head-kidney.

Janosik has described, in an embryo eighteen to nineteen
days old, what he thinks may prove to be a rudimentary pro-
nephros, in the form of a couple of peritoneal funnels just in
front of the anterior end of the Wolffian duct ; the anterior
funnel having close to it a structure not unlike an external
glomerulus. The early development and subsequent fate of
these structures have not yet been determined.

THE DEVELOPMENT OF THE REPRODUCTIVE
ORGANS.

1. The Essential Reproductive Organs.

These have already been described, in the introductory portion

II of this chapter (pp. 449 to 457) ; but a few further details may
conveniently be added here.
In embryos thirty-two days old (o/. Fig. 205), the genital
Ml

592 THE HUMAN EMBRYO.

ridges are present as a pair of bands of epithelium, many cells
thick, and lying along the inner sides of the Wolffian bodies.
Primitive ova are already present, and, according to Nagel, are
found not only in the genital ridges themselves, but also beyond
their limits, and especially in the thickened epithelium in
the neighbourhood of the Miillerian ducts. This may perhaps
be taken as an indication that the genital epithelium was
originally less sharply circumscribed than at present.

Nagel has shown that distinct differences may be detected in
the genital ridges of the two sexes from as early a period as
thirty-two days ; and he is inclined to doubt whether there is
absolute identity at any time, even in the earliest stages.

In the male, the genital ridge, at thirty-three days, shows a
fairly definite arrangement of the cells in strings ; these form a
network of tortuous anastomosing cords, arranged somewhat
regularly, and bound together by connective tissue. Embedded
in the cellular cords are the primitive sperm cells. These are
comparatively few in number ; their formation ceases at an
early stage, in embryos of about six or seven weeks, on the com-
pletion of the tunica albuginea ; but in the later stages, although
no new primitive sperm cells are formed from the germinal
epithelium, those which are already present increase freely by
division. The cellular cords themselves become converted into
the seminal canals, which are thus derived directly from the
germinal epithelium.

In the female, the primitive ova, in embryos of thirty-three
days, are much more numerous than the primitive sperm cells of
the male. They are found in various phases of development, and
the formation of new primitive ova continues until about the
close of gestation. It is very doubtful whether any new primitive
ova are formed after birth, and by some authorities their formation
is believed to stop about the seventh month. The tendency for
the smaller cells to become grouped around the primitive ova,
and so form follicles, is evident even in the fifth week, and affords
a good clue by which a young ovary may be distinguished from
a young testis, and the sex of the embryo thus determined.

2. The Accessory Reproductive Organs.

As in the rabbit, the chick, and indeed the great majority of
Vertebrates, the genital ducts of the human embryo are formed

THE REPRODUCTIVE ORGANS. 593

from tubes which originally belong to the excretory system ; the
oviducts being formed from the Mullerian ducts, and the vasa
deferentia of the male from the Wolffian ducts ; while other por-
tions of the embryonic excretory apparatus persist in a modified
or vestigial form, as accessory organs in relation with the repro-
ductive system.

a. In the Male.

The Mullerian ducts begin to atrophy about the middle of
the third month, and ultimately disappear completely along the
greater part of their length. The anterior end of the Mullerian
duct may persist, and in connection with it the hydatids of
Morgagni are believed to be formed ; this name being given to
one or more small pedunculated bodies, lying between the testis
and the head of the epididymis. One of these bodies is of larger
size, and more constant occurrence, than the others.

It is stated that the posterior ends of the Mullerian ducts
unite together, and give rise to the uterus masculinus, a small
pocket-like diverticulum from the dorsal wall of the prostatic
portion of the urethra, a quarter to half an inch in depth, and
bearing on its margins the slit-like openings of the vasa
deferentia. The statement, however, needs confirmation.

The Wolffian body and Wolffian duct. The greater part of
the Wolffian body disappears, but the anterior end becomes
intimately connected with the testis, and persists throughout
life. From the Wolffian tubules of this anterior end tubular
outgrowths arise, which during the fourth month grow into the
substance of the testis, and give rise to the vasa efferentia ; these
soon become connected with the seminal tubes, which latter, ac-
cording to Nagel, are formed directly from the germinal epithe-
lium. The anterior Wolffian tubules become the coni vasculosi ;
and the Wolffian duct is converted, in front, into the extremely
tortuous epididymis, and further back into the vas deferens.

The structures known as the vasa aberrantia, a series of tor-
tuous tubular diverticula from the lower end of the epididymis ;
Ind the parepididymis, or organ of Giraldes, are probably per-
istent portions of some of the hinder Wolffian tubules.
. In the Female.
The Mullerian ducts, at the beginning of the third month,
re still quite distinct from each other. Their anterior ends,
QQ

594 THE HUMAN EMBRYO.

with the abdominal openings, are widely separate ; their pos-
terior portions lie side by side, between and slightly dorsal to
the Wolffian ducts, and bound up with these by connective
tissue, to form what is spoken of as the genital cord. The
Miillerian ducts still end blindly behind.

Towards the end of the third month, the two Miillerian ducts
fuse together, opposite the middle third of the genital cord ; and
from this point the fusion extends rather rapidly forwards, and
much more slowly backwards. The fused portion, or utero-
vaginal canal, enlarges steadily, especially in its transverse
diameter. By the beginning of the fourth month, a distinction
appears between the uterine and vaginal portions of the canal ;
the proximal portion, or uterus, being lined by a columnar epi-
thelium, and the distal portion, or vagina, by a squamous
epithelium.

During the fourth month, the boundary line between the
uterus and vagina becomes a much sharper one. The uterus
becomes considerably dilated : the vagina, on the other hand, is
flattened dorso-ventrally ; and, by proliferation of its epithelial
cells, its lumen becomes completely blocked up for a time, re-
appearing in the course of the fifth month.

The two Miillerian ducts thus give rise to the whole length
of the female passages ; the anterior or proximal ends of the
ducts remaining distinct from each other, and forming the
oviducts or Fallopian tubes ; while the posterior or distal por-
tions fuse together, and give rise to the uterus and vagina.

The fusion of the two halves of the uterus is not comjoleted
until the latter part of the fourth month ; and the occasional
retention of a more or less complete uterine septum, even in the
adult, is due to imperfect fusion of the two originally distinct
ducts.

The cervix uteri is established during the fifth month, at
the time when the lumen of the vagina is reappearing. The
folds of the wall of the cervix, spoken of as the arbor vita?,
appear during the fourth month; while the differentiation of
the muscular walls, and of the enormously developed muscularis
mucosas commences in the sixth month. The uterine epithelium
is devoid of cilia during the whole of foetal life ; and up to the
time of birth there are no glands in the body of the uterus.
Glands are, however, present in the cervix, and apparently

THE FEMALE EEPKODUCTIVE ORGANS. 595

secrete tlie plug of mucus which commonly occupies the os uteri
at the time of birth.

The Wolffian body. In the female, outgrowths from the
anterior Wolffian tubules into the ovary occur, similar to those
which in the male give rise to the vasa efferentia ; but they do
not give rise to any adult structure.

A number of the Wolffian tubules of the anterior end of the
Wolffian body persist throughout life, forming the structure
known as the parovarium (Fig. 249, a), sometimes called the

Fig. 240. — The adult Ovary, Parovarium, and Fallopian tube. From Quain
' Anatomy.' (After Kobelt.)

a, a, parovarium, epoophoron, or organ of RosenmUUer ; formed from the anterior end
of the Wolffian body, b, remains of some of the anterior Wolffian tubules, sometimes
forming hydatids, c, the longitudinal duct of the parovarium, formed from the
anterior end of the Wolffian duct, rf, rudimentary Wolffian tubules, e, atrophied
remains of the Wolffian duct, or duct of Gaertner. /, the terminal bulb or hydatid, ft,
Fallopian tube, i, hydatid attached to the end of the Fallopian tube. I, ovary.

epoophoron or organ of Rosenmiiller; a series of transverse
tubes which run, with a somewhat tortuous course, in the fold of
peritoneum between the ovary and the Fallopian tube, and are
connected with the anterior end of the ovary.

A small portion of the hinder part of the Wolffian body may
persist as a rudimentary structure, the paroophoron, lying in the
peritoneum opposite the hinder end of the ovary.

The Wolffian duct persists, in front, as the longitudinal
duct of the parovarium (Fig. 249, c), into which the transverse
tubules, a, open, and which corresponds to the epididymis of the
male. The hinder part of the Wolffian duct usually disappears,
but it may persist along a greater or less portion of its length as

o Q 2

596 THE JH'MAN EMBRYO.

the duct of Gaertner, running alongside the Fallopian tube (Fig.
249, e), and sometimes extending along the walls of the uterus,
or even as far as the vagina.

3. The External Genital Organs.

These are practically identical in the two sexes, in the
early stages of their development ; the distinction between male
and female, as regards the external genital organs, not being
evident until about the ninth or tenth week.

At the end of the fifth week (Fig. 234), the septum dividing
the rectum from the urino-genital passage has almost reached
the surface, but the two passages apparently open by a single
cloacal aperture. Immediately in front of this aperture is a

Fig. 250. Fig. 251.

Fig. 250. — The external genitalia of a Human Embryo of about the ninth
week (probably rather younger). (From Kolliker, after Ecker.) x 2.

c, genital tubercle, or clitoro-penis. /, groove, continuous with urino-genital passage.
hi, labio-scrotal folds, n, umbilical cord, s, coccygeal region.

Fig. 251. — The external genitalia of a Human Embryo of about the tenth
week. (From Kolliker, after Ecker.) x 2.
a, anus, e, genital tubercle, or clitoro-penis. /, genital groove, continuous with
urino-genital aperture, hi, labio-scrotal folds, s, coccygeal region.

small conical projection, sg, the genital tubercle or clitoro-penis.
The posterior surface of this tubercle is marked by a longi-
tudinal groove, which leads, through the cloacal aperture, into
the urino-genital passage ; the lips of the groove are slightly
swollen, and are continuous with the lips of the cloacal opening,
which form the inner sexual folds. The tip of the genital
tubercle is expanded into a small knob, the glans.

A little later, towards the end of the second month, the
septum between the urino-genital passage and the rectum
reaches the surface, dividing the cloacal aperture iuto two
separate openings, an anterior or urino-genital (Fig. 251,/),
and a posterior or anal (Fig. 251, a).

Up to this time the course of development is practically the

THE EXTERNAL GENITAL ORGANS. 597

same in all embryos, but from about the tenth week differences
become apparent between the two sexes.

In the male, the genital tubercle elongates, and forms the
penis. The lips of the groove, along the posterior surface of the
tubercle, meet and fuse to form the canal of the penis, or penial
urethra ; and, by a similar fusion of the lips of the urino-genital
opening, the penial urethra and urino-genital passage become
directly continuous with each other. The glans penis is at first
solid, but towards the end of the third month the groove extends
forwards along it, and, gradually closing from behiud forwards,
carries the opening of the urethral canal to the apex of the
glans. The prepuce appears, towards the end of the third

Fig. 252.— The external genitalia of a male Human Embryo towards the end
of the third month. (From Kolliker, after Ecker.)

a, anus, e, penis. /, raphe formed by union of lips of genital groove, hi, scrotum,
r, i aphe formed by union of the two halves of the scrotum. t, coccyx.

Fig. 253. — The external genitalia of a female Human Embryo towards the
end of the third month. (From Kolliker, after Ecker.)

a, anus, e, clitoris. /, genital groove, tiff, urino-genital aperture, hi, labia majora.
n, labia minora, or inner genital folds. *, coccygeal region.

month, as a fold of skin round the base of the glans, and is at
first interrupted ventrally by the urethral groove.

The scrotum is formed from a pair of folds of skin, the labio-
scrotal or outer genital folds, which arise at the sides of the
urino-genital opening, and ultimately unite with each, other in
the median plane behind the penis (Fig. 252, hi).

In the female the genital tubercle remains small and becomes
the clitoris (Fig. 253, e) ; and the genital groove remains open.
The inner genital folds (Fig. 253, ?i), at the sides of the urino-
genital opening, become the labia minora or nymphae ; while the
outer genital, or labio-scrotal folds, hi, become the labia majora,
and, in front, the mons Veneris.

598 THE III .MAN EMBEYO.

The urino-genital canal shortens considerably in the female,
so as to bring the aperture of the urethra close to the surface.

The above changes are usually completed in both sexes by
the end of the third month ; but they may be delayed until a
much later date.

THE FCETAL MEMBRANES AND THE PLACENTA.

1 . The amnion is the thin transparent membrane which
invests the embryo like a sac, covering its dorsal surface and
sides (cf. Fig. 197, an).-^M?^.

The mode of formation of the amnion in the human embryo

has not yet been determined. In Reichert's ovum, estimated

to be twelve or thirteen days old, there was no trace of an

[\> VV') amnion present (Figs. 172 and 173); while in the embryos

tt ^TI'E and SR (Figs. 176, 178, 179), which are believed to be of the

thirteenth day, the amnion is already fully formed. Figs. 18G

|v.u|$qto 188 show the mode in which the development of the amnion

is believed to occur, by growth backwards of a fold of the wall of

the blastodermic vesicle over the embryo ; but the figures are

purely hypothetical, and the intermediate stages which they

represent have not been seen.

Of the two layers of which the^ amnion consists, the outer
one (cf Fig. 188) is simply a part of the wall of the blastodermic
vesicle, and it is usual to limit the term amnion to the inner
layer, which more immediately invests the embryo. The space
between this inner layer, or amnion, and the embryo is spoken
of as the amnionic cavity, and is filled with fluid.

The rate of growth of the amnion, as compared with that of
the embryo itself, varies considerably at different periods of
development. On its first formation, about the thirteenth day,
the amnion invests the embryo fairly closely (Fig. 1 79). Dur-
ing the third week the amnion grows rather more rapidly, so
that the space between it and the embryo enlarges somewhat
(Fig. 196). During the fourth week, as at the corresponding-
stages in the rabbit or chick, the embryo grows considerably,
and at the end of the week the amnion invests it very closely.

During the second month the amnion enlarges much more
rapidly, and the amnionic cavity becomes a space of consider-
able size, filled by the liquor amnii (Fig. 254). Owing to this

THE AMNION. 599

increase in its dimensions, the amnion forms a sheath around
the umbilical cord, and also comes into close contact with the
wall of the blastodermic vesicle over the whole extent of its inner
surface.

The liquor amnii, which occupies the amnionic cavity, between
the amnion and the embryo, varies much in quantity at different
periods of gestation. It is apparently most abundant about the
fifth or sixth month. Its actual quantity is difficult to fix, as it
varies greatly in different cases ; when in excess, i.e. more than
about 1J litre, it constitutes the affection known as hydrops
amnii.

The liquor amnii contains urea, especially during the later
months of gestation ; this appears to be a true excretory pro-
duct, separated by the kidneys of the foetus, and discharged
through the urino-genital aperture into the amnionic cavity.

Structurally, the human amnion consists, like that of the
rabbit or chick, of a single layer of epiblast cells, supported on a
thin layer of mesoblast (cf. Figs. 182 Jo 184). The mesoblast
consists of a homogeneous matrix, with embedded cells ; while
the epiblastic epithelium is, according to Minot, noteworthy on
account of the distinctness with which the intercellular bridges
of protoplasm, connecting the several cells with one another, can
be made out; the boundaries between adjacent cells being
formed, not by divisional planes, but by lines of vacuoles,
between which the protoplasmic bodies of the cells are directly
continuous with one another.

2. The Umbilical Cord.

The umbilical cord, which connects the embryo with the
placenta (Fig. §54), is formed in the first instance by the
allantoic stalk (Fig. 179, 197, tz). This stalk, in the human
embryo, is directly continuous, from the first, with both the
embryo and the wall of the blastodermic vesicle (Figs. 186 to
188), and has the appearance of a direct prolongation back-
wards of the hinder end of the embryo. On the formation of
the tail, the allantoic stalk is gradually driven down to the
ventral surface of the embryo (cf. Figs. 179, 197, 198, tl and
tz), and acquires the position and relations characteristic of
it in rabbit or chick embryos at corresponding periods.

The chief purpose of the allantoic stalk is to afford a path,

600

THE HUMAN EMBRYO.

along which the allantoic vessels can pass from the embryo to
the placenta (Fig. 198, aa, va) ; and the explanation of the pre-

Fig. 254.— A diagrammatic section of the pregnant Human Uterus at the
seventh or eighth week. (From Quain's ' Anatomy,' after Allen Thomson.)

al, allantoic stalk, am, true amnion ; the part shaded horizontally, between the
amnion and the embryo, is the amnionic cavity, c, cavity of the uterus, c', plug of
mucus in the cervix uteri, ch, chorion, dr, decidua reflexa. ds, decidua serotina. r/i>,
decidua vera, i, intestine of embryo, u, allantoic arteries, y, yolk-sac. y', yolk-stalk.

cocious appearance of the allantoic stalk, and probably also of
the peculiarities of its development, in human embryos, is to
be found in the importance of establishing vascular relations

THE UMBILICAL COED. 601

between the embryo and the mother at as early a period as
possible.

In the later stages of development, the yolk-stalk, or pedicle
of the yolk-sac (Fig. 254, y'), becomes closely applied to the
allantoic stalk, and bound up with it in a sheath formed by the
spreading amnion, am ; and it is to the compound structure
formed of these elements that the name umbilical cord is given.

The umbilical cord increases considerably in length during
development. About the middle of gestation it is usually from
13 to 21 cm. long, and from 9 to 11 mm. thick. At the time of
birth, its average length is from 48 to 60 cm., and its thickness
11 to 13 mm. ; but it is liable to very great individual varia-
tions. It may be as short as 12 cm.; or, on the other hand,
may attain a length of 167 cm.

The umbilical cord is almost invariably twisted spirally on
itself, and the cause of this twisting, which commences about
the middle of the second month, has been the subject of much
discussion. If examined more closely, it is found that all the
constituents of the cord are not twisted to the same extent ; the
spirals described by the allantoic arteries being always more
numerous, and closer together, than those of the whole cord, or
than those of the veins round which the arteries appear to twist.
The twisting appears to be due to the allantoic arteries increas-
ing in length more rapidly than the other constituents of the
cord, and so being compelled to adopt a tortuous instead of a
straight course. The allantoic arteries may describe as many as
thirty or forty complete turns in passing from the foetus to the
placenta.

As the spiral growth involves the whole umbilical cord, and
this cord is fixed at its placental end, it is clear that, as the cord
twists, the embryo must rotate in the liquor amnii. The cord
may become twisted round the neck of the foetus, and may
even be tied in knots : these knots being produced by the
cord, at an early stage of development, becoming thrown into a
loop, and the embryo then floating through the loop.

Structure of the umbilical cord. The fully developed umbilical
cord consists of the following structures (Fig. 254) : —

1. ^e sheath formed around it by the amnion. This invests
the cord very closely, except at its insertion into the placenta.

G02 THE HUMAN EMBRYO.

2. The right and left allantoic arteries, u. These are usually
quite distinct from each other along the greater part of the
length of the cord, but just before reaching the placenta are
almost invariably united by an anastomotic branch.

L>. The allantoic vein. This has thinner walls than the
arteries, and has also, according to Kolliker, rudimentary valves.
There are at first two allantoic veins, but the right one is, almost
from the first, smaller than the left, and disappears completely
about the fourth week.

4. The epithelial lining of the allantoic cavity. During the
first and early part of the second month, the allantoic stalk is
hollow, its cavity extending from the cloaca of the foetus along
the whole length of the cord, as far as the wall of the uterus.
Later on, in the third or fourth month, the cavity becomes con-
stricted, or altogether obliterated. Isolated portions of it may,
however, persist, especially at the proximal or foetal end of the
cord, up to the time of birth.

5. The yolk-stalk and its vessels, the vitelline arteries and
veins. These usually disappear during development, and are
seldom to be distinguished in the cord at full time. The yolk-
stalk at first lies in a groove in the allantoic stalk ; but it soon
becomes completely surrounded by this latter, and then ceases to
be distinguishable.

6. TJxe_Whartonian jellv : this forms the matrix of the cord,
in which are embedded the various structures described above.
It consists of a complex network of branching connective-tissue
cells, embedded in a clear gelatinous matrix. Immediately
beneath the surface epithelium, and around the blood-vessels
and the allantoic cavity, the connective-tissue meshwork is rather
denser than elsewhere; and, in the matrix, fibres are developed,
especially during the later months of gestation.

7. Up to the close of the third month, the end of the umbilical
cord next the embryo contains, as already noticed, a loop of the
intestine (Fig. 254, ■£), but after this date the alimentary canal is,
as a rule, completely withdrawn into the body of the foetus.

8. The Chorion.

The term chorion has been used in very different senses by
writers on embryology. It is convenient to employ it for that part
of the blastoderm, or blastodermic vesicle, which is not directly

THE UMBILICAL COED AND THE CHORION. 603

concerned in the formation of the embryo. It is usual to exclude
the amnion from the definition ; but in the case of the human
embryo the outer layer of the amnion, or ' false amnion ' as it is
commonly called in other Vertebrates, is so directly continuous
with the wall of the vesicle that it is better to include it under
the same name. u , ^7 2->

Thus in Reichert's ovum (Fig. 174) the chorion is the whole
wall of the vesicle, except the embryonic area, a. In His'
embryo E (Fig. 188), the chorion forms the entire wall of the
vesicle, the embryo being now depressed within its cavity.

It is the chorion which comes in contact with the walls of
the uterus (Figs. 254, 255), and it i^fconL-tha chorion that the
foetal ..part of the placenta is developed.

The human chorion is remarkable for its very early and
complete separation from the yolk-sac (Figs. 186% 188); and
also for the very early period at which villi are developed from
its outer surface.

Structurally, the chorion consists of an outer layer of epiblast,
which from the first is two cells thick ; and an inner and thicker
layer of mesoblast, which very early becomes vascular, the blood-
vessels being derived from the allantoic arteries and veins,
which reach the chorion along the allantoic stalk, and which
are, of course, directly continuous with the blood-vessels of the
embryo.

In Reichert's ovum (Figs. 172, 173, 174), the villi are con-
fined to a broad margiual zone round the equator, the centres of
the two flattened surfaces forming bare patches. At a very
slightly later period, in His' embryo E, and in others of about the
thirteenth day (Figs. 175, 188), the villi cover the entire surface
of the chorion.

The chorionic villi consist at first entirely of epiblast. They
arise as solid buds of epiblast, which become hollow as they in-
crease in size ; and at a later stage the mesoblast grows into
them along their axes, carrying the blood-vessels with it. During
the fourth week the villi grow actively ; they branch freely, and
in a very irregular manner. They penetrate the decidua, or
modified mucous membrane of the uterus, to a slight depth ; but
dn not, as was formerly believed to be the case, grow into the
uterine glands. They become attached to the decidua at their
1 [pa, but remain free along the rest of their length. As in their

604 THE HUMAN EMBRYO.

first appearance, so also during the later stages of their growth,
the epithelial layer, or epiblast, is always in advance of the
raesoblastic connective-tissue core ; the villi presenting lateral
processes, or knobs, caused by local thickenings of the epithe-
lium, into which at a later stage the vascular connective tissue
penetrates.

The villi are at first of uniform size over the whole surface of
the chorion (Fig. 188) ; but towards the end of the second month,
or early in the third, they begin to develop unequally. Opposite
the decidua serotina, or part of the uterine wall to which the
ovum is directly attached (Fig.f 254, ds), the villi increase
greatly in size and in complexity, forming ultimately the foetal
part of the placenta. Over the rest of the surface of the chorion,
opposite the decidua reflexa, dr, the villi, on the contrary, begin
to shrink ; the blood-vessels which supply them undergoing at
the same time a gradual diminution in size.

In this way a distinction is established between the chorion
frondosum, opposite the decidua serotina, which is very vascular,
and beset with closely placed and richly branched villi ; and the
chorion laeve, opposite the decidua reflexa, which is a thin trans-
parent membrane, with no blood-vessels, and connected with the
decidua reflexa merely by a few scattered, slightly branched, and
inconspicuous villi. By the end of the fourth month, the villi
of the chorion lasve have almost completely disappeared, except
from a narrow fringe round the margin of the placenta, where
they persist until the close of gestation.

Up to the end of the third month, the villi can be fairly
readily withdrawn from the crypts of the decidua in which they
are lodged, and the foetal and maternal structures thus separated
from each other ; but, after the placenta is definitely established,
the connection between the foetal and maternal elements becomes
so intimate that complete separation is no longer practicable.

The epithelium of the chorion frondosum undergoes impor-
tant changes during the later months of gestation. Of the two
layers of cells of which it consists from the first, the inner
or deeper layer becomes thickened in irregular patches, very
variable in number and in size; the individual cells are also
very irregular, and show signs of degenerative changes. The
outer, or surface layer of epithelium undergoes more extensive
changes. The cell boundaries become lost, and the cell bodies

THE CIIOEION AND THE DECIDUA. 605

run together to form a dense stratum, in which the nuclei remain
visible for a time ; ultimately the nuclei disappear, and the
whole layer becomes modified into a hyaline, very refractive
substance, permeated by numerous channels, so as to present
a reticular appearance, and absorbing staining reagents very
readily. This substance, formed by degeneration of the surface
epithelial cells of the chorion, has been described, before its
epithelial origin was known, as canalised fibrin.

Over the villi, the deeper or cellular layer of the epithelium
disappears in great part, persisting only in isolated patches.
The surface cells become converted in great part into a fibrin
layer, similar to that of the chorion frondosum itself.

In the chorion lasve the epithelium retains its cellular
character, and no fibrin layer is formed.

•k The Decidua.

The decidua is the mucous membrane of the pregnant
uterus. The early stages in its formation are, so far as they
are known, identical with those by which the catamenial or
menstrual decidua is formed. The mucous membrane becomes
thicker and more pulpy than in its quiescent condition ; the
blood-vessels enlarge ; the glands elongate, and their deeper ends
become tortuous and dilated ; the deeper part of the mucosa
becomes crowded with modified, and apparently proliferating
connective-tissue cells ; and the surface epithelium, lining the
uterus, together with the immediately underlying connective
tissue, show a tendency to disintegrate.

Up to this point, the formation of the catamenial decidua and
of the decidua of pregnancy appear to be identical; the sole
difference between the two is that, in the former, the processes
having reached a certain point, stop and then become retro-
gressive, the decidua being broken up and discharged, together
with a certain amount of blood, as the menstrual fluid ; while,
on the other hand, in the case of the decidua of pregnancy,
development, after reaching the point mentioned, does not stop,
but proceeds to further stages of elaboration.

The difference between the two courses seems to depend
solely on the presence of a fertilised ovum within the uterus in
the latter case, and on the absence of such an ovum in the
former; so that the catamenial decidua may be viewed as a

G06 THE HUMAN EMBRYO.

preparation on the part of the uterus for an ovum which m
reaches it; the decidua, after waiting a certain time, becoriiino-
broken up and discharged. If, however, impregnation is effected,
and a fertilised ovum reaches the uterus, a new stimulus is set
up, and the developmental processes, instead of stopping, go on
to further stages, and so give rise to the decidua of pregnancy.

Prior to the arrival of the ovum in the uterus, the decidua
forms a complete lining to the uterus^ It does not cover the
orifices of the Fallopian tubes (Fig^ z54), which remain open
throughout the greater part or the whole of pregnancy ; neither
does it extend into the cervix uteri, but stops abruptly at the
os internum. With these exceptions, the decidua forms a layer,
of approximately uniform thickness and structure, covering all
parts of the uterine wall.

It seems to be, to a great extent, a matter of chance with what
part of the uterus the ovum will come in contact, on entering its
cavity ; and it is therefore important that all parts Of the surface
should be equally prepared to receive it. In the great majority
of cases, the attachment of the ovum is in the neighbourhood of
the fundus, usually rather to one side of the median line, and
more frequently on the dorsal than the ventral surface. It
may, however, be situated in almost any part of the uterus ; and
its position may become a point of much practical importance.
Ercolani has suggested that the ovum, on entering the uterus,
is prevented from at once sinking to the cervix, by the fluid
secreted by the utricular glands of the uterus, and that it floats
on the surface of this fluid, until it comes in contact with, and
adheres to, the wall of the uterus ; the actual place of contact
would in this case vary considerably, according to the amount of
fluid in the uterus at the time.

The youngest ovum yet found in situ within the human
uterus, that described by Reichert, was not simply attached to
the decidua, but completely embedded in this (cf. Fig. 175); a
relation which is retained throughout the whole period of gesta-
tion (Fig. 254).

There has been some discussion as to the mode in which
this encapsuling of the ovum is brought about ; there are no
direct observations on the point in the case of human embryos,
but the fact that the openings of the uterine glands occur on
both surfaces of the encapsuling layer of the decidua, together

THE DECIDUA. 607

with the known facts in regard to other Mammals, render it
practically certain that the view first advanced by Sharpey is
correct, and that, immediately after the ovum has attached itself
to the uterine wall, the decidua grows up as a fold around and
over it, so as to encapsule it ; the object being, partly to maintain
the ovum in contact with the uterine wall ; and partly, perhaps
mainly, to provide an increased extent of vascular surface from
which the embryonic villi can draw nutriment.

The fold of the decidua which incloses, or encapsules, the
ovum is spoken of as the decidua reflexa; it is at first very
thin (Fig. 175, dx, and Fig. 254, dr), but it has the same struc-
ture as the other parts of the decidua. In its early stages it is
exceedingly vascular, the vessels converging from its margin to
a small patch of a cicatricial appearance on its most prominent
part, which probably indicates the point of meeting, and fusion
of the folds by which it is formed.

The part of the decidua to which the ovum is directly
attached, and from which the decidua reflexa is developed, is
called the decidua serotina (Fig. 175, dw, and Fig. 254, ds) ;
while the term decidua vera is given to the whole of the rest of
the decidua, which lines the cavity of the uterus, but has no
direct relation with the. embryo (Fig. 175, DV, and Fig. 254, dv).

The decidua vera plays no part in the nourishment of the
embryo, and during the latter half of pregnancy becomes greatly
reduced in thickness, and undergoes degenerative changes ; that
it should be formed at all is due, as already noticed, to the fact
that, as it is quite uncertain with which particular part of the
uterine wall the ovum will come in contact, all parts must be
ready in the first instance to receive it.

The fact that the decidua vera, though lining the greater
part of the uterus, takes no share in the nutrition of the em-
bryo, and after attaining a certain stage, first stops, and then
undergoes retrograde development, renders the comparison be-
tween the menstrual decidua and the decidua of pregnancy a
still closer one ; it further helps to render intelligible the not
very uncommon cases in which menstruation takes place at least
once after conception has occurred ; and also those much rarer
cases in which it has been stated to occur regularly throughout
the greater part or even the whole of pregnancy.

The decidua reflexa and decidua serotina are at first of very

608

THE HUMAN EMBRYO.

sniitll extent (Fig. 175). However, as the chorionic vesicle
with it3 contained embryo increases in size, the deci'dua reflexa
necessarily grows with it. This growth is at first more rapid than

^mwi

Fig. 255. — A pregnant Human Uterus of about the twenty-fifth day. The
uterus has been cut open longitudinally from the ventral surface; the
decidua reflexa has been cut open, and the flap turned down to expose
the chorionic vesicle ; and the right ovary has been bisected to show the
large corpus luteum. The embryo that was taken from this chorionic
vesicle is shown in Fig. 199. (From Quain's ' Anatomy,' after Coste.)

d?\ decidua reflexa. 4v, decidua vera, o, cavity of the decidua reflexa, in which the
chorionic vesicle is lying, u, uterus.

THE DEGIDUA. 609

that of the uterus as a whole, and in consequence the decidua
reflexa ultimately comes in contact with the decidua vera, and so
completely obliterates the cavity of the uterus (of. Fig. 254). This
usually occurs about the sixth month : the two layers, decidua
reflexa and decidua vera, are generally described as not only
coming in contact, but as fusing more or less completely together,
so as to form a single membrane ; but according to Minot's
observations, the decidua reflexa, which early undergoes degene-
rative changes, is entirely absorbed by the sixth month, so that
the chorion comes into contact with the decidua vera.

So long as the uterine cavity remains an actual one, i.e. up
to the time when the chorion meets with the decidua vera, there
remains an open passage from the vagina, through the uterus and
along the Fallopian tube, to the ovary ; and it is, at least theo-
retically, possible for spermatozoa to reach the ovary, and for
what is termed super-fcetation to occur.

The decidua serotina is simply the part of the decidua with
which the impregnated ovum comes in contact, on entering the
uterus, and to which it adheres ; and it is at first, therefore,
identical in structure with the decidua vera. It very early,
however, acquires special characters, owing to the chorionic villi
of the ovum becoming intimately connected with it. For some
time longer, the decidua serotina and decidua reflexa still remain
closely similar to each other ; but towards the end of the second
month (Fig. 254), the chorionic villi opposite the decidua re-
flexa begin to diminish in size and importance, and to show
signs of degenerative changes ; while those in connection with
the decidua serotina become much larger and more complicated.
The relations between the fcetal villi and the maternal tissues
become still more intricate, and gradually the complex and
elaborate structure of the fully formed placenta is acquired.

With regard to the detailed changes that take place in the
different parts of the decidua during pregnancy, our knowledge
is still imperfect in many important respects.

In the region lined by the decidua vera, the mucous mem-
brane becomes greatly thickened ; and the uterine glands be-
come dilated and elongated, and acquire exceedingly tortuous
courses. By the end of the fifth month the mucous membrane

R R

f)10 THE HUMAN EMBRYO.

is nearly half an inch thick. The surface layer, about one-fourth
of the entire thickness, is spoken of as the stratum compactum :
in it the gland tubes remain comparatively straight and narrow,
while in the interglandular tissue numbers of large, epithelium-
like^ decidual cells appear, apparently formed by modification
of connective-tissue corpuscles. The deeper three-fourths of
the thickness of the mucous membrane, or stratum spongiosum,
has a different structure, the gland tubes being greatly dilated
and very irregular in shape, and their lining epithelium consist-
ing of flattened or cubical, in place of columnar cells.

After the fifth month, by which time the chorion has met
with the decidua vera so as to obliterate the cavity of the uterus,
the decidua vera gradually becomes thinner and less vascular,
and undergoes degenerative changes, leading ultimately to the
almost complete disappearance of the glands, with the exception
of their deepest or outermost ends.

The decidua reflexa goes through changes of a very similar
kind : the glands first become dilated and lengthened, and then,
as the decidua reflexa becomes more and more distended through
the enlargement of the chorionic vesicle, the glands gradually
atrophy, and the entire layer degenerates, and ultimately com-
pletely disappears.

In the decidua serotina, from which the maternal portion ot
the placenta is formed, the changes have been followed in more
detail, but are still only imperfectly known. There is the same
division into two layers as in the decidua vera : (i) an inner or
superficial layer, from which the surface epithelium and all traces
of the uterine glands disappear entirely, and in which decidual
cells are developed in large numbers ; and (ii) an outer or deeper
layer, in which the gland cavities remain as irregular clefts,
from which the epithelium has disappeared ; except in the very
outermost layer, in immediate contact with the muscular wall
of the uterus, where the outer or blind ends of the glands
persist, greatly compressed, but retaining their epithelium.
It is from these outer ends of the glands that the epithelial
lining of the uterus is regenerated, after the separation of the
placenta.

The further changes that occur in the decidua serotina, and
more especially the relations of the blood-vessels, are described
in the next section.

THE PLACENTA. 611

5. The Placenta.

The fully formed placenta, at the close of gestation, is a dis-
coidal or cake-shaped body, of spongy consistency, measuring
from 16 to 21 cm. in diameter and 3 to 4 cm. in thickness. It
is attached to, or rather forms part of, the inner wall of the
uterus (cf. Fig. 254), and to its inner or free surface, usually a
little distance from its centre, the umbilical cord is attached, the
opposite end of the cord being connected with the foetus.

The placenta consists of outer or maternal, and inner or
foetal layers, derived respectively from the decidua serotina, and
from the chorion. The distinction between the foetal and the
maternal elements is easily made in the early stages of develop-
ment ; but in the fully formed placenta, owing to the intricacy
of the relations between the chorionic villi and the maternal
blood-vessels, and the profound histological modifications which
almost all parts undergo, it becomes a matter of the greatest
difficulty to determine the real nature of the several structures
met with ; and there are important points, more especially in
regard to the relations of the maternal blood-vessels, on which
our knowledge still remains imperfect and unsatisfactory.

The placenta consists of three chief layers or strata : (i) an
inner layer formed by the chorion ; (ii) an outer layer formed
by the decidua serotina, or modified mucous membrane of the
uterus ; (iii) a middle or intermediate layer, which is much
thicker than the other two, forming four-fifths or more of the
entire thickness of the placenta, and which consists of the intri-
cately branched foetal villi, together with the maternal sinuses
with which these are in relation.

Of these three layers, the inner one is distinctly foetal, and
the outer one maternal in origin ; the middle layer is foetal as
regards the villi themselves, but the precise relations of the
maternal vessels are still undecided. This middle, or villous
zone is the characteristic, and the functionally active part of the
placenta. At the margin of the placenta it thins out and dis-
appears, and the inner and outer layers, chorionic and decidual,
come into contact with each other in the region of the decidua
reflexa.

The inner or chorionic layer of the placenta has already been
described (pp. (301 to 605). The amnion is closely united with its
inner or free surface over its whole extent. The outer surface,

R B 2

612 THE HUMAN EMBRYO.

1 < wards the villous zone, is characterised by the presence of patches
of the substance spoken of as canalised fibrin, and shown by
Minot to be produced by a peculiar mode of degeneration of
the surface epithelial cells of the chorion. Kound the margin of
the placenta, the chorionic and decidual layers are so intimately
fused that it is impossible to make out a boundary line between
them : the decidual cells invade the chorion, and the two layers
undergo degenerative changes in common.

The outer or decidual layer of the placenta, formed by the
decidua serotina (Fig. 254, ds), is about 1*5 mm. thick, and
shows the same distinction into inner or compact, and outer or
spongy layers already noticed in the decidua vera. Both layers
contain very numerous decidual cells, formed, as elsewhere, by
modification of connective-tissue cells. The decidual cells of the
compact layer are smaller and more crowded than those of the
spongy layer. Their actual size varies very greatly, and the
largest ones may contain as many as ten nuclei.

The uterine glands have disappeared from the compact layer
of the decidua, but their outer or blind ends persist in the
spongy layer as irregular slit-like cavities, filled for the most
part with fine granular matter, and retaining in- places their
lining of glandular epithelium. The outer surface of the spongy
layer is closely united with the muscular wall of the uterus ; and
groups of decidual cells may penetrate between the muscle fibres
at places.

The middle or villous zone of the placenta is much the
thickest of the three, and is also the most important, and the
most complicated. It may be described, roughly, as a huge
sinus filled with maternal blood, and divided up into a labyrinth
of intercommunicating loculi by a framework of fibrous bands
and partitions, which run across between the decidual and cho-
rionic walls ; the loculi being occupied by forests of arborescent
villi, which arise from the chorionic wall and are richly supplied
with capillaries derived from the allantoic vessels of the foetus.

The villi branch extraordinarily freely : their stems arise
from the chorion, and the majority of the branches end freely,
but many are attached either to the uterine decidua, or to the
partitions separating the loculi.

The foetal blood-vessels. The blood is carried from the foetus
to the placenta by the two allantoic arteries, which run in the

THE PLACENTA. 613

substance of the umbilical cord. On reaching the placenta the
arteries branch freely, and very irregularly ; the branches spread
over the surface of the placenta, between the amnion and the
chorion, and dip rather suddenly into the substance of the
placenta. Within this they branch freely, and approach the
uterine surface of the placenta by a series of terrace-like steps
spreading out horizontally, and then dipping suddenly towards
the surface, two or three times in succession ; ultimately they
enter the villi, and follow their branches to their finest rami-
fications.

The capillaries of the villi, into which the arteries finally
pass, are variable in diameter, and exhibit irregular dilatations
and constrictions ; their average size is, however, very large, as
they frequently admit from four to six red-blood corpuscles
abreast.

The capillaries unite at their further ends to form veins,
which follow generally the same paths as the arteries, and finally
leave the placenta as the allantoic vein, which, running along
the umbilical cord,- returns the blood from the placenta to the
foetus.

The foetal vessels thus form a closed set of blood-vessels,
which have proper walls of their own, of the usual structure,
along their entire course, and which show no special pecu-
liarities except the large size of the capillaries in the villi.

The maternal blood-vessels are derived directly from the
uterine arteries and veins. The arteries, which are of com-
paratively small size, run with a very tortuous course, from
which their name ' curling arteries ' is derived, through both the
spongy and compact layers of the decidua, and open suddenly
into the great sinuses or loculi of the placenta. From these
sinuses the blood is returned by veins, which run very obliquely
through the decidua, and eventually join the veins of the muscular
wall of the uterus.

The foetal villi are thus bathed by a slowly moving stream
of maternal blood ; and the necessary nutritive and respiratory
interchanges must be effected in this middle or villous zone of
the placenta. Nothing is known as to the exact mode in which
these are carried on ; whether by simple diffusion, or whether
aided or modified by active participation of epithelial or other
cellular elements. The one thing that is quite clear is, that in the

614 THE HUMAN EMBRYO.

pl;icfiif;i the fcctal and maternal blood streams are kept apart,
and that no actual mixing of blood from the two sources can
occur.

The real nature of the sinuses, or loculi, in which the maternal
blood lies has been much discussed, and is not yet determined
with certainty.

It was formerly held that they are produced by enormous
dilatation of the capillaries, which normally connect the uterine
arteries and veins together ; and Waldeyer has shown that these
sinuses have a distinct epithelial lining, continuous with that of
the uterine vessels.

Kolliker and Langhans point out that, on the fcetal side of the
placenta, the walls of the sinuses are formed by the chorion, and
show no trace of decidual structure ; they, therefore, suggest that
the sinuses are not really maternal capillaries, but are spaces
between the maternal and foetal portions of the placenta, i.e.
between the decidua and the chorion, into which the blood has
penetrated by extravasation, or by rupture of the uterine vessels,
consequent on the degeneration which the uterine mucous
membrane is known to undergo.

On the other hand, a comparison of what is known concerning
the human uterus, with the facts ascertained in regard to the
formation of the placenta in the rabbit and in other Mammals,
leads to the belief that the sinuses may prove to be spaces formed
by absorption, not within the maternal tissues, but in the
chorionic epithelium itself; in which case the whole thickness
of the villous zone of the placenta would be of fcetal origin.

Separation of the Placenta at Birth.

In parturition, the contraction of the muscular walls of the
uterus, and the consequent pressure on the uterine contents,
more especially upon the amnionic fluid, causes the investing
membranes of the embryo, i.e. the combined decidua vera and
decidua reflexa, the chorion, and the amnion, to bulge through
the os uteri. On rupture of the membranes, the amnionic fluid
first escapes, and subsequently the foetus is expelled.

The further contraction of the uterus detaches the placenta
from the uterine wall, the plane of separation passing through
the outer or spongy layer of the decidua, in which the deeper
parts of the uterine glands still persist ; and the placenta, with

THE PLACENTA. 615

the decidua, the chorion, and the amnion forming a membranous
fringe around it, is in its turn expelled as the after-birth. The
continuation of the uterine contraction, after the expulsion of
the placenta, checks, and in normal cases reduces to a minimum,
the haemorrhage that necessarily results from the tearing across
of the maternal vessels along the plane of separation of the
placenta.

The deepest part of the spongy layer of the decidua, in
which are the blind ends of the uterine glands, remains in the
uterus as a thin lining to the muscular walls ; and, from the
epithelium of these persistent parts of the glands, the entire
uterine epithelium is speedily regenerated.

List of the more important Publications dealing with, or bearing
on the Develoj)ment of Man.

Ackeren, F. v. : ' Beitriige zur Entwicklungsgeschichte der weiblichen Sexual-

organe des Menschen.' Zeitsohrif t fiir wissenschaftliche Zoologie, xlviii.

1889.
Allen, F. J. : 'On the Cause of the Twisting of the Umbilical Cord, illustrated

by Mechanical Models.' Journal of Anatomy and Physiology, xxvi.

1892.
Born, G. : ' Ueber die Derivate der embryonalen Schlundbogen und Schlund-

spalten bei Saugethieren.' Archiv fiir mikroskopische Anatomie, xxii.

1883.
Bowen, J. T. : 'The Epitrichial Layer of the Human Epidermis.' Anatomischer

Anzeiger, iv. 1889.
Broca, A : * Contribution a l'Etude du Developpement de la Face.' Annales de

Gynecologie, xxviii. 1887.
Chiarugi, G. : ' Anatomie d'un Embryon Humain de la Longueur de mm. 2-6

en ligne droite.' Archives Italiennes de Biologie, xii.
Coste, M. : ' Histoire Gen^rale et Particuliere du Developpement des Corps

Organises.' 1847-1859.
Cunningham, D. J. : ' The Complete Fissures of the Human Cerebrum, and

their Significance in connection with the Growth of the Hemisphere

and the Appearance of the Occipital Lobe.' Journal of Anatomy and

Physiology, xxiv. 1890.
Darwin, C. : ' The Descent of Man.' 2nd edition. 1874.
Dursy, E. : ■ Zur Entwickelungsgeschichte des Kopfes des Menschen und der

hoheren Wirbelthiere.' 1869.
Ecker, A. : ' Icones Physiological 1851-59.
Ercolani, G. B. : ' Nuove Ricerche di Anatomia Normale e Patologica sulla

Placenta dei Mammiferi e della donna.' Mem. d. Accademia d. Scienz.

d. Bologna. 1883.
Erdl, M. P. : 'Die Entwickelung der Leibesform des Menschen.' 1846.

G16 THE HUMAN EMBRYO.

Farre, A.: Article 'Uterus and its Appendages.' Todd's Cyclopaedia of

Anatomy and Physiology, v. 1858.
Fol, H. : ' Description d'un Embryon Humain de cinq millimetres et six

dixiemes.' Recueil Zool. Suisse. 1884.
Froriep, A. : ' Ueber ein Ganglion des Hypoglossus und Wirbelanlagen in der
Occipitalregion.' Archiv fiir Anatomie und Physiologie : Anatomische
Abtheilung. 1882.
Gottschalk: 'Ein Uterus gravidus aus der fiinften Woche der Lebenden
entnommen.' Archiv fiir Gynakologie, xxix. 1887.

' Beitrag zur Entwickelungsgeschichte der menschlichen Placenta.'
Archiv fiir Gynakologie, xxxvii. 1890.
Heinz : • Bau und Entwicklung der menschlichen Placenta.' Archiv fiir

Gynakologie, xxxiii. 1888.
Hensen, V. : ' Die Physiologie der Zeugung.' Hermann's Handbuch der
Physiologie, vi. 1881.

Beitrag zur Morphologie der Korperform und des Gehirns des

menschlichen Embryos.' Archiv fiir Anatomie und Physiologie. 1877.

Hertwig, 0. : ' Lehrbuch der Entwicklungsgeschichte des Menschen und der

Wirbelthiere.' Dritte Auflage. 1 890.
His, W. : ' Anatomie menschlicher Embryonen.' 1880-85.

1 Mittheilungen zur Embryologie der Siiugethiere und des Menschen.'
Archiv fiir Anatomie und Physiologie : Anatomische Abtheilung. 1881.

' Ueber das Auf treten der weissen Substanz und der Wurzelfasern
am Ruckenmark menschlicher Embryonen.' Archiv fiir Anatomie und
Physiologie : Anatomische Abtheilung. 1883.

' Zur Geschichte des menschlichen Ruckenmarkes und der Nerven-
wurzeln.' Abhandlungen der mathematisch-physischen Classe der
konigl. sachsischen Gesellschaf t der Wissenschaf ten.' Bd. xiii. No. 6.
1886.

■ Ueber den Sinus prascervicalis und iiber die Thymusanlage.'
Archiv fiir Anatomie und Entwickelungsgeschichte. 1 886.

' Zur Bildungsgeschichte der Lungen beim menschlichen Embryo.'
Archiv fiir Anatomie und Entwickelungsgeschichte. 1887.

' Die Entwickelung der ersten Nervenbahnen beim menschlichen
Embryo.' Archiv fiir Anatomie und Entwickelungsgeschichte. 1887.

' Zur Geschichte des Gehirns sowie der centralen und peripherischen
Nervenbahnen beim menschlichen Embryo.' Abhandlungen der mathe-
matisch-physischen Classe der konigl. sachsischen Gesellschaft der
Wissenschaften. Bd. xiv., No. 7. 1888.

' Ueber die embryonale Entwickelung der Nervenbahnen.' Anato-
mischer Anzeiger, iii. 1888.

' Schlundspalten und Thymusanlage.' Archiv fiir Anatomie und
Entwickelungsgeschichte. 1889.

1 Die Neuroblasten und deren Entstehung im embryonal en Mark.'
Archiv fiir Anatomie und Entwickelungsgeschichte. 1889.

* Zur Anatomie des Ohrlappchens.' Archiv fiir Anatomie und
Entwickelungsgeschichte. 1889.

1 Die Formentwickelung des menschlichen Vorderhirns vom Ende
des ersten bis zum Beginn des dritten Monats.' Abhandlungen der
mathematisch-physischen Classe der konigl. sachsischen Gesellschaft
der Wissenschaften. Bd. xv. No. 8. 1889.

BIBLIOGKAPHY. 617

' Histogenese und Zusammenhang der Nervenelemente.' Archiv fiir
Anatomie und Entwickelungsgeschichte.' Supplement-Band. 1890.

' Der Tractus thyreoglossus und seine Beziehungen zum Zungen-
bein.' Archiv fiir Anatomie und Entwickelungsgeschichte. 1891.

1 Die Entwicklung der menschlichen und thierischen Physiognomien.'

Archiv fiir Anatomie und Entwickelungsgeschichte. 1892.

His, W., junior : ' Zur Entwickelungsgeschichte des Acustico-Facialgebietes

beim Menschen.' Archiv fiir Anatomie und Entwickelungsgeschichte,

Supplement-Band. 1889.

Janosik, J. : ' Zwei junge menschliche Embryonen.' Archiv fiir mikroskopische

Anatomie, xxx. 1887.
Kanthack, A. A. : ' The Thyreo-Glossal Duct.' Journal of Anatomy and

Physiology, xxv. 1891.
Kastschenko, N. : 'Das menschliche Chorionepithel und dessen Eolle bei den
Placenta.' Archiv fiir Anatomie und Entwickelungsgeschichte. 1885.
Keibel, F. : ' Zur Entwicklungsgeschichte der menschlichen Placenta.' Anato-
mischer Anzeiger, iv. 1889.

1 Em sehr junges menschliches Ei.' Archiv fiir Anatomie und
Entwickelungsgeschichte. 1890.

• Ein menschlicher Embryo mit scheinbar blascherjformiger Allan-
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' Ueber den Schwanz des menschlichen Embryo.' Archiv fiir Anatomie

und Entwickelungsgeschichte. 1891.
Klaatsch, H. : ' Zur Morphologie der Mesenterialbildungen am Darmkanal der

Wirbelthiere, ii. Theil, Saugethiere.' Morphologisches Jahrbuch, xviii.

1892.
Kolliker, A : ' Entwicklungsgeschichte des Menschen und der hoheren

Thiere.' Zweite Auflage. 1879.

' Zur Entwickelung des Auges und Geruchsorganes menschlicher

Embryonen.' Festschrift der Schweizer Universitat Zurich gewid-

met. 1883.
Kollmann, J. : • Die menschlichen Eier von 6 Mm. Grosse.' Archiv fiir Anatomie

und Physiologie : Anatomische Abtheilung. 1879.

• Die Korperform menschlicher normaler und pathologischer Em-
bryonen.' Archiv fiir Anatomie und Entwickelungsgeschichte, Supple-
ment-Band. 1889.

1 Die Entwickelung der Chorda dorsalis bei dem Menschen.' Anato-

mischer Anzeiger, v. 1890.

'Die Kumpfsegmente menschlicher Embryonen von 13 bis 35

Urwirbeln.' Archiv fiir Anatomie und Entwickelungsgeschichte. 1891.
Krause, W. : ' Ueber die Allantois des Menschen.' Archiv fur Anatomie und

Physiologie, 1875 and 1876 ; and Zoologischer Anzeiger, iv. 1881.
Kundrat und Engelmann : * Untersuchungen iiber die Uterusschleimhaut.'

Strieker's Med. Jahrbuch. 1873.
Kuppfer, C. : 'Decidua und Ei des Menschen am Ende des ersten Monats.-'

Miinchener medizinische Wochenschrift, xxxv. 1888.
Langhans, Th. : 'Untersuchungen iiber die menschliche Placenta.' Archiv

fiir Anatomie und Physoiologie, 1877.

1 Ueber die Zellschicht des menschlichen Chorion.' Henle's Festgabe.

1882.
Lannelongue et Menard : ' Affections Congenitales.' 1891.

618 THE HUMAN EMBRYO.

Lenhossek, M. v.: 'Die Entwickelung der Ganglionanlagen bei dem mensch-
lichen Embryo.' Archiv fiir Anatomie und Entwickelungsgeschichte.
1891.
Leopold, G. : ' Studien iiber die Uterusschleimhaut wahrend Menstruation,
Schwangerschaft und Wochenbett.' Archiv fiir Gyniikologie, xi. xii.
1877.

1 Ueber den Bau der Placenta.' Archiv fur Gyniikologie, xxxv. 1889.

Lockwood, C. B. : ' The Development and Transition of the Testis, Normal and

Abnormal.' Journal of Anatomy and Ph) siology, xxi. and xxii. 1887

and 1888.

Mall, F. : 'A Human Embryo twenty-six days old.' Journal of Morphology, v.

1891.
Marshall, C. F. : < The Thyro-glossal Duct, or Canal of His.' Journal of

Anatomy and Physiology, xxvi. 1891.
Meuron, P. de : ' Recherches sur le Developpement du Thymus et de la Glande

Thyro'ide.' 1886.
Meyer, H. : * Die Entwicklung der Urniere beim Menschen.' Archiv fiir

mikroskopische Anatomie, xxxvi. 1890.
Minot, C. S. : ' The Early Stages of Human Development.' The New York
Medical Journal. 1885.

' Uterus and Embryo. II. Man.' Journal of Morphology, ii. 1889.
4 On the Fate of the Human Decidua Refiexa.' Anatomischer
Anzeiger, v. 1890.

Articles ' Chorion,' ' Placenta,' etc., in Buck's Reference Handbook
of the Medical Sciences.

1 Human Embryology.' 1892.
Nagel, W. : ' Das menschliche Ei.' Archiv fiir mikroskopische Anatomie, xxxi.
1888.
• Ueber den Wolff'schen Korper des menschlichen Embryo.' Zeit-
schrift fiir Geburtshiilfe und Gyniikologie. 1889.

1 Ueber die Entwicklung des Urogenitalsystercs des Menschen/
Archiv fiir mikroskopische Anatomie, xxxi v. 1889.

1 Ueber das Vorkommen von Primordialeiern ausserhalb der Keim-
driisenanlage beim Menschen.' Anatomischer Anzeiger, iv. 1889.

1 Ueber die Entwickelung des Uterus und der Vagina beim Menschen/
Archiv fiir mikroskopische Anatomie, xxxvii. 1891.

1 Ueber die Entwickelung der Urethra und des Dammes beim
Menschen.' Archiv fiir mikroskopische Anatomie, xl. 1892.
Phisalix, C. : ' Etude d'un Embryon Humain de 10 millimetres.' Archives de

Zoologie experimentale et generale. 1888.
Reichert, C. B. : ' Beschreibung einer friihzeitigen menschlichen Frucht im

blaschenformigen Bildungszustande.' 1873.
Schafer, E. A. : Quain's • Elements of Anatomy.' 10th edition, vol. i. part 1 :

Embryology. 1890.
Schlesinger, W. : • Ein menschliches Ei aus sehr fruher Zeit der Schwanger-

schaft.' Internationale klinische Rundschau, ii. 1888.
Schwalbe, G. : • Das Darwin'sche Spitzohr beim menschliches Embryo.' Anato-
mischer Anzeiger, iv. 1889.
Selenka, E. : ' Zur Entstehung der Placenta des Menschen.' Biologisches

Centralblatt, x. 1891.
Spee, F. • Beobachtungen an einer menschlichen Keimscheibe mit offener

BIBLIOGRAPHY. 619

Medullarrinne und Canalis neurentericus.' Archiv fur Anatomie und

Entwickelungsgeschichte. 1889.
Tait, L. : 'Menstruation and the Ovaries.' Lancet. 1888.
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med. Wochenschrift. 1879.

' Bau und Wachsthumsveranderungen der Gekrose des menschlichen

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* Die Darmgekrose und Netze im gesetzmiisssigen und im gesetz-

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chez le Foetus Humain dans les deux sexes, avec quelques remarques

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1' Anatomie et de la Physiologie, xxv. 1889.
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Archiv fur Anatomie und Entwickelungsgeschichte, Supplement-Band.

1890.
Viti, A. : ' L' amnios umano nella sua Genesi e Struttura ed in Rapporto

all' Origine del Liquido amniotico.' 1886.
Vrolik, W. : * Die Frucht des Menschen und der Saugethiere.' 1854.
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1 Ueber den Placentakreislauf des Menschen.' Sitzb. d. kgl. preuss.

Akad. d. Wiss. zu Berlin. 1887.

'Bemerkungen iiber den Bau der Menschen- und Aff en-Placenta,'

Archiv fur mikroskopische Anatomie, xxxv. 1890.
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1889.

INDEX

Accessory auditory apparatus : Frog, 143-144 ; Chick, 281 ; Babbit, 397-399 ;

Man, 544
Accessory genital organs : Chick, 319-320 ; Babbit, 426 ; Man, 592-596
Accessory thyroid body: Frog, 182 ; Chick, 286
Acetabulum : Frog, 213
Adolescent period : Amphioxus, 87-89
Adrenal body : Chick, 320
After-birth: Man, 615

Age of embryo, estimation of : Chick, 223 ; Babbit, 352 ; Man, 467-470
Air cell : Babbit, 408
Air sac : Chick, 292
Alas nasi : Man, 498
Alecithal eggs : 7, 1 7
Alimentary canal : Amphioxus, 69-71 ; Frog, 105-109, 145-157 ; Chick, 281-

296 ; Babbit, 399-404 ; Man, 544-565
Allantoic stalk : Man, 482, 484, 496, 599, 602
Allantois: Chick, 227, 246-247, 294-296; Babbit, 370, 435; Man, 484, 485,

545
Amnion: Chick, 245-247 ; Babbit, 369-370 ; Man, 477, 479, 496, 598-599
Amphioxus, 37-89 : structure of adult, 37-44 ; morphological importance,
44-46 ; general account of development, 46-48 ; early em-
bryonic development, 48-61 ; condition at hatching, 61 ;
later embryonic development, 61-72 ; condition at close of
embryonic period, 72-73; larval period, 73-87; adolescent
period, 87-89 ; bibliography, 89
Anatomy : Amphioxus, 37-44
Anterior cerebral vesicle : Chick, 252 ; Babbit, 374
Anterior commissure of brain : Babbit, 379 ; Man, 518
Anterior gut diverticula : Amphioxus, 70
Anterior nares : Frog, 136 ; Chick, 275, 289 ; Man, 498
Antihelix : Man, 500
Antitragus : Man, 500
Antrum : Man, 538
Anus : Amphioxus, 71 ; Frog, 149 ; Chick, 291-292 ; Babbit, 404 ; Man, 546,

549
Anus of Kusconi: Frog, 107, 110
Aorta, cardiac : Amphioxus, 41, 71

dorsal: Amphioxus, 42; Frog, 169, 179-182; Chick, 307; Babbit, 417 ;
Man, 573, 576-578
Aortic arch: Frog, 178-179; Chick, 303-307; Babbit, 416-417; Man, 573-

576
Aortic trunk: Babbit, 416

Appendicular skeleton : Frog, 212-215 ; Chick, 332-335 ; Babbit, 433
Apteria : Chick, 337
Aqueous humour : Chick, 280

622 INDEX.

Arch of aorta: Chick, 306; Rabbit, 417; Man, 576

Archenteron : 22 ; Amphioxns, 54, 60

Area opaca : Chick, 224, 2:55

Area pellucida : Chick, 224, 235-236

Area vasculosa : Chick, 224, 242

Arm : Man, 492, 496, 500

Arteries: Frog, 178-183 ; Chick, 303-307; Rabbit, 416-417; Man, 573-578

Artery, allantoic: Chick, 307; Rabbit, 417; Man, 577, 602, 612

anterior cerebral : Frog, 180

anterior commissural : Frog, 180

anterior palatine: Frog, 180

ascending pharyngeal : Man, 575

basilar : Frog, 180 ; Man, 578

carotid: Frog, 180; Chick, 305; Rabbit, 416; Man, 575

caudal : Rabbit, 417

central, of retina : Rabbit, 389

coeliac axis : Man, 578

cutaneous : Frog, 183

ductus arteriosus : Chick, 307, 314 ; Rabbit, 417; J/arc, 576, 585, 586

external carotid: Chick, 305 ; Rabbit, 416; Man, 575

external maxillary : Man, 575

hypogastric : J/icm, 577

iliac : Man, 577

internal carotid : Chick, 305 ; Rabbit, 416 ; J/aw, 574

internal maxillary : Man, 575

intervertebral : Man, 578

lingual: Frog, 180; 67u<?&, 305; Man, 575

mandibular : Chick, 305

occipital : Jffaft, 575

pharyngeal : Frog, 180

posterior auricular : Man, 575

posterior commissural : Frog, 180

pulmonary : Frog, 182 ; Cfeiai, 306 ; Rabbit, 417 ; i!/a?i, 575

subclavian : Chick, 306; Rabbit, 417 ; Jfiwt, 576, 578

temporal : Man, 575

thyroid : Frog, 180

umbilical: Chick, 307

vertebral : J/aw, 576, 577, 578
Asymmetry: Amphioxus, 47, 68, 72, 87
Atlas vertebra : Chick, 327 ; Rabbit, 430
Atrial cavity : Amphioxns, 41, 82-84
Atrial pore : Amphioxus, 41, 83

Attachment of blastodermic vesicle to uterus : Rabbit, 371, 437
Attachment of ovum to uterus : Man, 606-607

Auditory capsule : Frog, 143, 204 ; Chick, 328, 331 ; Rabbit, 396 ; Man, 543-544
Auditory organ : Frog, 139-144 ; Chick, 280-281 ; Rabbit, 393-399 ; Man,

641-544
Auditory ossicle : Frog, 144, 209 ; Chick, 281, 330 ; Rabbit, 398-399
Auditory pit: Frog, 139, 140; Chick, 280 ; Rabbit, 393; Man, 489, 541
Auditory vesicle : 'Frog, 139, 140; Chick, 280; Rabbit, 393; Man, 492, 495,

541
Auditory vestibule : Frog, 139; Rabbit, 396
Auricle, of heart : Frog,' IKS', Chick, 300-303; Rabbit, 414-415; Man, 566,

567,'568-571
Auricular septum: Frog, 168; Chick, 301 ; Rabbit, 415; Man, 568-571
Axis cylinder, of nerve': Man, 524, 526-527, 528
Axis vertebra : Chick, 327 ; Rabbit, 430

Barb : Chick, 336
Barbule : Chick, 336
Basibranchial : Chick, 331

INDEX. 623

Basihyal: Frog, 206 ; Chick, 330

Basilar membrane: Rabbit, 397

Basilar plate: C%w>&, 329 ; Rabbit, 4S2

leak : /'Vo/7, 151 ; flfciofc, 267, 289

Bibliography: Introductory chapter, 35-36 ; Amphioxus, 89; ifyo?, 216-218-

r//vr/,', 337-340; /.V////>/Y, 4 15-447: a/im, 616-619
Bile duct: i''/w/, 166 ; r///V/\ 293; Rabbit, 410; £/««, 563
Bladder: i''w/> lr>,;; ^». 590

Blastocoel : Amphioxus, 51, 52 ; 7'Vw/, 103; Chick, 234
Blastoderm: tfkfoifc, 224, 234, 236-237
Blastodermic vesicle : Rabbit, 354-356 ; Man, 474
Blastomere, 19 ; Amphioxus, 49-52 ; Frog, 102 ; Rabbit, 352-354
Blast ophore, 12
Blastopore: Amphioxus, 54-66, 59 ; 7'W>/7, 107,110-111,114-115,149; Chid,

239,212
Blastula : Amphioxus, 52 ; 7'7n><7, 103-104
Blood corpuscles: Frog, 105
Blood-vessels : Amphioxus, 41-42, 71-72 ; JV<#, 165-186; £%&?&, 296-315;

AV/ ////-//, 412-421 ; Man, 566-687
Blood-vessels of placenta: Man, 01 2-614
Body cavity : Amphioxus, 41, GO, 68, 84-86; Chick, 243-244, 321-322; Rabbit,

427-428
Bones of skull : Frog, 210 ; Chick, 331-332
Brachial plexus : Man, 527

Brain : Frog, 117-12(5 ; Chick, 252-261 ; Babbit, 372-381; Man, 509-521
Brain vesicles : Frog, 118 ; Chick, 252 ; Rabbit, 371 ; Man, 481, 491
Branchial aortic arch: Chic/,, 305 ; Rabbit, 416-417; Man, 574-576
Branchial arch: Frog, 159; Chick, 284, 330; Rabbit, 401; Man, 495, 499,

550-552
Branchial bar : Frog, 20(5-207, 208-209 ; Chick, 330, 331 ; Rabbit, 433
Branchial blood-vessels : Frog, 169-179

of first branchial arch, 171-174

of second branchial arch, 174

of third branchial arch, 174

of fourth branchial arch, 174

of hyoid arch, 175-176

of mandibular arch, 176

changes at time of metamorphosis, 176-179
Branchial cleft : Frog, 92, 157-160; Chick, 285 ; Man, 495
Branchial groove : Rabbit, 400 ; Man, 490
Branchial membrane : Rabbit. 400

Branchial pouch : Frog, 157-160; Chick, 283; Rabbit, 100 ; Man, 550-551
Bridge of nose : Man, 497
Bronchi : Rabbit, 408

Buccal cavity : Amphioxus, 40, 81 ; Frog, 150 ; Rabbit, 400-403
Buccal hood: Amphioxus, 40, 81
Buccal orifice : Amphioxus, 39
Buccal tentacles : Amphioxus, 81
Bulbus olfactorius : Man, 517
Bursa Fabricii : Chick, 291

Caecum : Man, 548

Calcarine sulcus : Man, 519

Canalis auricularis : Man, 566, 567, 570

Canalis reunions: Rabbit, 396

Canalised fibrin : Man, 605, 612

Capitulum of rib : Rabbit, 430

Capsule of lens : Chick, 277 ; Man, 539-540

Cardiac aorta : Amphioxus, 41, 71

Carotid arch : Frog, 178

Carotid gland: Frog, 181-182

624 INDEX.

Carpus : Chick, 333

Cartilage-bone : Frog, 209 ; Chick, 324, 331

Catamenial flow : Man, 458

Cavity of amnion : Chick, 246 ; Man, 598

Cauda : Man, 499

Cement : Babbit, 407 ; Man, 560

Cenogenetic characters, 28

Central canal of spinal cord : Chick, 250 ; Rabbit, 382

Central nervous system: Frog, 113-126; Chick, 247-261; Rabbit, 363, 371-
382; Man, 509-528

Centrifugal cranial nerves : Man, 528-530

Centripetal cranial nerves : Man, 528-530

Centrolecithal egg, 7, 22

Ceratobranchial : Chick, 330

Ceratohyal : Frog, 206 ; Chick, 330

Cerebellum: Frog, 122 ; Chick, 254-256; Rabbit, 380; Man, 510, 521

Cerebral hemisphere: Frog, 125-126; C%ic7«;, 260-261 ; ito&H*, 376-379; Many
512, 513, 514, 516-520

Cervical flexure- Man, 496, 510, 511

Cervical plexus : Man, 527

Cervical rib : Chick, 327

Cervix uteri : Man, 594

Chalaza, 231

Changes in circulation at birth : Rabbit, 419-420 ; Man, 586-587

Changes in circulation on hatching : Chick, 314-315

Cheek : Man, 504

Chiasma, optic: Frog, 139; Chick, 259; Rabbit, 390; Man, 514, 520, 540

Chick, 219-340 : general account, 219-228 ; the egg, 228-232 ; the early
stages of development, 232-247; the nervous system,
247-274 ; the sense organs, 274-281 ; the alimentary canal,
281-296; the heart and blood-vessels, 296-315; the urinary
organs, 315-320 ; the body cavity and the muscular system,
321-323 ; the skeleton, 324-335 ; the feathers, 335-337;
bibliography, 337-340

Chin : Man, 502

Chorion : Man, 602-605, 611

Chorion frondosum : Man, 604

Chorion lasve : Man, 604

Chorionic villi : Man, 603, 609

Choroid : Rabbit, 392 ; Man, 540

Choroidal fissure : Chick, 278 ; Rabbit, 388 ; Man, 539

Choroid plexus of fourth ventricle : Frog, 122 ; Chick, 254 ; Rabbit, 381

Choroid plexus of lateral ventricle : Rabbit, 378 ; Man, 516

Choroid plexus of third ventricle : Frog, 124 ; Chick, 258 ; Rabbit, 375-376 ;
Man, 520

Chromatin loops : 14, 15

Ciliary ganglion : Chick, 268 ; Man, 536

Circulation in yolk-sac : Rabbit, 420-421

Circulatory system : Amphioxus, 41-42, 71-72 ; Froq, 165-185 ; Chick, 296-
315 ; Rabbit, 412-421 ; Man, 565-587

Circumvallate papilla : Man, 557

Clavicle : Frog, 213 ; Chick, 332 ; Rabbit, 433

Claw : Rabbit, 434

Cleft palate : Man, 555

Clitoris : Man, 597

Clitoro-penis : Man, 596

Cloaca : Rabbit, 411-412 ; Man, 545, 549, 596

Cloacal papilla : Rabbit, 411

Cloacal septum : Rabbit, 412

Club-shaped gland : Amphioxns, 70-71, 72, 79-80

Cochlea : Frog, 141 ; Rabbit, 393, 397 ; Man, 541-544

INDEX. 625

Ccfilom: Amphioxus, 41, 60, 68, 70,84-86; Chiok, 243-244. 321-3'>2- 7?/7^7
427-428 -omuuu,

Coelomic canal : Amphioxus, 41

Coloboma iridis : Man, 540

Colossal cells : Rabbit, 444

Columella : Frog, 209 ; C%M>/fc, 330

Columnar earner : Chick, 303

Commissures of brain : Rabbit, 378-379; Man, 517-518

Commissures of spinal cord : Man, 525

Comparison of early stages in Amphioxus and in Frog, 112

Comparison of early stages in Rabbit and in Man, 475-476

Comparison of Mammalian development with that of other Vertebrates 341-

Complete segmentation, 19, 22, 353

Concrescence of lip of blastopore : Frog, 111

Condensation of ancestral history, 30

Coni vasculosi : Man, 593

Conjunctiva : Chick, 280

Convolutions of hemispheres : Rabbit, 379 ; Man, 518-520

Coracoid: Frog, 213; Chick, 332

Cornea : Chick, 280 ; Rabbit, 392 ; Man, 539-540

Coronary sinus : Man, 568, 582

Corpora adiposa : Frog, 198

Corpora quadrigemina : Rabbit, 380 ; Man, 514

Corpora striata : Chick, 260; Rabbit, 378; Man, 516-517

Corpus albicans : Man, 518

Corpus callosum : Rabbit, 379 ; Man, 518

Corpus luteum : Rabbit, 349 ; Man, 455-456

Corpus luteum spurium: Man, 456

Corpus luteum verum : Man, 456

Corpus mammillare : Man, 520

Corpus spongiosum: Rabbit, 412

Corti, organ of: Rabbit, 397; Man, 543

Coste's embryo : Man, 487-488

Course of circulation in embryo: Chick, 311-314; Rabbit, 418-421; Man,

583-586
Cranial flexure: Frog, 118, 120; Chick, 260; Rabbit, 372; Man, 489
Cranial nerves : Frog, 128-134; Chick, 262-271; Rabbit, 382-385; Man, 528-

538
Cranium : Frog, 203-205 ; Chick, 328-330
Critical stage : Amphioxus, 47, 75

Crura cerebri : Frog, 123; Chick, 256; Rabbit, 380; Man, 521
Cutaneous sense organs : Frog, 144-145
Cutis layer : Amphioxus, 85

Decidua of menstruation : Man. 458, 463-465

Decidua of pregnancy : Man, 605-610

Decidua reflexa : Man, 607-609, 610

Decidua serotina: Man, 473, 607-609, 610, 611, 612

Decidua vera : Man, 607, 609-610

Decidual cells : Rabbit, 440; Alan, 612

Deciduous dentition : Rabbit, 404 ; Man, 561

Degeneration : Amphioxus, 45

Dental papilla: Frog, 211 ; Rabbit, 406; Man, 559-561

Dentine : Rabbit, 406-407 ; Man, 560

Descent of testis : Rabbit, 426 ; Man, 507, 508, 509

Deutoplasm : 4

Development: general account of, 1-3, 24-34 ; Amphioxus, 37-89; Frog, 90-

218; Chick, 219-340; Rabbit, 341-447 ; Man, 448-619
Diaphragm : Rabbit, 428

S S

626 INDEX.

Digestive system: Amphioxus, 69-71; Frog, 105-109, 145-157; Chick, 281-

296 ; Rabbit, 399-412 ; Man, 544-565
Discoidal segmentation : 22
Discus proligerus : Rabbit, 349 ; Man, 454
Distortion of ancestral characters : 30
Dorsal fissure of spinal cord : Chick, 250 ; Man, 527
Dorsal roots of spinal nerves: Amphioxus, 43; Frog, 127-128; Chick, 271-

273 ; Rabbit, 385-386 ; Man, 525-528
Down feathers : Chick, 335
Duct of Gaertner: Man, 596

Ductus arteriosus: Chick, 307, 314; Rabbit, 417, 419; Man, 576, 585, 586
Ductus Botalli : Chick, 307 ; Rabbit, 417, 419
Ductus endolympbaticus : Frog, 142

Ductus venosus : Chick, 310, 314 ; Rabbit, 419; Man, 581, 585, 586, 587
Duodenal loop of intestine : Chick, 290 ; Rabbit, 403 ; Man, 547
Dura mater : Rabbit, 381
Duration of development : Amphioxus, 48 ; Frog, 91-94 ; Chick, 221 ; Rabbit,

350 ; Man, 449, 465-467
Duration of pregnancy : Man, 465-467

Ear : Frog, 139-144 ; Chick, 280-281 ; Rabbit, 393-399 ; Man, 541-544
Early stages of development : Amphioxus, 48-61 : Frog, 100-112 ; Chick, 232-

247 ; Rabbit, 351-362 ; Man, 470-486
Ectoplacenta : Rabbit, 438
Ectoplacental lobule : Rabbit, 441
Egg: structure, 3-7; maturation, 7-11; fertilisation, 11-13; unicellular

character, 32
Egg: Amphioxus, 43-44, 49; Frog, 94-100; Ben, 220-222, 228-232; Rabbit,

344-351 ; Man, 448-455
Egg column : Man, 450-451
Egg shell : Hen, 220, 229
Eighth month : Man, 508

Embryo, formation of : Clack, 222-228 ; Rabbit, 362-367 ; Man, 472-486
Embryonal area : Rabbit, 357-361 ; Man, 473
Embryonic period : AmpMoxus, 48-73
Enamel: Frog, 211 ; Rabbit, 406 ; Man, 560
Enamel organ : Rabbit, 406 ; Man, 559-561
Endostyle : Amphioxus, 40, 79
Enterocoelic cavity : Amphioxus, 60, 243
Epiblast : 22, 23-24 ; Amphioxus, 53, 56 ; Frog, 104-105 ; Chick, 234, 236 ;

Rabbit, 358, 361 ; Man, 475, 486
Epiblastic buds of yolk-sac : Rabbit, 369
Epibranchial groove : Amphioxus, 40
Epicoracoid: Frog, 213
Epidermic layer of epiblast : Frog, 105, 113
Epididymis : Chick, 320 ; Rabbit, 426 ; Man, 593
Epigenesis : 220
Epiglottis : Man, 556
Epiphysial ossification : Man, 509
Epipleural cavity: Amphioxus, 41
Epoophoron : Man, 595
Equal segmentation : 19,22

Estimation of age of embryo : Chick, 223 ; Rabbit, 352 ; Man, 467-470
Ethmoidal plate : Chick, 329, 381 ; Rabbit, 432
Ethmoidal sinus : Man, 538

Eustachian tube : Frog, 143 ; Chick, 285 ; Rabbit, 397 ; Man, 544
Eustachian valve : Chick, 303 ; Rabbit, 418 ; Man, 569, 583, 584, 585
Excretory organ : Frog, 185-197 ; Chick, 315-320 ; Rabbit, 421-426 ; Man, 588-

591
External auditory meatus : Chick, 285 ; Rabbit, 398, 402 ; Man, 499-500, 544
External ear: Rabbit,398 ; Man, 499-500, 502, 544

INDEX. 627

External genital organs : Man, 506, 596-598

External gills : Frog, 160-161

Eye: Amphwxus, 43, 63; Frog, 136-139 ; Chick, 275-280; Rabbit, 387-392-

Man, 495, 539-541
Eyebrow : Man; 607
Eyelash : Man, 507
Eyelid: Chick, 2^0; Rabbit, 392; Man, 540-541

Face : Chick, 287-289 ; Man, 497-498, 502, 503

Fallopian tube : Rabbit, 426 ; Man, 594

False amnion : Chick, 246

Falx cerebri: Rabbit, 379 ; Man, 516

Fang of tooth : Man, 560

Fat body: Frog, 198

Featner : Chick, 335-337

Feather papilla : Chick, 335-336

Features : Man, 497-498, 502, 503

Fenestra ovalis : Frog, 204, 209 ; Chick, 330 ; Rabbit, 398

Fenestra rotunda : Rabbit, 397

Fertilisation: 11-13, 15-17,32; Amphioxus, 49 ; Frog, 91, 99-100; Hen, 222,
231-232; Rabbit, 351 ; Man, 448, 466-467, 471

Fifth month : Man, 506-507

Fifth ventricle : Rabbit, 379 ; Man, 517

Fifth week : Man, 496-501

Fin : Amjfhioxvs, 37, 85

Finger : Man, 500, 503

First week : Man, 471-472

Fissura arcuata : Man, 519

Fissure of spinal cord : Chick, 250 : Man, 527

Flexure of embryo : Chick, 226 : Rabbit, 364-365 ; Man, 478, 481, 488-491

Floccular lobe of cerebellum : Rabbit, 380

Foetal membranes : Man, 598-605

Foetal vessels of placenta : Rabbit, 442 ; Man, 612-613

Foetal villi : Man, 609, 611, 612, 613

Follicle cells : Chick, 229 ; Rabbit, 346 ; Man, 452-455

Food yolk : 4-7, 19, 29 ; Frog, 100-101 ; Chick, 221-222

Foot : Chick, 335 ; Man, 500

Foramen cascum : Man, 556

Foramen incisivum : Man, 555

Foramen of Monro : Chick, 261 ; Rabbit, 377 ; Man, 516

Foramen ovale : Chick, 303, 315 ; Rabbit, 415, 419 ; Man, 570, 584-587

Fore-brain : Frog, 118, 119 ; Chick, 252; Rabbit, 374 ; Man, 510

Fore-gut : Chick, 281 ; Rabbit, 399 ; Man, 545

Fore-limb : Frog, 215 ; Chick, 332-334; Man, 500

Formative cell : Chick, 235

Fornix : Rabbit, 379 ; Man, 518

Fourth month : Man, 506

Fourth ventricle : Frog, 122 ; Chick, 254 ; Rabbit, 381

Fourth week : Man, 492-496

FBOG, 90-218: general account of development, 90-94; the egg, 94-100;
the early stages of development, 100-112; the nervous
system, 112-134; the sense organs, 134-145; thealimentary
canal, 145-157; the gills and gill clefts, 167-164; the
heart and blood-vessels, 165-185 ; the urinary and reproduc-
tive organs, 185-198; the skeleton and the teeth, 198-215 ;
bibliography, 215-218

Frontal lobe of brain : Man, 517

Frontal sinus : Man, 538

Fronto-nasal process : Chick, 275, 287 ; Man, 497, 553

Froriep's ganglion : Man, 531

Furcula: Chick, 332; Man, 556, 562

s s 2

628 INDEX.

Gaortner, duct of: Man, 596

(Jail bladder : Frog, 156 ; Chiok, 293 ; Babbit, 410; Man, 563

Ganglion, auditory : Man, 536

ciliary : Chick, 268 ; Man, 536

cochlear : jtfrtw, 536, 543

Froriep's: Man, 531

Gasserian: Frog, 133; C7tw>A, 269 ; Man, 536

geniculate : Man, 536

of glossopharyngeal nerve : Man, 535

of pneumogastric nerve : Man, 535

olfactory : Man, 537

otic : Man, 536

sphenopalatine : J/#», 536

spinal : Chick, 272 ; Man, 525-526

submaxillary : Man, 536

vestibular : Man, 536, 543
Gasserian ganglion : Frog, 133 ; Chick, 269 ; Man, 536
Gastrocoel : Amphioxus, 54
Gastrula: Amphioxus , 53-57
General account of development of embryo : Amphioxus, 46-48 ; -Fro^, 90-

94; CM**, 219-228; Rabbit, 362-371 ; Jfow, 470-509
Genital cord : Jfan, 594

Genital ducts: Frog, 193-197; Chick, 319-320; Rabbit, 426; Man, 592-595
Genital organs, external: Man, 596-598
Genital papilla : Rabbit, 412; Mem, 596

Genital ridge: Frog, 94, 197 ; Hen, 229 ; ito&Jitf, 345; Man, 450, 592
Germinal area : Man, 473
Germinal cell : Man, 523
Germinal disc : 19 ; Chick, 222, 232-235

Germinal epithelium : Hen, 229 ; Rabbit, 345 ; Jiim, 450-451
Germinal layers: 22-24 ; Amphioxus, 53 ; i^Vo#, 104-110; CMcZ:, 235-242;

Rabbit, 356-361 ; Man, 486
Germinal vesicle : Amphioxus, 49 ; Frog, 95-96 ; Hen, 230 ; Man, 453
Gestation : Rabbit, 344 ; Man, 465-467
Giant cell : Rabbit, 444
Gill: Frog, 92, 160-163
Gill arch : Amphioxus, 40; i^'o^, 159
Gill cleft: Amphioxus, 40,71,75-78, 87-88; Frog, 92, 157-160; £%&£, 283,

285; Man, 490, 492,495, 551
Gill pouch : Chick, 283 ; Man, 490, 550-551
Giraldes, organ of : Man, 593
Gizzard : Chick, 290
Gland, carotid : Frog, 181-182

club-shaped : Amphioxus, 70-71, 72, 79-80
lacrymal : Chick, 280 ; Rabbit, 392 : Man, 541
mammary : Rabbit, 434
of Lieberkiihn : Mam, 549
of stomach : Man, 549
salivary : Man, 559

uterine : Rabbit, 437; i!/«», 458, 459, 610, 612
Glans penis : Man, 59(5, 597
Glenoid cavity : Frog, 212

Glomerulus : Frog, 192 ; Chick, 318 ; Rabbit, 422 ; 3/a», 589
Glomerulus of head kidney : Frog, 182, 190-191
Glottis: Frog, 154 ; Rabbit, 408; Man, 556, 562
Glycogenous cells : Rabbit, 442
Gonoblast: 12; Frog, 95, 197
Gonotome : Amphioxus, 89

Graafian follicle : Rabbit, 348-349 ; .a/iwi, 453-455
Grey matter of brain : Rabbit, 382
Gut diverticula : Aviphioxus, 70

INDEX. 629

Hair : Babbit, 433-434 ; Man, 506, 509

Hand : Man, 500

Hare-lip : Man, 554

Hatching: Chick, 228

Head cavity : A mphioxus, 70

Head fold : Chick, 226 ; Rabbit, 363 ; Man, 478

Head kidney: Frog, 185-191; Chick, 318-319; Rabbit, 421; Man, 591

Heart: Frog, 167-169; Chick, 298-303; Rabbit, 413-416 ; 3Ian,ill 478 48*

490, 492, 496, 565-573 ' ' '

Helix : Man, 499
Hemisphere, cerebral : Frog, 125-126 ; Chick, 260-261 ; Rabbit, 376-379 •

Man, 512, 513, 514, 516-520
Hepatic cell : Frog, 156 ; Rabbit, 410
Hilum folliculi : Man, 454

Hind-brain : Jbrog, 118, 119 ; Chick, 252-256 ; Rabbit, 380-381 ; Man, 510
Hind-gut : Chick, 281 ; Rabbit, 399 ; Man, 545
Hindlimb : Frog, 215 ; Chick, 335 ; Man, 501, 503
Hippocampal sulcus : Man, 519
Hippocampus major : Rabbit, 378
His' embryo E : Man, ill
His' embryo Lg : Man, 488-490
His' embryo Lr : Man, 490-492
His' embryo SR : Man, 477-479
Histological development of brain and spinal cord : Rabbit, 381-382 ; Man,

521-527
Holoblastic segmentation : 19, 22, 353
Horny jaw : Frog, 151-162
Horny teeth : Frog, 150-151
Human embryo : first week, 471-472

second week, 472-486

third week, 486-492

fourth week, 492-496

fifth week, 496-501

sixth week, 501-503

second month, 503-505

third month, 505-506

fourth month, 506

fifth month, 506-507

sixth month, 507

seventh month, 507-508

eighth month, 508

ninth month, 508-509
Hydatid of Morgagni : Man, 693

Hyoid arch: Frog, 159 ; Chick, 284, 330; Man, 490, 495, 550-552
Hyoid bar: Frog, 206, 208 ; Chick, 330, 331 ; Rabbit, 433
Hyoidean aortic arch : Chick, 305 ; Rabbit, 416 ; Man, 574-575
Hyomandibular gill-cleft : Frog, 159 ; Chick, 285

Hyomandibular gill-pouch : Frog, 159-160 ; Rabbit, 397-398 ; Man, 544
Hypoblast: 22, 23,24: Amphioxus, 53,56; Frog, 109; Chick, 237; Rabbit,

358, 361 ; Man, 475, 486
Hypothetical stages between Reicherts ovum and His' embryo E : Man,
483-485

Ilium : Frog, 214 ; Chick, 334

Impregnation : 12 ; Chick, 232 ; Rabbit, 350-351

Incubation : Chick, 223

Incus : Rabbit, 398-399

Inf undibulum : Frog, 124 ; Chick, 258; Rabbit, 376; Man 510, 520

Inner amnion : Chick, 245 ; Man, 4:11

Inner nasal process : Chick, 288

Internal gills : Frog, 161-163

630 INDEX.

Interorbital plate ~ Chick, 330

Intestine: Amjthioxus, 40 ; i'Vr^, 147-148 ; C%*cA, 290-291 ; Babbit, 403-404 *

.Maw, 646-M9
Iris : CAiflft, 279 ; Babbit, 389
Ischium: JVop, 214 ; Chick, 334
Isthmus: Man, 510

Jacobson, organ of: Frog, 13G; Babbit, 387 ; Jlfcwi, 495

Kidney: !?><#, 191-193; Chick, 319; JSte&Mt, 424-425; 3/aw, 590

Kollmann's embryo : Man, 481-482

Labia majora : Man, 597

Labia minora : Man, 597

Labial cartilage : Frog, 203, 205, 207

Labial cavity: Frog, 150

Labio-scrotal fold : Man, 597

Lacrymal duct : Chick, 280 ; Babbit, 392 ; Man, 541

Lacrymal gland : Chick, 280 ; Babbit, 392 ; Man, 541

Lacrymal groove : Chick, 288 ; Man, 498

Lamina terminalis : Chick, 256 ; Babbit, 374, 376 ; J/#w, 517

Larval period : Anvphioxus, 73-87

Laryngeal chamber : Babbit, 408 ; Man, 562

Lateral frontal process : Man, 497

Lateral plate : Aniphioxus, 84 ; Chick, 244

Lateral ventricle: J?Vy#, 126; o7iick, 260; Babbit, 377; Jfo», 516

Leg : i^0#, 215 ; Chick, 335 ; Jftwi, 492, 496, 501

Length of embryo : Man, 470-471

Lens, of eye: Frog, 137; Chick, 277; Babbit, 390-392; Man, 495/539

Limbs : Frog, 92-93, 212-215 ; Chick, 332-335 ; Man, 492, 496, 500, 502, 505

Lingual duct : Man, 558

Lip : Frog, 150-151 ; Man, 502, 553-554

Liquor amnii : Man, 598

Liquor folliculi : Man, 454

Liver: Aniphioxus, 40; Frog, 154-156; Chick, 292-294; Babbit, 410-411;

Man, 496, 563-564
Lobule of ear : Man, 500
Lower layer cells : Frog, 104 ; Chick, 234, 236
Lumbo-sacral plexus : Man, 527

Lung: Frog, 154; Chick, 292; Babbit, 408-410; Man, 562-563
Lymphatics : Frog, 185

Malleus : Babbit, 398-399

Malpighian body : Frog, 192; Chick, 318; Babbit, 422; Man, 589

Mammalian development, general characters of : 341-344

Mammalian placenta, a chorionic structure : 445

Mammary gestation : 344

Mammary gland : Babbit, 434

Man, 448-619 : preliminary account, 448-449 ; the human ovum, 449-470
general history of the development of the human embryo
470-509 ; nervous system. 509-538 ; sense organs, 538-544
digestive system, 544-565 ; circulatory system, 565-587
urinary organs, 588-591 ; reproductive organs, 591-598
foetal membranes and placenta, 598-615; bibliography, 615-
619

Mandibular aortic arch : Chick, 305 ; Babbit, 416 ; Man, 574, 575

Mandibular arch: Chick, 283, 287, 330; Babbit, 401; Man, 490/495, 499,
550-551

Mandibular bar : Frog, 205; Chick, 330, 331 ; Babbit, 433

Mantle layer of spinal cord : Man, 524, 528

Manubrium sterni : Chick, 332

INDEX. 631

Manns: Frog, 215; Chick, 333-334

Maternal vessels of placenta: Babbit, 441 ; Man, 618-614

Maturation of egg : 7-11 ; Frog, 96-99 ; Hen, 230-231 ; Babbit, 349-350 : Man,

471
Maxillary arch : Chich, 288 ; Babbit, 401 ; Man, 490, 495, 553-555
Measurement of embryo : Man, 470-471
Meatus venosus : Chick, 293, 309

Meckel's cartilage : Frog, 205, 206, 208 ; Chich, 330 ; Babbit, 399, 433
Meconium : Man, 506, 507, 508, 564

Medulla oblongata: Frog, 122; Chich, 253; Babbit, 380-381; Man, 521
Membrana limitans interna : Man, 523
Membrane bones: Frog, 210; Chich, 324, 332
Membrane of Reissner : Babbit, 397
Menstrual cycle : Man, 459-460
Menstrual decidua : Man, 458, 463-465
Menstrual discharge : Man, 458-459
Menstruation : Man, 457-465

Menstruation, connection with ovulation : Man, 462-465
Meroblastic segmentation : 22 ; Chich, 232
Mesencephalic flexure: Man, 511, 513
Mesenteron : Amphioxus, 69 ; Frog, 105-109, 145-148 ; Chick, 281-282 ;

Babbit, 399 ; Man, 544-549
Mesentery : Frog, 148 ; Chick, 291 ; Babbit, 400 ; Man, 547, 564-565
Mesoblast: 22,23, 24; Amphioxus, 60-61: Frog, 109-110; Chich, 237, 239-

242, 297-298 ; Babbit, 361-362 ; Man, 476, 481, 483, 486
Mesoblastic somite : Amphioxus, 60-61, 65-69, 84-87 ; Chich, 242-244, 322-

323, 324 ; Man, 482, 490, 495
Mesobranchial area : Man, 555, 556
Mesogaster : Man, 564
Metacarpal: Chich, 333-334
Metamorphosis : 30 ; Frog, 93-94
Metanephros : Chich, 319 ; Babbit, 424-425
Metapleural fold : Amphioxus, 37, 82-84
Metatarsal : Chich, 335
Metazoa, characters of : 1, 2, 3, 27
Metencephalic flexure : Man, 511, 513
Mid-brain: Frog, 118, 119; Chich, 252, 256; Babbit, 379-380; Man, 510,

520-521
Middle cerebral vesicle: Chich, 252; Babbit, 379
Mid-gut : Chich, 281 ; Babbit, 399 ; Man, 545
Milk dentition : Babbit, 404 ; Man, 561
Mitral valve : Man, 573
Modiolus of cochlea : Man, 544
Mons Veneris : Man, 597

Morphological importance of Amphioxus, 44-46
Motor cranial nerves : Man, 528-534

Motor roots of spinal nerves: Frog, 128 ; Chich, 273 ; Babbit, 385; Man, 524
Mouth : Amphioxus, 40, 71, 80-81; Froq, 148 ; Chich, 287; Man, 495, 545
Mullerian duct: Frog, 195-197; Chich, 318, 319, 320; Babbit, 425, 426;

Man, 590-591, 593-594
Muscle plate: Chick, 322-323; Babbit, 429
Muscular system : Chich, 322-323 ; Babbit, 429
Musculi papillares : Ch ick, 303
MyelospoDgium : Man, 523, 528
Myocoel : Amphioxus, 85-86, 89
Myotome : Amphioxus, 38, 67 ; Frog, 199 ; Chich, 32:5

Nail : Man, 506, 507

Nasal groove : Chich, 288 ; Man, 497, 553

Neck : Man, 502

Nephrostome of head kidney : Frog, 187-190

682 INDEX.

Nephrostome of Wolffian body : Frog, 192 ; Chick, 317-318
Nerve, chorda tympani : Man, 533

cranial: Frog, 128-134; Chick, 2G2-271 ; llabbit, 382-385; Man, 628-

538
dorsal : Amphioxus, 43
early development of: Frog, 126-128; Chick, 2G1-2G2; Man, 521-

528
lateral line: Frog, 130

mandibular: Frog, 132, 133 ; Chick, 269 ; Babbit, 384 ; Man, 536
maxillary: Frog, 133 ; Chick, 269 ; Babbit, 384; Afa», 536
ophthalmic: 7^.-7, 132, 133; 6'Mc&, 269; Babbit, 384; Jlfaw, 536
phrenic : J/aw, 527
recurrent laryngeal : Man, 515

spinal: 7V^, 128; 67/itf&, 271-274; Babbit, 385-386 ; Man, 524-528
sympathetic : Frog, 134 ; Chick, 274 ; Babbit, 386 ; J/cm, 538
ventral : Amjihioxus, 43
1st cranial (olfactory) : Frog, 133-134 ; Chick, 260, 266-267 ; Babbit

382 ; Man, 537-538
2nd cranial (optic): Frog, 133, 138-139; Chick, 267-268,279; Babbit

383, 389 ; Man, 536, 540
3rd cranial (oculomotor) : Frog, 133 ; Chick, 268 ; Babbit, 383 ; Man

534
4th cranial (pathetic) : Frog, 133 ; Chick, 268 ; ifoJWi, 383 ; Man, 534
5th cranial (trigeminal): Frog, 132-133; Chick, 269; Babbit, 384

J/tfw, 534, 536, 557
6th cranial (abducent): Frog, 132 ; Chick, 269; Babbit, 383 ; Jl/im, 533
7th cranial (facial); Frog, 132; CVm'cZ;, 264-266, 269; Babbit, 384

il/arc, 533
8th cranial (auditory) : Frog, 131, 140; Chick, 270; Babbit,384:; Man

535-536, 542
9th cranial (glossopharyngeal) : Frog, 131; 6%/<?7^, 270; Babbit, 384

J/«», 533, 535, 557
10th cranial (pneumogastric or vagus): Frog, 129-131; Chick, 270-

271 ; Babbit, 384 ; Man, 533, 535
11th cranial (spinal accessory): Chick, 271; Babbit, 385; Man, 532-

533
12th cranial (hypoglossal): Chick, 271; Babbit, 385; Jfl&», 531-532
Nerve cell : Man, 526
Nerve fibre : Man, 526-527
Nervous layer of epiblast : Frog, 105, 113
Nervous system: Amphioxus, 42-43, 57-60, 62-63; Frog, 112-134; Chick

247-274 ; 22aJ«£, 371-387 ; Man, 509-538
Neural canal : Amphioxus, 58, 62-63 ; Man, 481, 510
Neural crest: Frog, 127; Chick, 263; Jl/iwi, 526
Neural fold: Av'iphioxus, 58-59; 7>^, 113-114; Chick, 248-249; Babbit

363, 371 ; Man, 477
Neural groove: Frog, 113-114; Chick, 247; Babbit, 363, 371; i/iw, 477, 480

481, 509
Neural plate : Amphioxus, 58 ; Frog, 113, 127 ; Chick, 247
Neural ridge: Frog, 127; C7m'<?&, 261, 263; Man, 526

Neural tube : Amphioxus, 59-60, 63 ; Frog, 114-116, ; Chick, 248; Man, 510
Neurenteric canal : Amphioxus, 59, 63, 69 ; Frog, 115, 149, 156 ; Chicle, 250-

252 ; J/<m, 480
Neuroblast : Frog, 139 ; Chick, 249 ; Jf<m, 523-524, 526, 528, 537
Neuropore : Amphioxus, 60, 63, 69
Nictitating membrane : Chick, 280 ; Babbit, 392
Ninth month : Man, 508-509

Nose : Frog, 135-136 ; Chick, 274-275 ; Babbit, 387 ; Man, 497-498, 502, 5SK
Nostril: Froq, 136; Chick, 275, 289; Man, 498

Notochord: Amphioxus, 37, 61, 63-65 ; Froq, 109, 198-201; Chick, 242, 324-
327 ; Babbit, 430 ; Afafc, 480

INDEX. 633

Nuclear skein : 8, 15 ; Frog, 97

Nutrition of embryo : Amphioxus, 57, 62 ; Frog, 92 ; Rabbit, 370, 435 ; Man,

Nymphse : Man, 597

Obplacental folds of uterus : Babbit, 436

Occipital lobe of brain: Man, 517

Odontoblast : Rabbit, 407 ; Man, 560

Oesophagus: Frog, 153-154; Chick, 289-290; Rabbit, 403; Man, 545, 546,

548
Olfactory capsule : Frog, 204 ; Chick, 331 ; Rabbit, 432

Olfactory lobe: Frog, 126 ; Chick, 260, 267 ; Rabbit, 379 ; Man, 513, 517, 537
Olfactory organ : Amphioxus, 43 ; Frog, 135-136 ; Chick, 266-267, 274-275 •

Man, 492, 494-495, 497, 538
Omentum : Man, 565
Omission of ancestral stages : 28, 30
Omosternum : Frog, 213
Opercular cavity : Frog, 161
Opercular fold : Frog, 92, 161
Opercular spout : Frog, 161
Optic bulb: Frog, 137

Optic chiasma : 'Frog, 139 ; Chick, 259 ; Rabbit, 390 ; Man, 514, 520, 540
Optic cup : Frog, 138 ; Chick, 211-21$ ; Rabbit, 388-389 ; Man, 539
Optic lobe: Frog, 123; Chick, 256
Optic stalk : ^V^, 125, 137-139 ; CAiflA, 267, 279 ; Rabbit, 387 ; Man, 510,

536
Optic thalami : Frog, 123 ; Chick, 256 ; Rabbit, 374 ; JH/aw, 520
Optic vesicle: Frog, 125, 136-137; <?/«c&, 252, 260, 267, 275-276; Rabbit,

374, 387 ; Man, 510, 539
Ora serrata : Chick, 279
Organ of Corti : Rabbit, 397 ; Man, 543
Organ of Giraldes : Man, 593

Organ of Jacobson : Frog, 136 ; Rabbit, 387 ; Man, 495
Organ of Rosenmiiller : Man, 595
Outer amnion : Chick, 246
Outer nasal process : Chick, 288
Ovary : Amphioxus, 43 ; ^7W7, 197 ; Hen, 228 ; Rabbit, 345-349 ; Man, 450-

453
Oviduct: Frog, 195-197; Hen, 228-229; Chick, 319, 320; Rabbit, 426; Man,

594
Oviposition : Amphioxus, 44, 46 ; jF/w7, 90-91, 99
Ovulation : Rabbit, 350-351 ; Man, 456-457
Ovulation, connection with menstruation : Man, 462-465
Ovum : structure, 3-7 ; maturation, 7-11 ; fertilisation, 11-13 ; unicellular

character, 32
Ovum : Amphioxus, 43-44, 49 ; Frog, 94-100 ; Hen, 220-222, 228-232; Rabbit,

344-351 ; Man, 448-455

Palate : Chick, 275 ; Rabbit, 403 ; Man, 554-555

Palatopterygoid bar : Frog, 205, 206, 207-208 ; Rabbit, 433

Palingenetic characters : 28

Pancreas : Frog, 156 ; Chick, 294 ; Rabbit, 411 ; Man, 564

Pancreatic duct : Chick, 294; Rabbit, 411 ; Man, 564

Parachordal cartilage : Frog, 203, 204, 207 ; Chick, 328, 331 ; Rabbit, 432

Parepididymis : Man, 593

Parietal lobe of brain : Man, 517

Parieto-occipital sulcus : Man, 519

Paroophoron: Man, 595

Parovarium: Chick, 320; Rabbit, 426; Man, 595

Parthenogenesis : 15, 16, 17, 32

Partial segmentation : 22

634 INDEX.

Parturition: Rabbit, 444-445 ; Man, 614-015

Pecten : Chick, 279

Pectoral girdle : Frog, 212-213 ; Chick, 332

Pelagic life of Amphioxus, 47

Pelvic girdle : Frog, 213-215; Chick, 334

Pelvic symphysis : Frog, 213

Penis : Man, 597

Pericardial cavity : Frog, 167, 169; Chick, 321-322 ; Rabbit, 427-428

Perilymph : Frog, 142 ; Man, 543

Periotic capsule : Frog, 143, 204 ; Chick, 328, 331 ; Rabbit, 396, 432 ; Man,

543-544
Peripheral nervous system : Frog, 126-134; Chick, 261-274 ; Rabbit, 382-386 ;

Man, 521-538
Periplacental folds of uterus : Rabbit, 435
Peritoneum : Chick, 321
Perivascular cell : Rabbit, 438
Perivitelline fluid : Frog, 98, 100
Permanent dentition : Rabbit, 404 ; Man, 561

Permanent ova : Frog, 95 ; Chick, 230 ; Rabbit, 346-347 ; Man, 453
Pes: Chick, 335
Peyer's patch : Man, 549
Pharynx: Amphioxus, 40, 75-80; Frog, 152; Chick, 283-286; Rabbit, 400-

403 ; Man, 545, 546, 549-553
Phrenic nerve : Man, 527
Pia mater : Rabbit, 381

Pineal body: Frog, 123; Chick, 256-257; Rabbit, 374 ; Man, 520
Pinna : Rabbit, 398 ; Man, 499-500, 544

Pituitary body : Frog, 124-125 ; Chick, 259, 289 ; Rabbit, 376, 400
Placenta : Rabbit, 343, 370-371, 434-445 ; Man, 611-615
Placental area of blastodermic vesicle : Rabbit, 437
Placental folds of uterus : Rabbit, 435
Pleural cavity : Chick, 322 ; Rabbit, 428 ; Man, 563
Plica semilunaris ; Man, 541
Polar body: 8-11, 15-17; Amphioxus, 49; Frog, 97-99; Hen, 231; Rabbit,

349, 350 ; Man, 471
Polar mesoblast cells : Amphioxus, 55, 60, 65
Pons Varolii : Rabbit, 381 ; Man, 514, 521
Position of embryo : Chick, 225, 228 ; Rabbit, 438
Postanal gut : Frog, 156-157 ; Rabbit, 404
Postbranchialbody : Frog, ]64

Posterior nares : Frog, 136, 148 ; Chick, 275, 289 ; Man, 497, 553-554
Prseoral pit : Amphioxus, 70, 81-82
Precoracoid : Frog, 213
Preformation : 219

Pregnancy, duration of : Man, 465-467
Prepubic process : Chick, 334
Prepuce : Man, 597
Primary gill-slits : Amphioxus, 75-78
Primary reproductive cells: 12
Primary sulci : Man, 518-519

Primitive groove : Frog, 111 ; Chick, 238 ; Rabbit, 361
Primitive ova : 12; Frog, 95; Chick, 229-230; Rabbit, 345-346; Man, 451-

453, 592
Primitive sperm cells : Man, 592
Primitive streak: Frog, 111, 149; Chick, 237-239, 240, 252; Rabbit, 361;

Man, 479
Pro-amnion : Chick, 241, 245 ; Rabbit, 369
Processus globularis : Man, 497, 504, 553-554
Proctodeum: Frog, 145, 149-150; Chick, 283, 291-292; Rabbit, 404; Man,

546, 549
Pronephros: Frog, 185-39} ,• Chick, 318-319 ; Rabbit, 421

INDEX. 635

Pronucleus, female : 8, 13, 14; Frog, 100 \9Bdbbit, 351

Pronucleus, male : 13, 14 ; Frog, 100

Protovertebra : Amphioxus, 84^85 ; Chick, 244,*_322, 324 ; Man, 482, 490, 495

Protovertebral segmentation : Chick, 326

Protozoa: characters of , 1-2 ; reproduction of, 34

Pterylia : Chick, 337

Pubes : Frog, 214 ; Chick, 334

Pulmo-cutaneous arch: Frog, 179

Pulmonary arch : Rabbit, 417

Pulmonary trunk : Chick, 306 ; Babbit, 416; Man, 572, 573, 575

Pulp cavity of tooth: Froq, 211

Pupil : Chick, 279

Pygostyle : Chick, 328

Pylangium : Frog, 168, 169

Pyramid of thyroid : Man, 558

Quadrate cartilage : Frog, 205-206, 207-208 ; Chick, 330
Quill : Chick, 336

Rabbit, 341-447: preliminary account, 341-344; the egg, 344-351; the
early stages of development, 351-362 ; general history of the embryo,
362-371 ; nervous system, 371-387 ; sense organs, 387-399 ; digestive
system, 399-412 ; heart and blood-vessels, 412-421 ; excretory system,
421-426; coelom, 427-428; muscular system, 429; skeleton, 429-
433 ; skin, 433-434 ; placenta, 434-445 ; bibliography, 445-447

Kate of development : Amphioxus, 48 ; Frog, 91 ; Rabbit, 342, 367

Rauber's layer : Rabbit, 358, 361

Recapitulation theory : 24-34, 45, 90

Recessus labyrinth! : Man, 541-542

Recessus vestibuli ; Frog, 142 ; Rabbit, 393, 395 ; Man, 541, 542

Rectal diverticula ; Chick, 291

Rectification of cranial flexure : Frog, 120

Rectum : Chick, 291 ; Man, 549

Regeneration of uterine epithelium : Rabbit, 445 ; Man, 615

Reichert's ovum : Man, 472-476

Rejuvenescence : 17

Reproductive organs: Amphioxus, 88-89; Frog, 94-96, 197-198; Chick, 228-
230, 319-320; Rabbit, 344-346, 426; Man, 449-455, 591-
598

Respiration of embryo : Chick, 228, 294 ; Rabbit, 435 ; Man, 613

Retina: Frog, 138 ; Chick, 278; Rabbit, 389 ; Man, 540

Rib: Chick, 327-328; Rabbit, 430-432; Man, 549

Ripening of egg : 7-11; Froq, 96-99; Hen, 230-231 ; Rabbit, 349-350 ; Man,
471

Roots of cranial nerves : Man, 531, 534-535

Roots of spinal nerves : Man, 524-526

Rosenmuller, organ of : Man, 595

Sacculus: Frog, 141 ; Rabbit, 396 ; Man, 541-542

Saccus endolymphaticus : Frog, 142

Sacral vertebra : Chick, 328

Salivary gland : Man, 559

Scala media : Rabbit, 397 ; Man, 543

Scala tympani : Rabbit, 397 ; Man, 543

Scala vestibuli : Rabbit, 397 ; Man, 543

Scapula : Frog, 213 ; Chick, 332

Schizoccel : Chick, 243

Sclerotic : Rabbit, 392

Scrotal sac : Rabbit, 426

Scrotum : Man, 597

Second month: Man, 503-505

036 INDEX.

Second week : Man, 472-486

Secondary gill-slits : Amphioxus, 76-78

Secondary sulci : Man, 519-520

Segmental duct : Frog, 186-190, 193-194

Segmentation of egg": 14-15, 17-22,27; Amphioxus, 49-52; 1< rog, 100-104 ;

Chick, 232-235 ; Babbit, 352-354 ; Man, 471-472
Segmentation cavity : Amphioxus, 51-52 ; Frog, 103, 108-109 ; Chick, 234
Segmentation nucleus : 13, 14; Frog, 100, 102
Semicircular canal: Frog, 141 ; Rabbit, 393, 395 ; Man, 541-542
Semilunar valve : Ck&oTt, 802; Babbit, 416 ; Man, 573
Sense capsule : Frog, 204-205 ; Chick, 328-330
Sense organs: Frog, 134-145; Chick, 274-281; Rabbit, 387-399; Man,

538-544
Sensory cranial nerves : Man, 528-530, 534-538
Sensory roots of spinal nerves: Amphioxus, 43 ; Frog, 127-128; Chick, 271—

273 ; Rabbit, 385-386 ; Man, 525-528
Separation of placenta : Rabbit, 444 ; Man, 614
Septum inferius : Man, 571, 573
Septum narium : Man, 555
Septum of truncus arteriosus : Frog, 168 ; Chick, 301, 302, 305 ; Rabbit, 416 ;

Man, 571-572
Septum spurium : Man, 570
Septum superius : Man, 569
Seventh month : Man, 507-508
Sex, origin of : 34-35

Sexual differentiation : Frog, 95 ; Rabbit, 346 ; Man, 453
Shell membrane : Hen, 220, 229
Sinus prsecervicalis : Man, 495, 502, 553, 559
Sinus terminalis: Chick, 310; Rabbit, 368, 420; Man, 483
Sinus venosus : Frog, 168, 183 ; Chick, 309 ; Rabbit, 413, 414 ; Man, 568, 579
Sinuses of placenta: Man, 612, 614
Sixth month : Man, 507
Sixth week : Man, 501-503
Size of eggs : 342

Skeleton: Frog, 198-215; Chick, 324-335; Rabbit, 429-433
Skin : Rabbit, 433-434

Skull: Frog, 201-210; Chick, 328-332 ; Rabbit, 432-433
Somatic layer of mesoblast : Chick, 243 ; Rabbit, 427
Somatopleure : Chick, 243

Somite : Amphioxus, 60-61, 65-69, 84-87 ; Chick, 244
Spawn : Frog, 91, 99

Spawning period : Amphioxus, 46 ; Frog, 90
Spee's embryo : Man, 479-481

Spermatozoon: 12, 13; Amphioxus, 44 ; Frog, 99-100 ; Chick, 232; Man, 466
Sphenoidal sinus : Man, 538
Spina vestibuli : Man, 569-571

Spinal cord: Frog, 116-117 ; Chick, 249-252 ; Rabbit, 381 ; Man, 521-528
Spinal nerves: Frog, 128; Chick, 271-274; Rabbit, 385-386; Man, 521-528
Splanchnic layer of mesoblast : Chick, 243 ; Rabbit, 427
Splanchnocoel : Amphioxus, 85-86
Splanchnopleure : Chick, 243
Spleen : Frog, 185

Splitting of mesoblast: Chick, 243-244, 246 ; Rabbit, 427 ; Man, 483
Spongioblast : Man, 523-524
Stapes : Frog, 204, 209 ; Chick, 330 ; Rabbit, 398
Sternum: Frog, 213; Chick, 332 ; Rabbit, 432
Stomach: Chick, 290; Rabbit, 403 ; Man, 545,546, 548
Stomatoda3um: Frog, 145, 148-149; 67/^,259,281, 286-289; Rabbit, 100

Man, 482, 490, 492, 545
Stratum compactum of decidua : Man, 610
Stratum spongiosum of decidua : Man, 610

\

INDEX. (537

Bubgerminal cavity : Chick, 235-236

Sucker : Frog, 92

Sulci: Man, 518-520

Superficial segmentation : 22

Super-foetation : Man, 609

Supra- plexus : Fro//, 124

Suprarenal body : Chick, 320 ; Rabbit, 386-387

Suprascapula : Frog, 213

Sylvian aqueduct: Frog, 123 ; Chick, 256; Man, 520

Sylvian fissure : Man, 514, 516, 519

Sympathetic nerve svstem : Frog, 134; Chick, 274; Rabbit, 386-387- Man

538
Synangium : Frog, 169
Systemic arch : Frog, 178 ; Rabbit, 417
Systemic trunk : Chick, 306 ; Rabbit, 416 ; Ifaw, 672, 573, 575

Tadpole: Frog, 92-94

Tail : Man, 492, 496, 503

Tarsus : Chick, 335

Teeth: Tadpole, 150-151; Frog, 210-211; Rabbit, 404-407; Man, 559-561

Telolecithal egg : 7, 19 ; Frog, 101

Temporo-sphenoidal lobe of brain : Man, 517

Testis: Amphioxus, 44; Rabbit, 426 ; il/fl^.gQ?} COO, 50tK ^ «T*

Thalamencephalon : i^-^, 123-124 ; CAioJfe, 256-260; Rabbit, 374-376; Man

510, 520
Third month : Man, 505-506

Third ventricle : Frog, 123-124 ; Chick, 256 ; Rabbit, 374
Third week : Man, 486-492

Thymus: Frog, 164; Chick, 286; Rabbit, 408; Man, 658-559
Thyroglossal duct : Man, 557
Thyrohyal : Frog, 209
Thyroid body: Frog, 152-153; 6^&, 285-286 ; Rabbit, 407-4C8 ; Man, 557-

558
Thyroid cartilage : Man, 562
Toe : Man, 501, 503

Tongue : Frog, 152 ; Chick, 286; Rabbit, 402-403; J/a??, 555-557
Tongue-bar : Amphioxus, 78, 88
Tonsil: Frag, 163
Tooth sac: Rabbit, 407

Trabecular cranii : 7'7?w/, 203-205 ; C7nc&, 329, 331 ; Rabbit, 432
Trachea: Chick, 292; Rabbit, 409; Jfrwi, 562
Tract us olfactorius: Jfan, 517
Tragus : Man, 500
Tricuspid valve : Man, 573
Trigonum olfactorium: Man, 517
True amnion: Chick, 245 ; JMiwa, 477
Truncus arteriosus: Frog, 168-169; Chick, 300-303, 305-306; Rabbit, 416;

Man, 5tf6, 567, 571-573, 574
Tuberculum anterius helicis : Man, 499
Tuberculum anthelicis : Man, 499
Tuberculum antitragicum : Man, 499
Tuberculum impar : Man, 555-557
Tuberculum intermedium helicis : Man, 499
Tuberculum lobulare : Man, 499
Tuberculum of rib : Rabbit, 430
Tuberculum tragicum : Man, 499

Tubuliferous tissue: Frog, 197; Rabbit, 345, 426 ; Man, 150
Tunica albuginea: Rabbit, 346 ; Man, 452
Tunica fibrosa folliculi : Man, 454
Tunica granulosa : Rabbit, 349 ; Man, 451
Tunica propria folliculi: Man, 454

688 INDEX.

Twisting of umbilical cord : Man, 601
Tympanic cartilage : Frog, 143-144

Tympanic cavity : Frog, 143 ; Chick, 285 ; Rabbit, 398 ; Man, 544
Tympanic membrane : Frog, 143 ; Chick, 285 ; Rabbit, 398 ; Man, 544, 550
Tympano- Eustachian passage: Frog, 143; Chick, 285; Rabbit, 397; Man,
544

Umbilical cord : Man, 505, 599-602

Umbilicus : Man, 506, 507, 509

Unequal segmentation : 19, 22

Urachus : Man, 590

Ureter: Frog, 193-195; Chick, 319; Rabbit, 425; Man, 590

Urethral canal: Man, 597

Urinary organs: Frog, 185-197; Chick, 315-320; Rabbit, 421-426; Man,

588-591
Urinary tubules : Man, 590

Urinogenital passage : Rabbit, 412, 425 ; Man, 549, 596
Urohyal : Frog, 206
Urosacral vertebra : Chick, 328
Urostyle : Frog, 199-201 '

Uterine gestation : 344

Uterine glands: Rabbit, 437; Man, 458-459, 609-610, 612, 614-615
Uterus : Hen, 231 ; Rabbit, 426 ; Man, 594
Uterus masculinus : Rabbit, 426 ; Man, 593
Utriculus : Frog, 141 ; Rabbit, 396 ; Man, 541-542

Vagina : Rabbit, 426 ; Man, 594

Valves of heart : Frog, 167-169 ; Chick, 301-303 ; Rabbit, 414-416 ; Man, 573

Vasa aberrantia : Man, 593

Vasa efferentia : Frog, 197 ; Chick, 320 ; Rabbit, 426 ; Man, 593

Vas deferens: Chick, 320; Babbit, 426 ; Man, 593

Veins: Frog, 183-185 ; Chick, 308-311 ; Rabbit, 418; Man, 578-583

Vein, afferent hepatic : Chick, 309 ; Man, 579

allantoic : Chick, 310, 314 ; Rabbit, 414 ; Man, 568, 581, 602, 613

anterior abdominal : Frog, 184

anterior cardinal : Frog, 183 ; Chick, 308 ; Man, 582

anterior vena cava : Chick, 308-309 ; Rabbit, 414

coronary sinus : Man, 582

Cuvierian: Frog, 184; Chick, 308; Rabbit, 414; Man, 568, 582

ductus venosus : Chick, 310, 314 ; Man, 581, 585, 586, 587

efferent hepatic : Chick, 310 ; Man, 579

external jugular : Man, 582

facial: Frog, 183

hepatic: i'Voy, 183

hepatic portal : Man, 580, 581

iliac: Man, 581, 582

inferior jugular: Frog, 184

internal jugular : J/<2», 582

jugular: i'Vw/, 183 ; Chick, 309; Jfim, 582

meatus venosus : Chick, 293, 309, 311

mesenteric: Chick, 310

pectoral : Chick, 309

posterior cardinal : Frog, 183; C7wc&, 308; Rabbit} 423; Jlfo>?, 582

posterior vena cava: Frog, 184; Chick, 309-311; Rabbit, 414; JV/<s?i,
581

pulmonary : Frog, 185 ; Man, 582

renal portal: ifyY^, 184

sinus annularis : Man, 580, 581

subclavian : Man, 582

vena Arantii : Man, 581

vena ascendens : Man, 581

INDEX. 639

Vein, vertebral : Chick, 309

vitelline : Frog, 183 ; Chick, 300, 309, 310, 314 ; Rabbit, 413 414 4<>o •

Man, 568, 579-681, 602 : '

Velar plates : Frog, 163
Velar tentacles : Amphioxus, 80, 81
Velum : Amphioxus, 40, 80
Velum medullas anterius : Rabbit, 380
Velum medullas posterius : Rabbit, 381
Venous valves of heart: Rabbit, 414
Ventral commissure of spinal cord : Man, 525
Ventral fissure of spinal cord : Chick, 250 ; Man, 527
Ventral roots of spinal nerves : Frog, 128 ; Chick, 273 ; Rabbit, 385 : Man

524
Ventricles of brain : Frog, 116, 122-123; see also Third, Fourth, Fifth, and

Lateral Ventricles
Ventricle of heart : Frog, 168-169; Chick, 300-303; Rabbit, 415; Man, 566

567, 571
Ventricular septum: chick, 301, 302; Rabbit, 415; Man, 572-573
Vermis : Rabbit, 380
Vernix caseosa : Man, 508

Vertebral column: Frog, 198-201 ; Chick, 324-328 ; Rabbit, 430
Vertebral plate : Chick, 244
Vertebral segmentation : Chick, 327

Vesicle of brain : Frog, 118 ; Chick, 252; Rabbit, 371 ; Man, 481, 491
Vesicle of hemispheres : Frog, 126 ; Chick, 260; Rabbit, 376; ilfiw, 510, 516
Vesicula seminalis : Frog, 194-195
Villi of intestine : Man, 549
Villi of placenta : Rabbit, 441
Villous zone of placenta : Man, 612
Visceral arch : Frog, 159; Chick, 283-284 ; Rabbit, 400-401 ; Man, 487, 495,

498-499, 550-553
Visceral clef t : Frog, 92, 157-160; Chick, 283, 285 ; Man, 490, 492, 495, 551
Visceral groove : Chick, 283 ; Rabbit, 400 ; Man, 550, 551
Visceral pouch : Frog, 157-160; Chick, 283; Rabbit, 400; Jfore, 550,551
Visceral skeleton : Frog, 205-207 ; 67«'.6'/«:, 330-331
Vitality of ovum : Man, 466
Vitelline circulation : Rabbit, 420-421

Vitelline loop of intestine : Chick, 290 ; Rabbit, 403 ; Man, 547, 548
Vitelline membrane: Amphioxus, 49; Frog, 96 ; Hen, 229 ; Rabbit, 349
Vitreous body : Rabbit, 392 ; Jl/im, 539
Vocal cord : Man, 562
Vulva: Rabbit, 412

Whartonian jelly : Man, 602

White matter of brain : Rabbit, 382

White matter of spinal cord : Chick, 250 ; Rabbit, 382

White of egg : Hen, 220, 229, 231

White yolk : Hen, 230

Wing : Chick, 332-334

Wolffian body: Frog, 191-193 ; Chick, 316-318,320; Rabbit, 422-424; Man,

588-590, 593, 595
Wolffian duct: Frog, 193-195; Chick, 315-316, 320; Rabbit, 421-422; Man,

588, 593, 595-596
Wolffian ridge : Man, 492, 496, 501
Wolffian tubules : Frog, 191-193; Chick, 31(5-318; Rabbit, 422; Man, 589,

593

Xiphisternum : Frog, 213

Yellow yolk : Hen, 230

Yolk: 4-7, 19, 29 ; Frog, 96 ; Chick, 220-222, 229

640 INDEX.

Yolk-cells : Frog, 104, 105, 146

Yolk-plug: Frog, 107, 111, 147

Yolk-sac: Chick, 222, 22G; Rabbit, 367-369; Man, 476, 483, 496, 544

Yolk-spheres : Hen, 230

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DICTIOMRYof NATIONAL BIOGRAPHY.

Edited by LESLIE STEPHEN and SIDNEY LEE.

Volume I. was published on January I, 1885, and a volume has been issued
every three months since that date.

A further Volume will be published on January I, April i,July 1, and
October I of each year tint il the completion of the work.

EXTRACTS FROM A FEW PRESS NOTICES OF THE WORK.

Truth.— ' I am glad you share my ad-
miration for Mr. Stephen's magnum opus —

THE MAGNUM OPUS OF OUR GENERATION —

"The Dictionary of National Biography."
A dictionary of the kind had been attempted
so often before by the strongest men pub-
lishers and editors— of the day that I hardly
expected it to succeed. No one expected
such a success as it has so far achieved.'

The Athen/eum. — 'The latest volumes
of Mr. Stephen's Dictionary are full of

IMPORTANT AND INTERESTING ARTICLES.

Great names strike the eye constantly as we
turn the pages. . . . Altogether the volumes
are good reading. What is more important,
the articles, whether they are on small or
great personages, are nearly all up to the
high standard which has been set in the
earlier portions of the work, and occasion-
ally above it.'

Saturday Review. — 'From the name
we have cited it will be seen that great pain
have been taken with that portion of the
Dictionary which relates to modern times,
and this has been rightly done ; for often
nothing is more difficult than to rind a concise
record of the life of a man who belonged to
our own times or to those just preceding
them. Consistently enough, the Editor has
been careful to keep the work reasonably up
to date.'

The Manchester Examiner and
Times.— 'This magnificent work of reference
has now left the tentative stage of its existence
far behind, and Mr. Leslie Stephen and his
contributors are well under weu;h. ... We
extend a hearty welcome to the latest instal-
ment of a most magnificent work, in which
both the editing and the writing appear still
to improve.'

The Quarterly Review.— ' A "Dic-
tionary of National Biography,' or

WHICH THE COUNTRY MAY BE JUSTLY

proud, which, though it may need correct-
ing and supplementing, will probably never
be superseded, and which, in unity of con-
ception and aim, in the number of the
names inserted, in fulness and accuracy of
details, in the care and precision with which
the authorities are cited, and in the biblio-
graphical information given, will not only be
immeasurably superior to any work of the
kind which has been produced in Great
Britain, but will as far surpass the German
and Belgian biographical dictionaries now
in progress, as these two important under-
takings are in advance of the two great
French collections, which until lately reigned
supreme in the department of Biography.'

The Spectator. — 'As each volume of
the Dictionary appears, its merits become
more conspicuous. . . . The book ought to
commend itself to as wide a circle of buyers
as the " Encyclopaedia Britannica." '

The Lancet. — 'Its contents show no
falling off in accuracy and completeness, so
far as by a critical examination we have been
able to discover, from those of the previous
volumes, of which we have on all occasions
spoken with praise. When completed, the
Dictionary will be well nigh invalu-
able.

Tm-. Pall Mall Gazette.— 'As to the
general execution, we can only repeat the
high praise which it has been our pleasing
duty to bestow on former volumes. To find
a name omitted that should have been inserted
is well-nigh impossible.'

London: SMITH, ELDER, & CO., 15 Waterloo Place.

WORKS by the late PROFESSOR MARSHALL, F.R.S.

THIRD EDITIOX. [imperial 8vo. price £x. i.is. 6d.

ANATOMY FOR ARTISTS.

BY JOHN MARSHALL, F.R.S., F.R.C.S.

Professor of Anatomy Royal Academy of Arts ; late Lecturer on Anatomy at the

Government School of Design, South Kensington ; Professor of Surgery in University

College, London ; Senior Surgeon to the University College Hospital, &c, &c.

Illustrated with 220 Original Drawings on Wood, by J. S. Cuthbert.
Engraved by J. & G. Nicholls.

BY THE SAME AUTHOR.

Folio, price 8.y. in wrapper ; or in portfolio, price g.y.

A RULE OF PROPORTION FOR THE HUMAN FIGURE.

Illustrated by JOHN S. CUTHBERT.
FOURTH EDITION. 4to. with folio Atlas, 21s.

A DESCRIPTION OF THE HUMAN BODY :

ITS STRUCTURE AND FUNCTIONS.

Illustrated by Physiological Diagrams.

Designed for the use of Teachers in Schools, and of Young Men destined
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The Work contains 260 quarto pages of Text bound in cloth, and 240 Coloured Illustra-
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prepared for tbe department of Science ano Srt.

A SERIES OF LIFE-SIZED ANATOMICAL DIAGRAMS

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To Teachers and Students of Artistic Anatomy this Series of Diagrams
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PHYSIOLOGICAL DIAGRAMS.

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Eleven Diagrams, Life-size, each on paper 7ft. by 3ft. gin., coloured in fac-simile of the

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In the new Edition each subject has been re-drawn on the zinc, and,
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V* A detailed list of the Diagrams will be sent Post free on application.

London: SMITH, ELDER, & CO., 15 Waterloo Place.

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