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TEXT-BOOKS OF ANIMAL BIOLOGY

Edited by Julian S. Huxley, M.A.

Fullerian Professor of Physiology, Royal Institution

Honorary Lecturer in Experimental Zoology, King's College, London

VERTEBRATE ZOOLOGY

TEXT-BOOKS OF ANIMAL

BIOLOGY

Edited by Julian S. Huxley

COMPARATIVE PHYSIOLOGY. By L.

T. HOGBEN.

ANIMAL ECOLOGY. By Charles Elton.

Other Volumes in Preparation.
LIFE IN INLAND WATERS. By
Kathleen E. Carpenter. fn the Press.

EXPERIMENTAL ZOOLOGY. By Julian
S. Huxley.

THE DETERMINATION OF SEX IN
VERTEBRATES. By F. W. Rogers
Brambell.

INVERTEBRATE ZOOLOGY: An In-
troduction to Animal Morphology and
Bionomics. By W. Garstang.

VERTEBRATE
ZOOLOGY

AN INTRODUCTION TO THE COMPARATIVE

ANATOMY, EMBRYOLOGY, AND EVOLUTION

OF CHORDATE ANIMALS

BY

G. R. de BEER, M.A., B.Sc, F.L.S.

FELLOW OF MERTON COLLEGE, JOHN WILFRED JENKINSON MEMORIAL

LECTURER IN COMPARATIVE AND EXPERIMENTAL EMBRYOLOGY

IN THE UNIVERSITY OF OXFORD

WITH AN INTRODUCTION BY

JULIAN S. HUXLEY, M.A.

ULLERIAN PROFESSOR OF PHYSIOLOGY ROYAL INSTITUTION

NEW YORK

THE MACMILLAN COMPANY

1928

PRINTED IN GREAT BRITAIN BY
WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES

TO

KITTEN and ANNE

AUTHOR'S PREFACE

There are two methods of teaching Zoology. One method is
to deal with a limited number of selected types one by one,
and the other is to compare corresponding parts of a number
of different animals. Each method has its advantages and
its drawbacks. The type method is essential for gaining
acquaintance with actual animals, and is of fundamental
importance from the fact that it permits of practical study of
the complete animals themselves. It cannot be too much
emphasised that Zoology is the study of animals, and not the
study of books written about them. That being so, it is
obviously more convenient to dissect and study one type
thoroughly before passing on to the next, than to have a
number of dissections of corresponding portions of several
animals all going on at the same time. The first two parts of
this book are devoted to a study of types carefully selected so
as to be of the greatest utility in the interpretation of other
forms. Part I deals with the adult structure, and Part II with
the modes of development.

While the type method is necessary for a start, it is attended
with certain dangers. Too much attention may be paid to the
types themselves and too little to the other animals of which they
are but only in a general way typical. There is also the danger
that "... a multitude of facts overcrowd the memory if they
do not lead us to establish principles. ..." I have sought to
remedy this with the help of a comparative treatment of the
various organ-systems, which forms the subject of Part III.
In this part, the information obtained in Parts I and II is woven
into a framework, and other animals of interest are interpolated,
so as to present a general view of the organ-systems from the

viii PREFACE

evolutionary and functional points of view. By this means,
it is possible to mention the significant points of certain animals
which are unsuited to be taken as types in themselves. In
many cases these interpolated animals are fossils, from the
fragmentary knowledge of v/hich it would be impossible to
construct a sufficiently instructive type.

The use of this comparative treatment following upon the
descriptions of types entails a certain amount of repetition,
and this is intentional. Unfamiliar facts, which by themselves
may be devoid of any particular interest, acquire an added
attractiveness and significance when they are introduced under
more than one setting.

Lastly, in Part IV the types and comparisons are woven
together into a whole, and treated as a history of the chief
groups of vertebrate animals. It is hoped that the general
nature of the treatment of the characteristic features of
vertebrates, and the inclusion of a section dealing with the
affinities and evolution of the human race, may not be without
interest for the human anatomist.

A few words may be added with regard to the types. They
are selected and treated not only for their intrinsic importance,
but also as introductions to the next types. The description
of each type is therefore to some extent based on previous
types. So the dogfish is not only an example of a primitive
fish, but it also provides the material on which the disposition
of the arterial arches and cranial nerves may be studied, and
the knowledge so obtained is used in the interpretation of all
higher types. Similarly, Gadus serves as an introduction to
the bones of the skull, and Triton introduces the limb of the
land- vertebrate. This must explain what may appear to be a
lack of balance in the treatment of certain types.

Apart from the more ordinary dissections and observations
which I have been able to make personally, I am indebted for
sources of information chiefly to the teaching of the Oxford
school of Zoology, and in particular to Professor E. S. Goodrich,
F.R.S., whose principles I have largely attempted, however
unsuccessfully, to follow. I wish to record my gratitude to
him for his general guidance in many matters, and for the

PREFACE ix

facilities which I have enjoyed in the Department of Zoology
and Comparative Anatomy of the Oxford University Museum.

On occasion, I have had the privilege of discussing certain
matters with Professor G. Elliot Smith, F.R.S., Professor C,
Judson Herrick, Professor J. P. Hill, F.R.S., Professor Sir
Charles Sherrington, O.M., F.R.S., Professor W. J. Sollas,
F.R.S., Professor A. Thomson, and Professor D. M. S. Watson,
F.R.S. To all of them I wish to make due acknowledgment
for the help which their information and advice have afforded
me. To my friend and colleague Mr. B. W. Tucker I am
especially indebted for reading the MS., and for making
several valuable and helpful suggestions. I take great pleasure
in recording my thanks to Professor Julian Huxley, without
whose suggestion, interest, and persistent encouragement this
book would have remained unwritten. It goes without saying
that these gentlemen are not responsible for the errors which
this book contains.

I have thought it inadvisable to burden the text with
references. Instead, a short list of works is appended at the
end of most of the chapters. I may mention here certain easily
accessible works of great general utility in the study of
vertebrates :

Abel, O. Die Stamme der Wirbeltiere. Berlin and
Leipzig. 1 9 19.

Brachet, A. Traite d'Embryologie des Vertebres. Paris.
1921.

Graham Kerr, J. Text-Book of Embryology. Vol. II.
London. 1919.

Hyman, L. H. A Laboratory Manual for Comparative
Vertebrate Anatomy. Chicago. 1925.

Ihle, j. E. W., and others. Vergleichende Anatomie der
Wirbeltiere. Berlin. 1927.

Jenkinson, J. W. Vertebrate Embryology. Oxford.

!9i3-

Kellicott, W. E. Chordate Development. New York.

Kingsley, J. S. Comparative Anatomy of Vertebrates.
London, 1927.

G 2

x PREFACE

Kingsley, J. S. The Vertebrate Skeleton. London.

1925.

Lull, R. S. Organic Evolution. New York. 1917.

von Zittel, K. A. Grundziige der Palaontologie. Vol. II.
Berlin. 191 8.

With regard to the figures, I have preferred to illustrate a
few points thoroughly, rather than attempt to provide a picture
of all the structures that are worthy of attention. In any case,
a picture is but a poor substitute for the structure itself. The
majority of the figures were drawn specially for this book from
dissections and laboratory preparations, and my intention has
been to show that the student should have no difficulty in
examining for himself the structures here figured (and many
more besides which are not figured in this book), in any fairly-
well equipped laboratory. I am indebted to my wife for pre-
paring the figures for press, and to the following authors and
publishers for permission to reproduce figures : to the Dele-
gates of the Clarendon Press for figs. 73, 74, 75, 88, 89, 92, 94,
95, 96, 101, 102, 109, no, 114, 115, 116, from J. W. Jenkinson's
Vertebrate Embryology, and for permission to copy figs. 87,
90, 91, 93, 97, 98, 108, 113, from the same work ; to Professor
G. Elliot Smith and Mr. Humphrey Milford for figs. 180, 181,
183, from Essays on the Evolution of Man ; to Professor Boule
for fig. 182 ; to the Editor of the Quarterly Journal of Micro-
scopical Science for permission to copy fig. 118 ; to Professor
W. K. Gregory and the American Museum of Natural History
for permission to copy part of fig. 158 ; to Professor L. Bolk
for permission to copy fig. 123 ; to Professor W. J. Sollas for
permission to copy fig. 182 ; and to Mr. B. W. Tucker and
Mr. J. Z. Young for the loan of drawings of dissections.

In conclusion, I wish to express to Messrs. Sidgwick and
Jackson my appreciation of the care and skill which they have
so kindly shown in the preparation of this book.

G. R. de B.

Oxford,
February, 1928.

EDITOR'S INTRODUCTION

It is the aim of this series to provide a number of text-books
covering different fields of animal biology, in order to obviate
as far as possible the pedagogically unfortunate habit of trying
to introduce subjects of more recent development as appendages
to a single morphological theme. We may deplore the way
in which morphology arrogates to itself an unfair share of
the time-table in many zoological laboratories in this country *,
but the fact remains that morphology, well taught and well
linked-up with other branches, is still educationally the best
discipline in zoology, and, more surely than any other branch
of the subject, throws open windows on to those long vistas
that enlarge the mind and satisfy intellectual aspirations.
But, even if its own fascination is brought out, it can be taught
so as to leave it isolated among the later-developed branches
of biology, like a foreign body encapsulated in living,
growing tissues.

Mr. de Beer and I had many talks over this book. In the
first place, we felt that the average zoology student to-day
was being expected to absorb far too much detail in a given time,
and that as a result, he was often overtaxed and prevented from
seeing the wood for the trees. The teacher's aim should
be to use no more fact-material^ than is needed to embody
the architectural design which intellectual vision has planned ;
but enough to build it firm, on lasting foundations.

Our second point was the need for linking up the various
branches of biology. Morphology has such merits as a self-
contained discipline that these efforts at liaison are not always
made. I should like here to point out some of the ways
in which an isolated morphology comes up against a blank
wall, but through which she can advance to new view-points

xii EDITOR'S INTRODUCTION

once she has reached out her hand to sister branches of
biology.

Morphology's central conception, Homology, is being
modified by Genetics. Identical but independent mutation
of genes, as is now recorded from several different species
of Drosophila, shows that the conception of a common
ancestor is no longer fundamental to the idea of homology.

Here we obviously approach orthogenesis. On the other
hand, other orthogenetic ideas derived directly from mor-
phological study melt away in the light of developmental
physiology. Such phenomena as the progressive phylo-
genetic horn-development of various mammals, sometimes
occurring independently in parallel stocks, need not after all
imply orthogenesis in its strict sense of steady, determinate
change of the germ-plasm. A study of the mechanism
of the relative growth of parts shows, as Mr. de Beer
points out (Chap. XLIII), that natural selection for increased
size will automatically bring out the horn-growth, as what
Darwin called a correlated variation.

Recapitulation too must be viewed differently as the result
of studies on growth and on genetics. As D'Arcy Thompson
pointed out in his Growth and Form, differences in propor-
tion between related animals must be due primarily to differ-
ences in the growth-rates of the parts concerned. Later work
has shown that the characteristic proportion of a part gene-
rally depends on the part continuing to grow at a different
rate from the rest of the body for a long period. This being
so, many cases of recapitulation are due solely to this differen-
tial growth. Schultz has shown that the foetuses of primates
still show the limb-proportions characteristic of their adults,
but less strongly marked. This is recapitulation : but it
is also a direct consequence of long-continued differential
growth. The same principles can be applied to the recapitu-
lation of shell-form shown in the ontogeny of many Nauti-
loids, Ammonites, etc., and to the fact that vestigial organs
are often of greater relative size in young stages.

Such studies also bear on the systematic side of morph-
ology ; for where the growth-rates of two parts are markedly

EDITOR'S INTRODUCTION xiii

different, the animal has no fixed form. This is frequent in
Crustacea and Insects, and may even occur in mammals, as
Hinton has shown for voles. This obviously demands a
revision of certain taxonomic ideas on the value of precise
measurements of proportion.

The fact, first emphasised by Goldschmidt, that Mendelian
factors frequently act by altering the rates at which develop-
mental processes occur and the times at which they begin and
end, rather than effecting qualitative changes ab initio, also
bears on the problem. The eye of Gammarus is first
scarlet, then darkens (at different rates according to the
genes present) to or towards black. This is no proof that
the ancestral eye was red, but depends on the physiology of
melanin-deposition. Bolk, as a result of morphological
study, has shown how frequently characters of early stages
become prolonged into later life in the course of evolution ;
his analysis enlarges the old concept of neoteny, and shows
how much more common it is than usually supposed. This,
however, is what the developmental physiologist would
expect. If the time of appearance and relative importance
of an organ depends upon the rate of some process, we are
just as likely to have that rate altered in one direction as in
the other, and therefore just as likely to have an embryonic
character spread on to later stages as an adult character
pushed back into earlier stages. The latter is recapitulation,
the former the reverse ; and both depend, not upon some
mysterious evolutionary urge, but upon simple develop-
mental laws.

Other cases of recapitulation also become more intelligible
in terms of other aspects of Entwicklungsmechanik. Why,
for instance, are notochord, gill-slits, and arterial arches of
amniote vertebrates recapitulated, while their limbs never
recapitulate fins, and their gill-slits never recapitulate gills ?
The answer seems simple. The recapitulated ancestral organs
are necessities, as formative stimuli, for the production of
adult structures ; the non-recapitulated ones are not.

" Racial senescence," so-called, is another morpholo-
gical-evolutionary concept which looks very different when

xiv EDITOR'S INTRODUCTION

morphology makes contact with physiology, but lack of space
forbids a discussion of this point here.

Finally, there is the bearing on morphology of functional
modification. Most biologists do not seem to realize
the extent to which functional modification occurs in the
normal vertebrate body. It appears to be true, not only that
the size of every muscle in the body depends upon function,
but the size, direction, and structure of every tendon and
bone ; the detailed conformation of the blood-system depends
largely, or perhaps wholly, on hydrodynamic considerations ;
the size of every gland is regulated by its function ; and
even the nervous system does not escape. It is only in
earliest development that structure precedes function : later,
structure is the resultant of function.

The recognition of these facts demands a new attitude
towards the genetic and evolutionary bases of structural change.
To take but one example, it is disturbing but true to find
that the differences in form and minute architecture of the
human heel-bone which distinguish it from that of apes are
due to functional modification in each generation — to the fact
that we put our weight on it in a different way owing to our
walking upright. It is also disturbing to realize that in other
groups, function does not play this important role in mould-
ing structure. In all holometabolous insects, the size and
form of all hard parts come into being once and for all,
without previous function, since they have not existed in the
larva, or been used in the pupa, and without the chance
of being later modified by function, since there is no further
moult. Thus definitive form — the morphologist's raw
material — is arrived at by quite a different method in the
two highest groups of animals.

I have, I hope, said enough to show that certain aspects of
vertebrate morphology will bear restating ; and Mr. de Beer's
pages are themselves the best evidence of his success in
achieving that restatement without abandoning any of the
essentials which give morphology such value as a discipline
in its own right.

JULIAN S. HUXLEY.

0c7Tx

CONTENTS

PART I
Morphological Types illustrating the Different Stages
of Organisation and the Trend of Vertebrate
Evolution

PAGE

1. The Vertebrate Type as contrasted with the Invertebrate . I

2. Amphioxus, a primitive Chordate ..... 6

3. Petromyzon, a Chordate with a skull, heart, and kidney . 20

4. Scyllium, a Chordate with jaws, stomach, and fins . . 37

5. Gadus, a Chordate with bone ...... 64

6. Ceratodus, a Chordate with a lung ..... 80

7. Triton, a Chordate with 5-toed limbs ..... 89

8. Lacerta, a Chordate living entirely on land . . . .103

9. Columba, a Chordate with wings . . . . .117
10. Lepus, a warm-blooded, viviparous Chordate . . . 133

PART II

Embryological Types illustrating the
Modes of Development

11. The development of Amphioxus .

12. The development of Rana (the Frog) .

13. The development of Gallus (the Chick)

14. The development of Lepus (the Rabbit)

Different

161
172
198

227

PART III

Comparative Zoology of Chordates
Outline Classification of the main groups of Chordate animals

showing the value and extent of comprehensive terms . 237

15. The Blastopore 240

16. The Embryonic Membranes ...... 247

■J09Z3

XVI

CONTENTS

17. The Skin and its derivatives ....

. 256

18. The Teeth

. 261

19. The Coelom and Mesoderm ....

. 270

20. The Skull

. 279

Table of Vertebrate Bones .....

. 300

21. The Vertebral Column, Ribs, and Sternum .

. 302

22. Fins and Limbs ......

. 310

23. The Tail ........

• 324

24. The Vascular System .....

. 327

25. The Respiratory system .....

. 337

26. The Alimentary system .....

• 344

27. The Excretory and Reproductive systems .

. 348

28. The Head and Neck

• 354

29. The functional divisions of the Nervous system .

• 363

30. The Brain and comparative Behaviour

. 372

31. The Autonomic Nervous system

• 384

32. The Sense-organs .......

• 39i

33. The Ductless glands

. 398

34. Regulatory mechanisms ......

. 404

35. Blood-relationships among the Chordates .

. 410

PART IV

Evolutionary Morphology

36. The bearing of Physical and Climatic factors on Chordates .

37. The origin of Chordates, and their radiation as aquatic

animals .........

38. The evolution of the Amphibia : the first land-Chordates

39. The evolution of the Reptiles ......

40. The evolution of the Birds .......

41. The evolution of the Mammalia ......

42. The evolution of the Primates and Man ....

415

422

431
436
445
452
460

PART V

43. Conclusions ......... 477

Classification of the animals and groups of animals mentioned

in this book ......... 487

Index

493

LIST OF ILLUSTRATIONS

FIG.
I.
2.

3-

4.

5-
6.

7-
8.

9-
10.
11.
12.
13.
14.
15.
16.

17.
18.
19.
20.
21.
22.

23.
24.
25.
26.
27.
28.
29.
30.
3i.
32.
33-
34-
35-
36.
37.
38.

39-
40.

41.

Schematic chordate showing gill-slits ..... 3

Section through typical chordate ..... 3

Amphioxus, general view ....... 6

Amphioxus, anterior end ....... 7

Amphioxus, hinder end ....... 9

Amphioxus, vascular system ...... 9

Amphioxus, section through endostyle .... 10

Amphioxus, section through gill-bars ..... 10

Amphioxus, nephridia . . . . . . .11

Amphioxus, transverse sections ...... 14

Petromyzon, general view . . . . . . .21

Petromyzon, anterior region of adult . . . . . 21

Petromyzon, anterior region of Ammoccete . ... 21

Origin of chordate eyes . .... 23

Petromyzon, section through brain ..... 25

Petromyzon, endostyle of Ammoccete ..... 29

Method of kidney formation in chordates .... 32

Petromyzon, urinogenital aperture and anus ... 34

Scyllium, general view ....... 38

Scyllium, section through skin . . . . • 39

Scyllium, auditory sac ....... 40

Scyllium, section through brain ...... 42

Scyllium, ventral view of brain ...... 43

Scyllium, cranial nerves ....... 44

Scyllium, skull and visceral arches ..... 48

Scyllium, pectoral and pelvic girdles ..... 50

Scyllium, alimentary system, and afferent branchial vessels . 52

Kidneys in Gnathostomes ....... 54

Scyllium, male urinogenital system ..... 56

Scyllium, female urinogenital system ..... 57

Scyllium, venous system . . . . . . • 59

Scyllium, arterial system ........ 60

Gadus, general view ........ 65

Gadus, skull and pectoral girdle ...... 67

Gadus, palato-pterygo-quadrate ...... 68

Gadus, neurocranium ....... 69

Gadus, dorsal view of skull ...... 70

Gadus, dissection ........ 76

Ceratodus, general view ....... 80

Ceratodus, skeleton of pectoral fin . . . . .83

Ceratodus, lung, heart and vascular system .... 84

xvm

LIST OF ILLUSTRATIONS

42.
43-
44.
45-
46.

47-
48.
49.
50.

5i-
52.

53-
54.
55-
56.

57-
58.

59-
60.
61.
62.

63.
64.
65.
66.

67.
68.
69.
70.

71-

72.

73-
74.
75-
76.

77.
78.

79-
80.
81.
82.
83.
84.
85.
86.
87.

90.
91.
92.

Triton, pectoral girdle and fore limb .

Triton, sacrum, pelvic girdle, and hind limb

Triton, dorsal view of skull .

Triton, ventral view of skull

Triton, dissection

Salamandra, dissection

Varanus, skull seen from behind

Lacerta, pectoral girdle and fore limb

Lacerta, sacrum, pelvic girdle and hind limb

Lacerta, pectoral girdle and sternum

Lacerta, dissection

Kidney formation in amniotes

Feather, structure

Columba, pectoral girdle, wing and sternum

Columba, hind limb .

Columba, pelvic girdle

Columba, air-sacs

Columba, vascular system .

Dog, skull showing foramina

Lepus, vertebrae and ribs .

Lepus, vascular system

Diagram of arterial arches and vagus nerve

Lepus, female urinogenital system

Lepus, male urinogenital system

Lepus, brain in section and ventral view

Diagram of mammalian ear

Development of Amphioxus, early stages

Amphioxus, origin of notochord, nerve-cord

Amphioxus, young embryo and larva

Amphioxus, development of mouth, gill-slits

Amphioxus, formation of atrium

Rana, egg showing grey crescent

Rana, formation and closure of blastopore

Rana, stages of gastrulation seen in section

Closed blastopore of Rana and primitive streak

Rana, origin of mesoderm .

Rana, origin of notochord and neural folds

Rana, formation of nerve-tube

Rana, section showing growth in length

Rana, origin of the kidneys.

Rana, origin of the lungs .

Rana, formation of the gill-slits .

Rana, origin of the heart

Rana, development of the eyes .

Rana, formation of the ears

Gallus, blastoderm incubated 12 and 15 hours

Gallus, primitive streak at 10 and 15 hours,

verse section ......

Gallus, primitive streak at 10 and 15 hours,

tudinal section .....
Gallus, blastoderm incubated 20 hours
Gallus, blastoderm incubated 24 hours
Gallus, origin of notochord and nerve-cord .

and

, and

of

seen

seen

mesoderm
endostyle

Gallus

in trans-
in longi-

LIST OF ILLUSTRATIONS xix

FIG. PAGE

93. Gallus, embryo incubated 30 hours ..... 205

94. Gallus, embryo incubated 30 hours seen in longitudinal

section ......... 206

95. Gallus, embryo incubated 30 hours seen in transverse section

(posterior region) ....... 207

96. Gallus, embryo incubated 30 hours seen in transverse section

(anterior region) ........ 208

97. Gallus, embryo incubated 36 hours ..... 209

98. Gallus, embryo incubated 60 hours . . . . .210

99. Gallus, formation of amnion . . . . . .212

100. Gallus, 4 days incubated, showing allantois . . .214

101. Gallus, formation of amnion, chorion, yolk-sac, and allantois 215

216
217
219
221

223

225
228
229
230
232

235
241
242

244
249
250
251

252

259
262
263
264
266
272
273
275
280
281
282
284
285
286
287
287
289
289
289
289
290

102. Gallus, relations of embryonic membranes

103. Gallus, face of embryo

104. Development of metanephros

105. Section through metanephros

106. Gallus, skeleton of limbs and girdles of embryos incubated

5 and 9 days .......

107. Development of feather ......

108. Lepus, early stages of development ....

109. Lepus, formation of the amnion ....
no. Lepus, relations of embryonic membranes and placenta
in. Lepus, section through placenta ....

112. Development of hair ......

113. Developing dogfish, showing blastopore

114. Hypogeophis, embryos showing blastopore

115. Reptile, sections through blastoderm showing origin of

blastopore .......

116. Human embryo, relations of embryonic membranes

117. Perameles, section through placenta .

118. Cow, section through placenta ....

119. Cat, section through placenta ....

120. Scyllium, development of denticles

121. Development of teeth, in mammals .

122. Development of teeth in the dogfish .

123. Diagram showing methods of tooth-succession .

1 24. Types of teeth .......

125. Lacerta, embryo showing relations of ccelom

126. Bird, showing relations of ccelom

127. Mammal, embryo showing relations of ccelom .

128. Trout, development of chondrocranium

129. Schematic chondrocranium ....

130. Relations of splanchnocranium to neurocranium

131. Osteolepis, skull

132. Stegocephalian (Loxomma), skull

133. Seymouria, skull

134. Chelone, skull ....

135. Testudo, skull ....

136. Cotylosaur (Captorhinus), skull .

137. Ichthyosaurus, skull .

138. Varanus, skull ....

139. Sphenodon, skull

140. Columba, skull

LIST OF ILLUSTRATIONS

of the

and

FIG.

141. Theromorph (Mycterosaurus), skull

142. Theromorph (Cynognathus), skull

143. Dog, skull ....

144. Primate (Chimpanzee), skull

145. Snake, streptostylic skull .

146. Chelone, hind view of skull

147. Ornithorhynchus, hind view of skull

148. Varanus, palatal view of skull .

149. Dog, palatal view of skull

150. Lower jaws of Varanus and dog

151. Reptilian and mammalian methods of articulation

lower jaw, and auditory ossicles .

152. Scyllium, development of vertebral column

153. Vertebral column of Scyllium, Amia, Embolomeri

154. Vertebral column of bony fish, crocodile, and Sphenodon

155. Crocodile, anterior region of vertebral column .

156. Dorsal and ventral ribs .....

157. Cladoselache, pectoral fin . ....

158. Sauripterus, fin, and pentada.ctyl limb

159. Sphenodon, fore limb .....

160. Sphenodon, gastralia and pectoral girdle .

161. Ornithorhynchus, pectoral girdle

162. Evolution of tetrapod limb ....

163. Wings of bird, Pterodactyl, and bat .

164. Fore limbs of Ichthyosaurus, Plesiosaurus, pengui

dolphin .......

165. Homocercal tail ......

166. Heart and aortic arches ; comparative diagram .

167. Segmentation of the head .....

168. Development of eye- muscles ....

169. Section through spinal cord showing relations of nerve-roots

and functional components .

170. Diagram of functional nerve components

171. Sections through end-brain of dogfish, frog, Chelonian, and

shrew ........

172. Dorsal view of brains of Petromyzon, Scyllium

Ceratodus, Triton, Lacerta, Columba, Lepus

173. Autonomic nervous system

174. Pituitary body of cat

175. Radiation of lower chordates

176. Radiation of amphibia

177. Radiation of reptiles .

178. Radiation of birds

179. Radiation of mammals

180. Piltdown skull .

181. Rhodesian skull

182. Neanderthal and modern man : skeletons compared

183. Comparison of sections of skulls

184. Comparison of brains of Macroscelides, Tupaia, Tarsius and

marmoset . . . . . . . .

185. Comparison of embryos of dogfish, lizard, chick, rabbit

and man

Gadus

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47i

VERTEBRATE ZOOLOGY

PART I
MORPHOLOGICAL TYPES

CHAPTER I

THE VERTEBRATE TYPE AS CONTRASTED WITH THE
INVERTEBRATE

Although Vertebrate animals form the subject of this book,
it must be said at once that, strictly, the term Chordate would
be more correct as a title. The chorda dorsalis or notochord,
from which the name is derived, made its appearance earlier
in evolution than the vertebral column. There are therefore
some animals which have a notochord but no vertebral column.
On the other hand, all animals with a vertebral column also
have a notochord at some time in their lives.

The term Vertebrate is used here partly because it is
equivalent in importance to Invertebrate, and the most usual
division of the animal kingdom lies between these two, and
partly because attention is here paid particularly to the higher
groups of " true " vertebrates. The lowly and peculiar
Balanoglossids as well as the degenerate Ascidians will be
left largely out of account, since they are not of much assistance
in tracing the evolutionary history of the higher forms.
Amphioxus as representative of the Cephalochordates, how-
ever, must be carefully considered on account of the help
which it gives in interpreting and understanding various
matters in higher forms.

B

2 MORPHOLOGICAL TYPES

The first necessity is to be clear as to what a chordate or
vertebrate animal is, and how it differs in plan of structure
from invertebrate animals (as typified, say, by Annelids or
Arthropods).

A vertebrate is bilaterally symmetrical and moves typically
in one direction with one side constantly presented upwards.
A ccelom is present and the body, which is elongated from
front to rear, is made up of a linear series of more or less
similar blocks or segments. This repetition of parts or
metameric segmentation affects tissues derived from all three
of the primary layers from which the animal develops (see
Chapter XI).

The gut on its way from mouth to anus is suspended in a
fold of ccelomic epithelium forming a dorsal mesentery.
The coelomic cavity can be separated into the following
regions. The dorsal parts of the coelomic epithelium form
the somites, which are segmentally arranged, and give rise
to plates of muscle, or myotomes, one pair to each seg-
ment. The portion of ccelomic space associated with each
myotome is a myocoel. The myoccel is bounded mesially by
the myotome, and laterally by the cutis-layer of the ccelomic
epithelium. Slightly ventral to each myotome is a region of
ccelomic epithelium which gives rise to excretory tubes
(coelomoducts). This region is the nephrotome and its cavity
the nephrocoel, also metamerically segmented. The ventral
region of the ccelom is lined by epithelium (peritoneum) which
forms the splanchnopleur where it is applied to the gut, and
the somatopleur applied to the outer wall of the body. In
this region the cavity is called the splanchnoccel.

The splanchnoccel is continuous from end to end of the
animal, and uninterrupted by partitions or septa, except for
that which separates an anterior pericardial from a posterior
perivisceral space. This amounts to saying that the segmenta-
tion of the mesoderm does not persist in the ventral region.
In higher forms much use is made of the free and uninter-
rupted space afforded by the splanchnoccel for the accommo-
dation of longitudinal excretory and genital ducts, extensions
of the liver and lungs, and coilings of the gut.

VERTEBRATE AND INVERTEBRATE

Fig. i (upper block). — Schematic view of a typical chordate animal in the
region of the gill-slits.

Part of the wall of the body is represented as removed in order to reveal

the interior.

Fig. 2 (lower block). — Transverse section through a typical chordate
animal (an embryo dogfish).

c, splanchnocoel ; d, cutis-layer or dermatome ; da, dorsal aorta ; dn,
dorsal nerve-root ; g, gut ; gs, gill-slit ; m, myotome ; mc, myocoel ; ms,
mesentery ; n, notochord ; nc, nerve-cord ; nl, nephroccel ; nr, neural
crest ; nt, nephrotome ; o, opening of gill-slit ; sc, sclerotome ; si, sub-
intestinal blood-vessel ; so, somatopleur or body-wall ; sp, splanchnopleur
or gut-wall ; v, blood-vessel running in the gill-arch between the gill-slits ;
vn, ventral nerve-root.

4 MORPHOLOGICAL TYPES

It is customary to refer to the dorsal segmented regions of
the mesoderm as vertebral plate, and to the ventral unsegmented
portions as lateral plate.

The gut is primitively straight, leading from a mouth at
the anterior end to an anus ; the latter is not at the extreme
posterior end of the animal but some distance in front of it.
Behind the anus is a well developed tail containing tissue
derived from all three germ-layers. The possession of such
a structure is one of the characteristics of the type as opposed
to most invertebrates.

The blood flows in well-marked channels, and the direction
of flow is forwards ventrally and backwards dorsally, which is
the reverse of the invertebrate condition. The heart is
ventral. Blood is led from the intestine to the liver by a
hepatic portal vessel. A " portal " vessel is a vein which
differs from others in that it not only starts from capillaries,
but breaks up into capillaries again at the other end. The
hepatic portal vein therefore runs from the capillaries of the
intestine to those of the liver, and " carries " digested food-
products thither. Ordinary veins do not break up into
capillaries again, but connect with other veins and lead to the
heart.

The nervous system is in the form of a hollow tube which
runs all the way down the dorsal side of the animal, and
contrasts sharply with the chief invertebrate type of two solid
ventral nerve-cords, swelling out into ganglia in each segment.
In vertebrates, the nerves are of two kinds, issuing from the
nerve- tube by dorsal or by ventral roots.

The primitive respiratory system of vertebrates is equally
distinctive, and consists of a number (usually five or six) of
pairs of openings which lead from the front part of the gut to
the outside — the gill-slits with their contained gills. These
structures are among the most important, not only on account
of their distinctiveness, but also because of the modifications
which they undergo and the consequences which follow from
their possession in evolution. The higher vertebrates breathe
by means of lungs, which are sacs pushed out from the gut.

Running down the back of the animal, above the gut and

VERTEBRATE AND INVERTEBRATE 5

beneath the nerve-tube, is a slender elastic rod which acts as
a primitive skeleton. This is the notochord, which in higher
forms is more or less obliterated and replaced by the backbone
or vertebral column. The main skeleton of vertebrates is
internal and not outside the body as in many invertebrates.

The higher forms have appendages, either fins or limbs,
four in number arranged in two pairs. They are composed
of tissue derived from several segments, not from one only as
in invertebrates.

The differences and similarities between this fundamental
plan and that of invertebrates may conveniently be set out in
tabular form.

Vertebrates and most higher Invertebrates agree in being :

bilaterally symmetrical ;

coelomate ;

metamerically segmented.
Vertebrates differ from Invertebrates in having :

a notochord ;

a dorsal and tubular nerve-cord ;

gill-slits ;

a postanal tail ;

a ventral heart through which blood flows forwards ;

main skeleton internal ;

appendages formed from several segments ;

a hepatic portal system ;

dorsal and ventral nerve-roots.

CHAPTER II

AMPHIOXUS, A PRIMITIVE CHORD ATE

A thorough knowledge of the type which forms the subject
of this chapter is fundamental for the study of vertebrates.

Externals. — Amphioxus lanceolatus is a small animal
(about 2 inches long) found in shallow seas with a sandy
bottom in which it burrows. It is elongated and pointed at
each end, from which fact it derives its name. The body is
compressed from side to side and is capable of rapid move-
ment by swimming, though it usually stays embedded in
sand, feeding with only the mouth protruding. Both ends of

be. n. gs. g -^ i f.

Fig. 3. — Amphioxus seen from the left side.
a, anus ; ap, atriopore ; be, buccal cirrhi ; g, gonad ; gs, gill-slits ; n,
notochord ; t, tail.

the animal are expanded into thin vertical fins, which are
joined by a shallow fin running all along the middle line of
the back. The fin also extends a short distance forwards
from the hind end on the ventral side. This median fin is of
simple structure ; it is supported by a row of stifTeners known
as " fin-ray boxes." These are made of connective tissue,
and they are more numerous than the segments of the body.
Along the dorsal fin the fin-ray boxes are arranged in a single
row, but along the short ventral fin there is a double row
of fin-ray boxes.

6

AMPHIOXUS

The epidermis is only one-cell thick, as in many inverte-
brates. Underlying the epidermis is the mesodermal dermis,
which, in Amphioxus, takes the form of gelatinous connective

■s s s

tissue. Epidermis and dermis together form what is ordi-
narily known as the skin. Beneath the skin the myotomes of
the body are arranged in a continuous row from front to rear.

8 MORPHOLOGICAL TYPES

Seen from the side and visible through the skin they are
markedly V-shaped, with the apex pointing forwards. Those
of one side alternate with those of the other.

The ventral side of the front end of the animal is expanded
to form the oral hood. The sides of the oral hood (which
are not quite symmetrical) bear a fringe of buccal cirrhi, each
one supported by a small jointed skeleton. The cirrhi bear
sense-organs. On the under surface of the oral hood the
epithelium is modified in places into a ciliated organ (the
organ of Miiller, or wheel-organ) whose function it is to
create a current of water flowing towards the mouth. Slightly
to the right of the middle line, a small depression opens into
the cavity of the oral hood, known as Hatschek's pit. (The
development of this interesting structure is described on
p. 167.)

The mouth is situated at the hind end of the oral hood,
and is a circular aperture pierced through a vertical transverse
plate, the velum. The size of the opening is regulated by a
circular sphincter muscle. In addition, there are twelve
velar tentacles arising from the rim of the mouth, provided
with sense-organs. Their function is to act as a strainer
across the mouth-opening.

The anus opens not at but near the hind end of the body
on the ventral surface, slightly to the left of the middle line
owing to median position of the ventral fin. Just in front of
the ventral fin is another aperture, the atriopore, the signifi-
cance of which will be understood with a knowledge of the
structure of the atrium.

Other external features to be noticed are the olfactory (or
Kolliker's) pit on the left side of the body very near the front
end, and the metapleural folds of the atrium. The gonads
can also be seen from the outside, as a row of sacs between
the mouth and the atriopore.

Alimentary System.— The gut leads straight from the
mouth to the anus without any loops or kinks. The anterior
half of it is the pharynx, the posterior is the intestine. A
blind outpushing is given off on the right side from the front
of the intestine, forming the so-called liver- diverticulum.

AMPHIOXUS

10

MORPHOLOGICAL TYPES

The gut is suspended by a dorsal mesentery, and its lining is
ciliated. It is surrounded by a thin coat of smooth muscle.

The side walls of the pharynx are perforated by a large
number of gill-slits, openings which slant forwards from
below up. For this reason several gill-slits will be cut in a
single transverse section.

Ciliary Mode of Feeding. — Along the whole length of the
dorsal wall of the pharynx runs a ciliated groove known as the

b.«gEE

Fig. 7. — Amphioxus : transverse
section through the endostyle
showing the cilia, the four
tracts of glandular cells (gc),
the subendostylar coelom (se),
and the ventral aorta (va).

Fig. 8. — Amphioxus : transverse section
through two primary (pg) and one
secondary (sg) gill-bars.

ae, atrial epithelium ; b, blood-vessel ;
c, coelomic cavity in the primary gill-
bars ; sk, skeletal rods of the gill-bars.

hyperpharyngeal groove. Anteriorly this groove connects
with two tracts of ciliated cells, the peripharyngeal bands
which pass round one on each side behind the mouth and
down to the floor of the pharynx. There they join the endo-
style, which extends all the way back through the pharyngeal
region. The endostyle consists of four tracts of glandular
cells, separated by tracts of ciliated cells. The glandular cells
secrete a sticky mucus which, by the action of the cilia, is

AMPHIOXUS

ii

driven forwards and sideways up the gill-bars, and round the
peripharyngeal bands, where food particles become entangled
in it. The food particles have been swept into the mouth
with the current of water made by the cilia of the wheel-organ
and gill-bars. Mucus and food then get carried into the
hyperpharyngeal groove and back to the intestine. By this
means the food is carried safely back through the pharynx as
by a moving stairway, and is not lost with the water which
streams out through the gill-slits.

The ciliary method of feeding is primitive. From the
nature of its mechanism it can only supply particles of food

Fig. 9. — Amphioxus : view of the dorsal portion of the pharynx and gill-
slits (gs), showing the nephridia («).

on, opening of the nephridium into the atrium ; pg, primary gill-bar ; s,
synapticulum ; sg, secondary gill-bar.

of small size, and therefore it can only occur in smallish
animals. In higher forms in which other methods such as
biting or sucking have been adopted for procuring food, the
endostyle is no longer required to secrete a mucus " fly-
paper " ; it becomes modified in a most striking way and
gives rise to the thyroid gland (see p. 399).

Atrium. — The gill- slits do not open directly to the outside
world but into a cavity known as the atrium, which in its
turn opens to the exterior near its posterior extremity by the
atriopore. On the right side of the body (but not on the left)
the atrium extends back behind the atriopore as a blind sac

12 MORPHOLOGICAL TYPES

nearly as far as the anus. The atrium has been formed by
folds of the body- wall above the gill-slits growing down on
each side (the metapleural folds) and meeting underneath
what is the true ventral surface of the animal. The space of
the atrium therefore represents a portion of the outside world,
and is lined entirely by ectoderm. The low ridges running
along the ventro-lateral edge of each of the atrial folds are the
metapleural folds, and between them, meeting in the middle
line, are the epipleurs, in the form of horizontal shelves. These
close off the atrial cavity (see p. 170). The atrium is closed
in front so that all the water which enters it does so through
the gill-slits and passes out of the atriopore. Where the
pharynx passes into the intestine, a pair of conical outpushings
of the atrium project into the dorsal ccelomic cavities, one on
each side, forming the so-called " brown funnels " (see p. 15).
The function of the atrium is to protect the pharyngeal
region, which is very vulnerable owing to the gill-slits.

Respiratory System. — At early stages the gill-slits corre-
sponded to the segmentation of the body, but more and more
of them are formed (up to 180) and the correspondence is
lost. The gill-slits are separated from each other by gill-bars,
the inner surface of which is covered by endodermal, the
outer by ectodermal tissue (forming the inner wall of the
atrium). There are two kinds of gill-bars : primary, and
secondary or tongue-bars. All the bars have a skeletal rod
(composed of a chitin-like substance) passing down them and
stiffening them. The rods of the secondary bars end simply
at their ventral ends, while those of the primary bars bifurcate.
Another difference is that the primary bars contain a portion
of ccelomic cavity while the secondary bars do not. The bars
are strongly ciliated, and by the activity of these cilia water is
forced through the slits into the atrium. As the water passes
between the gill-bars, the blood circulating in the blood-vessels
of the latter becomes oxygenated. There are three vessels in
each primary bar and two in each secondary bar ; the vessels
in the secondary bars are connected with those in the primary
bars by vessels running in the synapticula, or connecting
struts.

AMPHIOXUS 13

Vascular System. — Running forward under the floor of the
pharynx beneath the endostyle is the ventral aorta. There is
no specialised heart, but this aorta is contractile, and propels
the blood into the afferent branchial arteries which run to the
primary gill-bars. At the base of the bars these arteries swell
into little contractile bulbils and divide into the three vessels
which run up the bars. The secondary bars obtain blood in
their two vessels indirectly from the primary bars through the
vessels in the synapticula. Branches are sent to the excretory
organs (nephridia, see p. 15) which are thereby enabled to
extract the excretory products from the blood.

From the gill-bars the blood is collected into the efferent
branchial vessels which run to the lateral dorsal aortae, one on
each side of the mesentery, just above the hyperpharyngeal
groove. Behind the pharynx they join to form the single
dorsal aorta, which carries blood back to the posterior regions
of the body. In the septa separating each pair of adjacent
myotomes, segmental vessels leave the aorta and distribute
blood locally. The blood is collected up again into the sub-
intestinal vessel which runs forwards beneath the intestine
from the hind end of the body. It breaks up into capillaries
in the region of the liver- diverticulum, and so forms a hepatic
portal system. From the liver the vessel runs forwards
beneath the endostyle of the pharynx as the ventral aorta.
There are also paired cardinal veins running in the body-wall
at the level of the gonads, and extending forwards in the
region of the pharynx and backwards to the tail. These veins
connect with the subintestinal vessel by transverse veins, the
ductus Cuvieri, on each side, which bridge across the ccelom.
The blood is colourless.

Ccelom. — The relations of the ccelom are of great import-
ance. The myotomes of the body are separated by septa
(between the segments), but they do not fit the septa closely.
Small spaces are left which are remnants of the myoccels.

Behind the pharynx the relations of the ccelom are quite
simple and typical. The gut is suspended by a dorsal
mesentery in a spacious splanchnoccel. In the pharyngeal
region, however, the relations are slightly complicated by the

-df. #--

AMPHIOXUS 15

presence of the gill-slits. Since these slits are openings from
the gut to the outside (morphologically, ignoring the atrium)
they form connexions between the gut-wall and the body- wall,
and thereby necessarily obliterate the ccelom in places. The
ccelom, therefore, is restricted to the regions between the
slits, i.e. to the gill-bars. The coelom is perfectly normal
above and below the level of the gill-slits. Accordingly, there
are a pair of dorsal coelomic cavities, separated from one
another in the middle line by the dorsal mesentery ; and a
ventral coelomic cavity known from its position as the sub-
endostylar coelom. The latter is in open communication with
the dorsal ceeloms on each side by means of the coelomic
canals in the primary gill-bars. There are no coelomic canals
in the secondary or tongue bars, for they are later developments
which divide the original gill-slits into two. The relations of
the coelom are not difficult to understand when it is remembered
that relicts are left between the slits in the primary gill-bars.

Into each dorsal coelomic cavity a conical outpushing of
the atrium projects, from the region just behind the gill-slits.
The apex of the cone points forwards, and so lies dorsal to
the hindmost gill-slits. These structures are the so-called
" brown funnels," of doubtful significance (see p. 12).

Excretory System. — The excretory organs of Amphioxus
are remarkable in that they are nephridia. In all other
chordates the excretory organs are coelomoducts or meso-
dermal kidneys (see p. 31). The nephridia lie over the gill-
slits, project into the dorsal coelomic cavities and extend a
short way down the coelomic canals in the primary bars in the

Fig. 10. — Amphioxus : transverse sections through the body in the regions
of A, Kolliker's pit ; B, Hatschek's pit ; C, the anterior region of the
pharynx ; D, the posterior region of the pharynx ; E, between pharynx
and atriopore ; F, the atriopore ; G, between atriopore and anus ;
H, the anus.

a, anus ; op, atriopore ; at, atrium ; be, buccal cirrhi ; bf, brown funnel ;
c, ccelom ; da, dorsal aorta ; dc, dorsal coelomic canal (in the region of the
pharynx) ; df, dorsal fin ; dn, dorsal nerve-root ; e, eye-spot ; en, endostyle ;
ep, epipleur ; fr, fin-ray box ; g, gonad ; gs, gill-slit ; hg, hyperpharyngeal
ciliated groove ; Hp, Hatschek's pit ; i, intestine ; Kp, Kolliker's pit ; /,
liver ; Ida, lateral dorsal aorta ; m, myotome ; mc, myocoel ; mp, meta-
pleural fold ; n, notochord ; nc, nerve-cord ; oh, oral hood ; p, pharynx ;
pa, extension of the atrium behind the atriopore ; sc, subendostylar coelomic
canal ; vf, ventral fin ; vn, ventral nerve-root.

16 MORPHOLOGICAL TYPES

form of small bent tubes. Each nephridium bears bunches
of flame-cells or solenocytes, like hollow pins with a whip or
flagellum hanging down inside from the head, and serving to
flush out the contents. There is no internal opening to
the nephridia, which derive the products which they excrete
from the blood-vessels and ccelomic fluid by diffusion. The
nephridia open into the atrium (that is, morphologically to
the outside) by small pores situated near the top of the
secondary gill-bars. They are segmental in origin.

There is another nephridium at the front of the animal,
lying dorsal to the oral hood near the middle line. It opens
into the pharynx just behind the mouth, and is known as
Hatschek's nephridium. No chordates other than Amphioxus
are known to possess nephridia.

Genital System. — The sexes are separate, but very similar
in appearance. The gonads are pouches of germ-cells
arranged in a row on each side of the body from about the
ioth to the 36th segments, in the region of the gill-slits.
When these pouches are full they bulge into the atrium ; but
they must not be considered as lying in the atrium, for they
are separated from it by the whole thickness of the body- wall.
The segmental arrangement of the pouches is more or less
preserved. When ripe, the germ-cells burst out of the
pouches and pierce the body- wall, thus finding themselves in
the atrium. From here they make their way to the outside
through the atriopore. The cavity of the pouches is, of
course, ccelomic.

Skeleton. — Reference has already been made to the skeletal
supports of the buccal cirrhi and to those of the gill-bars.
The most important skeletal structure of Amphioxus is, of
course, the notochord. This elastic rod extends from end to
end of the animal, dorsal to the gut and ventral to the nerve-
cord. Its extreme extension, almost to the tips of the anterior
and posterior fins, is noteworthy.

Nervous System. — The central nervous system consists
of a straight tube running all the way down the back of the
animal, dorsal to the notochord and ventral to the fin-ray
boxes. Kolliker's pit on the left side of the snout represents

AMPHIOXUS 17

the spot where the cavity of the tube opened to the exterior
at earlier stages (the neuropore).

The cavity of the nerve-tube is enlarged at its front end
forming the cerebral vesicle. At the same time the external
diameter of the tube remains the same ; its walls are here
therefore thinner. This specialisation is in Amphioxus the only
indication of a brain. At the front end of the nerve-tube is a
pigment-spot, to be regarded as a visual organ. Other such
pigment-spots, or primitive " eyes," are to be found further
back near the central canal.

On each side of the body the nerve-cord gives off nerves,
which are of two kinds, dorsal and ventral. In each segment
on each side of the body there is one dorsal nerve-root and
one bunch of ventral nerve-roots. The ventral roots are
distributed solely to the muscle-fibres in the myotome of that
segment, and are " motor " nerves. The dorsal roots are
concerned with transmitting impulses received from the
sense-organs all over the skin (especially numerous on the
buccal cirrhi), and with innervating the smooth musculature
of the gut and atrium. The axons which go to make up the
sensory or afferent fibres of the dorsal nerve-roots are derived
directly from the sensory cells in the skin. The sensory cells
therefore convey their impulses direct to the central nervous
system on the plan characteristic of many invertebrates. In
all forms above Amphioxus, the impulses are collected from
the sensory cells by axons derived from other nerve-cells,
whose nuclei lie in swellings or ganglia on the dorsal nerve-
roots. Amphioxus is therefore primitive in not possessing
these ganglia or nerve- cells.

The most anterior two pairs of roots are dorsal, and have
no ventral roots corresponding to them. They innervate the
sense-organs of the snout, oral hood, and buccal cirrhi.

While reviewing the foregoing description of Amphioxus
an important analysis can be made. In the light of knowledge
of other forms, the characteristics of an animal can be divided
into two classes : primitive and specialised. There are those
characters which are developed and perfected in the process
of evolution to the next stage, and which are therefore simpler

C

1 8 MORPHOLOGICAL TYPES

at the stage in question (in this case Amphioxus). A primitive
character of this kind is shown by the vascular system of
Amphioxus. There are also negative characters, for the later
evolutionary stage may possess structures which the present
stage lacks. The absence of a specialised head in Amphioxus
is an example of a primitive negative character of this kind.
Then there are characters of which it cannot be said that they
are simpler than those of the next evolutionary stage, nor that
they lead on to them, but which can be considered as his-
torically primitive in the sense that they occur at early stages
but not at later ones. The ciliary method of feeding is an
historically primitive character of this kind : it preceded
the jaw-method of feeding in time, but was not simpler than
the latter method, nor did it lead up to it.

All primitive characters imply the possibility of progress
in evolution. On the other hand, there are certain characters
which have not only not contributed to the progress in evolu-
tion to the next stage, but have debarred their possessors
from ever evolving to that stage. Such specialised or
secondary characters are typified by the atrium of Amphioxus.

The analysis may conveniently be set out in tabular form :

Amphioxus

Primitive Characters.

Ciliary mode of feeding, with endostyle ;

Epidermis one-cell thick ;

Afferent nerve-fibres derived from sensory cells ;

Complete row of segmented myotomes from front to

rear ;
Very slight specialisation of brain ;
No specialised head ;
No paired limbs or paired sense-organs ;
No specialised heart ;
Gonads segmental, without special ducts ;
Nephridia ; segmentally arranged ;
Simple and unbranched liver diverticulum.

AMPHIOXUS 19

Specialised Characters.
Atrium ;

Extra large number of gill-slits, having lost corre-
spondence with the segmentation of the body ;
Tongue-bars ;

Asymmetry of oral hood and early development ;
Extreme anterior extension of the notochord.

The large number of its primitive characters show that
Amphioxus is a primitive animal, i.e. related to the original
ancestors from which all chordates evolved. The secondary
characters which Amphioxus possesses, however, show that
it is not on the direct line of chordate descent.

Literature

Bourne, G. C. An Introduction to the Study of the Comparative Anatomy
of Animals. Vol. 2. Bell, London, 191 5.

Delage, Y., et Herouard, E. Zoologie Concrete. Vol. 7. Les
Procord£s. Schleicher Freres, Paris, 1898.

Goodrich, E. S. On the Structure of the Excretory Organs of Amphioxus.
Quarterly Journal of Microscopical Science. Vol. 45, 1902 ; and
Vol. 54, 1910.

Orton, J. H. The Ciliary Mechanisms on the Gill and the Mode of
Feeding in Amphioxus. Journal of the Marine Biological Association.
Vol. 10, 1913.

Willey, A. Amphioxus and the Ancestry of the Vertebrates. Columbia
University Press, 1894.

CHAPTER III

PETROMYZON, A CHORD ATE WITH A SKULL, HEART, AND KIDNEY

Externals. — Petromyzon, commonly known as the lamprey, is
an elongated animal not unlike a fish, but without paired fins
or jaws. Some species live in fresh water, and others in the
sea. Their length varies from a few inches to about four feet.

The slimy epidermis is about a dozen cells in thickness and
contains glands. In the middle line the skin is produced into
median fins, two on the back and one round the tail. These
fins are stiffened only by rays of cartilage.

At the front there is a circular mouth surrounded by horny
teeth. Behind the mouth on each side is a small deep-set eye,
and then seven apertures in a row. These are the external
openings of the gill-pouches. Dorsally, in the middle line
near the front, there is a small hole which is the single median
opening of the nasal sacs and the hypophysial cavity (see
p. 401). The anus is in the mid- ventral line, not far in front
of the ventral portion of the tail-fin.

Through the mouth there protrudes a rasping organ called
the tongue, which like the sides of the mouth is covered with
horny teeth. These teeth are little cones, formed from the
ectoderm, and replaced from underneath when worn away.
They must be carefully distinguished from the teeth of all
higher forms, which are of a different nature.

The lamprey fastens itself by means of its circular and
sucker-like mouth onto its food (mostly fish), and rasps at it
with its tongue. This method of feeding is very specialised
and almost degenerate ; and it has brought about several
specialisations in the structure of the animal. The horny
teeth are one of these.

PETROMYZON

21

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Head. — The outstanding advance which the lamprey shows
over the condition of Amphioxus is the possession of a definite
head. A head is a specialisation of the anterior region of the
body brought about in connexion with :

the development of paired sense-organs for perception

at a distance ;
the correlated specialisation of the nerve-tube into a

brain.

To these is added in higher forms the specialisation of the
most anterior gill-bars into jaws for the capture of food ; but
this need not be considered here as the lamprey has no jaws.
The organs of the head are protected by a special skeletal
structure called the skull.

Sense-Organs. — The nose consists of two sacs invaginated
from the skin, and whose epithelium is specialised for the
perception and detection of chemical substances. This
epithelium sends nerve-fibres to the brain, forming the
olfactory nerves. While the paired nature of the olfactory
organ is easily seen on dissection, it is outwardly obscured by
the great expansion and upgrowth on each side of the region
corresponding to the upper lip. This modification causes
both the nasal pits and the hypophysial sac to open to the
outside by a single common median dorsal pore.

The main rudiments of the paired eyes in development
grow out from the brain on each side, giving rise to the optic
vesicles. The outer side of each vesicle is pushed in so as to
convert it into a cup, and the lens (developed from the super-
ficial skin) fits into the mouth of the cup, just beneath its rim.
The inner layer of the cup contains the cells which are sensitive
to light, and form the retina ; the outer layer forms a backing
of pigment. Outside this again, two mesodermal layers are
laid on. The innermost of these is the choroid which contains
blood-vessels, the outer, which is also the outermost of the
whole eyeball, is the hard and protective sclerotic. The
sclerotic encloses the whole eyeball, but in front of the lens it
is transparent, forming the cornea. The cornea is in contact
with the epidermis, which is here also transparent, forming the

PETROMYZON

23

conjunctiva. The eye may then be regarded as a closed hollow
ball, with the enclosed chamber divided into two by the lens :

Fig. 14. — Diagram showing the method of origin of the eyes in chordate
animals and the relations of the sensory cells.

A, before the formation of the nerve-cord the sensory cells (sc) are on
the outer surface ; B, when the nerve-cord has been formed the sensory
cells line its cavity (c) ; C, formation of the optic vesicles (ov) ; D, origin
of the lens (/) from the epidermis, conversion of the optic vesicles into
optic cups (oc) and formation of the pineal vesicle (pv) ; E, condition with
completely formed eyes, ac, anterior chamber ; ch, choroid ; cm, ciliary
muscle ; en, conjunctiva ; cr, cornea ; e, epidermis ; i, iris ; /, lens ; on,
optic nerve ; p, pigment layer ; pc, posterior chamber ; pi, pineal lens ;
pr, pineal retina (erect) ; r, retina (inverted) ; s, sclerotic.

viz., a posterior chamber between the retina and the lens, and
an anterior chamber between the lens and the cornea. The
posterior chamber contains a jelly-like substance called the

24 MORPHOLOGICAL TYPES

vitreous humour, the anterior chamber contains the aqueous
humour.

There is an important point to notice in connexion with the
retina. In Amphioxus the cells which are sensitive to light
line the central canal of the nerve-tube ; in all higher forms,
that portion of the wall of the nerve-tube in which these cells
lie is bulged out sideways to form the eye. The sensitive cells
are, however, still morphologically on the inner side of the wall
of the brain (i.e. adjacent to the central cavity or the cavity of
the optic vesicle). The nerve-fibres which convey the im-
pulses away from the retina cannot go through this cavity, they
must run in the wall of the optic vesicle and of the brain. In
so doing the nerve-fibres must therefore pass between the
sensitive cells and the lens. This means that the image of
the seen object reaches these sensitive cells after passing
through the nerve-fibres. A retina of this kind is called in-
verted, and is characteristic of the paired eyes of all chordates.
The pineal eyes (described below in connexion with the brain)
have an erect retina, for here the nerve-fibres leave the retina
on the side away from the lens. With regard to the paired
eyes, it is essential to realise that the cavity of the primitive
optic vesicle is not the same as that of the definitive eyeball (or
posterior chamber). In the process of conversion of the optic
vesicle into the optic cup, the cavity of the vesicle has been
obliterated. The eyes are not immovable, but can be turned
in various directions. This is effected by the myotomes of
the first three segments, which are modified into so-called
eye-muscles. The description given above applies to the
paired eyes of all chordates ; the eyes of the lamprey are,
however, somewhat degenerate.

Petromyzon has so-called auditory organs or ears, but it
must be remembered that these organs primitively do not serve
for the purpose of hearing, but are organs of balance. They
take the form of sacs on each side of the brain behind the eyes,
giving off canals in the form of half- hoops, each end of which
opens into the sac. These are the semicircular canals ; each
one bears a swelling or ampulla containing a statolith or organ
of balance (see p. 395). In all vertebrates above Petromyzon

PETROMYZON

25

there are three such canals, in planes at right angles to one
another, but Petromyzon has only two.

Brain. — The anterior end of the nerve-tube is modified and
enlarged in connexion with the paired sense-organs to form the
brain. The brain can be divided into three main regions :
fore-, mid-, and hindbrain. The forebrain bears the olfactory
lobes in front, and on each side it connects with the eyes (which
are really part of it) by the optic nerves. The roof bears the
pineal and parapineal eyes. These lie in the middle line, the
pineal above the parapineal which is degenerate. The pineal
eye is a vesicle of which the dorsal wall forms a lens, and the
ventral wall a retina backed with pigment. The nerve-fibres

ns. 00. Pf- Ff ■

hioc- Pb

Fig. 15. — Petromyzon : view of median longitudinal section through the

brain.

c, cerebellum ; cp, choroid plexus ; hs, hypophysial sac ; n, notochord ;
nc, nerve-cord ; ns, nostril ; oc, optic chiasma ; 00, olfactory organ ; pb,
pituitary body ; pe, pineal eye ; ppe, parapineal eye.

lead away from the underside of this retina, which is therefore
not inverted but erect. Above the pineal eye, the skull is thin
and the tissues are more or less transparent. Beneath the
forebrain is a simple pituitary body (see p. 399), the pars
intermedia of which is apposed to the feebly developed in-
fundibulum of the brain. The pituitary body in Petromyzon
has lost connexion with the hypophysial cavity ; the latter
extends backwards beneath the brain forming the hypophysial
sac, and connects with the exterior through the median dorsal
pore.

The midbrain bears the optic lobes, and the roof .of the
hindbrain is modified into a rudimentary cerebellum. It is

26 MORPHOLOGICAL TYPES

important to notice that the roof of the brain in Petromyzon
is very thin and membranous ; with the exception of the
transverse commissures in the forebrain, the optic lobes, and
the cerebellum, it contains scarcely any nerve-cells at all.

Although at first sight the brain differs considerably from
the more posterior part of the nerve- tube or spinal cord, it is
easy to see how it was derived from the latter.

In each segment on each side there is a ventral nerve
supplying the segmented muscles formed from the myotomes
of the vertebral plate (somatic muscles), and a dorsal nerve
supplying the sense-organs. In the gill-region, the dorsal
nerves also supply the muscles formed from the unsegmented
lateral plate (visceral or splanchnic muscles). In the region
of the trunk these segmental nerves are called spinal nerves,
those which emerge from the brain are called cranial nerves.

Nerves. — The ventral nerves of the first three segments
innervate the muscles which actuate the eyeball. They are
respectively the oculomotor, trochlear, and abducens. The
dorsal nerves corresponding to them are the profundus,
trigeminal, and facial ; the auditory nerve is a branch of the
facial. The ventral root of the 4th segment supplies the
most anterior complete myotome, and its corresponding
dorsal nerve, the glossopharyngeal, passes down in the arch
behind the first gill-slit (and in front of the second). The
next dorsal nerve, the vagus, is a composite one, formed by
the aggregation of portions of several other posterior dorsal
nerves. It sends a branch down behind each of the remaining
gill-slits, as well as to the " lateral-line " organs (see p. 38),
and to the heart and gut. The ventral roots corresponding
to the vagus supply the 5th and following myotomes, and the
muscles beneath the gills. The muscles of the tongue are
supplied by the trigeminal nerve.

In Amphioxus, the dorsal or sensory nerves are formed of
fibres produced inwards from the sensory cells themselves.
In Petromyzon and all higher forms this method of formation
applies only to the olfactory nerves. All the other sensory
nerves are formed in a different way. There are special nerve-
cells which send one fibre to the sensory cell and another

PETROMYZON 27

into the central nervous system. These nerve-cells are not
in the nerve-tube but just outside it. They lie on the track
of the sensory dorsal nerves and form swellings or ganglia.
In Petromyzon and all higher forms, on every dorsal nerve-
root, whether cranial or spinal, there is a ganglion. The
ventral nerves consist of nerve-fibres which are formed from
nerve-cells which lie inside the nerve-tube ; they therefore
do not have ganglia.

In Petromyzon there are two important primitive features
to note in connexion with the nerves. One is that the dorsal
and ventral roots of each segment do not join together, but
remain separate. The other is that the nerves are simple and
uncovered by insulating material, i.e. they are non-medullated.

The nerves which innervate striated muscles go straight
from the central nervous system to the muscle. On the other
hand, those nerves which supply the smooth muscle-fibres of
the gut and of the arteries (and in higher forms certain other
structures also) do not run direct from the central nervous
system to the muscle. Instead, they run to other nerve-cells,
and these run to the muscle. These latter nerve-cells form
part of the autonomic nervous system (see Chapter XXXI).
In Petromyzon this system is only feebly developed. It is
represented by some groups of nerve-cells along the gut,
supplied by the intestinal branch of the vagus (see p. 47), and
also by some cells close to blood-vessels near the spinal cord.

Skull and Skeleton. — The brain, paired sense-organs, and
roots of the cranial nerves are protected by a case of cartilage
forming the skull. This is characteristic of all forms above
Amphioxus which are therefore referred to as Craniata. The
brain is surrounded by the cranium proper ; the sense-organs
are protected by capsules. The nasal capsules are fixed on to
the front of the cranium by connective tissue ; the auditory
capsules are firmly fused on to the sides of the cranium by
cartilage. The spinal cord enters the skull at the hind end
through the foramen magnum, but in Petromyzon the nerves
of the fourth and following segments (glossopharyngeal and
vagus) come out from the brain behind the hindmost limit of
the skull. This shows that the process of cephalisation or

28 MORPHOLOGICAL TYPES

specialisation of the skull has not extended very far. The
walls of the skull are very incomplete, as is the roof. The
notochord extends forwards as far as the forebrain. In the
trunk-region, in each segment on each side of the notochord,
is a pair of cartilaginous pegs, one behind the other. The
anterior peg in each segment (interdorsal) is in front of the
ventral nerve-root ; the hinder peg (basidorsal) is in front of
the dorsal root. These pegs are the rudiments of the vertebral
column, as yet very incomplete and not in any way constricting
or interrupting the notochord.

Between every two gill-slits, behind the last and in front
of the first, are cartilaginous rods, the branchial arches or gill-
arches. Together they constitute the branchial basket. The
most anterior branchial arches, together with cartilages belong-
ing perhaps to vanished gill-clefts and others in connexion
with the rasping tongue, form a skeletal framework attached
to the skull and termed the splanchnocrarsium. The brain-
case and sense-capsules are called the neurocranium. In
higher forms the term " skull " is usually applied to both these
structures, but it should be realised that they are fundamentally
distinct. Petromyzon has no biting jaws ; instead, its mouth
is round, for which reason the group to which it belongs is
known as the Cyclostomes. The splanchnocranium of the
Cyclostome is unimportant from the present point of view,
because it is so much specialised that it can throw little light
on the skulls of higher forms.

The fins are supported by rays of cartilage.

Alimentary System. — In order to understand the structure
of the alimentary canal and associated organs more easily, it is
necessary to leave the adult Petromyzon and to turn to its
larval form, which is known as the Ammoccete. The mouth
is situated in a buccal cavity separated from the pharynx by a
velum. The side walls of the pharynx are pierced by seven
pairs of gill-slits.

Along the floor of the pharynx runs a groove which is con-
tinuous anteriorly with a pair of peripharyngeal bands. These
rise up on each side of the mouth, behind the velum. Posteriorly
the groove runs into a ventral hollow downgrowth of the

PETROMYZON

29

pharynx. The floor of this downgrowth is folded and con-
tains four rows of glandular cells. It is obvious that this
structure is practically identical with that of the endostyle of
Amphioxus. Behind the pharynx, the gut leads straight back
through an intestine to the anus.

During the metamorphosis from the Ammocoete larva to
the adult, certain important changes take place. The buccal
cavity develops into a sucker with horny teeth, and the rasping
tongue is formed in the floor of the pharynx. The endostyle
closes up and its glandular and ciliated cells disappear but

Fig. 16. — Petromyzon

transverse section through the endostyle of an
Ammocoete larva.

gc, the four tracts of glandular cells ; fp, floor of the pharynx ; va, branch of
the ventral aorta. (Compare with Amphioxus.)

its duct gives rise to the thyroid gland. From its habit of
adhering closely to its food with its sucking mouth, water
cannot easily pass through the animal's mouth to its gills. The
latter are modified into pouches which take water in through
their external apertures and then expel it again. Inside, the
pharynx becomes divided into two parts, one above the
other. The upper portion becomes the definitive passage
from the mouth to the intestine ; the lower becomes the
branchial tube. The latter is blind behind, receives the
inner openings of the seven pairs of gill-pouches, and opens

3o MORPHOLOGICAL TYPES

in front into the buccal cavity guarded by the remnants of the
velum.

These changes which take place during the life of Petro-
myzon show that it is a form descended from ancestors which
practised the ciliary mode of feeding, since when, it developed
a specialised and somewhat degenerate method of feeding of
its own.

Behind the pharyngeal region the gut runs straight as the
intestine to the anus : there is no indication of a curved and
enlarged region known in all higher forms as the stomach.
In the intestine the surface of absorption is slightly increased
by a small inwardly projecting ridge which, as it winds heli-
coidally down the intestine, is known as a " spiral valve."
The lining of the gut is ciliated.

Ventral outgrowths from the front of the intestine form the
liver. It is more or less degenerate in the adult, and it is said
that its communication with the intestine by means of the
bile-duct becomes lost, so that its only communication is with
the blood-vessels. The pancreas is very rudimentary, and
represented only by scattered packets of cells along the
intestine.

Vascular System. — Running forwards beneath the intestine,
and therefore in the splanchnopleur, is a subintestinal vessel.
It runs to the liver where it breaks up into capillaries forming
a hepatic portal system. From the liver, the vessel proceeds
forwards as the hepatic vein, and soon swells out beneath the
pharynx and becomes specialised to form a muscular pump :
the heart.

The heart is composed of the following structures : a sinus
venosus, into which the hepatic and other veins enter ; leading
on to a thin- walled auricle and a thick- walled ventricle. The
entry to and exit from the ventricle, which does the propelling
of the blood, are guarded by valves so that blood cannot flow
in the reversed direction. The length of the structures com-
posing the heart is greater than that of the space (pericardium)
in which they lie ; consequently the heart is slightly bent on
itself into the form of an S.

From the ventricle the ventral aorta runs forward beneath

PETROMYZON 31

the branchial duct, and gives off paired afferent branchial
vessels to the gill-arches, where they break down into the
capillaries of the gills. Paired efferent branchial vessels then
gather up the oxygenated blood and lead it to the dorsal aorta,
which runs back just beneath the notochord, and is continued
forwards into the head as the internal carotid artery.

On each side of the dorsal aorta are paired anterior and
posterior cardinal veins which lie of course in the body-wall
or somatopleur. At the level of the sinus venosus, the
cardinals of each side communicate with the heart by means
of the ductus Cuvieri. But as the cardinals are in the body-
wall and the sinus venosus is in the gut- wall, the ductus
Cuvieri have to cross the ccelom. This they do by means of a
bridge of ccelomic epithelium called the transverse septum.
The coelom is thus divided into an anterior region surrounding
the heart : the pericardium ; and a posterior perivisceral
splanchnocoel. This division is incomplete in the Ammo-
ccete larva, but complete in the adult. In the adult there
are peculiar median ventral veins in connexion with the
tongue, and the ductus Cuvieri on the left side disappears.

The blood is red owing to the presence of haemoglobin in
corpuscles. There is no spleen.

Excretory System. — No nephridia are found in any Craniate.
The excretory organ is derived from the segmented nephro-
tomes, between the myotomes and the lateral plate. Typically,
each nephrotome contains a cavity, the nephrocoel, which opens
into the splanchnocoel by ciliated funnels (ccelomostomes), one
in each segment. The nephroccels swell out into little cavities
known as Bowman's capsules, into each of which a glomerulus
projects. Each glomerulus is formed from an arteriole from
the dorsal aorta and a venule leading to the posterior cardinal
vein of its side. Glomerulus and Bowman's capsule together
form what is known as a Malpighian corpuscle. From each
capsule, a tubule grows backwards and into the tubule of its
next posterior neighbour. A collecting duct is thus formed
on each side, and it grows back, meets its fellow in the middle
line, and opens behind the anus on a small papilla. This is
the typical structure of the vertebrate kidney, and it is to be

32

MORPHOLOGICAL TYPES

^3t3ZJJC-3C3

Aic4?i

A c'c , mc. n'c.

Pf- d.a- Be.

pt. T- pc. dmtaa

■/ \ ! ^ -r-

J^rjc^i iL-f^v-— " n n ,L~^V\

V-' Pd c ** V

feft&^gg^l

I I

DT. 'pd. D mcl. m

[Fig. 17.— Method of formation of the kidneys in chordate animals.

A, early stage showing the cavity of the splanchnoccel (sc) M"™^**
with those of the myocoels (tnc) by means of the nephrosis (nc). 1, intes-
tine B hypothetical archinephros. The nephrocoels now known as
Bowman' s capsules (Be) preserve their connexions with the . jp anc fcnoccd
by means of ciliated funnels (,/). From each capsule 1 tubu le Ogrow
back and joins that formed from the next posterior cfPs^%\0sof^edth^
archineohric duct (ad). With each Bowman's capsule is associated a
SoSlu" torfomed from an arteriole from the dorsal aorta (da) and a

PETROMYZON 33

noted that the tubules which form it are of mesodermal origin,
coming from the ccelomic epithelium, and are sharply to be
distinguished from nephridia, which are ectodermal in origin.

Originally these coelomostomes must have served to free the
genital products (as indeed the spermatozoa are freed in all
higher forms), and excretion also took place through them from
the coelom to the exterior. Then the excretory products were
brought to the tubules by blood-vessels and the coelom lost its
excretory function. The tubules lose their connexion with the
splanchnocoel. In Petromyzon the coelomostomes were never
open. In other forms, the coelomostomes may persist. In
Myxine, a close relative of Petromyzon, the Malpighian
corpuscles retain their segmental arrangement, but in all other
forms they become increased in number, and the segmental
correspondence is lost.

The tubules arise in two sets. First an anterior lot, opening
into the pericardium, form the pronephros, and their duct (the
pronephric duct) grows right back to the papilla. The
pronephros nearly disappears in the adult Petromyzon, and is
replaced in function by an identical but more posterior set of
tubules which form the mesonephros. The mesonephric
tubules grow into the pronephric duct which they find ready-
made for them, and which becomes known as the mesonephric
duct. The mesonephric tubules develop later than the
pronephric tubules, but are of essentially the same nature.
This is illustrated in Bdellostoma, another relative of Petro-
myzon, in which a continuous series of tubules arises, the more
anterior of which become the pronephros, and the posterior

venule to the posterior cardinal vein (pc). C, condition in most young
chordates, in which an anterior set of pronephric tubules (pt) gives rise to
a pronephric duct (pd) before the more posterior mesonephric tubules
(dmt) are properly formed, pf, pronephric funnels ; mf, mesonephric
funnels. D, condition in older chordates, and retained throughout life in
Myxine. The pronephros has degenerated, and the mesonephric tubules
(mi) have joined the pronephric duct which now bears the name mesonephric
duct (md). The latter is no longer continuous with the anterior portion of
the pronephric duct. The capsules of the mesonephros lose their connexion
with the splanchnocoel, but they retain their simple segmental arrangement.
E, condition in Petromyzon. The pronephros has degenerated, and the
number of capsules in the mesonephros has been increased by the formation
of secondary Bowman's capsules (sBc).

D

34

MORPHOLOGICAL TYPES

the mesonephros. Such a primitive kidney approaches very
closely to the hypothetical Archinephros, with its archinephric
duct. It is important to note that in the Cyclostomes there is
but one kidney-duct on each side. The kidneys and their
ducts hang down in the ccelom as the so-called nephric fold.

Genital System. — The gonads are situated in a ridge hang-
ing down from the roof of the coelom. Originally paired, the
gonad is single and median in Petromyzon. In both sexes the
genital products are shed freely into the splanchnoccel. In
front of the place where the two mesonephric ducts join, each

a u<3

FlG. i 8. — Petromyzon : view of a dissection from the left side of the anus (a)
and urinogenital (ug) aperture.

df, dorsal fin ; i, intestine ; kd, kidney duct ; n, notochord ; nc, nerve-
cord ; pv, perivisceral coelomic cavity. The arrow is passed through the
genital pore from the coelom.

duct has a small pore opening into the splanchnoccel, and it is
through these pores that the genital products escape to the
exterior ; they have no special ducts.

Ccelom and Mesoderm. — The eventual division of the
coelom by the transverse septum into pericardial and peri-
visceral cavities has already been mentioned. The myoccels
are completely obliterated, and the only other portions of
coelomic cavity are the kidney- tubules and ducts. The first
three somites are drawn off into the service of the eyeballs,
and the fourth forms the first proper trunk-muscle. The

PETROMYZON 35

series of somites is therefore complete, and no segment
has lost its somite. The myotomes are W-shaped, which
condition can easily be derived from the V-shaped myotomes
of Amphioxus by the turning forwards of their upper and lower
ends. They are not subdivided into dorsal and ventral
portions as in higher forms.

The characters of Petromyzon and Cyclostomes can be
analysed under three headings : those which show an advance
from the condition of Amphioxus, those which are primitive
when compared with higher forms, and those which are
secondary and specialised.

Characters shown by Petromyzon, absent in Amphioxus and
typical of Craniata :

Formation of a distinct head, brain, and skull ;

Formation of a distinct heart ;

Formation of pro- and mesonephric kidneys ;

Epidermis several cells in thickness ;

Dorsal nerves with ganglia ;

Rudimentary vertebral column ;

Myotomes W-shaped ;

Rudimentary sympathetic nervous system.

Characters shown by Petromyzon {and Cyclostomes) which
are primitive when compared with higher forms :

Endostyle, ciliated groove and velum of the Ammocoete

larva ;
Emergence of glossopharyngeal behind the cranium ;
Dorsal and ventral nerves separate and unconnected in

each segment ;
Fourth segment forming a complete myotome : no

myotomes lost ;
Notochord unconstricted by vertebral column ;
Kidney-tubules segmental in Myxine ;
Myotomes not divided into dorsal and ventral portions ;
Persistence (although slight) of the pronephros ;
Absence of biting jaws ;
Absence of paired fins ;

36 MORPHOLOGICAL TYPES

Absence of dermal skeleton (fin-rays or teeth) ;
Absence of special stomach ;
Absence of special genital ducts ;
Absence of medullated nerves.

Specialised characters of Petromyzon :

Rasping tongue ;
Sucking mouth ;
Horny teeth ;
Sac-like gill-pouches ;
Separate branchial duct ;
Large hypophysial sac ;

Single median dorsal pore for nasal organs and hypo-
physial sac.

Literature.

Goodrich, E. S. Vertebrata Craniata, Cyclostomes and Fishes. Black,
London, 1909.

Parker, T. J. A Course of Instruction in Zootomy (Vertebrata). Mac-
millan, London, 1884.

CHAPTER IV

SCYLLIUM, A CHORD ATE WITH JAWS, STOMACH, AND FINS

Externals. — The dogfish possesses an elongated body with a
distinct head and tail, the latter provided with a tail-fin of which
the ventral lobe is larger than the dorsal (heterocercal). There
are two median dorsal fins. The most obvious advance over
the Cyclostome condition is the possession of paired fins, of
which there are two pairs : a pectoral and a pelvic.

The head has paired nasal sacs and eyes. The mouth
is situated some distance behind the anterior end of the snout.
Behind the mouth on each side are six openings into the
pharynx. The first pair of these is small and more dorsally
situated than the others ; it is the spiracle. The remaining
five are the gill-slits, numbered i to 5. The anus lies in a
cloaca (joint opening of the alimentary and urino-genital
systems) in the midventral line behind the pelvic fins, and on
each side of it is a small pore (the so-called abdominal pore)
communicating with the coelom. In the male, there is a pair
of claspers on each side of the cloaca.

Denticles. — The body is covered all over with small sharp
spikes, with the points directed backwards. These are the
placoid scales or denticles. They are made of dentine covered
over with a cap of enamel. Dentine is a hard substance pro-
duced by mesodermal cells beneath the epidermis ; and it is
identical with the substance of which the teeth of all verte-
brates are made. It consists of a calcified ground-substance
in which filamentous processes of cells are to be found, but no
cells themselves. In this particular it differs from bone. The
enamel is formed from the ectoderm. On the inner rim of the
jaws just inside the mouth, denticles are also found. They

37

38

MORPHOLOGICAL TYPES

get pushed out to the
edge of the jaws, and act
as biting teeth. When
worn out, they are re-
placed by others which
are pushed up in their
turn to the biting edge
(see Fig. 122, p. 263).

Lateral Line. — In vari-
ous places over the body,
there are peculiar organs
belonging to what is
known as the lateral-line
system. Essentially, they
take the form of canals
sunk beneath the skin,
and opening to the ex-
terior at intervals. In
these canals are sense-
organs whose probable
function it is to appre-
ciate low-frequency vibra-
tions in the water. One
of these canals runs along
the side of the body from
the tail to the head, and
is the true " lateral-line
canal." At about the
level of the first gill-slit,
it gives off a transverse
occipital canal which runs
over the top of the head
and meets its fellow from
the opposite side. It con-
tinues forwards over the
spiracle as a short tem-
poral or postorbital canal,
and divides into two. One

SCYLLIUM

39

portion goes forwards over the eye as the supraorbital canal,
the other beneath it as the infraorbital canal. In addition there
is typically a canal which runs down behind the spiracle
(hyomandibular canal) to the lower jaw (mandibular canal) ;
but these two are more or less interrupted in the dogfish.

Close to the lateral-line canals in various places there are
little pits leading from pores on the surface down narrow tubes
to ampullar at the bottom , where there are sense-organs . These

Fig. 20. — Scyllium : section through the skin showing the ampullar of
Lorenzini (aL), denticles (d), nerve-fibres (w), opening of a lateral-
line canal (oe), and a sense-organ (so) in the canal.

are the pit-organs, or ampullae of Lorenzini. Together with
the lateral-line system they constitute the neuromast, or
acustico-lateralis organs, to which the ear also belongs. These
organs are also present in Petromyzon, but not so definitely
arranged.

Ear. — The ear consists of a pit sunk in from the skin and
forming a sac, which communicates with the exterior by a long
tube and a fine pore ; the ductus endolymphaticus. The sac
is divided into a more dorsal utricle, and a more ventral saccule.

4o

MORPHOLOGICAL TYPES

The utricle bears three semicircular canals, at the base of each
of which is a swelling or ampulla containing an organ of
balance. The whole ear is to be regarded as a very much
enlarged lateral-line organ.

Eye. — The eyes are hollow cups with sensitive retinal
layer, iris, pigment layer, vascular choroid, and protective
cartilaginous sclerotic. Fitting into the aperture of the eye-

- - -de.

FlO, 21. — Scyllium : view of the outer side of the left auditory sac.

a, ampulla ; 06, 2c, and /v. anterior, lateral, and posterior semicircular

canals ; Jc. ductus endolymphaticus : s, saccule ; If, utricle.

cup is the spherical lens, which is attached to the cup by a
ventral ciliary muscle.

The movements of the eyeball are effected by six muscles.
Four oi these (superior, internal, inferior, and external rectus
muscles') exert straight pulls on the four cardinal points of the
eyeball, and turn it upwards, forwards, downwards, or back-
wards respectively. The two others (superior and inferior
oblique muscles) pull it obliquely either forwards and upwards
or forwards and downwards.

Nose. — The nose is formed by a pair of pits on each side

SCYLLIUM 41

of the under surface of the snout just in front of the mouth,
and connected with it by grooves which run to its corners.
Inside the pits, the sensory olfactory epithelium is thrown into
a number of folds.

Nervous System. — As in the Cyclostome, the brain is
divisible into fore-, mid-, and hind-regions. Further, the
fore- and hind-brains can also be divided into two for facilitat-
ing description. There are therefore five sections of the
brain, whose names from front to rear are : telencephalon,
diencephalon (also called thalamencephalon), mesencephalon,
metencephalon, and myelencephalon. The first two divisions
together form the forebrain or prosencephalon, the last two
form the hindbrain or rhombencephalon. The sides of the
telencephalon (or end-brain) are greatly expanded, and bear
the olfactory bulbs. On the floor is the optic chiasma, where
the optic nerves cross over from one side to the other. In
front of this is the lamina terminalis ; the thickened lower
portions of the side walls are the corpora striata.

A transverse fold in the roof, the velum transversum, marks
the beginning of the diencephalon (or between-brain). The
sides are thickened and known as the optic thalami, the floor
is depressed to form the infundibulum to which the pituitary is
attached. The roof bears a projection : the epiphysis, vestige
of the pineal eye.

The floor, sides, and roof of the mesencephalon are
thickened, so that its cavity is reduced and is known as the
aqueduct of Sylvius. The roof forms the paired optic lobes.

The roof of the metencephalon is thick and forms the
cerebellum, that of the myelencephalon is thin. To the sides
of and behind the cerebellum are the restiform bodies.

The cavity of the forebrain is called the 3rd ventricle ;
that of the hindbrain the 4th ventricle. The brain is sur-
rounded by a membrane carrying blood-vessels (the pia mater),
and this dips down in folds from the roof of the 3rd and of the
4th ventricles to form a choroid plexus. Connecting one side
of the brain with the other there are tracts of fibres called
commissures. Of these, the habenular and the posterior are
in the roof of the between-brain and midbrain respectively

42 MORPHOLOGICAL TYPES

There is also an (anterior) transverse commissure in the
lamina terminalis, but on the whole there is little inter-
connexion between the two sides of the brain.

The myelencephalon, or medulla oblongata, passes back
gradually into the spinal cord. This is a tube with thick walls
and a small central cavity, continuous of course with that of the
brain. The nerve-cells are grouped round the centre of the

he pcop. Rf.
I Li i

oU- .

CP3.

vi.

>b

Fig. 22. — Scyllium : median view of a longitudinal section through
the brain.

The various regions of the brain are separated by broken lines across
the central cavity, and indicated by the letters : T, telencephalon (end-
brain) ; D, diencephalon (or thalamencephalon, between-brain) ; Ms,
mesencephalon (midbrain) ; Mt, metencephalon (anterior part of hind-
brain) ; My, myelencephalon (posterior part of hindbrain) ; ac, anterior
commissure ; c, cerebellum ; cp 3 and 4, choroid plexus of the third and
fourth ventricle ; he, habenular commissure ; i, infundibulum ; //, lamina
terminalis ; mo, medulla oblongata ; oc, optic chiasma ; ol, olfactory lobe ;
op, optic lobe ; pb, pituitary body ; pc, posterior commissure ; pe, pineal
stalk ; Rf, Reissner's fibre ; rn, recessus neuroporis ; sc, spinal cord ;
v 3 and 4, cavity of the third and fourth ventricle ; vt, velum transversum.
(Partly after Nicholls.)

cord, and form the " grey matter." Outside them and
occupying the remaining space are the ascending and descend-
ing tracts of nerve-fibres, provided with medullary sheaths,
and forming the " white matter." This arrangement of
central grey matter and peripheral white matter holds also in
the brain. Only in the cerebellum and in the optic lobes are
there some superficial nerve-cells ; i.e. grey matter outside
white.

SCYLL1UM

43

Attention may here be called to Reissner's fibre. It is a
wire-like structure of unknown function which runs from the
posterior commissure in the roof of the midbrain, through
the cavity of the nerve-tube right down to its hind end where it
is attached. Reissner's fibre is present in most chordates,
but not in Amphioxus nor in man.

The pia mater has already been mentioned. It encircles

Fig. 23. — Scyllium : ventral view of the brain showing the pituitary

body.

al, anterior lobe ; nil, neuro-intermediate lobe ; vl, ventral lobe of the
pituitary body ; il, floor of the diencephalon ; on, optic nerve ; sv, saccus
vasculosus.

the whole nerve-tube, and corresponds to the choroid layer of
the eye. Outside it is a tougher membrane, the dura mater,
protective in function. It is applied to the inner wall of the
skull and corresponds to the sclerotic layer of the eye.

Spinal Nerves. — In each segment on each side, behind the
head, the spinal cord gives off a ventral nerve, and a dorsal
nerve with a ganglion on it. These two nerves join to form a
mixed spinal nerve. Soon after joining, their components

44

MORPHOLOGICAL TYPES

— gsi,

SCYLLIUM 45

separate out again to their various destinations. The ventral
roots are distributed to the muscles of the myotomes along
the trunk and in the fins, the dorsal roots to the sense-organs.
In addition, each spinal nerve sends a branch to the sympathetic
ganglia, which are joined to one another by nerve-fibres which
form two longitudinal chains, one on each side of the dorsal
aorta.

The nerve-fibres are medullated, except those of the
sympathetic system.

Cranial Nerves and Head-Segmentation. — The cranial nerves
are of importance in unravelling the segmentation of the
head. Some of them are dorsal roots, and some are ventral,
but they never join to form mixed nerves as in the region
of the trunk.

The olfactory nerve (No. I) is not a true nerve like the
others, for it is formed of the fibres produced by the cells of
the nasal epithelium which grow in to the forebrain (in the
manner characteristic of all the nerves in Amphioxus). The
small nervus terminalis which accompanies it for some distance
is also devoid of segmental significance.

Similarly the optic nerve (No. II) is not segmental, for the
whole optic cup and stalk are really parts of the brain itself.

The dorsal root of the first (or premandibular) segment is
the profundus (No. V i), which unfortunately disappears in
Scyllium. In the closely related Squalus it is present, and
runs forwards through the socket for the eye (the orbit) under
the superior oblique muscle, and innervates the skin of the
snout.

Fig. 24. — Scyllium : dorsal view of a dissection of the cranial nerves,
from a drawing by Mr. B. W. Tucker.

II, optic ; III, oculomotor ; IV, trochlear ; V md, mandibular branch
of trigeminal ; V and VII op, superficial ophthalmic branches of trigeminal
and facial ; V mx and VII b, maxillary branch of trigeminal and buccal
branch of facial ; VII p, palatine branch of facial ; VII h, hyomandibular
branch of facial ; VIII, auditory ; IX, glossopharyngeal ; X, vagus ; X b 2,
second branchial branch of vagus ; X /, lateral-line branch of vagus ;
X v, visceral branch of vagus ; ac, auditory capsule ; c, cerebellum ; ey
eye ; ery external rectus eye-muscle ; gs 1 and 5, first and fifth gill-slits ;
h, hypoglossal nerve ; io, inferior oblique eye-muscle ; ir, inferior rectus
eye-muscle ; ol, olfactory lobe ; s, spiracle ; sc, spinal cord ; so, superior
oblique eye-muscle ; sry superior rectus eye-muscle ; t, terminalis nerve.

46 MORPHOLOGICAL TYPES

The corresponding ventral root of the first segment is the
oculomotor (No. Ill), which supplies the following four eye-
muscles : superior, internal, and inferior rectus, and the
inferior oblique. It is also connected with the sympathetic
ciliary ganglion.

The second or mandibular segment has as its dorsal root
the trigeminal (No. V, 2 and 3). This nerve is composed of a
superficial ophthalmic branch running forwards over the eye,
a maxillary branch in the upper jaw and a mandibular branch
in the lower jaw. These nerves are distributed to sense-
organs in the skin, and also to the muscles which move the
jaws.

The corresponding ventral root of the second segment is
the trochlear (also called pathetic ; No. IV), which innervates
the superior oblique eye-muscle.

The dorsal root of the third or hyoid segment is the facial
nerve (No. VII). It is made up of the following branches : —

superficial ophthalmic, running forwards over the eye
in company with that of the trigeminal, and inner-
vating the supraorbital lateral-line organs ;

buccal, running forwards beneath the eye and inner-
vating the infraorbital lateral-line organs ;

hyomandibular, passing down behind the spiracle to
innervate the lateral- line organs of the lower jaw, and
the muscles of the hyoid arch ;

palatine, innervating the taste-organs on the roof of the
mouth ;

pretrematic, running down in front of the spiracle,
innervating sense-organs.

The auditory nerve (No. VIII), which innervates the sense-
organs of the ear, is really an enlarged and specialised branch
of the facial nerve.

The corresponding ventral root of the third segment is the
abducens (No. VI), which supplies the external rectus eye-
muscle.

The dorsal root of the fourth segment is the glossopharyn-
geal (No. IX). It has a branch to the temporal region of the

SCYLLIUM 47

lateral-line canal, a pharyngeal branch to the gut, and a branchial
branch which divides ; a small branch running in the hyoid
arch in front of the first gill-slit, and the main branch running
behind the ist gill-slit in the ist branchial arch. The glosso-
pharyngeal thus bears the same relations to the ist gill-slit as
the facial does to the spiracle.

There is no ventral root to the fourth segment ; the
somite which it would innervate disappears.

The fifth segment has also lost its myotome and ventral
root during development. To each of the remaining gill-
slits, 2nd to 5th, there corresponds a branchial nerve, the
main branches running behind the slits and pretrematic
branches in front of them. These nerves are the dorsal roots
of the fifth, sixth, seventh, and eighth segments, which have
joined together to form the vagus (No. X). The lateral-line
organs in the occipital region of the head and all along the side
of the trunk to the tail, are innervated by branches of the vagus.
In addition, the vagus sends a visceral branch to the heart and
stomach, forming part of the parasympathetic system.

The ventral roots of the sixth and following segments
innervate the myotomes of their segments, and also contribute
to a nerve — the hypoglossal — which runs back over the gill-
slits, down behind them and forwards beneath them to inner-
vate some muscles under the pharynx.

The ninth is the first segment to have a fully formed mixed
spinal nerve.

Skull. — The skull and all the skeleton is made of cartilage.

The glossopharyngeal and vagus nerves emerge well in
front of the hind end of the skull. The latter therefore
occupies a larger number of segments than in Petromyzon,
namely seven. The skull encloses the brain completely except
for an aperture in its roof. The cranial nerves all emerge
through special holes or foramina. The auditory and olfactory
capsules are firmly fused on. The notochord disappears in
the skull-region, and a definite joint is formed between the
hind end of the skull and the front of the vertebral column.

The jaws are formed by the skeleton of the first or mandi-
bular visceral arch, which separates the mouth from the

48

MORPHOLOGICAL TYPES

spiracle. The upper and lower portions of this arch's skeleton
move on one another and form the upper and lower jaws.
The skeleton of the upper jaw is the ptery go- quadrate, that of
the lower is Meckel's cartilage. The possession of jaws is the
criterion of the group Gnathostomata, to which Scyllium and
all higher forms belong. The arches between the gill-slits
also have cartilaginous rods. That of the second or hyoid

i ,fj Svqc.

Fig. 25. — Scyllium : view of skull and visceral arches.

a, foramen for efferent pseudobranchial artery ; ac, auditory capsule ;
c, centrum of vertebra ; ch, ceratohyal ; cb 2, ceratobranchial of second
arch ; d, foramen for dorsal spinal nerve-root ; eb 4, epibranchial of fourth
arch ; hb 2, hypobranchial of second arch ; hm, hyomandibula ; id, inter-
dorsal cartilage ; Mc, Meckel's cartilage ; o, orbit ; oc, olfactory capsule ;
pb 1 , pharyngobranchial of first arch ; pg, pterygoquadrate cartilage ;
v, foramen for pituitary vein ; vr, foramen for ventral spinal nerve-root ;
II, optic nerve foramen ; III, oculomotor nerve foramen ; IV, trochlear
nerve foramen ; V and VII, trigeminal and facial foramen ; V and VII o,
foramina for ophthalmic branches of trigeminal and facial.

visceral arch (separating the spiracle from the 1st gill-slit) is
composed of a dorsal portion, the hyomandibula ; and a
ventral portion, the ceratohyal, and basihyal. The following
visceral arches are made up of four pieces on each side, which
are from above downwards, the pharyngobranchial, epi-,
cerato-, and hypobranchial. There is also a median basi-
branchial. The pterygo-quadrate and the hyomandibula
represent the " epi " elements of their respective arches,

SCYLLIUM

49

Meckel's cartilage and ceratohyal the
There is no difficulty in recognising the fact that the jaws
are simply slightly modified visceral arches. These carti-
laginous arches lie in the splanchnopleur. Stiffening the
partitions between the gill-slits are extrabranchials and
branchial rays.

It is important to notice that these jaws and branchial
arches, which together constitute the splanchnocranium, are
not fused on to the neurocranium or attached to it otherwise
than by ligaments.

The pterygo-quadrate is slung from the skull by the
hyomandibula, the upper end of which is attached to the
auditory capsule. This method of suspension of the upper jaw
is called hyostylic. The upper jaw does not touch the neuro-
cranium itself. In addition to its attachment by the hyomandi-
bula, there are two ligaments, the ethmoid and the post-
spiracular, which tie the upper jaw to the brain-case.

The ordinal numbers by which the arches, segments, and
slits are known are unfortunately liable to lead to confusion,
for which reason they are tabulated below :

Slits or Clefts.

i st visceral = spiracle
2nd visceral = i st gill slit
3rd visceral = 2nd gill slit
4th visceral =3rd gill slit
5th visceral =4th gill slit
6th visceral = 5th gill slit

Vertebral Column. — Corresponding to each septum between
two segments, there are paired basidorsal and basiventral
cartilages, surrounding the notochord. The sheath of the
notochord is penetrated by these cartilages which, together,
form a bobbin-like ring or centrum, which constricts and
interrupts the notochord. The centra articulate on one
another end to end, and in this way a vertebral column is

Segments.
1 st = premandibular
2nd = mandibular

Arches or Bars.
1st visceral = mandibular

3rd=hyoid

2nd visceral =hyoid

4th

3rd visceral = 1 st branchial

5th

4th visceral = 2nd branchial

6th

5th visceral = 3rd branchial

7th

6th visceral = 4th branchial

8th

7th visceral = 5 th branchial

50

MORPHOLOGICAL TYPES

formed. Rising up from the centra are the neural arches,
which enclose the spinal cord in a canal. Alternating with
these are interdorsal cartilages. The ventral nerves emerge
behind the neural arches, and the dorsal roots behind the

Fig. 26. — Scyllium : ventral view of, A, pectoral, and B, pelvic girdle.

by basipterygium ; c, coracoid region ; d, skeleton of clasper (present in
the male) ; ms, mesopterygium ; mt , metapterygium ; n, nerve foramen ;
p> pelvic cartilage ; pr, propterygium ; r, radials ; s, scapular region.

interdorsal cartilages. Ventral extensions of the basiventrals
beneath the centra in the tail-region form haemal arches, in
which blood-vessels run. Lateral extensions of the basi-
ventrals give rise to the ribs. They extend in the septum

SCYLLIUM 51

that divides the myotomes horizontally, and are called " dorsal "
ribs (see p. 307).

Fins. — The median fins are supported by jointed cartila-
ginous rods or radials. These were originally direct con-
tinuations of the neural and haemal arches, but as a result of
the shortening of the bases of the fins (or concentration), the
radials no longer correspond with the vertebras, except in
the ventral lobe of the tail. In addition to the cartilaginous
radials, the web of the fin is supported by horny dermal fin-
rays, close under the skin on each side of the radials. These
rays, or ceratotrichia, are more numerous than the radials.

The paired fins also have an internal skeleton of cartila-
ginous radials, and are anchored to the body by girdles lying
in the body- wall. The pectoral girdle is a half-hoop of cartilage
set transversely to the long axis of the body, with the free ends
pointing upwards. On each side is a hollow, the glenoid
cavity, into which the cartilages of the fins fit. The latter
cartilages are the most proximal radials, which form three
large cartilages, the pro-, meso-, and metapterygia. The
ventral portion of the pectoral girdle is termed the coracoid
region ; from the glenoid cavity to the free tips which project
dorsally, the cartilage is known as the scapular region.

The pelvic girdle is formed by a transverse cartilage, at
each end of which an elongated backwardly-directed basi-
pterygium is articulated. This basipterygium forms the axis
of the pelvic fin, and bears a number of cartilaginous radials
on its anterior border.

The pectoral and pelvic fins, as well as the median fins,
have their webs supported by horny dermal fin-rays, the
ceratotrichia.

Alimentary System. — The mouth leads into the pharynx,
the sides of which are pierced by the spiracle and the gill-slits.
The food consists of fair-sized pieces of prey, seized by the
jaws, and in no danger of being lost through the gill-slits.
Behind the pharynx is the oesophagus which leads into a large
stomach. In its formation, the gut has been kinked to the left,
so that the stomach is a well-defined region. Ventral to it is a
large liver with a gall-bladder from which a bile-duct leads

52

MORPHOLOGICAL TYPES

va.

ab5. ■

R

bcUy/

pv-yC

SCYLLIUM 53

to the intestine. In the U-shaped bend which the stomach
makes with the intestine lies the pancreas, the duct from which
enters the intestine close to the bile-duct. The intestine bends
backwards and runs straight to the rectum, which has a small
diverticulum (the rectal gland), and leads to the cloaca. The
gut is considerably longer than the distance from the mouth
to the cloaca, and the " slack " is accounted for by the
asymmetry of the stomach. This asymmetry persists through
all the higher vertebrates. The internal surface of the
intestine is increased by a fold forming the spiral valve.

Coelom and Myotomes. — The gut is suspended in the
splanchnocoel by a dorsal mesentery. Anteriorly the splanch-
nocoel is almost completely cut off from the cavity of the
pericardium by the transverse septum, which leaves only small
pericardio-peritoneal canals. Posteriorly, the splanchnocoel
is in communication with the exterior by the pair of abdominal
pores.

The series of somites is not complete. The first three
give rise to the eye-muscles, but the myotomes of the fourth
and fifth segments disappear during development, leaving the
sixth to form the first complete myotome. Each myotome is
divided into a dorsal and a ventral portion by a horizontal
septum. Into this septum the ribs (known as dorsal ribs)
extend. The ventral portions of the most anterior myotomes
send muscles forwards beneath the pharynx, in the midventral
line. These hypoglossal muscles lose connexion with their
original myotomes, and connect the ventral ends of the skeleton
of the visceral arches with the coracoid region of the pectoral
girdle. The fins contain muscles attached to the radials.
These muscles are derived from the myotomes. All muscles

Fig. 27. — Scyllium : ventral view of dissection showing the alimentary
system and the afferent branchial vessels (male).

ab 1 and 5, first and fifth afferent branchial artery ; bd, bile-duct ; clt
clasper (present in the male) ; /, fold overlying groove running from the
nasal sac to the mouth ; gi, first gill-slit ; 1, intestine ; /, liver ; m, mouth ;
n, nasal sac ; py pericardium ; pg, pectoral girdle cut ; pn, pancreas ; pv,
hepatic portal vein ; r, rectum ; rg, rectal gland ; s, stomach ; sp, spleen ;
sv, sinus venosus ; t, testis ; th, thyroid gland ; ug, urinogenital papilla ;
v, ventricle of heart ; va, ventral aorta.

Fig. 28. — Structure and relations of kidneys and ducts in Gnathostomes.

A, larval condition with a Miillerian duct (Md) as well as a Wolffian
duct (Wd). The Bowman's capsules (Be) of the mesonephros communi-
cate with the splanchnocoel (sc) through the mesonephric funnels (mf).
i, intestine. B, condition in the adult female. The Miillerian duct persists
and functions as an oviduct. The eggs freed from the ovary (0) enter the
mouth of the oviduct (oM), and go down it, past the oviducal gland (og)
where the shell is secreted. The Wolffian duct is purely excretory in function.
C, typical condition of the adult male . The Miillerian duct has disappeared
except for vestiges of its opening (rM), and the sperm-sac (ss). Sperms pass
from the testis (t) through the vasa efferentia (ve) corresponding to meso-
nephric funnels, to the Wolffian duct. The latter is not only excretory
but also genital in function , and is also called the vas deferens . D , condition
of the adult male Scyllium. The Wolffian duct connects as usual with the
anterior mesonephric tubules and vasa efferentia, but the more posterior
part of the mesonephros (e) is solely excretory in function ; its tubules run
into a collecting duct (cd) which has separated off from the Wolffian duct.
The base of the latter or vas deferens is thickened to form the vesicula
seminalis (vs), and it and the collecting duct open into the sperm-sac.

SCYLLIUM 55

derived from myotomes are striated, voluntary, and innervated
by ventral nerve-roots. The muscles of the jaws and branchial
arches, although visceral, are striated and voluntary ; they are,
however, not innervated by ventral roots, but are supplied by
dorsal cranial nerves. The remainder of the visceral muscles
are all smooth and involuntary, and are to be found in the walls
of the gut, blood-vessels, and oviducts. They are innervated
by the autonomic system (sympathetic and parasympathetic).

Urino-genital System. — The kidney of the adult dogfish
is a mesonephros, similar to that of Petromyzon. Here,
however, the excretory and genital systems are closely associated,
and it is necessary to treat them together. In Scyllium and all
Gnathostomes, in place of the single mesonephric duct on each
side, there are typically two. One of these, the Wolffian duct,
can be regarded as the original mesonephric duct, and it con-
tinues to receive the tubules from the Bowman's capsules.
The other is the Mullerian duct which opens into the coelom
by the conjoined openings of the degenerated pronephric
tubules, and leads straight back to the cloaca without any
connexion with the mesonephric tubules. The degree of
development which these ducts show depends on the sex of
the animal.

In the male, the testes are paired, and are connected by
their anterior ends to the mesonephric tubules by means of
the vasa efferentia. These correspond to the original
ccelomostomes. Through them the sperms reach the
Wolffian duct, which becomes known as the vas deferens ;
its posterior end swells to form the seminal vesicle. The
anterior portion of the mesonephros therefore is concerned
with the evacuation of the genital products in the male. The
posterior portion (sometimes and incorrectly called the meta-
nephros) is solely excretory in function. Its tubules run into a
collecting duct which connects with the Wolffian duct, both
running into a sperm-sac. The two sperm-sacs, one on each
side, join to form a urino-genital sinus which opens into the
cloaca by a urino-genital papilla. The Mullerian ducts in the
male are reduced to a pair of funnels on the ventral side of the
oesophagus, and the sperm-sacs.

56

MORPHOLOGICAL TYPES

In the female the mesonephros is entirely excretory in
function. The Wolffian ducts are swollen posteriorly to form
urinary sinuses which open to the cloaca by a urinary papilla.

Fig. 29. — Scyllium : ventral view of a dissection of the urinogenital
system of a male adult.

cd, collecting duct ; cl, clasper ; ek, excretory portion of the mesone-
phros ; ^alimentary canal cut ; p, pericardium ; pp, pericardio-peritoneal
canal indicated by an arrow ; ri, rudimentary opening of the Miillerian
ducts ; ss, sperm-sac ; t, testis ; ng, urinogenital papilla ; ve, vasa efferentia ;
vs, vesicula seminalis ; Wd, Wolffian duct or vas deferens.

The right ovary hangs in the ccelom covered by a fold of
coelomic epithelium ; the left ovary disappears. The large
eggs when they are ripe drop free into the ccelom. They enter

SCYLLIUM

57

the funnel-shaped openings of the Mullerian ducts on the
ventral side of the oesophagus, and pass down them. These

Fig. 30. — Scyllium : ventral view of a dissection of the urinogenital
system of a female adult.

ab, abdominal pore through which an arrow is passed ; t, oesophagus ;
io, internal opening of the Mullerian ducts ; k, mesonephros ; Md, Mullerian
duct ; o, ovary ; og, oviducal gland ; 00, opening of Mullerian duct into the
cloaca ; p, pericardium ; pp, pericardio-peritoneal canal through which an
arrow is passed ; r, rectum ; so, sinus venosus cut ; up, urinary papilla ;
Wd, Wolffian duct.

ducts or oviducts swell out into the oviducal glands, where the
horny egg-case is secreted, and open separately into the cloaca.

58 MORPHOLOGICAL TYPES

The Miillerian duct is always genital, the Wolffian duct is
always excretory, but in the male it is genital in function as
well. In some dogfish the mesonephric tubules retain their
funnels, opening into the splanchnoccel.

Vascular System. — The vascular system is built on the
same plan as that of Petromyzon. The subintestinal vein,
which forms the hepatic portal vein, runs to the liver in a
portion of mesentery in company with the bile-duct. The veins
in the body-wall (or somatic veins) consist of a pair of cardinal
veins running parallel with and on each side of the dorsal
aorta. They connect with the sinus venosus of the heart by
the ductus Cuvieri, which cross the ccelom from the body- wall
to the gut- wall, in the transverse septum. The anterior
cardinals bring the blood back from the head (orbital sinus and
jugular), and from the ductus Cuvieri backwards the veins are
known as posterior cardinals. In addition there are paired
inferior jugular sinuses bringing blood back from the ventral
regions of the head, and paired lateral abdominal veins draining
the ventral posterior regions of the body- wall. All these lead
into the ductus Cuvieri. The hyoid sinus is in the hyoid arch.

The heart in its pericardium is bent on itself, and is in the
form of an S. The sinus venosus, which receives the ductus
Cuvieri and the hepatic sinus from the liver, opens into the
auricle whence the blood passes through an opening guarded
by valves to the thick- walled ventricle. This lies beneath and
behind the auricle. In front of the ventricle is a muscular
conus arteriosus with two rows of valves which prevent the
blood from flowing back into the ventricle. The conus leads
through a bulbus (see p. 330) to the ventral aorta from which
five pairs of afferent branchial arteries are given off. These
break down into the capillaries of the gill-lamellae in the hyoid
and four branchial arches. Each set of lamellae on one wall of
a slit is called a demibranch. There are two demibranchs in
each gill-slit except the last, which has only an anterior one.

The oxygenated blood is collected up into four efferent
branchial arteries which correspond to gill-slits 1 to 4. They
lead to the median dorsal aorta. Each efferent branchial
artery is made up of two collecting vessels one on each side of a

Fig. 31 .— Scyllium ; diagrammatic view of the venous system.
ac, anterior cardinal ; bv, brachial vein ; c, caudal vein ; dC, ductus
Cuvieri ; hp, hepatic portal vein ; hs, hyoid sinus ; hv, hepatic vein ; ij,
inferior jugular sinus ; iv, iliac vein ; k, mesonephnc kidney ; I, liver ;
la, lateral abdominal vein ; Ic, lateral cutaneous vein ; os, orbital sinus ;
pc, posterior cardinal ; pv, pituitary vein ; rp, renal portal vein ; sv, sinus
venosus ; v , ventricle of the heart.

6o

MORPHOLOGICAL TYPES

SCYLLIUM 61

gill-slit and joining above it. Each gill- arch therefore contains
two such vessels and one afferent vessel. The demib ranch
of the 5th gill- slit is drained by the 4th efferent branchial
artery. The demibranch of the spiracle receives already
oxygenated blood from the anterior demibranch of the 1st
gill-slit. Since it does not oxygenate this blood itself it is
called a pseu dob ranch. The vessel leading from it to the
internal carotid inside the skull is the efferent pseudobranchial
artery, which can be seen at the back of the orbit or eye-socket.

The anterior prolongation of the dorsal aorta is the internal
carotid artery which pierces the base of the skull and supplies
the brain. The dorsal aorta gives off paired subclavian
arteries to the pectoral fins. Into the mesentery suspending
the gut it sends the following : cceliac, lieno-gastric, anterior
and posterior mesenteric arteries, which between them
vascularise the stomach, liver, intestine, and the well-developed
spleen. Farther back the dorsal aorta gives off pelvic arteries
to the pelvic fins and renal arteries to the kidneys as well as
branches to the gonads, and continues into the tail as the
caudal artery. The blood from the tail is returned forwards
by the caudal vein which forks, one branch going to each
kidney as the renal portal vein. From there the blood is
removed by the posterior cardinals.

Ductless Glands (see Chapter XXXIII). — The spleen has
already been mentioned ; it lies in the mesentery near the
stomach (see p. 152).

With the development of jaws, the ciliary mode of feeding
has been abandoned. The wheel- organ of Amphioxus is
represented by an ectodermal ingrowth — the hypophysis,
which comes into intimate contact with the infundibulum of
the brain to form the pituitary body.

Fig. 32. — Scyllium : diagrammatic view of the arterial system, and of the
sympathetic, supra-renals, and inter-renal (female specimen). Partly
from a drawing by Mr. J. Z. Young.

c, caudal artery ; Ca, cceliac artery ; da, dorsal aorta ; eb 2, second
efferent branchial artery; ep, efferent pseudobranchial artery; gi, first
gill-slit ; ia, iliac artery ; tc, internal carotid artery ; ir, inter-renal body ;
k, mesonephric kidney ; s, spiracle ; sa, subclavian artery ; sg, sympathetic
ganglia, some of which are interconnected by the longitudinal sympathetic
nerve-chains ; sr, supra-renal bodies.

62 MORPHOLOGICAL TYPES

The endostyle is not present as such, but it has been trans-
formed into the thyroid gland, situated beneath the floor of the
pharynx, as in the adult Petromyzon. In some dogfish, its
cells still bear cilia.

The lining of the top of the gill-slits grows upwards to
form a number of paired glandular masses ; in close association
with the anterior cardinal veins they form the thymus glands.
Lying on the course of the sympathetic nerve-chains, there are
a number of bodies of the same origin and nature as the
sympathetic ganglion-cells. They are the supra-renal bodies
and originate from the nerve-tube. Posteriorly, between the
kidneys is an elongated structure, the inter-renal, which is
formed from the coelomic epithelium. It is interesting to
find these two sets of structures separate, for in higher forms
they combine to form the adrenals.

Characters present in Scyllium, which are lacking in
Petromyzon :

Biting jaws ;

Paired fins ;

Denticles (" placoid scales ") ;

Dermal fin-rays (ceratotrichia) ;

Definite stomach and pancreas ;

Mixed spinal nerves (dorsal and ventral roots joined) ;

Vertebral column constricting the notochord ;

Dorsal ribs ;

Myotomes separated into dorsal and ventral portions ;

Miillerian and Wolffian ducts ;

Seven segments included in the skull.

Characters of Scyllium which are primitive when compared
with higher forms :

Absence of bone ;

Absence of swim-bladder ;

Gill-slits opening separately to the outside ;

Heart with single auricle and single ventricle ;

Separate supra-renals and inter- renals.

SCYLLIUM 63

Literature

Bourne, G. C. An Introduction to the Study of the Comparative Anatomy
of Animals. Vol. 2. Bell, London, 191 5.

Cahn, A. R. The Spiny Dogfish. A Laboratory Guide. Macmillan,
New York, 1926.

Daniel, J. F. The Elasmobranch Fishes. University of California Press,
1922.

Goodrich, E. S Vertebrata Craniata. Cyclostomes and Fishes.
Black, London, 1909.

Nicholls, G. F. The Structure and Development of Reissner's Fibre and
the Subcommissural Organ. Quarterly Journal of Microscopical
Science. Vol. 58. 1913.

CHAPTER V

GADUS, A CHORD ATE WITH BONE

Externals. — The genus Gadus includes the cod, whiting, and
haddock. In shape, Gadus differs from the dogfish in being
relatively shorter and more compressed from side to side.
Gadus belongs to the group of higher bony fish known as the
Teleostei, and in these the tail is typically forked and outwardly
symmetrical, a condition called homocercal. In Gadus the
tail is also outwardly symmetrical, but the tail-fin differs from
that of other Teleostei. It represents the hind ends of the
dorsal and ventral median fins. It is therefore not a homo-
cercal but a pseudocaudal fin (see p. 325).

There are three dorsal and two ventral median fins. Of the
paired fins the pelvic pair is actually anterior to the pectoral
pair in position.

The mouth is bounded by tooth-bearing jaws. On the
upper side of the snout, slightly behind the mouth, are the
nasal pits. Each of these is a cavity communicating to
the exterior by two openings, but not in any way connected
with the mouth.

The eyes are large. The gill-slits do not open separately
to the exterior, but they are covered over by an operculum.
The water which emerges from the gill-slits passes between
the hind and lower edges of the operculum and the body.
There is no open spiracle. The cloaca is shallowed out, so
that the anus and the urino-genital apertures are separate ;
the former in front of the latter.

Scales.' — Scales form one of the most obvious features of
the fish ; they are arranged in W-shaped rows, overlapping
from head to tail. Each row primitively corresponds to the

64

GADUS 65

underlying myotome, which is also W-shaped. It is important

to notice that the scales are not external, but lie in the meso-
dermal tissue (from which they are formed) beneath the

66 MORPHOLOGICAL TYPES

epidermis. The scales are thin, flat plates of material akin to
bone. They are kept throughout life, and enlarge by con-
centric additions. These scales have nothing to do with the
denticles or placoid " scales " of the dogfish.

Fin-rays. — The fins are supported by fin-rays, but these,
instead of being horny and unjointed like the ceratotrichia of
the dogfish, are bony and jointed, and are called lepidotrichia.
In the more highly-developed bony fish like Gadus, the
lepidotrichia correspond in number to the radials of the axial
skeleton, in the dorsal and ventral median fins. There is a
lepidotrichium on each side of the tip of each radial, and a
joint between them enables the web of the fin to be raised or
lowered. At the edge of the fin, between the lepidotrichia,
there are some small unjointed horny rays called actinotrichia.
These correspond to the ceratotrichia of the dogfish.

Skeleton.- — The cartilaginous skeleton corresponding to
that of the dogfish is present in early stages of development in
the bony fish. In the adult, most of this cartilage is replaced
by an altogether different skeletal material, viz. bone. Bones
which arise in this manner, i.e. replacing pre-existent cartilage ,
are called cartilage-bones or replacing bones. Some bones, on
the other hand, have no cartilaginous precursor at all. These
arise independently, as more or less flat plates in relation to
the surface of the body, though they may sink deeper. These
are dermal or membrane-bones. There is no difference in
structure between cartilage-bones and membrane-bones, the
distinction applies only to the method of origin. Sometimes
a bone which develops as a cartilage-bone in one animal may
arise as a membrane-bone in another, and vice versa, though
these cases are rare. It is to be noted that as a rule a cartilage-
bone represents an ossification in a cartilaginous structure
which exists in the dogfish, whereas a membrane-bone is a
structure which is wholly unrepresented in the dogfish. There
is no doubt that the scales, fin-rays, and bones are kindred
structures.

Skull. — As in other forms, the skull can be divided into the
neurocranium or brain-case, and the splanchnocranium or
jaws.

GADUS

67

T? O*

68

MORPHOLOGICAL TYPES

On the floor of the brain-case are the basioccipital, pre-
vomer, and parasphenoid ; in front is the mesethmoid. The
roof is formed by the paired nasals, frontals, parietals, and the
supraoccipital. The foramen magnum, through which the
spinal cord enters the skull, is bounded below by the basi-
occipital, above by the supraoccipital, and on each side by the
paired exoccipitals.

The auditory capsules are well ossified, and each contains
five bones. The lower part of each capsule is made of a prootic

me.

i
l

Is hm.

f. P^-mp. i sp.

<l. sy.

Fig. 35. — Gadus : view of a portion of the skull from the left side, after
removal of the lachrymal and suborbital bones, in order to show
the palato-pterygo-quadrate arch. {For lettering see p- 70.)

in front and an opisthotic behind. Above these are the
sphenotic, pterotic, and epiotic bones.

The sides of the brain-case are very incomplete. Anteriorly,
there are the paired prefrontals, between the frontal and the
parasphenoid on each side. Farther back the paired latero-
sphenoids are situated beneath the edge of the frontal and in
front of the prootic on each side. A large window is left open
in the side of the brain-case, through which many nerves and
blood-vessels pass from the skull to the space in which the eye
is lodged, called the orbit. Quite at the side there is a string

GADUS

69

of little bones which bound the orbit below and behind. These

bones touch the prefrontal anteriorly and the frontal behind.

The most anterior of the

string is the lachrymal,

and the following ones

are the suborbitals and

postorbitals.

Before leaving the
neurocranium, mention
must be made of the
post-temporal, which
touches the epiotic and
pterotic behind. It will
be noticed again in con-
nexion with the shoulder
girdle.

The splanchno-
cranium consists of the
bony supports of the
visceral arches. In the
branchial arches there are
four elements on each
side : pharyngo-, epi-,
cerato-, and hypo-
branchial. The pharyngo -
branchials of the anterior
branchial arches are
fused together ; the
skeleton of the posterior
branchial arches is less
well developed and
ossified. The h y p o-
branchials of the anterior
three branchial arches
articulate with a median
and ventral basibranchial. The fused pharyngobranchials
and the ventral elements of the last arch bear teeth.

The skeleton of the first two visceral arches (mandibular

7o

MORPHOLOGICAL TYPES

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GADUS 71

and hyoid) is somewhat modified. In the dogfish, the palato-
ptery go- quadrate cartilage forms the margin to the upper jaw,
but in the bony fish this is no longer the case. Here
it is formed by the paired premaxilla and maxilla. Lying
median to these are the paired palatine (which touches the
brain-case), the three pterygoids (ecto-, endo-, and meta-
pterygoid), and, farther back, the quadrate. These bones
arise in relation to the palato-quadrate arch, and they therefore
no longer form the upper boundary to the mouth, but lie at
the sides of its roof. The quadrate articulates with a bone
of the lower jaw called the articular, and which corresponds to
Meckel's cartilage in the dogfish. Here again a new margin
to the jaw is formed, by the dentary. The ventro-posterior
part of the lower jaw is formed by the angular.

The suspension of the jaws is hyostylic, i.e. the quadrate
is connected with the neurocranium by the hyoid arch. The
hyoid-arch skeleton consists of an upper hyomandibula which
articulates above with the auditory capsule, and is pierced by
a foramen for the hyomandibular branch of the facial nerve.
Beneath the hyomandibula is the symplectic to which the
quadrate is attached. The ventral portion of the arch is made
up of the epihyal, ceratohyal, and hypohyal. The epihyal is
connected with the symplectic by the interhyal. Below and
between the hypohyals is a median basihyal. The ceratohyal
bears branchiostegal rays.

The operculum is a posterior extension of the hyoid arch,
and it is supported by four bones which are fixed on to the
hind edge of the hyomandibula and symplectic. These are
the preopercular, opercular, sub-, and interopercular bones.

Pectoral Girdle. — The primitive girdle corresponding to
that of the dogfish is formed by a dorsal scapula and a ventral
coracoid, on each side. Here they are ossified. With these
the pectoral fin articulates by means of the radials. These
radials are short and fused, and the web of the fin is supported
by the lepidotrichia. Another series of bones is plastered
on to this primitive girdle from in front. These bones are
the cleithrum, post-cleithrum and supra-cleithrum. The
supra-cleithrum articulates with the post-temporal so that the

72 MORPHOLOGICAL TYPES

pectoral girdle is connected with the skull. There is no
clavicle, though this bone is present in more primitive bony
fish.

Pelvic Girdle. — The pelvic girdle is in the form of a pair
of Y-shaped bones lying in the body- wall. The anterior
forks of the Y of the bones on each side are joined by a median
cartilage. The pelvic fins articulate with the outer side of
these bones. The radials in the pelvic fins are even more
reduced than those in the pectorals.

Vertebral Column. — The centra of the vertebral column
are bony discs, concave on both sides, and the vertebrae
articulate with one another by means of facets or zygapophyses.
Dorsally, each centrum bears a pair of processes which join to
form the neural arch. This arch is produced farther into the
neural spine. The radial surmounts this and, in the regions
of the dorsal fins, is articulated with the lepidotrichia. The
radials scarcely project at all into the fins. The spinal nerves
emerge between the neural arches.

In the trunk-region, each centrum bears a pair of ventro-
lateral processes, to which the (" ventral," see p. 83) ribs are
attached. In the region of the tail these processes are directed
downwards, and join to form the haemal arches. These are
prolonged into the haemal spines, which support the ventral
lobe of the tail-fin. The neural arches correspond to basi-
dorsals, and the ventro-lateral processes to basiventrals. The
notochord is of course obliterated by the centra.

Before leaving the skeleton, it remains to sort out the
various bones into cartilage-bones and membrane-bones, and
those whose constituents arise in both ways and which may
therefore be called mixed bones.

Cartilage-bones.

Membrane-bones.

Mixed bones,

Neuro-cranium.

Basioccipital

Nasal

Sphenotic

Exoccipital

Frontal

Pterotic

Supraoccipital

Parietal

Prefrontal

Prootic

Parasphenoid

Opisthotic

Prevomer

GADUS

73

Cartilage-bones. j

Membrane-bones. Mixed bones

Neuro-cranium.

Epiotic (a tendon-bone)

Lachrymal

Mesethmoid

Suborbitals

Laterosphenoid

Postorbitals

Basisphenoid : not

always present

Splanchno-cranium .

Quadrate

Endopterygoid Palatine

Metapterygoid

Ectopterygoid

Articular

Premaxilla

Hyomandibula

Maxilla

Symplectic

Dentary

Epihyal

Angular

Ceratohyal

Opercular

Hypohyal

Preopercular

Interhyal

Subopercular

Basihyal

Interopercular

Pharyngobranchial

Branchiostegal rays

Epibranchial

Ceratobranchial

Hypobranchial

Basibranchial

Vertebral Column.

Centra

Ra dials

Appendicular Skeleton.

Scapula

Cleithrurh

Coracoid

(Clavicle)

Pelvic

Post- clei thrum

Radials

Supra-cleithrum

Post-temporal

Dermal Skeleton.

Lepidotrichia

Scales

74 MORPHOLOGICAL TYPES

It will be obvious from this table that the majority if not
all the cartilage-bones are ossifications in cartilage which
itself is represented in the dogfish. On the other hand, there
are no structures of any kind in the dogfish which have any
connexion with the membrane-bones. The membrane-bones
are of interest from two points of view. A number of them
bear teeth : premaxilla, maxilla (not in Gadus, however), pre-
vomer, dentary. Others enter into relations with the lateral-
line canal system, and these relations are of importance, for
owing to their constancy they enable homologies to be made
between bones in fish and in higher Vertebrates. The lateral-
line canal of the trunk runs forwards from the tail, and in so
doing it pierces the scales. On reaching the head it is pro-
tected by a few " lateral-line ossicles," and then passes through
the post-temporal to the pterotic and sphenotic. The supra-
orbital canal runs forwards over the eye through the frontal
and nasal, the infraorbital canal pierces the chain of bones
formed by the post-orbitals, infraorbitals, and lachrymal.
The hyomandibular canal runs down through the preopercular
to the dentary.

Teeth. — The teeth are fundamentally similar to the denticles
or placoid scales of the dogfish, but instead of being scattered
all over the surface of the body, they are restricted to the
mouth. They are composed of a core of dentine containing
a pulp cavity, and are covered over with a cap of enamel.
The bones which bear teeth have been enumerated above.

Nervous System. — The brain and spinal cord lie in the
long tubular cavity provided by the skull and neural arches of
the vertebrae. The spinal cord is essentially similar to that
of the dogfish, and calls for no special description. The spinal
nerves, each composed of a dorsal and a ventral root, emerge
between the neural arches.

- In the brain, the medulla oblongata is not very different
from the spinal cord. The cerebellum is well developed and
projects downwards and forwards beneath the roof of the
midbrain forming the valvula, a structure which is peculiar
to bony fish. The roof of the midbrain is produced into optic
lobes. The floor of the forebrain projects downwards as the

GADUS 75

infundibulum, and is attached to the pituitary body and the
saccus vasculosus. The latter structure, which is of doubtful
significance, is peculiar to fish. It is a region of the brain-
floor where the wall is thin, thrown into folds, and very richly
supplied with blood-vessels. It has been supposed that its
function is to secrete the cerebro-spinal fluid which fills the
cavity of the brain and spinal cord, or to estimate the pressure
of this fluid. The olfactory lobes are peculiar in that they are
situated far forwards, close behind the nasal pits. They are
connected with the rest of the brain by long olfactory tracts.

The olfactory nerves are short, which fact is correlated
with the length of the olfactory tracts. The optic nerves have
no chiasma. The three eye-muscle nerves, oculomotor,
trochlear, and abducens, are similar to those of the dogfish,
and call for no special comment. The profundus is reduced,
the trigeminal has the usual maxillary and mandibular branches.

The facial nerve has ophthalmic, buccal, and hyomandibular
branches, innervating respectively the supraorbital, infra-
orbital, and hyomandibular lateral-line canals. In addition,
the facial nerve has a cutaneous branch which runs upwards
and backwards, and divides into three nerves which can be
seen immediately underneath the skin. One of these runs
along the base of the median dorsal fins ; another runs
obliquely down across the side of the body to the median
ventral or anal fin ; the last branch runs to the pectoral and
pelvic fins.

The auditory nerve calls for no comment. The relations
of the glossopharyngeal and vagus nerves to the gill-slits is the
same as in the dogfish. The vagus supplies the heart and
viscera, and also the lateral line of the trunk. This nerve
supplying the lateral line branches, one portion remaining
close to the lateral-line canal, and the other runs a little below,
at the level of the septum which divides the myotomes into
dorsal and ventral portions.

Sense-organs. — The nose is represented by paired nasal
sacs on the upper side of the snout, each with two openings,
and without connexion with the mouth. The eye is similar in
structure to that of the dogfish ; but there is in addition a

76

MORPHOLOGICAL TYPES

vascular process extending into the cavity of the eyeball known

as the campanula Halleri, which is attached to the retractor
lentis muscle.

In the ears, the ductus endolymphaticus no longer maintains

GADUS 77

its persistent opening to the outside. In the saccule there are
two large calcareous concretions or otoliths ; otherwise the
structure of the organ is similar to that of the dogfish.

The lateral-line canals have already been mentioned, and
their course and innervation described.

Alimentary Canal. — The mouth leads into the pharynx, out
of which the five pairs of gill-slits open. There is no open
spiracle. The gill-arches between the slits are smaller than
those of the dogfish, and do not form a broad septum as in
that fish. This is correlated with the fact that they are covered
over by the operculum. The gills are supported by two rows
of branchial rays on each arch.

Behind the pharynx, the oesophagus leads to the stomach
which bears a number of blind tubes, the pyloric coeca. The
liver has a gall-bladder from which the bile-duct runs to the
intestine. The latter receives the pancreatic duct from the
pancreas, makes a loop forwards and back again and runs to
the rectum, which opens at the anus.

The swim-bladder is to be regarded as a derivative of the
alimentary canal, and in many forms it retains its connexion
with it by an open duct. This connexion has, however, been
lost in Gadus, and the swim-bladder is a closed sac which
occupies the dorsal portion of the coelomic cavity, close up
against the under side of the vertebral column. Its ventral
wall is thick and is covered with the coelomic epithelium ; its
dorsal wall is very thin. Inside the bladder is a rete mirabile,
a concentration of small blood-vessels forming a gland which
secretes oxygen into the bladder. This " red " gland, as it
is called, is supplied with blood by the mesenteric artery, like
the other viscera. The function of. the bladder is hydrostatic,
for by varying the amount of gas which it contains (by passage
of gas from the blood to the bladder or vice versa) the fish can
accommodate itself to any given depth of water and maintain
itself there without muscular exertion. As will be seen later,
it probably corresponds to the lung of the air-breathing
vertebrates.

Excretory System. — A pair of mesonephric kidneys extend
longitudinally, dorsal to the swim-bladder and below the

78 MORPHOLOGICAL TYPES

vertebral column. At their posterior ends they join, and the
single median excretory duct runs ventrally, behind the swim-
bladder to a urinary sinus. This opens to the exterior at the
urinary aperture.

Genital System. — Gadus (and the higher bony fish) differs
from the dogfish in that the urinary and genital systems are
not intimately connected. The testes are elongated structures
suspended in the coelomic cavity on each side of the gut. They
do not connect with the kidney, but join one another posteriorly,
and send a single duct to open at the genital aperture. In the
female, the ovaries correspond in position to the testes. The
remarkable thing is that the ovaries are enclosed in sacs which
lead by a single duct to the genital aperture. Here, therefore,
the eggs are never shed free into the ccelom, to enter the open
mouths of oviducts.

Vascular System. — The heart consists of sinus venosus,
single auricle, single ventricle, and bulbus arteriosus. It is
to be noticed that the muscular conus which was present in
the dogfish has disappeared, and has only left its valves as a
vestige. The ventral aorta gives off four pairs of afferent
branchial arteries, one ascending each of the first four branchial
arches. From these arches the blood is collected up into the
efferent branchial arteries which run to the lateral dorsal
aorta of their side. The lateral dorsal aortae are joined together
behind the gill-region to form the single dorsal aorta, and they
also join in front of the gills, so that a ring is formed called the
circulus cephalicus. Anteriorly the internal carotids run to
the head ; behind, the dorsal aorta gives off the subclavian
arteries to the pectoral fins, the cceliac and mesenteric arteries
to the viscera, and continues backwards between the kidneys
to the tail.

The venous system does not differ in essentials from that of
the dogfish.

Gadus is a type of one of the most successful group of
marine animals. It shows certain important advances over
the condition of the dogfish, but when compared with higher
forms most of its characters are seen to be specialised and
secondary.

GADUS 79

Characters of Gadus which show an advance over the con-
ditions in Scyllium :

Bone ;

New marginal skeleton to the jaws ;

New elements added to the pectoral girdle ;

Swim-bladder.

Characters of Gadus which are secondary when compared
with other forms :

Continuity of the gonads with their ducts ;
Loss of optic chiasma ;
Loss of clavicle ;
Loss of conus arteriosus ;

Swim-bladder closed, and adapted to hydrostatic
function.

Literature

Goodrich, E. S. Vertebrata Craniata : Cyclostomes and Fishes. Black,
London, 1909.

Parker, T.J. A Course of Instruction in Zootomy (Vertebrata) . Macmillan ,
London, 1884.

CHAPTER VI

CERATODUS, A CHORD ATE WITH A LUNG

Externals. — Ceratodus is the Australian lung-fish, a group of
great importance, whose only other living representatives are
Protopterus in Africa and Lepidosiren in South America. It
is not an uncommon thing for ancient and primitive groups of
animals to have gone extinct everywhere except for definite,
small, and isolated regions of the earth. These animals are
therefore an example of discontinuous geographical distribution.
In shape, Ceratodus is typically a fish. The median fins

) )

) ) > ) > } ) ' ) ' i

. ; ) ; > ) > >

r»x>'

tf.

Fig. 39. — Ceratodus : view from the left side. (Partly after Goodrich.)

e, eye ; pc and pi, pectoral and pelvic " archipterygial " fins ; tf, tail-
fin (diphy cereal ?).

are remarkable in that the dorsal, caudal, and ventral fins are
all continuous with one another. Further, the tail-fin is
symmetrical and pointed, and resembles the primitive di-
phy cereal type, such as is found in Cyclostomes. There is,
however, a certain amount of doubt as to whether the tail-fin
of Ceratodus is primitive. When a tail such as this is derived
secondarily by simplification from another type (as is the case
in the eel, for example) it is called gephyrocercal.

The paired fins are elongated and leaf- like. They have a

80

CERATODUS 81

central axis bearing radials both in front and behind, a con-
dition known as biserial, and they conform to the type known
as " archipterygial." Both the paired fins and the dorsal
fin are covered with scales.

The gill-slits, five in number, are protected by an oper-
culum. The spiracle is closed. The anterior nostrils are on
the ventral surface of the snout, and behind them are the
posterior nostrils which open into the mouth. There is a
cloaca into which the alimentary, excretory, and genital systems
open, as also do a pair of abdominal pores.

Scales. — The scales are thin and covered with spines which
must not be mistaken for denticles. They overlap one another
from before backwards as in Gadus, and they also extend over
the dermal bones of the skull and the paired fins, and the
dorsal fin.

Fin-rays. — The dermal fin-rays are jointed and made of
fibrous substance. They differ from the lepidotrichia of
Gadus in being more numerous than the radials, and in being
covered over by scales. No actinotrichia are present, and it
is uncertain whether these rays, which are called camptotrichia,
represent the ceratotrichia or the lepidotrichia of Gadus.

Skull. — The skeleton is largely cartilaginous, and little
of this primitive skeleton is replaced by bone. The neuro-
cranium forms a complete case enclosing the brain, olfactory
and auditory capsules, and several vertebrae are plastered on
to its hind end. As some of these bear ribs, the latter appear
to articulate with the skull, and are called cranial ribs. The
only cartilage-bone in the neurocranium represents one of the
neural arches which have been incorporated as just described.
The membrane-bones which cover the dorsal surface of the
neurocranium are sunk beneath the surface of the skin and are
themselves overlain by scales. These bones are very modified
and secondary, there is a preponderance of median unpaired
bones, and as they cannot well be compared with those of
other forms, there is little advantage in studying them in
detail. On the underside of the neurocranium are to be found
a parasphenoid, prevomer, and paired ptery go-palatines.

The splanchnocranium is important because of the manner

G

82 MORPHOLOGICAL TYPES

in which the upper jaw is fastened on to the skull. The
quadrate is directly attached to the neurocranium by carti-
laginous processes : a basal process and an otic process.
There is also an ascending process. This method of suspension
of the jaws is called autostylic ; the hyomandibula plays no
part in it. The relations of the basal, otic and ascending
processes to the neighbouring nerves, veins, and arteries are
important, and most of them will be found to be identical
in all the remaining groups of vertebrates. The ascending
process lies between and separates the profundus from the
maxillary branch of the trigeminal {i.e. it is situated between
Y1 and V2) ; the palatine nerve runs down behind and forwards
beneath the basal process ; while the facial nerve (hyomandi-
bular branch) and jugular vein pass on the inner and under
side of the otic process.

The premaxilla and maxilla have disappeared, and conse-
quently there are no teeth round the edge of the jaws. In
the lower jaw the dentary is very much reduced. Teeth are
carried on the prevomer, pterygo-palatine, and splenial (mem-
brane-bone). An angular is present in the lower jaw.

In the hyoid arch there are a very small hyomandibula,
and well- developed ceratohyal, hypohyal, and basihyal. The
skeleton of the branchial arches also is not very well developed,
and the arches do not carry any branchial rays.

The operculum is supported by opercular and subopercular
bones.

Vertebral Column. — The vertebral column, which is con-
tinuous in front with the hind end of the skull, is made up of
paired basidorsal and basiventral cartilaginous elements, which
do not interrupt the notochord. The basidorsals rise up into
bony neural arches and neural spines which are attached to the
jointed radials supporting the median fin. The basiventrals
in the hinder region form haemal arches carrying haemal spines
and radials supporting the ventral median fin. Farther
forwards the basiventrals are produced into ribs. These do
not extend into the horizontal septum between the dorsal and
ventral portions of the myotomes, like the true or " dorsal "
ribs of Scyllium. Instead, they bend down and lie just

CERATODUS 83

outside the outer lining of the coelom. From their position
they are known as " ventral " or pleural ribs.

Limbs and Girdles. — The primitive pectoral girdle is
cartilaginous and composed of paired dorsal scapular regions,
and ventral coracoid regions, which latter are joined to one
another in the midventral line. Overlying this are the
membrane-bones, clavicle, clei thrum, and the post- temporal
which connects the girdle with the hinder part of the skull.

The pelvic girdle is formed of a median Y-shaped cartilage
with the prongs directed backwards and articulating with the
pelvic fins.

The fins are covered with scales. Their endoskeleton is

pr.

Fig. 40. — Ceratodus : skeleton of the pectoral fin, showing the " archiptery-
gial " structure, with an axis (a), bearing preaxial (pr) and postaxial
(po) radials.

cartilaginous and composed of a long central axis of about
twenty pieces, tapering away to the tip. On each side of this
axis are radials (pre- and postaxial). Beneath the scales are
the camptotrichia.

Teeth. — The plates of teeth, which are firmly attached to
the prevomer, pterygo-palatines, and splenials, are the result
of fusion of separate teeth.

Alimentary Canal. — In its main lines the alimentary canal
does not differ much from that of Scyllium, with a spiral valve
in the intestine. Its most interesting and important feature
is that in the floor of the oesophagus there is an opening (the
glottis) leading to a tube or trachea which passes up round the
right side of the gut to the lung. This is a large sac with

84 MORPHOLOGICAL TYPES

highly vascular walls surrounding a cavity which is subdivided

Fig. 41. — Ceratodus : diagram of the relations of the lung, heart, and
vascular system seen from the ventral side (combined from diagrams
after Baldwin Spencer, simplified).

aa, anterior abdominal vein ; ab, afferent branchial artery ; b, brachial
vein ; ea, efferent branchial artery ; ej, external jugular vein ; g, glottis ;
ijt internal jugular vein ; /, lung ; la, left auricle ; //, left lateral abdominal
vein ; Ipa, left pulmonary artery ; Ipc, left posterior cardinal vein ; o,
oesophagus ; p, pharynx ; pv, pulmonary vein ; rl, right lateral abdominal
vein ; rpa, right pulmonary artery ; rpc, right posterior cardinal vein
(remnant) ; rvcs, right vena cava superior ; t, truncus arteriosus ; vci,
vena cava inferior.

into little chambers or " cells." In the lung the blood can be
oxygenated when the water in which the animal lives becomes

CERATODUS 85

polluted, and the animal rises to the surface to take in air
through the nostrils. It is this capacity of breathing by means
of lungs and gills which is responsible for the name of the
group Dipnoi, to which Ceratodus belongs. The lung is
homologous with the swim-bladder of Gadus.

Vascular System. — Blood is supplied to the lung by
branches of the last (6th) efferent branchial artery, which can
now be called pulmonary arteries. The right pulmonary
artery runs direct to the lung, but the left passes down under
the gut and up again on the right side parallel with the windpipe
or trachea. This shows that the primitive position of the lung
was ventral, and that it moved up the right side, whither it is
followed by the left pulmonary artery. Blood leaves the lung
by the pulmonary veins, which unite to form one vein. This
vein also passes down on the right side of the gut and goes
right through the sinus venosus to open into the left side of
the auricle. The auricle itself is partially divided into two by
a septum, so that the blood (oxygenated) from the lung comes
in on the left, and that from the rest of the body (deoxygenated)
enters on the right from the sinus venosus. The ventricle is
single, but the conus arteriosus is nearly divided into two by
enlarged valves.

The ventral aorta leads forwards from the conus and gives
off four pairs of afferent branchial arteries. The two collecting
vessels in each gill-arch join to form the efferent branchial
arteries which combine to form the dorsal aorta. The arrange-
ment of the valves in the conus and truncus arteriosus is such
that the blood from the sinus venosus tends to go into the
posterior branchial arches (and so to the pulmonary arteries),
while that from the pulmonary vein gets into the anterior
arches. There is therefore an attempt to separate the circula-
tion of the freshly-oxygenated blood from that of the impure
blood which should go and be oxygenated.

There are two important points to notice in the venous
system. The posterior cardinal on the right side loses its
connexion with the ductus Cuvieri. Instead, it has developed
a new connexion with the sinus venosus, forming the inferior
vena cava. The ductus Cuvieri can also be called the superior

86 MORPHOLOGICAL TYPES

vena cava (right and left). The lateral abdominal veins unite
in the midventral line and so give rise to an anterior abdominal
vein, which runs into the right ductus Cuvieri close to its
connexion with the sinus venosus.

The hindmost portions of the posterior cardinal veins
bring blood from the posterior region of the body to the
kidneys, and so form renal portal veins. Blood leaves the
kidneys by the left posterior cardinal and the inferior vena
cava.

Urino-genital System. — The excretory system is similar
to that of Scyllium. The mesonephric kidneys are elongated,
and connect by means of Wolffian ducts with the cloaca. The
testis is connected with the kidney by vasa efferentia, and the
Wolffian duct functions as a vas deferens. In the female, the
eggs from the ovary are shed freely into the ccelom, and enter
the openings of the Mullerian ducts which lead them to the
cloaca.

Nervous System. — The most remarkable feature of the
nervous system is the formation of cerebral hemispheres in
the telencephalon (end-brain). They are hollow outgrowths
from the diencephalon projecting forwards side by side. A
transverse section in the region of the brain of Ceratodus
therefore would show a pair of cavities, not a single cavity as
in lower forms. The cavities of the cerebral hemispheres
are the so-called first and second ventricles of the brain ; they
communicate with the cavity of the rest of the forebrain (third
ventricle) through the foramina of Monro. This is the first
appearance in the vertebrate series of the organs which mean
so much in the supremacy of man over other animals. In
Ceratodus, the roof of the cerebral hemispheres is still
membranous. The cerebellum is small.

As regards the sense-organs, the eyes and ears present no
striking features. It must be remembered that the nasal
sacs each have two openings. The lateral line is somewhat
degenerate, and in some regions may consist of a groove
instead of a canal.

CERATODUS 87

Characters of Ceratodus which are lacking in other living
fish, but present in Amphibia :

Respiratory lung ;

Pulmonary arteries and veins ;

Divided auricle, and conus arteriosus ;

Vena cava inferior ;

Anterior abdominal vein ;

Anterior and posterior nostrils : the latter within the

mouth ;
Autostylic suspension of jaws ;
Ascending process ;
Cerebral hemispheres.

Characters of Ceratodus which are primitive when compared
with other fish :

Cloaca ;

Contractile conus ;
Uninterrupted notochord ;
Diphycercal tail ?
Spiral valve in intestine.

Characters of Ceratodus which are secondary and specialised :

Loss of Maxilla and Premaxilla ;

Median membrane-bones over the skull ;

Lack of ossification in the cartilaginous neurocranium ;

Specialised tooth-plates ;

Fusion of vertebrae on to the back of the skull ;

Ventral ribs ;

Rotation of the lung to the dorsal position.

It will be obvious from these tables that Ceratodus and the
Dipnoi generally are very remarkable animals. On the one
hand, they have a surprising number of characters which no
other fish possess, and which are typical of Amphibia ; yet,
on the other hand, their relationship with the Amphibia cannot
be very close, because of the large number of specialised
characters which they show.

88 MORPHOLOGICAL TYPES

Literature

Goodrich, E. S. Vertebrata Craniata: Cyclostomes and Fishes. Black,
London, 1909.

Huxley, T. H. Contributions to Morphology : Ichthyopsida. On
Ceratodus forsteri. Proceedings of the Zoological Society of London,
1876.

Spencer, W. B. Ceratodus : The Blood-vessels. Macleay Memorial
Volume, 1892.

CHAPTER VII

TRITON : A CHORDATE WITH 5-TOED LIMBS

Externals. — Triton, the newt, is sharply distinguished from
all the types so far described, because its limbs end in fingers
and toes instead of being fins. The foot has five toes, but in
the newt and allied animals, the number of fingers on the hand
has been reduced from five to four. Triton and all higher
vertebrates are typically land-animals, and are collectively
called the Tetrapoda. Some of them, however, have reverted
to the condition of living in water, in varying degrees. So the
whales, seals, and hosts of extinct marine reptiles have come to
live almost if not entirely in water, and the newt also spends
more of its time in water and is more adapted to it than its
ancestors were. This secondary return to aquatic conditions
is, however, easily and fundamentally distinguished from the
primitive aquatic habit of the fish. The possession of typically
5-fingered, " pentadactyl," limbs is a sure criterion of a
terrestrial animal, or of one whose ancestors were terrestrial.
As an example of the secondary readaptation to aquatic
conditions may be mentioned the webs of skin which in some
species of newts extend between the fingers, and are used for
swimming.

The skin is soft and slimy owing to the presence of glands,
and is used largely as a respiratory surface for the oxygenation
of the blood. There are no scales or fin-rays of any sort.
The tail carries a continuous median dorsal and ventral fin,
and in the male animals of some species, there is also a fin
along the back, which becomes enlarged at the breeding season.

The first part of the life is spent in the water in which the
eggs are laid and hatched, and since the early stages are aquatic

89

9o MORPHOLOGICAL TYPES

and the later ones terrestrial, these animals (newts, toads, and
frogs) are called Amphibia. In the early larval condition
there are external gills, and subsequently three pairs of gill-
slits. These disappear when the animals metamorphose and
come out on land.

The aquatic larvae have lateral-line organs, disposed in a
similar manner to those of fish, only they are sunk in a groove
instead of being in a canal. When the newt emerges from the
water in the summer, these organs degenerate somewhat.
They reappear when the newt returns to the water, as it does
at the next breeding season, if not before.

The mouth is wide, and the external nostrils are just above
it. The eyes are small. The alimentary, excretory, and
genital apertures are situated in a cloaca just in front of the
base of the tail.

Skull.- — The cartilaginous neurocranium is very similar to
that of Ceratodus. The suspension of the jaws is autostylic,
and besides the basal and otic processes, there is also an
ascending process. These processes have precisely the same
relations to the neighbouring blood-vessels and nerves as they
have in Ceratodus. Only a little of this cartilaginous brain
case and olfactory and auditory capsules is replaced by cartilage-
bone. On each side, anteriorly, are the orbitosphenoids.
Posteriorly are the prootics and exoccipitals, which form the
condyles with which the skull articulates with the first vertebra.

The membrane-bones covering the skull dorsally are paired
nasals, prefrontals, frontals, and parietals ; on each side of the
latter are the squamosals which overlie the quadrates. On the
under side are the paired prevomers, pterygoids, and the
parasphenoid.

Paired maxillae are present, and the two premaxillae have
fused together in front. The pterygoids (dermal bones)
extend freely forwards from the quadrates. In the lower
jaw, part of Meckel's cartilage ossifies as the articular, which
is encased anteriorly between two membrane-bones ; the
(lateral) dentary and the (medial) splenial. The surfaces by
which the quadrate and the articular are in contact are carti-
laginous. The ceratohyals and the ventral elements of the

TRITON

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92 MORPHOLOGICAL TYPES

first two branchial arches form a framework often called the
" hyoid," lying under the floor of the mouth, and of importance
in breathing. When this floor is lowered, air is drawn in
through the nostrils. These are then closed and the floor
raised, with the result that the air has to pass down the throat
and into the lungs.

Teeth are carried by the premaxilla, maxilla, prevomer,
and dentary. They are similar to the teeth of fish already
described, but now they are firmly fixed on to the bones which
carry them.

The vagus nerves emerge just in front of the back of the
skull, and the hypoglossal roots emerge behind it. Six
segments are included in the skull.

Vertebral Column . — The vertebrae are elongated cylinders,
articulating with one another by zygapophyses and cartilaginous
pads on their front and rear surfaces. The notochord is much
constricted. Each vertebra bears a pair of neural arches
above, and those of the tail also have haemal arches beneath.
The trunk- vertebrae (except the first) bear lateral transverse
processes, with which the ribs articulate. The latter are
" true " or dorsal ribs. The first vertebra is modified in
connexion with its articulation with the condyles of the skull.
The ribs belonging to one vertebra are modified and attached
to the ilia of the pelvic girdle, forming the sacrum.

Pectoral Limb and Girdle. — The pectoral girdle is
represented only by the primitive cartilaginous girdle of the
endoskeleton, and indeed it remains largely unossified. The
dermal or membrane-bones of the girdle of other forms have
disappeared. There is a dorsal scapula, and a ventral portion
in which these may be distinguished : an anterior cartilaginous
precoracoid and a posterior partly ossified coracoid. A sternum
is present as a median plate of cartilage, overlapping the
coracoid.

The fore limb is divisible into three regions : upper arm,
forearm, and hand. The skeleton of the limbs is derived from
the radials of the fins of fish, and consequently its bones are
cartilage-bones. The bone in the upper arm is the humerus,
which fits into the glenoid cavity of the girdle, proximally.

TRITON

93

Distally, it articulates with the radius and ulna of the forearm.
The radius is the anterior of the two, i.e. preaxial. These
two bones in turn articulate with the bones of the wrist or
carpus, which is composed of seven small " carpal " bones.
The radiale (scaphoid) and ulnare (cuneiform) articulate with
the radius and ulna respectively. (Typically there are three
bones in this proximal row, the additional one being the inter-
medium (lunar) which here is fused with the ulnare.) Next,
there is a centrale, and the distal row of carpals is formed of
four bones. The bones of the fingers are the metacarpals,
and two or more phalanges, according to the finger. The
first finger has been lost, so the others are numbered 2 to 5.
The digital formula by which the number of phalanges is
expressed, is : o, 2, 2, 3, 2.

Pelvic Limb and Girdle. — The pelvic girdle also consists
of a dorsal and two ventral elements. Dorsally, the ilium
leads from the acetabulum to the modified rib of the vertebra
forming the sacrum. The posterior ventral element is the
ischium : both ilium and ischium are bony. The anterior
element is the pubis which remains cartilaginous, and bears
another cartilaginous process akin to the sternum projecting
forwards. The ischium and pubis meet their fellows of the
opposite side in the middle line.

Like the fore limb, the hind limb can be divided into three
parts : thigh, shank, and foot. The bone of the thigh is the
femur, which articulates with the acetabulum of the girdle.
Distally, it joins the tibia (preaxial) and fibula of the shank.
The ankle or tarsus is very similar to the wrist. The three
proximal tarsal bones are separate : tibiale, intermedium
(together possibly equivalent to the astragalus of higher forms),
and fibulare (calcaneum). The second row is represented by
the centrale (navicular), and the distal row by four bones.
The proximal bones of the toes are the metatarsals, bearing
the phalanges. The digital formula is 2, 2, 3, 3, 2.

Alimentary System. — The floor of the mouth carries a
tongue, of which the hind edge is free.* Salivary glands are

* This tongue has nothing to do with the similarly named structure in
Petromyzon.

94

MORPHOLOGICAL TYPES

present, opening into the mouth. The glottis lies on the floor
of the pharynx and leads to the windpipe. The stomach is
typical and the intestine makes a few loops before leading to the

Fig. 46. — Triton : dissection of female seen from the ventral side.
b, bladder ; b d, bile-duct ; ca, carotid arch ; cl, cloaca ; da, dorsal
aorta; gb, gall-bladder; h, heart; i, intestine ; k, mesonephric kidney;
/, liver ; In, lung ; Md, Miillerian duct ; o, ovary ; p, pharynx ; pn, pan-
creas ; r, rectum ; s, stomach ; sa, systemic arch ; sp, spleen ; Wd, Wolffian
duct.

rectum, which opens into the cloaca. Ventral to the rectum
is a sac which is formed as an outgrowth from it, the bladder.
This sac is of great importance in the evolution of the higher

TRITON 95

vertebrates, where it gives rise to the allantois. For this reason
the bladder of Amphibia is called " allantoic," to distinguish
it from the swellings of the urinary ducts or " bladders " of
fish. The liver is divided into several lobes, a gall-bladder is
present and the bile-duct which leads from it joins the duct
from the pancreas to open into the intestine.

Respiratory and Vascular Systems. — The windpipe, or
trachea, leads back ventral to the gut and divides into the two
bronchi leading to the lungs. These differ from the lung of
Ceratodus only in that they are paired, and that they maintain
their primitive position ventral and lateral to the gut. In the
adult the gill-slits have disappeared, and the afferent and
efferent branchial arteries are directly continuous instead of
being separated by the capillaries of the gills. These arteries
are now called arterial arches, and they run round the pharynx
from the ventral to the dorsal side as if they were still separated
from one another by the gill-slits. The ventral aorta is shortened
up so much that the arterial arches come off from the truncus
arteriosus, close to the heart. The ist arterial arch runs in the
mandibular arch, and the 2nd arterial arch likewise ascends the
hyoid arch. Although present in the embryo, these arterial
arches disappear. The 3rd arch persists as the carotid. At
its base is the lingual artery which represents the anterior
prolongation of the original ventral aorta. The carotid then
passes through the carotid gland (see p. 401) and ascends the
3rd visceral (ist branchial) arch until it reaches a position
dorsal to the pharynx. Here it turns forwards and enters the
skull. It is the anterior prolongation of the original dorsal
aorta.

The 4th arterial arch is known as the systemic. It goes
up in what was the 4th visceral (2nd branchial) arch and turns
backwards. Arrived here, dorsal to the pharynx, this vessel
is exactly in the position of and corresponds to the lateral
dorsal aorta of fish, only it has lost its connexion with its
anterior prolongation which is now the internal carotid. This
connexion, when present, is called the ductus caroticus.

The 5th arterial arch disappears in Triton, though it is
present in the nearly related form Salamandra, where it leads

96

MORPHOLOGICAL TYPES

TRITON 97

from the truncus up the 5th visceral arch and joins the lateral
dorsal aorta.

The 6th arterial arch persists as the pulmonary. It also
runs up round the pharynx and joins the lateral dorsal aorta,
and, just as in Ceratodus, it gives off an artery which runs
backwards to the lungs, on each side. The lungs are sym-
metrical, and the pulmonary arteries do not twist round the
gut as in Ceratodus. The connexion between the pulmonary
artery and the lateral dorsal aorta is called the ductus arteriosus,
or ductus Botalli.

The lateral dorsal aortae join one another in the middle line
above the gut to form the dorsal aorta. On the way, several
arteries are given off : the subclavians to the fore limbs, the
cutaneous to the skin, and, near the point of junction, the
cceliaco-mesenteric which runs ventrally in the mesentery
suspending the gut, and supplies blood to the viscera. The
dorsal aorta continues running back, supplying the kidneys
on the way, and gives off the two iliac arteries which supply
the hind limbs ; thence it runs on into the tail.

As in Ceratodus, the sinus venosus receives three large
veins. These are the paired ductus Cuvieri or superior vena?
cavae, and the single inferior vena cava. Each superior vena
cava is made up of four principal veins; external jugular (from
the ventral regions of the head and tongue) ; internal jugular,
corresponding to the anterior cardinal (from the dorsal regions
of the head, brain, and skull) ; subclavian, itself made up of
the brachial from the fore limb and the cutaneous from the
skin ; and the posterior cardinal (anterior portion).

The inferior vena cava receives the hepatic veins from the

Fig. 47. — Salamandra : dissection of male seen from the ventral side.

The nerves are shown on the left and the urinogenital ducts on the right.
5a, fifth arterial arch ; aa, anterior abdominal vein ; ab, adrenal bodies ;
bp, brachial plexus ; c, cloaca ; ca, carotid arch ; cd, collecting ducts from
the excretory portion of the kidney ; da, coeliac artery ; cv, caudal vein ;
d, dorsal aorta ; da, ductus arteriosus ; ej, external jugular vein ; /, femoral
vein ; ij, internal jugular vein ; k, mesonephric kidney ; pa, pulmonary
artery ; pc, posterior cardinal vein ; pv, pelvic vein ; r, rectum ; rp, renal
portal vein ; sa, subclavian artery ; sv, subclavian vein ; sy, systemic arch ;
syc, sympathetic nerve-chain ; syg, sympathetic ganglion and supra-renal
body ; t, testis ; ta, truncus arteriosus ; vci, vena cava inferior ; vcs, vena
cava superior ; vd> vas deferens ; ve, vasa efferentia.

H

98 MORPHOLOGICAL TYPES

liver, and the renal veins from the kidneys. Blood returns from
the tail by the caudal vein which divides into two ; each portion
connects with two veins coming from the hind limb (femoral
and sciatic) and runs as the renal portal vein to the kidney of
its side. All the blood from the hind regions of the body is
not bound to take this course, for instead of engaging in the
renal portals it may enter the pelvic veins which run towards
one another, join in the midventral line, and proceed forwards
as the anterior abdominal. This vein connects with the
hepatic portal vein, which collects up the blood from the
intestine and takes it to the liver.

From the lungs, the pulmonary veins return the oxygenated
blood to the left auricle of the heart.

The heart consists of the two auricles and the single
ventricle. It is to be noticed, however, that the septum
separating the two auricles is perforated, allowing blood to pass
from one side to the other. Leading from the ventricle is the
conus arteriosus, which, as in Ceratodus, is provided with
valves. The truncus represents the ventral aorta very much
shortened up ; in its anterior region just before giving off the
arterial arches, it is divided into two by a horizontal septum,
forming a cavum pulmonale (leading to the pulmonary arch)
and a cavum aorticum (leading to the systemic and carotid
arches).

The circulatory system is on the whole very similar to that
of Ceratodus. It is to be noted that the separation of the venous
blood from the arterial is still far from complete. The blood
is oxygenated in the lungs and in the skin. It returns from the
lungs to the left auricle by the pulmonary veins, and from the
skin to the right auricle by the cutaneous, subclavian, superior
vena cava, and sinus venosus. The remainder of the blood
entering the right auricle is venous.

Urino-genital System. — The mesonephric kidneys project
downwards from the roof of the ccelomic cavity, covered over
by ccelomic epithelium ; they are therefore more easily visible
than those of fish. The tubules of the kidneys are drained
by the Wolffian ducts which lead into the bladder. In the
male, the vasa efferentia from the testis lead through the

TRITON 99

anterior tubules of the kidney and so into the Wolffian ducts
which become the vasa deferentia. The more posterior
tubules of the kidney are solely excretory in function, and they
do not connect with the Wolffian duct until the latter is close
to the cloaca. This is a step in the direction of separating the
genital from the excretory ducts, which would be effected if
the ducts from the purely excretory part of the kidney were to
move still farther down the Wolffian duct and eventually open
directly into the cloaca.

In the female, the Wolffian ducts are solely excretory in
function, and the Miillerian ducts or oviducts, which open
into the coelomic cavity anteriorly, receive the eggs and convey
them down to the cloaca into which they open separately.

The kidneys retain their open ciliated funnels, leading into
the coelomic cavity (ccelomostomes).

Nervous System. — The brain has large elongated cerebral
hemispheres, in the roof of which nerve-cells appear. The
floor and side of the hemispheres form the corpus striatum.
The cavities of these hemispheres (ist and 2nd ventricles)
communicate with that of the diencephalon (3rd ventricle) by
the foramina of Monro. The pineal projects upwards from
the roof, and the infundibulum down from the floor of the
diencephalon ; and a choroid plexus projects into the 3rd
ventricle. There is no saccus vasculosus. The roof of the
midbrain forms the optic lobes which are joined in the middle
line, and do not present a double appearance. The hind brain
has a cerebellum and a choroid plexus projecting into the 4th
ventricle.

The cranial nerves are similar to those of the dogfish,
except for the fact that the disappearance of the lateral-line
organs (or their very great reduction) entails the disappearance
of those nerves which supply them, viz. superficial ophthalmic,
buccal and mandibularis externus of the facial, and lateralis
of the vagus. There is a further simplification owing to the
closure of the gill-slits. The glossopharyngeal is distributed
to the tongue and pharynx. The vagus supplies the muscles
of the larynx, and also sends parasympathetic fibres to the heart,
stomach, and intestine. The hypoglossal comes out behind

ioo MORPHOLOGICAL TYPES

the skull and is counted as the ist spinal nerve ; it runs to the
muscles beneath the tongue which actuate the " hyoid " plate
for the purpose of breathing. The spinal nerves to the limbs
are grouped, forming the brachial and sciatic plexus for the
fore and hind limb respectively.

As in the dogfish, there are sympathetic nerve-chains on
each side of the dorsal aorta. They continue forwards
accompanying the internal carotids into the head. They join
the sympathetic ganglia to one another, each receiving in
addition a ramus communicans from its corresponding spinal
nerve.

Sense-organs. — The lateral-line sense-organs have already
been mentioned. The ears are in a degenerate condition in
Triton, for although they appreciate vibrations in air, i.e.
sound, their structure is not typical of land- vertebrates, and
will not be considered here. It may be mentioned, however,
that the tympanic cavity which is characteristic of the ears of
other Tetrapods and which is homologous with the spiracular
slit of the dogfish, is not developed ; and that the ear-drum
or tympanic membrane is also absent.

The lens of the eye is attached to a protractor lentis muscle,
contraction of which increases the distance between the lens
and the retina and accommodates the eye for near vision.
There is a retractor bulbi muscle which pulls the eyeball in,
and depresses the roof of the mouth, which action assists in
the process of swallowing.

Mesoderm and Ccelom. — An important feature is that the
wall separating the pericardium from the perivisceral ccelom
is very thin and membranous, and unlike the stiff partition
present in the dogfish and Gadus. The amphibian condition
is already foreshadowed in Ceratodus, and it results in the
fact that the heart and pericardium project back into the
perivisceral cavity, ventral to the gut.

The first three somites give rise to the eye-muscles. The
4th disappears during development, and the 5th produces
muscle-fibres which persist.

Ductless Glands . — The thyroid arises from a downgrowth
from the floor of the pharynx and afterwards divides into two,

TRITON 101

right and left. The groups of vesicles of which it is composed
are surrounded by connective tissue. Close to it are the
parathyroids, on each side, and developed from the ventral
region of the gill-slits which close up at metamorphosis. Their
origin is therefore segmental, and it is worth noticing that
parathyroids do not appear in vertebrates with persisting water-
breathing gills.

The thymus glands arise from the dorsal sides of the gill-
pouches, and are therefore also segmental in origin.

The adrenal bodies are in a very interesting condition.
They consist of islands of tissue overlying the ventral surface
of the kidneys and extending forward as isolated lumps at the
side of the dorsal aorta. The bodies consist of two kinds of
tissue : cortical, corresponding to the inter-renal of the dogfish,
and medullary or chromaffme tissue, corresponding to the
supra- renals of the dogfish, and like them derived from the
sympathetic nervous system. The cortex (and inter- renal) is
formed from the ccelomic epithelium. In the region of the
kidney, the adrenal bodies are composed of both cortical and
medullary tissue, as in the higher vertebrates. The bodies in
front of the kidneys, however, may consist entirely of medullary
tissue, as in the supra-renals of the dogfish. These animals
therefore provide a very interesting intermediate condition.

The pituitary consists of four parts. The pars nervosa
is formed from the floor of the infundibulum, the remaining
three (anterior, intermedia, and tuberalis) arise from the
hypophysis.

Characters of Triton which show an advance over the con-
ditions in Fish, and which are typical of Tetrapoda :

Limbs ending in digits (fingers and toes) ;

Formation of arterial arches, short-circuiting the

gill-capillaries ;
Interruption of dorsal aorta from internal carotid ;
Pelvic girdle composed of three elements : one dorsal

and two ventral ;
Presence of an allantoic bladder ;

102 MORPHOLOGICAL TYPES

Joining of cortical and medullary tissue to form the

adrenal bodies ;
Parathyroid glands ;
Salivary glands.

Characters of Triton which are specialised when compared
with higher forms :

Reduction of bone in the skull and girdles ;
Absence of membrane-bones in the pectoral girdle ;
Incompleteness of the interauricular septum ;
Degenerate condition of the ear.

Characters of Triton which are typical of Amphibia :

Heart with two auricles and single undivided ventricle ;
Skin naked, i.e. without horny scales ;
Aquatic larval stage.

CHAPTER VIII

LACERTA : A CHORDATE LIVING ENTIRELY ON LAND

Externals. — In general shape, the lizard is not very dissimilar
from the newt, but it differs from it in one very important
respect, which is characteristic of all the animals (Reptiles) of
the group to which the lizard belongs. The body is covered
with scales formed from the epidermis, and therefore totally
different from the true scales of fish, which are always formed
from the (mesodermal) dermis. To mark this distinction,
the scales of reptiles are called corneoscutes. They cover the
whole body including the limbs and head, and on the latter
their arrangement does not correspond with that of the under-
lying bones. On the last phalanges of the fingers and toes,
the scales form horny claws. Underlying the corneoscutes
of the head there are ossifications of the dermis forming osteo-
scutes, which fuse with the underlying bones of the skull.

The skin is dry and devoid of glands. The eyes have upper
and lower eyelids, and also a so-called " third eyelid " or nicti-
tating membrane. Behind the eye is a circular area sunk
slightly below the level of the skin, and covered over like a
drum by the tympanic membrane or ear-drum. The external
nostrils are on the side of the snout ibove the mouth. At the
base of the tail is the cloaca, which in the male is provided
with a pair of protrusible copulatory organs.

Skull. — The skull of the lizard, although well ossified,
has several holes in it, separating the membrane-bones. The
nasal aperture is bounded by the premaxilla, nasal and maxilla.
The orbit is limited by the prefrontal, frontal, and postfrontal
above, and the lachrymal, jugal, and postorbital beneath.
Behind the orbit is an aperture called a temporal fossa, in

103

104 MORPHOLOGICAL TYPES

between the parietal, supratemporal, squamosal, postorbital,
and postfrontal. In Lacerta this fossa is covered over by the
osteoscutes mentioned above. This is not the case in Varanus,
a form related to Lacerta, and in which the relations of the
temporal fossa may be conveniently studied. There is a
postorbital bar formed from the postfrontal, postorbital, and
jugal, separating the orbit from the temporal fossa ; and a
horizontal temporal bar formed by the postorbital and squa-
mosal, forming the lower border of the temporal fossa. (It

Fig. 48. — Varanus : view of the skull from behind.

bo, basioccipital ; bp, basipterygoid process of basisphenoid ; bs, basi-
sphenoid ; fm, foramen magnum ; p, parietal ; pf, post-temporal fossa ;
pp, paroccipital process (opisthotic and exoccipital) ; pt , pterygoid ; q, quad-
rate ; s, supratemporal ; so, supraoccipital.

should be mentioned that there remains an element of doubt
concerning the homologies of the bones here called supra-
temporal and squamosal. See p. 440.) On the floor of the
nasal capsules are the septomaxillaries, which overlie Jacob-
son's organs.

The quadrate abuts against the fused opisthotic and exoc-
cipital which form the paroccipital process. The quadrate also
articulates by a loose joint with the pterygoid, and is movable
relatively to the squamosal and brain-case (a condition known

LACERTA 105

as streptostylic, see p. 292) ; in connexion with this arrange-
ment the upper jaw can be raised relatively to the brain-case.

The foramen magnum is bounded by the basioccipital,
supraoccipital, and exoccipital bones. In front of the basi-
occipital, on the floor of the skull, is the basisphenoid, which
has a pair of basipterygoid processes for articulation with the
pterygoids. In front of the basisphenoid is the parasphenoid.

On the palatal surface the pterygoids are long bones lying
to each side of the middle line. Behind, they connect with the
quadrate, their inner surfaces articulate with the basipterygoid
processes of the basisphenoid, and in front each pterygoid is
connected with two bones : the transpalatine laterally and the
palatine medially. The transpalatine is the representative of
the ectopterygoid of Gadus. In front of the palatines are the
prevomers. The margin of the upper jaw is made by the
premaxilla? and maxillas. The ascending process is present
and ossified as the epipterygoid, which rises as a slender pillar
from the pterygoids. As in other animals, it separates the
ophthalmic from the maxillary branches of the trigeminal
nerve (see Figs. 138, 148, and 150).

In the lower jaw, the posterior region of Meckel's cartilage
is ossified as the articular ; in addition there are the following
membrane-bones : dentary, angular, supra-angular, splenial,
and coronoid. Teeth are carried on premaxilla, maxilla,
palatine, and dentary.

The ventral portions of the hyoid and branchial arches form
a " hyoid " skeleton beneath the tongue. The hyomandibula
is represented by the columella auris, a slender rod which
connects the ear-drum or tympanic membrane with the fenestra
ovalis in the side of the auditory capsule. This change of
function of the hyomandibula from the condition in Scyllium
and Gadus, where it supports the quadrate, is made possible
by the autostylic method of suspension of the quadrate.

It is to be noted that the skull articulates with the vertebral
column by one median condyle, and that in the formation of
the skull, two more segments have been incorporated than in
the Amphibia. This accounts for the fact that the hypoglossal
nerves emerge from the skull, instead of behind it.

106 MORPHOLOGICAL TYPES

Vertebral Column. — In Reptiles and all higher vertebrates,
the first two vertebrae are peculiarly modified. The first is
called the atlas, and its anterior surface is hollow to receive the
condyle of the skull. Its centrum has, however, been separated
from it and attached to that of the second vertebra, forming the
odontoid peg. Round this peg the atlas and skull are free to
rotate. The second vertebra is called the axis. The subse-
quent vertebrae of the neck and thorax are normal, and consist
of centra with neural arches and spines, and zygapophyses.
The centra are concave anteriorly, and convex posteriorly, a
condition described as proccelous.

The vertebrae of the tail are peculiar in that they are split
transversely, and when the lizard sheds (autotomises) its tail
the break occurs at one of these splits. Under some of the
tail-vertebrae are Y-shaped haemal arches, ossified as " chevron-
bones."

Ribs are carried by all vertebrae in front of the sacrum except
the first three ; they articulate with the centra of their
respective vertebrae. The ribs belonging to the vertebrae
of the neck (cervical) are short, those of the anterior region of
the thorax are attached ventrally to the sternum (five pairs).
The more posterior ribs (" floating ") do not touch the sternum.
The two sacral vertebrae bear stout transverse processes which
are attached to the ilia of the pelvic girdle.

Pectoral Girdle and Limb. — The cartilage-bones of the
pectoral girdle are the scapula and coracoid, both contributing
to the glenoid cavity into which the head of the humerus fits.
The anterior borders of these bones are characteristically
indented. The membrane-bones consist of a pair of clavicles,
and a median Y-shaped interclavicle. The forelimb is typical,
and similar to that of Triton except that it has five fingers,
each ending in claws.

Pelvic Girdle and Lirnb. — The acetabulum is bordered
by ilium, ischium, and pubis. The ilium points backwards
towards its articulation with the sacral vertebrae. Both the
ischium and the pubis meet their fellow-bones of the opposite
side in the middle line, forming symphyses. On each side, the
pubis and ischium are separated by the ischio-pubic foramen.

LACERTA

107

The hind limb is similar to that of Triton, but the tarsal
bones undergo a modification. The proximal bones are fused
into one which is attached to the tibia and fibula ; the distal
bones are reduced to two, which become attached to the meta-
tarsals. The result is that the ankle can only bend in one
place, at the so-called mesotarsal joint. The 5th metatarsal is

Fig. 49. — Lacerta : pectoral-
girdle and forelimb, seen
from the left side.

Fig. 50. — Lacerta : sacrum, pelvic-
girdle and hind limb, seen from the
left side.

c, coracoid ; ca, carpals ; cl, clavicle ; /, femur ; fi, fibula ; h, humerus ;
ic, interclavicle ; il, ilium ; ip, ischio-pubic foramen ; is, ischium ; mc,
metacarpal ; mts, metatarsal (note hook-like shape of that of fifth digit) ;
p, phalanges ; pu, pubis ; r, radius ; s> sternum ; sc, scapula ; sv, sacral
vertebrae ; t, tibia ; ta, tarsals ; tp, transverse processes of sacral vertebrae ;
u, ulna.

worthy of notice on account of its peculiar hook-shaped
appearance.

Alimentary System.- — The tongue is long, bifid at the tip,
and protrusible. It is supported by the " hyoid " skeleton.
On the roof of the mouth the palatal folds appear, extending
inwards from the sides. At the back of the mouth-cavity,
the Eustachian tubes open. These represent the cavity of the

io8

MORPHOLOGICAL TYPES

spiracle of the dogfish, and each is closed laterally by the
ear-drum or tympanic membrane. These cavities are also
called tympanic cavity, and " middle-ear," and will be referred
to again in connexion with that sense-organ. The mouth
is provided with salivary glands, which assist digestion.

The glottis leads to the larynx and lungs. The remaining

Fig. 51. — Lacerta : ventral view of the pectoral girdle and sternum.

c, coracoid ; cl, clavicle ; h, humerus ; ic, interclavicle ; r, ribs ;
s} sternum ; sc, scapula.

viscera do not differ sufficiently in detail from those of Triton
to necessitate a specific redescription.

Respiratory System. — The lungs are sacs with very vascular
walls, and they are the only respiratory organs, for the skin no
longer functions as such. Another change from the amphibian
condition is shown by the method of breathing. Instead of
raising and lowering the floor of the mouth, the ribs are pulled
forwards by muscles which run obliquely from rib to rib. At
rest, the ribs slope backwards, and when pulled forwards the

LACERTA

109

effect is to increase the volume of the thoracic cavity and of the
lungs. Air then rushes in.

Vascular System . — The heart consists of sinus venosus,
two auricles and a single ventricle. The truncus arteriosus
has been split into three right down to its base, so that the
ventricle opens directly into three arteries. The more ventral
of the three opens into the right side of the ventricle, and leads
to the lungs, dividing as it goes into two pulmonary arteries.
These no longer connect with the lateral dorsal aorta or systemic
arches. The pulmonary circulation is therefore distinct.
The other two vessels are the right and left systemic arches
(corresponding to the systemic arches of the newt and the 4th
arterial arches of fish). The right systemic arch springs from
the left side of the ventricle, and the left arch from the right
side of the ventricle. These vessels run up the 4th visceral
arch and join dorsal to the gut to form the dorsal aorta. Now,
the left side of the ventricle is occupied mostly by arterial
(oxygenated) blood from the left auricle and pulmonary veins ;
consequently the right systemic arch receives pure blood, more
or less. But the left arch and the pulmonary vessel are on the
right side of the ventricle, which contains venous blood from
the right auricle, sinus venosus, and the veins of the body.
In addition, there is an incomplete septum dividing the
ventricle, so that while the pulmonary artery receives venous
blood as would be expected, the left systemic arch receives
mixed blood. The carotid arches spring from the base of the
right systemic arch and therefore receive pure blood, as indeed
they need, for they supply the brain. The carotid arches run
dorsally in the 3rd visceral arch and when dorsal to the gut run
forwards into the head as the internal carotid arteries. In
addition, however, the carotid arteries connect back with the
systemic arches by what are really remnants of the lateral
dorsal aorta. These connexions are known as the ductus
caroticus.

The arteries given off to the viscera are on the whole similar
to those of Triton.

The venous system is likewise similar to that of Triton, but
it is necessary to mention three new points. In correlation

no

MORPHOLOGICAL TYPES

LACERTA in

with the lack of respiratory function on the part of the skin,
the cutaneous vein is not found. The posterior cardinal
veins are likewise much reduced ; that on the left disappears
altogether, that on the right is now known as the azygos vein.
Lastly, the renal portal system is less well developed, and this
is associated with the fact that the functional kidney in the
adult is a new structure, the metanephros.

Urino-Genital System. — The kidneys are paired structures
lying in the roof of the posterior part of the coelomic cavity,
which connect with the base of the allantoic bladder (where
the latter opens into the cloaca) by means of ducts called
ureters. These kidneys are not the same as the mesonephric
kidneys of the animals previously described ; they are meta-
nephric kidneys, and serve excretory functions only, never
connecting with the genital organs. Otherwise, they are
similar in structure to the mesonephros, and consist of Mal-
pighian corpuscles with glomeruli and tubules. The meta-
nephros develops later than the mesonephros, and out of the way
of the posterior cardinal veins. The mesonephros is present
in early stages of development, but does not function as an
excretory organ in the adult. As the renal portal system is
associated with the mesonephros, the disappearance of the one
is correlated with the reduction of the other.

In the female, the mesonephros disappears in the adult,
together with the Wolffian duct. The Miillerian duct persists
as the oviduct and serves to convey the eggs (which drop from
the ovary into the ccelom) to the exterior via the cloaca. Glands
in the oviduct secrete a shell round the egg, for it is laid on
dry land, and not in water. The embryo develops within a
membrane, the amnion, for which reason reptiles, birds,
and mammals are called Amniota. In the male, the Miillerian
duct is absent, but the mesonephros and Wolffian duct persist,
serving only to evacuate the sperms. The testis is connected
with the mesonephros by vasa efferentia in the ordinary manner,
and the tubules of the mesonephros, through which the sperms
pass, form the epididymis. The epididymis is really very
long, and when unravelled it forms a tube which in man is
over twenty feet long. During their passage through it, the

112

MORPHOLOGICAL TYPES

mc.

#S58fc /■

Fig. 53. — Method of formation of the kidneys in amniotes.

A, larval condition, with Mullerian duct (Md), Wolffian duct (Wd),
mesonephric funnels (mi) opening into the splanchnocoel (sc) ; i, intestine.
The ureter (u) arises from the base of the Wolffian duct and divides into a
number of tubes which eventually connect with the metanephric capsules
(mc). B, condition in the adult male. The Mullerian duct has disappeared
and the Wolffian duct persists as the vas deferens (vd), receiving the sperms
from the testis (t) by means of the vasa efferentia (ve). The ureter drains
the urine from the metanephros (m) to the bladder (b). C, condition in the
adult female. The Mullerian duct persists as the oviduct (od), the opening
of which into the splanchnocoel is the Fallopian tube (Ft) ; o, ovary. The
Wolffian duct has disappeared.

LACERTA 113

sperms are acted on by a secretion, as a result of which they
complete their development and acquire the power of individual
movement. The Wolffian duct is the vas deferens leading to
the cloaca. As already mentioned, the wall of the cloaca
bears two eversible copulatory organs, for since the egg is
surrounded by a shell when it leaves the oviduct, it is obvious
that fertilisation must take place in the oviduct itself.

Nervous System. — The brain is built on the same plan as
that oL Triton, but it shows an advance in the increased size
of the cerebral hemispheres. In these there is a small amount
of superficial grey matter or cortex ; in lower forms the grey
matter is almost entirely within the white. This is a very
important advance from the point of view of the evolution of
the human brain. The sides of the telecephalon (corpus
striatum) and of the diencephalon (thalamus) are enlarged, and
the cavities of the 1st, 2nd, and 3rd ventricles are consequently
reduced. There is a well-developed pineal eye, arising from the
diencephalon, and connected with the right habenular ganglion.
The cranial nerves are similar to those of Triton except for
the fact that the hypoglossus (12th nerve) is included among
them, and that there is a spinal accessory nerve (nth nerve).
The spinal accessory supplies the dorsal muscles of the shoulder
girdle, and represents a specialised portion of the vagus of
lower forms.

Sense-organs. — The lateral line sense-organs are no longer
present, and the sole representative of the system to which
they belong is the ear. In addition to being an organ of balance,
the ear is also stimulated by vibrations in air, or sound. (This
is also the case in Amphibia such as the frog, but not so typically
in Triton in which the ear is degenerate (see p. 100).) The
vibrations impinge on the ear-drum (tympanic membrane)
and are communicated to the columella auris (hyomandibula) ;
the latter conveys the vibrations across the cavity of the
middle-ear or tympanic cavity (spiracular visceral cleft) to
the fenestra ovalis in the wall of the auditory capsule. The
auditory capsule contains the auditory vesicle ; between the
latter and the wall of the capsule is a fluid called perilymph,
while the auditory vesicle itself contains endolymph. The

1

ii4 MORPHOLOGICAL TYPES

vibrations brought by the columella auris are imparted through
the fenestra ovalis to the perilymph, which in turn passes
them on through the wall of the auditory vesicle to the
endolymph. Here the vibrations stimulate the special sensory
cells. The wall of the auditory capsule has a second opening
(the fenestra rotunda), situated ventrally to the fenestra ovalis.
The fenestra rotunda is covered by a membrane separating the
perilymph from the tympanic cavity, and its function is to damp
down and deaden the vibrations in the perilymph when they
reach it. With regard to the auditory vesicle itself, the utricle
has the usual three semicircular canals, and the saccule, which
is better developed than in lower forms in connexion with the
perfecting of the sense of hearing, has a ductus cochlearis.

The retina of the eye contains mostly cones with very few
rods. The lens changes its degree of convexity, and thereby
its focal length, as a result of the contraction of the circular
iris-muscle. The sclerotic is strengthened by bony plates.
Mention has already been made of the three eyelids. The
lining of the lids is in places modified into glands. At the
inner side of the eye is the Harderian gland which lubricates
the " third eyelid " (nictitating membrane) ; at the outer
angle is the lachrymal gland. The transparent nictitating
membrane, which is really a fold of the conjunctiva, is activated
by a muscle derived from the retractor bulbi, and like it
innervated by a branch of the abducens. The lower lid is
depressed by a special muscle.

The pineal eye, already seen in Petromyzon, is remarkably
developed. Its stalk rises up from the roof of the diencephalon
and swells out into a vesicle of which the lower portion forms
the sensory layer, and the upper forms the lens. This eye
lies below the foramen between the parietals ; it is, however,
covered over by connective tissue and a corneoscute.

The cavity of the nose is enlarged, and a shelf projects
inwards from the side wall, increasing the surface of the nasal
epithelium and forming a so-called concha. Ventral to the
nasal cavities are a pair of pockets, originally formed from
the nasal cavities, and lying just above the prevomers. Each
opens into the mouth cavity a little way in front of the

LACERTA 115

choanae, or internal nostrils. These structures are known as
Jacobson's organ, and they probably serve to smell food in
the mouth.

Coelom. — The splanchnocoel is represented by the peri-
cardium and perivisceral coelomic cavities. The lungs project
backwards into the latter on each side of the stomach, supported
by the pulmonary folds of the coelomic epithelium (accessory
mesenteries). These folds also connect ventrally with the
liver, by the pulmo-hepatic ligaments. The gut is of course
suspended from the roof of the coelomic cavity by the dorsal
mesentery, and connected with the liver ventrally by the
so-called lesser omentum,* also a fold of epithelium. The
mesentery ventral to the liver mostly disappears, but persists
anteriorly as the falciform ligament. In this manner the
coelomic cavity becomes divided up into a number of inter-
communicating spaces. On each side of the gut and mesentery,
and median to the pulmonary folds and pulmo-hepatic ligaments,
is a pulmo-hepatic recess which ends blindly in front, and opens
posteriorly into the main cavity. Owing to the kinking of the
stomach to the left, the first portion of the intestine (duodenum)
recurves to the right, and the right pulmo-hepatic recess forms
part of a pocket, the omental cavity. This cavity communicates
with the main coelom on the right side by an opening, the upper
and front borders of which are formed by the right pulmonary
fold and pulmo-hepatic ligament ; the lower and hind borders
are formed by the lesser omentum running from the duodenum
to the liver, and by the mesentery supporting the duodenum.
The opening is the foramen of Winslow, and its relations are
important with regard to the inferior vena cava which runs
down its upper and anterior border, and the hepatic portal
vein, the hepatic artery, and the bile-duct which run along its
lower border (see Fig. 125).

* The lesser omentum is equivalent to the gastro-hepatic ligament and
the duodeno-hepatic ligament : portions of mesentery connecting the liver
with the stomach and the duodenum respectively.

n6 MORPHOLOGICAL TYPES

Characters of Lacerta > lacking in lower forms , and common to
Amniota :

Embryos develop on land within an Amnion and a
shell ;

Metanephros and ureter ;

Spinal accessory and Hypoglossal nerves emerge from
the skull ;

Superficial grey matter (cortex) in the cerebral hemi-
spheres ;

Breathing effected with the help of the ribs ;

Jacobson's organ ;

Development of saccule, tympanic cavity (spiracular
pouch), and conversion of the hyomandibula into the
columella auria. (Already present in the frog, though
feebly developed in Triton) ;

Atlas and Axis vertebrae differentiated ;

Copulatory organs developed ;

Ischio-pubic foramen present.

Characters of Lacerta which are typical of Reptiles :

Skin covered with horny scales ;

Ventricle of the heart incompletely divided (except in
Crocodiles).

Literature.

Parker, T. J. A Course of Instruction in Zootomy (Vertebrates).
Macmillan, London, 1884.

CHAPTER IX

columba: a chordate with wings

Externals. — The birds are principally distinguished by the
possession of feathers, and the modification of the fore limbs
into wings. The hind limbs continue to serve for terrestrial
locomotion, but it is important to note that although the birds
have evolved the habit of standing on two legs only, the body
is still carried in a horizontal position. The mouth is toothless
but bordered by a horny beak, the external nostrils are on each
side of the upper beak, a little way behind the tip. The eyes
have upper and lower lids, and also a " third eyelid," or
nictitating membrane. The ear-drum is no longer flush with
the surface of the skin, but sunk at the bottom of a tube, which
is the external auditory meatus. The alimentary and urino-
genital systems open at the cloaca. The tail is very much
shortened, and on its dorsal side is the uropygial gland.
This gland, which is the only one to be found in the skin of
birds, produces a secretion with which the bird preens its
feathers, and makes them waterproof. There are scales on
the legs, and claws at the ends of the toes (in a very few cases
also on the fingers), but no dermal ossifications of any kind are
present.

Feathers. — The feathers are arranged on the surface of the
body in definite tracts, called pterylae. Feathers are formed
by the epidermis (see p. 224), and are of different kinds in the
various regions of the body. Those visible on the outer
surface of the bird are called penna?, which include the quill
or flight-feathers and the contour feathers of the adult bird.
Their typical structure may now be described.

A penna consists of a stalk (quill or rachis) carrying a vane.

117

n8

MORPHOLOGICAL TYPES

Fig. 54. — View of a portion of a feather to show the structure and relations
of the barbs and barbules.

Two adjacent barbs (b) are represented cut off from the stalk. The
barbs bear distal barbules (db) on the side away from the base of the feather,
and proximal barbules (pb) on the opposite side. The hooks or hamuli (h)
on the distal barbules of one barb are attached to the groove on the edge of
the proximal barbules of the adjacent (distal) barb.

COLUMBA 119

The vane is made up of a large number of barbs on each side
of the central stalk or rachis, and each barb carries a number
of barbules on either side. The barbules bear hooks and
notches, by means of which the barbules of one barb are
attached to the barbules of adjacent barbs. In this way a stiff,
air-resisting plane is formed, which is especially well developed
in the flight-feathers. The flight-feathers on the wings are
called remiges, those on the tail rectrices. The remiges which
are carried on the " forearm " (radius and ulna) are the
" secondaries," those on the " hand " (carpals, metacarpals,
and phalanges) are called the " primaries." Contour feathers
cover the body and give it a smooth surface which presents
little resistance to the air during flight. They are smaller
than flight-feathers, and the hooks or hamuli on the barbules
are not so well developed. Contour feathers usually possess
an aftershaft, which is like a duplicate vane arising from the
base of the rachis. As a rule it is small, but in the cassowary
the aftershaft may be as long as the main shaft. The base
of the quill beneath the vane is a hollow cylinder, opening
below by the inferior umbilicus, and above at the base of the
vane by the superior umbilicus. The superior umbilicus is
between the main shaft and the aftershaft, which relations
become obvious from a study of the development of the
feather (see p. 224).

In addition to the pennae there are in most birds down
feathers or plumulae. In these the barbules and hamuli are
very degenerate so that there is no stiff vane at all. The down
feathers form a dense layer which prevents the movement of
the air in it, and therefore functions as a non-conductor of
heat. This is important because birds are warm-blooded,
and without this protection they would lose their heat rapidly
by radiation from the skin to the surrounding air.

Some feathers consist only of a slender stalk with scarcely
any barbs ; they resemble hairs and are known as filo-
plumes.

The kinds of feathers described above are characteristic
of adult birds, and may collectively be called teleop tiles. In
the young birds they are preceded by nestling- feathers, or

120 MORPHOLOGICAL TYPES

neossoptiles. Filoplumes, plumutee, and pennse are preceded
by preflloplumes, preplumulae, and prepennae, respectively.

Feathers are usually coloured, and since they are dead
structures, their colours are due either to pigment which they
contain, or to the optical properties of their texture. Feathers
are moulted and replaced periodically, which in many birds
enables plumages of different type and colour to succeed one
another. Flight-feathers are usually moulted in pairs, sym-
metrically right and left, as a result of which the bird is still
able to fly during the moulting period.

Skull. — The skull of the pigeon, as of birds generally, is
strongly ossified, so much so that the bones tend to fuse
together and the sutures between them to disappear. The
brain-case is much enlarged compared with lower animals.
Covering the roof are : nasals, prefrontals, frontals, parietals,
and squamosals (membrane-bones). The cartilage-bones of
the neurocranium are : basioccipital, supraoccipital, ex-
occipitals, basisphenoid, laterosphenoids, orbitosphenoids, and
ethmoid. The brain-case does not extend forward between
the eyes, which are only separated by an interorbital septum.
The bones of the auditory capsule fuse and form the periotic.

The parasphenoid of lower forms is represented by the
median rostral, and the paired basitemporals, which latter are
attached to the underside of the basisphenoid. There is a
single occipital condyle.

It is worth noticing that the squamosal now forms part of
the wall of the brain-case ; the latter is so much enlarged that
the cartilage-bones are insufficient to enclose it (see Fig. 140).

The quadrate articulates with the periotic by an otic process.
Stretching forwards from the quadrates are two strings of
bones on each side. On the outer side are the quadrato-jugal,
jugal, maxilla, and fused premaxillae, all membrane-bones
forming the margin of the upper jaw. Median to these, the
pterygoids run forwards from the quadrates, and articulate
with the basipterygoid processes of the basisphenoid, and with
the palatines, which run forwards to the maxillas. Median to
them are the small prevomers, fused together in the middle
line.

COLUMBA 121

In the lower jaw, Meckel's cartilage is represented by the
articular, and the angular, supra- angular, splenial, and dentary
are membrane-bones.

The hyoid skeleton is represented dorsally by the columella
auris, connecting the ear-drum with the fenestra ovalis of the
auditory capsule, passing behind the quadrate. Ventrally, the
" hyoid " consists of basihyal, ceratohyal, basibranchial, and
ceratobranchial of the ist branchial arch.

There are no teeth in the pigeon nor in any living
bird.

Vertebral Column. — The first two vertebras are the atlas
and axis. They are followed by twelve others, forming the
cervical region of the vertebral column. The articulation of
the centra with one another is of a peculiar saddle-like pattern
called heterocoelous, and giving the neck great flexibility.
The vertebrae have neural arches, zygapophyses, extra articular
facets called hypapophyses, and transverse processes. The
ribs articulate with the vertebrae by two heads ; a dorsal
tuberculum (fitting on to the transverse process) and a ventral
capitulum (touching the centrum). None of the ribs of the
cervical vetebrae reach the sternum, and the first ten, carried
by vertebrae 3 to 12, are actually fused with their respective
vertebrae. In this manner, each of these vertebrae has a little
(vertebrarterial) canal on each side. Cervical ribs of vertebrae
13 and 14 are free.

There are five thoracic vertebrae, of which the first four are
fused together, and the last is fused on to the next posterior
vertebra (ist lumbar). The thoracic ribs are jointed and are
attached ventrally to the sternum. All the free ribs except the
last bear processes (uncinate) which overlap the next posterior
rib, and help to give strength to the thoracic box.

The lumbar vertebrae are six in number, and they are fused
in front with the last thoracic, and behind with the two sacral
vertebrae, and the first five caudals. In this way an extensive
sacrum is formed, to which the ilia of the pelvic girdle are
attached, strong enough to stand the leverage on the ilia due
to the horizontal position of the bird's body with the legs at
the hind end.

122 MORPHOLOGICAL TYPES

After this come six free caudal vertebrae, and then four
more all fused up together to make the pygostyle.

Pectoral Girdle and Limb. — The shoulder girdle is formed
of scapula and coracoid (cartilage-bones), and a clavicle
(membrane-bone) which meets its fellow from the other side
in the middle line to form the furcula, or " merrythought.'*
The scapula extends backwards over the ribs ; the coracoid
is attached to the sternum. Where the scapula, coracoid, and
clavicle meet, they enclose a foramen (triosseum) between them,
which acts as a pulley through which the tendon of the minor
pectoral muscle passes, to be inserted on the humerus and so
raise the wing.

The sternum is remarkable for its relatively enormous
median keel or carina. On each side of it the pectoral muscles
are inserted. Of these, the minor pectoral muscles have been
mentioned above ; the major pectoral muscles pull the wing
down and in so doing lift the bird in the air. The difference
between " red meat " and " white meat " can be well shown
in the pectoral muscles of different birds. Muscles which
perform long-continued actions are rich in sarcoplasm and
haemoglobin, and are therefore red. Other muscles, the action
of which is not continuous, are poor in sarcoplasm, and their
fibres are therefore white in colour. The falcon is a bird
which spends long periods on the wing, during which its
pectoral muscles are in continuous activity. It is not sur-
prising to find therefore that these muscles are " red meat."
On the other hand, the domestic fowl does not use its pectoral
muscles continuously, and they are white.

The skeleton of the wing consists of humerus, radius, and
ulna. The wrist and hand are somewhat modified ; there are
two free proximal carpal bones, the radiale and ulnare ; but
the distal carpals have fused with the three fused metacarpals
to form a carpo-metacarpus. The first digit is represented by
a phalanx bearing feathers which form the " bastard wing."
The remaining two digits have two and one phalanges respec-
tively, and they, together with the carpo-metacarpus, bear
the primary remiges.

Pelvic Girdle and Limb. — The pelvic girdle is at first sight

COLUMBA

123

different from that of any animal so far described. The
acetabulum is perforated, and is formed from the usual three
bones, ilium, ischium, and pubis. The ilium extends forwards

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and backwards and is attached to the long sacrum. The
ischium is fused along the greater part of its length with the
posterior part of the ilium, leaving an ilio-sciatic foramen

124 MORPHOLOGICAL TYPES

through which the sciatic artery and nerve run to the hind
limb. The pubis is remarkable in that it points backwards
and runs along the ventral border of the ischium, from which
it is separated by the obturator fissure (corresponding to
the ischio-pubic foramen, and serving for the passage of the
obturator nerve). Neither the pubis nor the ischium extend
to the middle line ; they consequently do not meet their
fellows of the opposite side, and have no symphyses. The
absence of symphyses may be correlated with the fact that
birds lay relatively very large hard-shelled eggs.

The femur is short and thick ; covering the front side of its
lower extremity is the patella. The tibia is fused with the
fibula and with the proximal tarsal bones to form the tibio-
tarsus. The distal tarsals are fused on to the united metarsals
of the 2nd, 3rd, and 4th toes to form the tarso-metatarsus.
The 1st metatarsal is small and free ; the digital formula of the
phalanges is 2, 3, 4, 5, o (there being no 5th toe). The terminal
phalanges bear claws. The first toe is in birds usually directed
backwards, and it is opposable to the other digits. This
arrangement enables a bird to stand securely on a narrow
twig of a tree, with the first toe clasped round behind the twig
and the remainder in front of it. The joints of the toes are
bent by tendons which run back beneath the foot and up
behind the tarso-metatarsus. The more the tarsal joint is
bent (in squatting), the tighter these tendons are stretched,
and the more strongly are the toes bent. The bird can there-
fore ensure a tight grip on its perch without effort, and even
when asleep, simply by squatting.

It is obvious that the hind limb can bend only at the knee
and between what were the proximal and distal rows of tarsals :
this extreme form of the meso- tarsal joint is the rule in birds.

It is to be noted that the bones of the bird's skeleton are
very light, and that most of them are hollow ; some of these
spaces communicate with the air-sacs which will be described
in connexion with the respiratory system.

Alimentary System. — The tongue is small and pointed, and
behind it the two Eustachian tubes open into the mouth by a
single common aperture. The glottis in the floor of the gullet

COLUMBA 125

leads to the lungs. The gullet swells out into the crop at the
base of the neck. This is a thin- walled sac in which the food
is temporarily stored. The stomach is modified in that the
glands are restricted to an anterior chamber, the proventriculus.
Following on this is the gizzard, which has thick hard walls,
and in which the food is crushed up with the help of stones
and grit. The duodenum leads out from the gizzard, and
receives the three ducts from the pancreas, and the two bile-
ducts from the liver, which does not possess a special gall-
bladder. The intestine is coiled, and leads to the rectum,
which bears a pair of cceca. The rectum leads to the cloaca
which is peculiar in that it is subdivided into three regions.
That into which the rectum opens is called the coprodaeum ;
next comes the urodaeum, into which the ureter and genital
ducts lead ; and lastly the proctodeum which opens to the
exterior. Into the proctodeum opens the bursa Fabricii, a
blind sac-like organ of unknown significance.

Ccelom. — A knowledge of the relations of the ccelom is
necessary for a proper understanding of several of the organs
in birds. As in the lizard, the lungs are contained in folds of
the ccelomic epithelium which connect with the liver (forming
pulmo-hepatic ligaments), but in addition, these ligaments
make a connexion with the side wall of the general ccelomic
cavity. In so doing, they slant downwards and laterally from
the roof of the ccelomic cavity, and are called the oblique septa.
They separate a portion of the ccelomic cavity on their upper
and outer sides, from the remainder of the perivisceral ccelomic
cavity, forming the pleural cavities. Into these cavities the
lungs project. The gizzard is connected with the floor of the
ccelomic cavity by a post-hepatic septum, so that altogether the
cavity of the ccelom is considerably obstructed and divided up.
The pericardium is, of course, separated off from the rest of
the ccelom by the transverse septum, but it is important to
notice that in birds there is no diaphragm (see Fig. 126).

Respiratory System. — The glottis leads into a long trachea
or windpipe, strengthened by cartilaginous and bony rings.
This trachea divides into the two bronchi, and at the point of
division a membrane extends forwards and projects into the

126 MORPHOLOGICAL TYPES

trachea from the angle between the bronchi, forming the

pIG 58.— Columba : ventral view of a dissection to show the air-sacs.
a, abdominal air-sac ; at, anterior thoracic air-sac ; c, cervical air-sacs ;
co, coracoid ; die, diverticulum of the interclavicular air-sac ; /, iurcula ;
^/humerus (which contains a diverticulum from ic) ; i, intestine ; ic, inter-
clavicular air-sac ; pt, posterior thoracic air-sac ; r, rib ; s, sternum ; tr,
trachea.

syrinx, by the vibrations of which birds sing. The bronchi

COLUMBA 127

lead to the lungs which are closely pressed up against the ribs.
The cavity of the lungs is repeatedly subdivided, giving them
the appearance of being filled with a very vascular and spongy
material. They are no longer simple hollow sacs with large
undivided cavities as in the lower forms. The lungs of the
bird are peculiar in that they give off a number of pouches or
air-sacs, which extend into many parts of the body. There
are nine of these air-sacs, arranged in the following manner :
a pair of cervical sacs at the base of the neck on each side ; an
interclavicular sac in the region of the furcula ; two pairs of
thoracic sacs, and a pair of abdominal sacs. The bronchi lead
through the lungs, into which they give off a few air-passages,
to the air-sacs. The walls of the air-sacs are not vascular and
no respiratory exchange takes place in them ; they act as
reservoirs, and when the body cavity is compressed by raising
the sternum, the air in them is forced into the lungs and out
again. The efficiency of the respiratory system of the bird is
due to the fact that there are no blind ends, and the air in the
spaces of the lungs is completely refreshed at each expiration.
It is worth noticing that the temperature of the body of birds
is remarkably high (about 420 C), and this is connected with
the efficiency of the respiratory exchange. Expiration is the
active process, by dropping the sternum the air-sacs expand
and fill again. It may be remembered that several of the bones
are hollow, and diverticula of the air-sacs extend into them,
as for example those of the interclavicular sac into the humerus.
Vascular System. — The heart of the bird is not unlike that
of the lizard, but the ventricle is completely divided into two
by an interventricular septum. The pulmonary arch is
present in the bird just as it is in the lizard, and it leads from the
right ventricle to the lungs. The right aortic (or systemic)
arch is also present, arising from the left ventricle. The left
arch which in the reptile arises from the right side of the
ventricle and receives mostly venous blood, has disappeared
completely in the bird. All the venous blood returns to the
heart from the superior and inferior venae cava? direct into the
right auricle, there being no sinus venosus. All this blood
passes into the right ventricle and to the lungs, from which the

128 MORPHOLOGICAL TYPES

arterial blood returns to the left auricle. The systemic "arch
therefore receives nothing but pure arterial blood from the
left ventricle, into which it has passed from the left auricle.
There are therefore two completely separate circulations in the
heart, and this is rendered possible by the fact that the heart
is four-chambered, both auricle and ventricle being completely
divided longitudinally.

The right systemic arch gives off a pair of arteries which run
forwards for a short distance, known as the innominate arteries.
Each divides into two, forming the carotid arteries and the
subclavians. The carotids run forwards to the head ; the
subclavians supply the pectoral muscles and the wings. The
systemic arch runs up on the right side of the gut and reaches
a position dorsal to it where it is known as the dorsal aorta.
It gives off coeliac and mesenteric arteries to the alimentary
canal, and sciatic arteries to the legs, and then divides to form
the iliac arteries and the caudal artery which supply the hinder
regions of the body.

The superior venae cavae receive the jugular veins, and these
are peculiar and interesting in that the right and left veins are
connected by a cross-channel at the top of the neck. In the
twisting of the long and flexible neck, it may happen that the
vein on one side is squeezed, and the flow of blood in it
interrupted. This blood can, however, return to the heart by
passing across the connexion just described, and down the
jugular vein of the other side. The superior venae cavae also
each receive a subclavian vein made up of a brachial vein from
the wing and a pectoral vein from the pectoral muscles.

The inferior vena cava receives the hepatic veins, and is
formed by the junction of a pair of iliac veins. These receive
the femoral veins from the legs and the renal veins from the
kidneys. The blood in the hinder regions of the body is led
forwards in a caudal vein, which soon divides into three. Two
of these vessels represent the renal portal veins of the lower
vertebrates, but in the adult bird these veins connect direct
with the iliac veins and vena cava inferior, without breaking
down into capillaries in the kidneys at all. There is therefore
no renal portal circulation. The third vessel into which blood

cmv.

Fig. 59. — Columba : ventral view of a dissection of the vascular system ;
after a drawing by Mr. B. W. Tucker.

a, aorta ; ba, brachial artery ; bv, brachial vein ; c, carotid artery ; cv,
caudal vein ; cmv, cceliaco-mesenteric vein ; da, dorsal aorta ; ev, epigastric
vein ; fa, femoral artery ; fv, femoral vein ; h, heart ; hv, hepatic vein ;
ia, iliac artery ; /, jugular vein ; k, kidney ; /, liver ; li, left innominate
artery ; Ipa, left pulmonary artery ; Ives, left vena cava superior ; pa,
pectoral artery ; pt, posterior mesenteric artery ; pv, pectoral vein ; rpa,
right pulmonary artery ; rv, renal vein ; sa, sciatic artery ; t, testis ; vet,
vena cava inferior.

K

130 MORPHOLOGICAL TYPES

may flow from the caudal vein is the coccygeo-mesenteric vein,
which runs downwards and forwards in the mesentery support-
ing the intestine, and joins the (hepatic) portal vein. The
latter runs from the intestine and duodenum to the liver, as in
all vertebrates. There is one more vessel worthy of mention,
and that is the epigastric vein which runs forwards from the
mesentery, passes ventral to the liver, and joins the hepatic
vein. This epigastric vein represents the anterior region of
the anterior abdominal vein of lower forms : the hinder part
of this vein is represented in the bird by the coccygeo-mesen-
teric vein.

The chief difference, therefore, between the venous systems
of the bird and the lizard, is the direct connexion of the " renal
portal veins " with the inferior vena cava in the former. It
may also be noted how the great development of the pectoral
muscles has brought about a modification of the vascular
system, in the form of the well- developed pectoral arteries and
veins. These muscles, on which the flight of the bird depends,
are the most active in the body.

Urino-genital Systems. — The kidneys are metanephric, not
mesonephric. The kidneys lie in the roof of the coelomic
cavity, and each is divided into three lobes. Each kidney is
connected by a ureter with the urodseal division of the cloaca.
There is no bladder.

In the male, the testes are connected with the cloaca
(urodaeum) by the vasa deferentia, or Wolffian ducts. In the
female, the right ovary and right oviduct as a rule disappear ;
there is then only one ovary (the left) and one oviduct (also the
left) or Mullerian duct in the adult bird. The reason for the
suppression of one ovary and duct is presumably that if two
eggs were to be laid simultaneously (one by each oviduct),
their combined size would block the passage between the two
sides of the pelvic girdle. The Wolffian ducts are not present
in the female, and the male has no Mullerian ducts.

Nervous System. — In the brain, the cerebral hemispheres
are well developed, and considerably larger than in the lower
forms. This increase in size is due to the enlargement of the
corpus striatum, and not to the development of the cortex or

COLUMBA 131

superficial layer of grey matter forming the roof of the hemi-
spheres. The cortex of the brain in birds is thin, and markedly
different in this respect from that of the mammals. The
cerebellum is also well developed as is the rule among animals
which rely on a sense of balance, and its surface is thrown into
ridges. The median portion of the cerebellum is known as
the vermis, on each side of which is a conical projection known
as the flocculus. The front of the cerebellum is in contact
with the hinder surface of the cerebral hemispheres, and the
optic lobes which form the roof of the midbrain are thrown to
the side.

As in the reptiles, there are twelve pairs of cranial nerves,
the spinal accessory and hypoglossal being included in the
skull. The wing is supplied by the brachial plexus, composed
of nerves from the hind part of the neck and the front of the
thorax. The leg is supplied by a femoral nerve, and a sciatic
plexus and nerve, which runs through the ilio-sciatic fissure in
the pelvic girdle. The obturator nerve which pierces the
obturator foramen innervates the region of the acetabulum.

Sense-organs. — With regard to the sense-organs, there is
not much advance over the conditions in the reptiles. The
eye is elongated from cornea to optic nerve, instead of being
spherical. Projecting into the posterior chamber of the eye,
which is occupied by the vitreous humour (see p. 23), is an
upstanding vascular structure. This structure, known as the
pecten, arises from the spot (" blind spot ") where the optic
nerve and artery enter the eye ; it recalls the columella Halleri
which is found with similar relations in the eye of Teleost
fish. The function of the pecten is still dubious, but its
vascularity suggests that it is concerned with supplying oxygen
to the vitreous humour and the posterior chamber generally.

Accommodation in the eye of the bird is peculiar. The
junction between the cornea and sclerotic is covered on the
inside by a muscle which is striated (Cramp ton's muscle).
Contraction of this muscle results in an increase of the con-
vexity of the outer surface of the eye : cornea and conjunctiva,
which accommodates the eye for near vision. The convexity
of the lens is also increased by contraction of the circular

132 MORPHOLOGICAL TYPES

muscle of the iris. At the same time, the contraction of the
ciliary muscle pulls the hinder part of the eye forwards, and
this reduces the tension on the suspensory ligaments, which
are attached to the lens.

In the ear, the cochlear part of the saccule is better developed
than in reptiles, and is beginning to show the spiral winding.

With the exception of the warm-bloodedness, and the
complete subdivision of the ventricle of the heart, the characters
which birds show, and which are not yet developed in the
reptiles, are specialisations which do not appear in the mammals.
Birds represent a further development of reptiles in one direc-
tion, while the mammals evolved in another direction from
another group of primitive reptiles.

Characters of Columba which show an advance on the con-
ditions in lower forms {and which are at the same time specialisa-
tions not found in mammals) :

Feathers ;

Modification of the pectoral limbs into wings ;

Loss of teeth ;

Formation of air-sacs (foreshadowed in the

Chamaeleon) ;
Formation of oblique and post-hepatic septa ;
Loss of right ovary and oviduct ;
Very long sacrum ; and fusion of vertebrae ;
Extreme posterior position of pubis ;
Loss of left systemic arch.

Literature

Parker, T. J. A Course of Instruction in Zootomy (Vertebrate).
Macmillan, London, 1884.

CHAPTER X

LEPUS : A WARM-BLOODED, VIVIPAROUS CHORD ATE

Externals. — The most obvious characteristic of the rabbit and
of other mammals is the possession of hair, which, typically,
forms a complete covering to the body. Hairs are more or
less cylindrical epidermal structures, seated in little pits or
follicles at the base of each of which is a papilla. The
epidermal cells just above the papilla multiply actively and
contribute new material to the hair, which in this way grows
in length (see p. 234). The central axis of the hair is called the
medulla, and surrounding this is the cortex (which is often
pigmented), and a cuticle. The function of hair is to prevent
loss of heat from the body by radiation, for mammals are warm-
blooded (homothermous). It also serves for protection, and
sometimes as a sensory tactile organ, as in the case of the
vibrissas or " whiskers."

The fingers and toes end in claws, likewise epidermal
structures.

The skin is thicker than in the forms previously described.
In the epidermis there is a great difference between the
actively-growing cells at the base (stratum Malpighi), and the
flat, horny cells on the surface (stratum corneum) which are
continually being lost and replaced from the stratum Malpighi.

The dermis of the skin forms the basis of leather, and it
commonly contains fat forming a layer which assists the
animal in maintaining its internal heat. Beneath the skin are
muscles which serve to move and shake it. In the region of
the trunk these muscles form the panniculus carnosus ; in the
head the skin muscles are concerned with movements of the

133

134 MORPHOLOGICAL TYPES

eyebrows, lips, and external ears (platysma muscles) (see
p. 278).

The skin in mammals is well supplied with glands of
epidermal origin, and of which there are three kinds : sudori-
parous, sebaceous, and mammary. The sudoriparous or
sweat-glands, are small tubes which sink into the dermis from
the surface, and end blindly after a certain number of coilings.
They serve to excrete water which is obtained from the neigh-
bouring blood-vessels, and in so doing they play an important
part in the regulation of the temperature of the body. The
water excreted is ordinarily converted into vapour, and thereby
absorbs the latent heat required for this conversion from the
body.

The sebaceous glands differ from the sweat-glands in that
they branch repeatedly, and that their secretion is not an
extracellular and liquid, but intracellular greasy substances
which are pushed out in the loaded cells themselves. These
glands are usually found opening into the hair-follicles, whence
the greasy secretion spreads over the hair. Other glands of
this type open to the surface along the edge of the eyelids
(Meibomian glands), and into depressions at the sides of the
anus (perineal glands). The secretion of the latter is
responsible for the smell of the rabbit.

Mammary glands or milk glands, are also characteristic of
the whole order Mammalia. They occur in both sexes, but
are normally functional only in the female. They are branched
tubes lying between the skin and the underlying muscles on
the ventral surface of the body, and opening to the surface by
nipples, of which there are in the rabbit about four pairs,
corresponding to the usual number of young born in a litter.

The eyes have upper and lower eyelids and a small nicti-
tating membrane. A noteworthy feature is the presence of
external ears, or pinnae, which assist the sense of hearing, by
concentrating the waves of sound.

The anus is at the root of the tail, and is separate from the
urinogenital aperture, which is situated in front of it, and takes
the form of a penis in the male or a vulva in the female. At
the sides of the penis in adult males are the scrotal sacs which

LEPUS 135

contain the testes. This ventral position of the testes is a new
feature, peculiar to most adult mammals.

Skull.— The skull has two occipital condyles, formed from
the exoccipitals. The floor is formed by basioccipital, basi-
sphenoid, presphenoid, and vomer, the latter representing the
anterior portion of the parasphenoid of lower forms. The
mesethmoid is perforated by a number of pores through which
branches of the olfactory nerve run to the nasal sacs ; it is
known as the cribriform plate. Anteriorly, the mesethmoid
extends as the vertical septum nasi, which separates the
cavities of the nasal capsules.

The roof of the skull is formed by the supraoccipital,
parietals, frontals, and nasals. The bones of the auditory
capsules are fused to form the periotics, which form the hinder
part of the side of the brain-case. The remainder of the side
is formed by the squamosals, alisphenoids (corresponding to
the epipterygoids of reptiles), and orbitosphenoids. There is
a small lachrymal bone near the front of the orbit.

The margin to the upper jaw is formed by the premaxillae
and the maxillae. From the maxillae the jugals extend back-
wards and meet a process (zygomatic) of the squamosal forming
(with the jugal) the " cheek bone." The roof of the mouth
is a false palate, formed by flat extensions of the maxillae and
palatine bones meeting their fellows of the opposite side
ventral to the true roof of the mouth, and enclosing the nasal
passage. The pterygoids are small bones behind the palatines,
and at the sides of the basisphenoid. The vomer representing
the parasphenoid is covered over by the false palate. Large
tympanic bullae lie beneath the periotic and contain the
tympanic cavity. The nasal cavities contain a number of
scroll- like turbinal bones (see Figs. 143, 149, and 150).

The wall of the skull is pierced by a number of holes or
foramina through which nerves and blood-vessels pass out
and enter. These foramina are commonly situated between
different bones, for the nerve or blood-vessel developed first
and the bones formed afterwards. At first these bones are
small, but as they grow they meet one another forming sutures,
and foramina are open sutures. Occasionally the bone grows

136

MORPHOLOGICAL TYPES

„sp.

>oc.

Fig. 60. — Skull of a dog, seen from the left side and slightly from
beneath.

The Roman figures close to the arrows indicate the cranial nerves which
emerge from the several foramina. II, optic nerve through the optic
foramen ; III, oculomotor nerve ; IV, trochlear nerve ; Vi, first or ophthal-
mic branch of the trigeminal nerve, and VI, abducens nerve, all emerging
through the foramen lacerum anterius ; V2, second or maxillary branch of
the trigeminal nerve, through the foramen rotundum ; V3, third or mandi-
bular branch of the trigeminal nerve, through the foramen ovale ; VII,
facial nerve, through the stylo-mastoid foramen ; IX, glossopharyngeal
nerve ; X, vagus nerve ; XI, spinal accessory nerve, all emerging through
the foramen lacerum posterius ; XII, hypoglossal nerve, through the
condylar foramen ; a, alisphenoid ; ac, alisphenoid canal (lodging the so-
called external carotid artery) ; bs, basisphenoid ; Et, opening into the
tympanic bulla for the Eustachian tube ; flm, foramen lacerum medius
through which the internal carotid artery enters the skull ; fp, false palate ;
j, jugal ; /, lachrymal ; mp, mastoid process ; tip, nasal passage ; oc, occipital
condyles ; os, orbitosphenoid ; p, parietal ; pi, palatine ; pm 4, fourth
premolar modified into the carnassial tooth ; pt, pterygoid ; s, squamosal ;
sp, styloid process ; tb, tympanic bulla.

LEPUS 137

all round the nerve or blood-vessel, and the foramen then
pierces that bone. The most important foramina are those in
the orbit and hinder region of the skull, and they are most
conveniently studied in the skull of a young dog, in which the
sutures between the bones are still plainly visible.

The optic nerve (II) emerges through the optic foramen in
the orbitosphenoid bone. Immediately behind this is the
foramen lacerum anterius between the orbito-sphenoid and
the alisphenoid. Through it pass the oculomotor (III),
trochlear (IV), abducens (VI), and the ophthalmic branch of
the trigeminal nerve (Vi). The maxillary branch of the tri-
geminal (V2) emerges through the foramen rotundum in the
alisphenoid, while the mandibular branch of the same nerve
(V3) passes through the foramen ovale between the alisphenoid
and the periotic. The Eustachian tube passes into the tympanic
bulla through an opening between the latter and the basi-
sphenoid. The foramen just median to this is the lacerum
medius (likewise between the tympanic bulla and the basi-
sphenoid), through which the internal carotid artery pierces
the skull. The so-called external carotid artery which is
given off from the internal carotid before the latter enters the
skull, runs forwards through the alisphenoid canal. This is
not a true foramen for it does not lead into the skull, but is a
short tunnel in the alisphenoid. Its hind entrance is below the
foramen ovale, and its anterior exit is confluent with the
foramen rotundum. The facial nerve (VII) emerges through
the stylo-mastoid foramen, between the hind face of the
tympanic bulla and the periotic. Between the periotic and
the exoccipital is a large elongated opening, the foramen
lacerum posterius. Through this, pass the glossopharyngeal
(IX), the vagus (X), and the spinal accessory (XI) nerves, and
the internal jugular vein. The hypoglossal nerve (XII)
passes through the condylar foramen in the exoccipital. The
large hole at the back of the skull for the spinal cord is the
foramen magnum.

The lower jaw consists of a single bone : the dentary,
which articulates with the skull by means of the squamosal.
This method of articulation is characteristic of mammals, and

138 MORPHOLOGICAL TYPES

differs from the articular-quadrate articulation of all other
Gnathostomes. Indeed, at first sight, the quadrate and
articular appear to be absent from the mammalian skull. On
the other hand, whereas other vertebrates have one single bone
connecting the tympanic membrane with the fenestra ovalis
of the auditory capsule, in the mammal there are three such
bones. The one nearest to the fenestra ovalis is perforated
and called the stapes (stirrup) ; it is homologous with the
columella auris and hyomandibula of lower forms. The next
bone is the incus, which is in reality the quadrate as can be
shown by its embryological development ; and the last bone
is the malleus which is in reality the articular. During the
course of the evolution of the mammals, these bones have
therefore undergone a remarkable change of function.

The " hyoid " is a small plate of bone, connected with the
periotic by a number of small bones representing the hyoid
arch (styloid process). The ceratobranchials of the 1st
branchial arch are represented by the posterior horns of the
hyoid (thyrohyoid). Elements of the remaining branchial
arches are possibly represented in the cartilages of the larynx
and of the trachea.

Teeth. — Equally distinctive of mammals are the teeth,
which are of different shape in the various regions of the
mouth, a condition termed heterodont as distinct from the
homodont condition of lower forms in which all the teeth are
alike. A further distinction lies in the fact that the teeth are
replaced once only in the mammal (diphyodont condition), and
not repeatedly as in lower forms (polyphyodont). There are
four kinds of mammalian teeth : incisors, canines, premolars,
and molars. The incisors or cutting teeth are situated at the
front of the mouth, those of the upper jaw are borne on the
premaxillae. The next kind of tooth is the canine or tusk, but
it is not present in the rabbit, which is a herbivorous animal.
In the dog the canines are well developed ; that in the upper
jaw is the most anterior tooth in the maxilla, and the canine of
the lower jaw lies in front of that in the upper when the mouth
closes. In the rabbit there is a long gap or diastema between
the incisors and the premolars. Premolars and molars are

LEPUS 139

often much alike, but their distinction lies in the fact that the
premolars appear in two sets : " milk teeth " arising first and
being replaced by permanent teeth as in the case of incisors
and canines ; but there is only one generation of molars.
Premolars and molars or grinding teeth, are the hindmost teeth
to be carried on the maxillae in the upper jaw.

Bearing in mind the four different sorts of teeth, it is possible
to describe the dentition of a mammal very simply and quickly
by means of a " dental formula " : that of the rabbit is :

.2033

1-, c-, p^,m>.

1023

The dental formula of the dog, on the other hand, is :

.3142
v>, c-, p?, m-.

3 14 3

Most mammalian teeth grow to a certain size and then
cease, as a result of the closing of the entrance into the pulp-
cavity by the formation of " roots " or fangs. Some, however,
retain the open pulp-cavities which are continuously supplying
food material to the odontoblasts, as a result of which the tooth
can go on growing throughout life. Such teeth are called
" rootless," or " with persistent pulps," and examples are to
be found in the incisors of the rabbit. As a rule, teeth which
are subjected to perpetual wearing down owing to grinding or
gnawing, or which can grow out of the mouth for unlimited
distances such as the tusks of the elephant, are of this kind.
In the rabbit, the lower incisors are kept in check by the upper
ones, and vice versa ; but if one tooth through accident is lost
or destroyed, the opposing tooth in the other jaw is no longer
resisted in its growth. Under such circumstances it grows
continuously and eventually kills the rabbit by preventing it
from shutting its mouth. In a sense, it may be compared with
the unruly growth of a tumour.

Skeleton. — The skeleton of mammals has a peculiarity in
many of its bones which is not found in any other vertebrates.
Several of the cartilage-bones, and especially the vertebrae and
the bones of the limbs are composed of three pieces : a central

140

MORPHOLOGICAL TYPES

shaft or diaphysis and an epiphysis at each end. Between the
epiphyses and the diaphysis there are, in young mammals,
portions of cartilage, and the bone is able to increase in length
by adding on new bone to the diaphysis at each end. Eventu-
ally, however, the epiphyses become firmly fused on to the
diaphysis, and no further growth of the bone is then possible.

Fig. 61. — Lepus : thoracic vertebras and ribs.

A, seen from in front ; B, seen from the left side, c, centrum of the
vertebra ; ca, capitulum of the rib ; na, neural arch ; ns, neural spine ; r,
rib ; t, tuberculum of the rib ; tp, transverse process.

The epiphyses can still, however, be recognised as distinct
from the diaphysis.

Vertebral Column. — In all mammals the number of cervical
vertebrae is seven (three species only form an exception to this
rule). The first is the centrum-less atlas, and the second is
the axis bearing the centrum of the atlas in the form of the
odontoid peg. The cervical vertebrae have vertebrarterial
canals formed by the fusion of the tuberculum of the rib to the

LEPUS 141

transverse process of the vertebrae, and the capitulum of the
rib to the centrum. The thoracic vertebrae are usually a
dozen in number, and each is related to a pair of ribs with
which it articulates by tubercular and capitular facets. Behind
the thoracic region are the lumbar vertebrae, usually seven in
number, and characterised by their large transverse processes.
Next comes the sacral region which is attached to the ilium of
the pelvic girdle, and the caudal region with vertebrae which
become simpler in structure as they approach the tip.

Fore Limb and Girdle. — The pectoral girdle is formed by the
scapula, which bears a ridge, the acromion, and a small coracoid
process representing the coracoid of lower forms. There is
no separate coracoid. The clavicle is slender, and joins the
acromion to the sternum. The arm is made up of the usual
bones : humerus, radius, and ulna, three proximal carpals
(scaphoid or radiale, lunar or intermedium, cuneiform or
ulnare) ; one central carpal (centrale), and four distal carpals
(trapezium, trapezoid, magnum, and unciform) make up the
wrist. There are five metacarpals, and the phalanges are
2, 3, 3, 3, 3 in number on the respective fingers.

Hind Limb and Girdle. — The pelvic girdle is formed of the
usual three bones : ilium, ischium, and pubis, on each side.
The ilium runs forwards and upwards from the acetabulum to
the sacrum, instead of backwards as in reptiles. The pubis
meets its fellow from the opposite side in the middle line,
forming the pubic symphysis ; and a large obturator foramen
separates the pubis from the ischium of its own side.

The femur has a large head, which fits into the acetabular
cavity of the pelvic girdle, and three processes or trochanters,
which serve for the attachment of muscles. The tibia is large,
but the fibula is small and fused on to the tibia. Covering the
front side of the joint between femur and tibia is a small bone,
the patella or knee-cap.

The proximal tarsal bones are two in number : the astra-
galus, and the calcaneum (heel-bone). There is one centrale
or navicular, and three distal tarsals. The latter are, the
mesocuneiform (2nd tarsal), ectocuneiform (3rd tarsal), and
the cuboid (fused 4th and 5th tarsals). The rabbit is specialised

1 42 MORPHOLOGICAL TYPES

in having lost the endocuneiform (ist tarsal), otherwise its
tarsus is easily comparable to that of Triton. The foot has
four toes, the ist or hallux having disappeared. There are
consequently four metatarsals, and the digital formula for the
number of phalanges is o, 3, 3, 3, 3.

It is common to find small irregular bones on the under or
palmar side of the joints of several of the fingers and toes, and
covering certain joints of the arm and leg. These are the
sesamoid bones. They arise in connexion with the insertion
of tendons on to bones. Examples are the patella, and the
pisiform which underlies the joint between the ulna and the
cuneiform bone of the wrist. Sesamoids are important
functionally, but they have not much significance in com-
parative anatomy, since they are not constant from group to
group.

Sternum and Ribs. — The breast-bone or sternum is sub-
divided into six sections, called sternebrae, and a posterior piece
called the xiphisternum. The most anterior of these (manu-
brium) is attached to the clavicles. The first seven ribs
articulate ventrally with the sternum. The dorsal part of each
of these ribs is bony, the ventral part cartilaginous. The next
two ribs are attached ventrally not to the sternum but to the
seventh rib, and the remainder end freely and are not attached
to any skeleton.

Ccelom. — An important characteristic of mammals is the
fact that the perivisceral coelomic cavity is completely divided
into two by a transverse partition, anteriorly convex, the
diaphragm. The lungs are in front of this diaphragm, enclosed
in the pleural cavities. The pericardium is also in front of the
diaphragm, but its cavity is separated from that of the pleural
cavities. The remaining viscera lie in the general peritoneal
cavity behind the diaphragm (see Fig. 127).

The diaphragm, which is of course pierced by the alimentary
canal, the aorta, and the inferior vena cava, divides the trunk
effectively into thoracic and abdominal regions. It is muscular,
and plays an important part in the process of respiration,
assisting the ribs in increasing the capacity of the thoracic box,
and causing air to rush into the lungs. It is developed in

LEPUS

143

part from the transverse septum. The scrotal sacs contain a
portion of coelomic cavity, lined with ccelomic epithelium or
peritoneum termed tunica vaginalis.

The alimentary canal is supported by a dorsal mesentery.
The latter, in the region of the stomach, is called the great
omentum, and is pulled ventrally and backwards so as to
enclose a sac (the omental bursa) which communicates with the
general peritoneal cavity on the right side of the stomach by
the foramen of Winslow. In the rabbit the great omentum is
small, but in other forms it is extensive and laps over the
ventral side of several coils of the intestine. Fat is often
found deposited in the great omentum, and especially in the

Pig-

The pleural cavities each surround a lung, which is
suspended in them by a mesentery. The coelomic epithelium
of a pleural cavity is called the pleura, and it is divided into
visceral (or splanchnic) and parietal (or somatic) parts. The
parietal pleura lines the outer wall of the pleural cavity, the
anterior face of the diaphragm, and in the middle line comes
into contact with its fellow from the opposite side to form the
mediastinal septum. The visceral pleura continues from the
parietal and covers over the lung. When the ribs are lifted,
the pleural cavities increase their volume, and since the space
between the visceral and parietal pleura is a closed one,
expansion of the parietal pleura is necessarily accompanied by
expansion of the visceral pleura and of the lung which it covers.
If the thoracic box were punctured and air could get into the
pleural cavities, the visceral pleura and the lungs would fail
to expand. The pericardium lies ventral and median to the
pleural cavities.

Alimentary System. — The original edge to the mouth is
represented by the gums, in which the teeth are set. Outside
these, fleshy lips are developed. The roof of the mouth is
formed by the false palate, due to the extension inwards of a
shelf from the maxilla and palatine bone on each side. In this
region it is called the hard palate, and it is continued posteriorly
a short distance by the soft palate, in which there is no bone.
The false palate encloses the nasal passage between itself and

i44

MORPHOLOGICAL TYPES

LEPUS 145

the true roof of the mouth, and this passage opens into the
mouth behind the soft palate, by the secondary choana. The
floor of the mouth is occupied by a large soft tongue. Four
pairs of salivary glands secrete into the mouth. They are, the
parotid (just in front of the ear), the submaxillary (behind the
angle of the mouth), the infra-orbital (below and behind the
cheek-bone, and the sublingual (on the inner side of the lower
jaw).

The Eustachian tubes open into the mouth near the
opening of the nasal passage, and behind them are the tonsils
which are remnants of the 2nd pair of visceral (1st branchial)
gill pouches.

The pharynx connects with the larynx by the glottis, and
this opening is protected by a flap, the epiglottis. In breathing,
the soft palate is dropped, thus allowing air to come in through
the nasal passage and into the mouth, and the epiglottis is
raised, allowing the air to enter the larynx on its way to the
lungs. When swallowing is taking place, the soft palate is
raised, closing the nasal passage, and the epiglottis is forced
down and bars the way into the larynx.

The oesophagus runs through the diaphragm to the cardiac
portion of the stomach. The other or pyloric portion of the
stomach opens into the duodenum, the opening being sur-
rounded by a sphincter muscle. The duodenum receives the
bile-duct and the pancreatic duct. The liver is large, and fits
close up against the posterior face of the diaphragm. It is

Fig. 62. — Lepus : dissection of the vascular system seen from the ventral

side.

a, aorta ; ab, adrenal body ; al, anterior laryngeal nerve (branch of
vagus) ; am, anterior mesenteric artery ; c, carotid artery ; ca, cceliac artery ;
eg, anterior mesenteric sympathetic ganglion ; en, cervical nerve ; d,
diaphragm ; da, ductus arteriosus ; dn, depressor nerve (branch of vagus) ;
i, innominate artery ; isg, posterior cervical sympathetic ganglion ; /,
jugular vein ; /, larynx ; o, oesophagus ; p, pulmonary trunk ; pa, pulmonary
artery ; ph, phrenic nerve ; pv, pulmonary vein ; ra, renal artery ; rd,
ramus descendens (branch of hypoglossal nerve) ; rl, recurrent laryngeal
nerve (branch of vagus) ; rv, renal vein ; s, stomach ; sa, subclavian artery ;
ssg, anterior cervical sympathetic ganglion ; sv, subclavian vein ; sy,
sympathetic nerve-chain ; t, thymus gland ; th, thyroid gland ; tr, trachea ;
v, vagus nerve ; vet, vena cava inferior ; vg, vagus ganglion ; XII, hypo-
glossal nerve.

L

146 MORPHOLOGICAL TYPES

connected with the stomach by the lesser omentum (mesen-
tery), and with the floor of the peritoneal cavity by a small
ventral mesentery, the falciform ligament. The gall-bladder
is green in colour. The pancreas lies in the mesentery which
stretches between the two arms of a loop formed by the
duodenum.

The small intestine is lined with countless finger-shaped
processes called villi which absorb the products of digestion.
Along the wall of the intestine are masses of lymphatic tissue
known as Peyer's patches, from which lymphocytes pass into
the cavity of the intestine. In the rabbit the small intestine
is over two yards long, and it ends in a chamber called the
sacculus rotundus, with which the ccecum and the large
intestine connect. The ccecum or blind gut ends blindly as
its name implies, and at its extremity is the vermiform appendix
which contains much lymphatic tissue. The ccecum is a
structure commonly found in herbivorous animals, for in it
cellulose is digested with the help of bacteria. It is usual to
find it reduced or absent in carnivorous forms. The large
intestine or colon connects with the caecum near the opening
of the sacculus rotundus, and leads to the rectum and anus.

Respiratory System.— Most of the structures concerned with
respiration have already been described in connexion with
the mouth and the pleural cavities. The larynx is protected
by a number of cartilages (thyroid, cricoid, and arytenoid) to
which muscles are attached. Internally, it contains the vocal
cords. The trachea which is kept open by cartilaginous rings,
leads from the larynx to the point where the two bronchi arise.
Each bronchus leads to a lung, and becomes subdivided into
larger and larger numbers of increasingly smaller air-spaces.
The mammalian lung is not a vascular hollow sac such as the
lung of the newt or the lizard ; its cavity is repeatedly sub-
divided so that it appears to be filled with spongy tissue in
which blood-capillaries circulate, surrounded on all sides by
the minute air-spaces. The surface of contact between air-
spaces and blood-vessels is very great ; in man, for example, it
is about thirty times the area of the body-surface.

Vascular System.- — The heart contains four chambers, two

LEPUS 147

auricles and two ventricles. The truncus arteriosus has been
split into two, right down to its base. One of these vessels
opens out of the right ventricle and leads to the lungs ; it is
the pulmonary artery. The other opens out of the left ventricle
and is the aorta which leads to the carotid arteries and the
systemic arch. The two superior venae cavae and the inferior
vena cava open directly into the right auricle ; there is no sinus
venosus. The pulmonary veins open into the left auricle.
Guarding the opening between the right auricle and right
ventricle is the tricuspid valve ; the corresponding opening
between the left auricle and left ventricle is guarded by the
mitral valve.* The openings of the aorta and pulmonary
artery are guarded by semi-lunar valves.

The systemic (4th arterial) arch persists only on the left
side. On the right, it is represented only by the short in-
nominate artery from which the right carotid and right sub-
clavian arteries arise. On the left side these two arteries arise
from the systemic arch, which, passing back and up round the
left side of the gut, becomes the dorsal aorta. The dorsal
aorta gives off the following arteries : coeliac (to stomach,
liver, duodenum, and spleen) ; anterior mesenteric (to small
intestine and colon) ; posterior mesenteric (to rectum) ; all of
which run ventrally in the mesentery to the several viscera.
Between the anterior and the posterior mesenteric arteries,
the dorsal aorta also gives off the renal arteries to the kidneys,
and the genital arteries to the gonads. In the case of males
in which the testes have descended into the scrotal sacs,
the latter arteries are of considerable length. Posteriorly the
dorsal aorta divides into the iliac arteries which supply the
hind legs, and the caudal artery. .The 5th arterial arch dis-
appears, but the 6th is represented by the pulmonary.
Originally, as in the fish, the 6th arterial arch communicated
with the lateral dorsal aorta, and this communication is present
in the mammalian embryo, on the left side, in the form of the
ductus arteriosus. In the adult the ductus arteriosus loses
its function (which is important in the embryo) and degenerates

* The number of flaps which these valves possess should be obvious :
the tricuspid has three, and the mitral two.

148

MORPHOLOGICAL TYPES

The ductus arteriosus is also called the

into a ligament
ductus Botalli.

The vena cava superior of each side is made up of the
jugular and subclavian veins, and opens into the right auricle.
In some forms the left superior vena cava is connected with the
right by a transverse innominate vein, and so loses its own
opening into the right auricle. The left vena cava superior
also receives at its base the thoracic duct which connects with

bvi

Fig. 63. — Diagram showing the relations of the arterial arches and the
branches of the vagus nerve in : A, Scyllium, and B, Lepus ; seen
from the left side.

aly anterior laryngeal nerve ; bv 1, 3, 4, and 6, blood-vessels (arterial
arches) running in the first, third, fourth, and sixth visceral arch ; cay
carotid arch ; d, ductus arteriosus ; da, dorsal aorta ; Et, Eustachian tube ;
g 1, first gill-slit ; h, heart ; m, mouth ; pa, pulmonary artery ; rl, recurrent
laryngeal nerve ; s, spiracle ; sa, systemic arch ; t, tonsil ; v, vagus nerve ;
vb 1,4, first, fourth branch of the vagus nerve.

the system of lymphatic vessels. The posterior cardinal veins
are represented by the azygos (right) and hemiazygos (left)
veins of the wall of the thorax. The hemiazygos connects
with the azygos, which opens into the right superior vena cava.
The connexion between the hemiazygos and the left superior
vena cava has been lost.

The walls of the heart itself are drained by veins, called
coronary veins, which open into the right auricle.

LEPUS 149

The veins from the hind legs (iliac and femoral veins) run
into the inferior vena cava, which also receives the genital vein
from the gonads, the renal veins from the kidneys and the
hepatic veins from the liver, and runs into the right auricle.
Blood from the stomach and intestine is carried to the liver
by the hepatic portal vein : there is no renal portal vein. The
blood of the mammals differs from that of all other animals
in that in the adult, the red blood-corpuscles have no nuclei.
Instead of being biconvex, the red corpuscles here are bicon-
cave. The source of supply of new blood-corpuscles in late
embryonic and in adult life is in the red marrow which is
situated in the central cavity of a number of bones. In
addition, lymphocytes are produced in the lymph- glands,
which also serve as blood-filters. It is possible that blood-
corpuscles may also be formed in the spleen.

Like birds, mammals are warm-blooded, or homothermous.

Urino-genital System. — The kidneys are asymmetrically
placed. They are metanephric structures, connected by the
ureters with the urinary bladder.

In the female the Mullerian ducts persist while the Wolffian
ducts disappear together with the mesonephros (traces of the
latter may persist as the epoophoron and paroophoron). The
ovaries are close to the anterior end of the Mullerian ducts or
oviducts, which open into the peritoneal cavity by the Fallopian
tubes. The base of each oviduct is enlarged and specialised
to form the uterus, in which the young embryos develop, for
mammals are viviparous. The two uteri are close together,
and they open into the single median vagina. The bladder is
just ventral to the vagina and connects with it to form the
vestibule which communicates with the exterior by the vulva.

The vestibule is dorsal to the pubic symphysis, and ventral
to the anus, with which it has no connexion. There is therefore
no cloaca.

In the male, the Mullerian ducts disappear except for the
uterus masculinus, which lies dorsal to the bladder. The
testes are connected with the epididymis, representing the
mesonephros of their own side. From the epididymis the vas
deferens or Wolffian duct leads to the base of the bladder on its

i5o

MORPHOLOGICAL TYPES

dorsal side, close to the prostate gland. The bladder and
vasa deferentia lead into a tube, the urethra, which runs
through and opens to the exterior at the end of the penis.

Fig. 64. — Lepus : dissection of the female urinogenital system seen from
the ventral side.

For explanation of lettering, see Fig. 65.

The testes arise near the roof of the peritoneal cavity,
suspended by mesenteries. When here, they are said to be
in the abdominal position, for later on they descend ventrally
and backwards into the scrotal sacs. The spermatic cords,
containing the artery from the dorsal aorta, show the path

LEPUS

J51

taken by the testes in their descent ; they passed median and
ventral to the ureters as is shown also by the course of the

ab._

Fig. 65. — Lepus : dissection of the male urinogenital system seen from the

ventral side.
a, anus ; ab, ad, adrenal body ; b, bladder ; da, dorsal aorta ; e, epi-
didymis ; g, gubernaculum ; ia, iliac artery ; k, kidney ; o, ovary ; oa,
ovarian artery ; od, oviduct ; p, penis ; r, rectum ; ra, renal artery ; sa,
spermatic cord ; ss, scrotal sac ; t, testis ; u, ureter ; ut, uterus ; v, vesti-
bule ; va, vagina ; vet, vena cava inferior ; vd, vas deferens.

vas deferens. The epididymis is connected with the scrotal
sac by an elastic cord, the gubernaculum, which in early stages

i52 MORPHOLOGICAL TYPES

grows down into the scrotal sac and guides the testis thither
in its descent.

It may be mentioned that the ovary in mammals is peculiar
in possessing Graafian follicles (see p. 227).

Ductless Glands (see Chapter XXXIII). — The spleen is
situated in the mesentery near the stomach. It is related to
the lymphatic glands, and its function is to act as a filter or
purifier of the blood. This it does by destroying worn-out
blood-corpuscles, and foreign bodies which may have got into
the blood.

The thyroid is two-lobed, and lies across the ventral side
of the larynx. It is associated with the parathyroids.

The thymus lies close in front of the heart, and is smaller
in older than in younger animals. The adrenals are small
compact bodies lying anterior to the kidney on each side.
Each consists of a cortex (corresponding to the inter-renal of
Scyllium) and a central medulla (supra- renal). The pituitary
lies in a depression in the floor of the skull, called the sella
turcica. The gland is composed of four parts : anterior,
intermedia, tuberalis, and nervosa. The pineal gland is on
the roof of the between-brain and between the cerebral hemi-
spheres. The pancreas has already been noticed on account
of its external secretion into the duodenum, but it also has a
very important internal secretion formed by the islets of
Langerhans. The gonads produce an internal secretion which
is responsible for the differentiation of the sexual characters
of their particular sex, but it is not yet clear which tissue is
responsible for this effect. In the pregnant female, the follicle
from which the egg was liberated becomes a corpus luteum,
the internal secretion of which plays an important part in the
development of the embryo in the uterus.

Nervous System. — The most important characteristics of
the mammalian nervous system are to be found in the brain.

The medulla oblongata or myelencephalon is not very
different from that of lower forms, but in the metencephalon
the cerebellum is much enlarged and divisible into a number of
lobes. Its surface is thrown into a number of folds, which
increases the quantity of superficial grey matter or cortex

LEPUS

i53

which it contains. There is also a band of nerve-fibres which
join the two sides of the cerebellum to one another passing
ventral to the rest of the hindbrain ; this is the pons Varolii,

Fig. 66. — Lepus : the brain, seen, A, from the inner side of a longitudinal
vertical section ; B, from the ventral side.
ac, anterior commissure ; c, cerebellum ; cc, corpus callosum ; ch,
cerebral hemisphere ; cq, corpora quadrigemina ; cr, crura cerebri ; fM,
foramen of Monro (shown by an arrow) ; h, hippocampal commissure ; he,
habenular commissure ; It, lamina termihalis ; Iv, lateral ventricle (cavity
of cerebral hemisphere) ; mo, medulla oblongata ; oc, optic chiasma ; ol,
olfactory lobe ; p, pineal body ; pb, pituitary body ; pc, posterior com-
missure ; pi, pyriform lobe ; pV, pons Varolii ; sc, soft commissure. The
roman figures indicate the roots of I, olfactory ; II, optic ; III, oculomotor ;
IV, trochlear ; V, trigeminal ; VI, abducens ; VII, facial ; VIII, auditory ;
IX, glossopharyngeal ; X, vagus, and XII, hypoglossal nerves.

peculiar to mammals. The cavity of the 4th ventricle does not
extend into the cerebellum, which is solid. The roof of the
midbrain bears, not two, but four prominences. That is to

154 MORPHOLOGICAL TYPES

say, that in place of the two optic lobes of lower vertebrates,
there are now four corpora quadrigemina.

The sides of the between-brain are thickened to form the
optic thalami, so much so indeed that the two sides touch one
another across the constricted 3rd ventricle, forming the
" soft commissure." The roof of the between-brain bears the
pineal stalk, the floor is depressed to form the infundibulum
to which the pituitary body is attached. Posterior to the
pituitary, the corpora mammillaria form two prominences
depending from the floor. The main bundles of fibres which
pass up and down from the brain and spinal cord run in the
ventral portion of the hindbrain, dorsal to the pons Varolii,
and diverge right and left in the region of the infundibulum
forming the crura cerebri.

The cerebral hemispheres, or roofs of the lateral ventricles
forming the end-brain, are enormous and extend backwards
covering over the between-brain and midbrain. The super-
ficial layer of nerve-cells or grey matter forming the cerebral
cortex, which was slightly developed in reptiles, is in the
mammals thick and well formed. The surface is thrown into
a few folds, forming sulci and gyri ; but these are not so
numerous in the brain of the rabbit as in higher mammals.
The body of the hemispheres is marked out into a number of
lobes by fissures (frontal, parietal, occipital, and temporal
lobes). The two hemispheres are separated by a deep cleft
or median fissure, but the cortex of each side is connected with
that of the opposite side by a broad band of transverse fibres
forming the corpus callosum, likewise peculiar to mammals.
The cavities of the hemispheres are the lateral ventricles, which
communicate with the 3rd ventricle by the foramina of Monro.

Beneath the temporal lobes are the pyriform lobes which
correspond to part of the roof of the end-brain of reptiles, and
which communicate with the olfactory lobes in front. The
floor of the end-brain is marked by the optic chiasma and the
corpus striatum.

The various regions and centres of the brain in mammals are
extensively connected with one another by tracts of fibres.
Most of these connexions can only be made out by detailed

LEPUS 155

study, but the transverse tracts or commissures are easily seen
in a longitudinal section of the brain. Of these, the corpus
callosum (connecting cerebral cortex) and the pons Varolii
(connecting cerebellar cortex) have already been mentioned.
In addition there are the following : the hippocampal com-
missure, which connects the two hippocampal lobes, running
ventral and posterior to the corpus callosum and dorsal to the
3rd ventricle ; the anterior commissure, connecting the two
halves of the corpus striatum, and running in the anterior wall
of the 3rd ventricle or lamina terminalis ; the habenular
commissure, connecting the optic thalami, running across the
roof of the 3rd ventricle just beneath the pineal body ; the
posterior commissure, in the roof of the midbrain ; the inferior
commissure, crossing the floor of the 3rd ventricle close to the
optic chiasma. The " soft commissure " is not really a com-
missure, since it does not transmit a transverse tract of fibres.

Meningeal Membranes. — The brain is surrounded by the
vascular pia mater, which projects into the lateral ventricles,
the 3rd and the 4th ventricles, forming in each a choroid plexus.
Outside the pia mater is the arachnoid membrane, and outside
this again is the protective and hard dura mater. The cerebro-
spinal fluid which fills the canal of the spinal cord and the
ventricles of the brain communicates with the space contained
by these meningeal membranes through an opening in the roof
of the 4th ventricle, the foramen of Magendie.

Nerves. — The distribution of the peripheral nerves in the
head is not dissimilar from that in lower forms, but attention
may be paid to the conditions in the region of the neck. On
each side of the neck, just lateral to the trachea, there are a
number of nerves running parallel with the carotid artery and
jugular vein. The vagus is one of these : it emerges from the
skull (through the foramen lacerum posterius) and swells into a
ganglion from which a nerve runs backwards. It soon gives off
an anterior laryngeal nerve which runs to the larynx, and a small
depressor nerve which accompanies the vagus in its course
backwards to the heart. The vagus passes ventral to the aortic
arch on the left, and ventral to the innominate artery (which
corresponds to the aortic arch) on the right. Immediately after

156 MORPHOLOGICAL TYPES

passing the artery, the vagus gives off a posterior or recurrent
laryngeal nerve which loops round the artery, passes dorsal to
it, and runs forwards again along the side of the trachea.
On the left side the loop of the recurrent laryngeal passes
behind the ductus arteriosus. This peculiar course of the
recurrent laryngeal nerve is easily understood on referring to
the nervous system of Scyllium. The anterior laryngeal nerve
corresponds to part of the first branch of the vagus which runs
in the 4th visceral (2nd branchial) arch. The posterior or
recurrent laryngeal nerve corresponds to part of the 4th branch
of the vagus which runs in the 7th visceral (5th branchial) arch.
Now the aortic arch, and its representative on the right the
innominate artery, are the blood-vessels of the 4th visceral
(2nd branchial) arch ; and the ductus arteriosus is the vessel
of the 6th visceral (4th branchial) arch. In development
these arches are displaced backwards to a considerable extent.
But this backward movement of these arteries necessarily pulls
back the nerves of the next posterior visceral arch, and this is
why the recurrent laryngeal nerves have to loop round the
arteries before they can reach their destination. The main
branch of the vagus continues backwards to the heart, stomach,
and intestine and corresponds to the visceral branch of the
vagus of Scyllium. It transmits fibres which belong to the
parasympathetic (autonomic) nervous system.

Parallel with the vagus in the neck is the trunk of the
sympathetic nervous system. It swells into the anterior
cervical ganglion, on a level with the ganglion of the vagus,
and continues forwards into the head accompanying the internal
carotid artery. Farther back, the sympathetic trunks have a
posterior cervical ganglion, and run backwards accompanying
the aorta, swelling into ganglia in most of the segments of the
thorax and abdomen. From some of these ganglia, fibres
run to the anterior mesenteric ganglion on the root of the
anterior mesenteric artery, and to the posterior mesenteric
ganglion, which is situated near the root of the posterior
mesenteric artery. From these ganglia, fibres are distributed
to the smooth muscles of the gut, bladder, and other viscera.
It may be repeated that the feature which distinguishes the

LEPUS

157

autonomic (sympathetic and parasympathetic) nerves from the
remainder, is the fact that in the autonomic system the muscles
and glands are not connected directly with the brain or spinal
cord by a single nerve-cell, but by two, one reaching from the
brain or cord to the sympathetic (or parasympathetic) ganglion,
and the other continuing from this ganglion to the muscle or
gland in question. Such muscles are always smooth and

de.~-

SCL.— i

c:

EC

Fig. 67. — Diagram showing the structure of the ear in mammals.

ac, auditory capsule ; c, cochlea ; de, ductus endolymphaticus ; Et,
Eustachian tube ; jo, fenestra ovalis ; fr, fenestra, rotunda ; i, incus (quad-
rate) ; m, malleus (articular) ; oe, external auditory meatus (outer ear) ;
s, stapes (columella auris, hyomandibula) ; sa, saccule ; sc, semicircular
canal ; tc, tympanic cavity (middle ear) ; tm, tympanic membrane (ear-
drum) ; «, utricle.

involuntary. Striped (voluntary) muscles are innervated
direct from the brain or cord by nerve-cells which go all the
way without interruption.

As in lower forms, the ganglia of the sympathetic trunk are
connected with the spinal ganglia by rami communicantes.

The diaphragm contains muscles of somatic origin which
are innervated by the phrenic nerves. These nerves are

158 MORPHOLOGICAL TYPES

formed from the 4th and 5th cervical spinal nerves, and run
back to the diaphragm on each side of the heart. The length
of their course shows the amount which the diaphragm,
together with the heart and aortic arches, have moved back-
wards during development ; a movement which has already
been noticed in connexion with the recurrent laryngeal nerves.

Sense-organs. — The sense-organs of the mammal show
certain peculiarities. The sensory surface of the olfactory
organs is increased by the formation of folds supported by the
turbinal bones. Jacobson's organ opens into the mouth in
some forms, but it disappears in others. The ear is remarkable
for the external pinna, and the inclusion of the articular and
quadrate as the malleus and incus, in the chain of bones which
together with the stapes (columella auris) connect the tympanic
membrane with the fenestra ovalis. The projection from the
saccule which forms the ductus cochlearis in lower forms, and
is responsible for hearing as apart from appreciating balance
(the function of the rest of the ear), is in the mammals very
highly developed. It is much elongated, and is coiled in a
spiral which enables it to be accommodated in the compara-
tively small cochlear part of the auditory capsule.

The eyelids are movable and muscular, and well supplied
with glands ; lachrymal and Harderian glands are present,
and a naso-lachrymal duct.

Characteristics of Lepus, typical of Mammals :

Hair ;

Bones with diaphysis and epiphyses ;

Two condyles to the skull ;

Loss of coracoid ;

Tympanic bulla ;

Lower jaw composed of dentary only ;

Teeth heterodont and diphyodont ;

Articulation of dentary with squamosal ;

Conversion of articular and quadrate into malleus and

incus ;
Diaphragm ;
Single left aortic arch ;

LEPUS 159

Non-nucleated red blood-corpuscles ;

Great expansion of cortex in cerebral hemispheres ;

Corpus callosum ;

Pons varolii ;

Turbinals ;

Cochlea spirally wound ;

Descent of testes into scrotal sac ;

Mammary glands ;

Uterus and placenta (allantoic) ;

Graafian follicles.

Literature

Bradley, O. C. A Guide to the Dissection of the Dog. Longmans,
Green, London, 1912.

Howell, A. B. Anatomy of the Woodrat. Bailliere, Tindall and Cox,
1926.

Marshall, A. M., and Hurst, C. A Junior Course in Practical Zoology.
John Murray, London, 1920.

PART II
EMBRYOLOGICAL TYPES

CHAPTER XI

AMPHIOXUS

Fertilisation. — The egg is surrounded by a vitelline membrane
secreted by itself, and contains yolk mostly aggregated at one
(the vegetative) pole. It is freed from the ovary and makes
its way to the outside via the atrium and atriopore, at a stage
shortly after the extrusion of the first polar body. In the water
a sperm penetrates into the egg, which then proceeds to give
off the second polar body ; the egg and sperm pronuclei then
fuse and fertilisation is effected. The second polar body
marks the animal pole of the egg, and it persists throughout
cleavage until the beginning of gastrulation, when it is possible
to see that the future anterior end of the embryo arises at a
point near the animal pole. Actually the axis of the egg (from
animal to vegetative pole) makes an angle of 300 with the
antero-posterior axis of the embryo. The egg-axis is deter-
mined in the ovary by the position of attachment of the egg to
the germinal epithelium. The dorso-ventral median plane of
symmetry of the embryo is marked by the point of entrance
of the sperm.

Cleavage. — The cleavage of the egg is total or holoblastic,
i.e. the amount of yolk present is insufficient to prevent cell-
division, but the cells of the vegetative pole are larger than
those at the animal pole. Up to the 8-cell stage, the cell
divisions keep pace with one another, but after that they

161 m

1 6:

EMBRYOLOGICAL TYPES

become irregular. As a result of cleavage a ball of cells or
morula is formed, and as the number of cells increases the
ball becomes hollow. The central cavity is the blastocoel,
surrounded by a single layer of cells which are smaller in the
future anterior region of the embryo, and larger posteriorly.
The embryo at this stage is a blastula.

Gastrulation. — The posterior side of the blastula, where

Fig. 68. — Amphioxus : early stages of development.

A, early blastula, showing the blastocoel (b) ; B, late blastula ; C,
beginning of gastrulation, the ectoderm (ec) can now be distinguished from
the endoderm {en) ; D, gastrula with primitive gut-cavity or enteron (e) ;
E, late gastrula, showing the blastopore (bl) or mouth of the enteron ; F,
stage in which growth in length has occurred as a result of the activity of
the cells round the rim of the blastopore.

the cells are relatively larger, becomes flattened, and at one
point (on the future dorsal side) actually tucked in beneath
the more anterior smaller cells. In this way a lip is formed
which soon extends right round the flattened region, which
sinks in towards the centre of the blastula. This process of
tucking-in is known as invagination, and the lip beneath
which this takes place is the rim of the blastopore. At the
same time as the flattened region is becoming invaginated, the

DEVELOPMENT OF AMPHIOXUS 163

rim of the blastopore is growing over towards the future
posterior pole of the embryo, a process known as epiboly.
Between them, the processes of invagination and epibolv
result in the conversion of the hollow single-layered ball (the
blastulaj into a double-layered hemispherical bowl. The
original cavity of the ball (the blastoccel) has been obliterated,
and the cavity of the bowl is the archenteron or primitive gut,
communicating with the exterior through the blastopore.
The embryo at this stage is known as a gastrula ; its outer
layer is the ectoderm which will give rise to the epidermis,
sense-organs, and nervous system ; its inner layer is the
endoderm which is destined to give rise to the lining of the
alimentary canal and its derivatives. The process of g-astrula-
tion therefore entails the separation of these germ-layers.

The overgrowth of the rim of the blastopore or epiboly
continues as a result of the activity and division of its cells,
and produces an elongation of the embryo along its antero-
posterior axis. New cells are contributed to the ectoderm
outside and to the endoderm inside, and the blastopore
diminishes in diameter. The cells of the ectoderm develop
cilia, but the embryo is still enclosed within the vitelline
membrane.

Mesoderm, Nerve-tube, and Notochord. — The cells along
the middle line of the roof of the archenteron are destined to
form the notochord. On each side of them is a band of cells
which will give rise to the third germ-layer, or mesoderm.
The cells along the mid-dorsal line of the ectoderm form a
flat band which sinks in beneath the surface, and is grown over
by the ectoderm on each side, which rises up to form the
neural folds. This flat band is the neural plate ; it soon
becomes V-shaped in section, and the two arms of the V join
so as to give rise to a long tube running all the way along the
back just beneath the ectoderm : the nerve-tube. In front,
this tube is open at the neuropore, a place where the neural
folds have not met, and which is indicated by Kolliker's pit in
the adult. Behind, the neural folds rise up at the sides of
and behind the blastopore. When they meet, they roof over
the blastopore, which thus no longer communicates direct to

164

EMBRYOLOGICAL TYPES

the exterior, but finds itself opening into the hind part of the
nerve-tube. In this manner the neur-enteric canal is formed.

nc n-

r 1 ms

,-sp.

-ec.

Fig. 69. — Amphioxus : transverse sections through young embryos,
showing the origin of the notochord, nerve-cord, and mesoderm.
A, early stage showing the enterocoelic pouches (ep) still in communica-
tion with the gut-cavity (g) ; the roof of the gut is giving rise to the noto-
chord (n) ; the nerve-cord (nc) although overgrown by the ectoderm (ec)
has not yet formed a tube ; b, blastocoel ; en, endoderm ; m, mesoderm.

B, later stage showing the enterocoelic pouches nipped off from the gut.

C, stage showing the extension of the ccelom (c) between ectoderm and endo-
derm, the formation of mesodermal somites (ms) ; the notochord is separate
from the gut, and the nerve-cord is rolling up. D, late stage, the nerve-
cord is a tube, the ccelom is divided into myocoel (ml) dorsally and splanch-
nocoel (sp) ventrally, the inner wail of the latter cavity being the splanchno-
pleur (sr) and its outer wall the somatopleur (so). The inner wall of the
myocoel is modified into a muscle-plate or myotome (my), and ventral to the
latter is the sclerocccl (se).

The notochord rises up from the rest of the roof of the
archenteron and forms a solid rod of cells extending all the

DEVELOPMENT OF AMPHIOXUS 165

way down the body, just ventral to the nerve-tube. As the
embryo grows in length, new cells are added on to the noto-
chord rudiment from behind by the activity of the rim of the
blastopore.

In each of the bands of cells which will give rise to the
mesoderm, a longitudinal groove develops ; the groove
opening widely into the cavity of the archenteron. The
grooves deepen, and their front portions become separated
from the more posterior region by a transverse partition on
each side. These front portions become cut off from the
archenteron altogether, and so a pair of mesodermal pouches
are nipped off, each containing a portion of coelomic cavity
which has been in communication with the archenteron and is
therefore called an enteroccel. This pair of pouches gives
rise to the first pair of somites, and it must not be mistaken for
the pair of anterior head-cavities or anterior gut- diverticula,
which develops farther forward and at a later stage.

Behind the first pair of somites, the grooves become nipped
off from the cavity of the archenteron anteriorly, while they
continue to communicate with it posteriorly. This means
that the mesoderm becomes separated from the wall of the
archenteron progressively from in front backwards ; and it
also becomes divided up by transverse partitions into somites
from in front backwards. These posterior somites (from the
second inclusive) differ from the first pair only in that the
mesoderm from which they are formed becomes separated
from the wall of the archenteron before being broken up into
somites, whereas the first pair of somites is demarcated before
losing connexion with the wall of the archenteron.

The mesoderm is therefore segmented very early, and each
segmental block of mesoderm or somite is separated from the
ones in front and behind by a septum. The somites in the
anterior region are derived from tissue which was invaginated
to form the original archenteron, and consequently they are
said to be formed from " gastral " mesoderm. The more
posterior somites owe their substance to the production of new
cells by the rim of the blastopore as the embryo elongates, and
such mesoderm is called peristomial. The difference between

1 66 EMBRYOLOGICAL TYPES

these kinds is solely one of origin. After the separation of
the mesodermal somites and of the notochord, the lateral
edges of the endoderm grow over and meet to reform a roof
over the gut- cavity.

The somites increase in size, and grow down between the
gut and the ectoderm on each side. Eventually they meet
beneath the gut and the wall separating them breaks down, so
that the ccelomic cavity of each somite communicates with
that of the corresponding somite on the opposite side of the
body. The layer of ccelomic wall or epithelium which touches
the endodermal wall of the gut is called the splanchnic layer,
that touching the ectoderm of the surface of the body is the
somatic layer. That part of the ccelomic wall which abuts
against the nerve-tube and notochord on each side becomes
thickened and gives rise to muscle-fibres forming the myotome :
one myotome to each somite on each side. The more dorsal
portions of the ccelomic cavity on each side, separating the
myotome from the outer (or cutis) layer, are called the myocoels ;
whereas the more ventral portion, into which the splanchnic
layer suspends the gut from above, is the splanchnocoel. The
myocoels become separated from the splanchnocoel of their
somite by a horizontal partition. The myocoels retain their
segmental arrangement, and remain separated by the septa
from the myocoels of the somites in front and behind. The
septa separating the splanchnocoels, however, break down, so
that there is a continuous splanchnocoelic or perivisceral cavity
from one end of the animal to the other.

The myotomes soon begin to show the V-shape character-
istic of the adult, and the alternation in position between right
and left sides.

In connexion with the mesoderm, there remain to be
described a pair of pouches which become nipped off from the
extreme front end of the wall of the gut. These are the
anterior head-cavities, or anterior gut- diverticula. They arise
symmetrically, but the right one soon occupies all the anterior
region of the embryo in front of the ist pair of myotomes,
and becomes the head-cavity. The left anterior gut-diverti-
culum remains small, and eventually acquires an opening to

DEVELOPMENT OF AMPHIOXUS

107

the outside at the bottom of an ectodermal inpushing called
the preoral pit ; in the adult this opening is Hatschek's pit.

There is no mesenchyme in Amphioxus, and the connective
tissue which surrounds the nerve-tube and notochord is derived
from hollow ingrowths from the myocoels, forming the
scleroccels, the walls of which are the sclerotomes. The fin-ray
boxes are also nipped off from the myoccels.

Portions of the myoccel persist in the adult between the
myotomes and the connective tissue which surrounds them.
Lastly, a downgrowth from each of the myoccels in the anterior

. 9.

B 3

Fig. 70. — Amphioxus : young embryo and larva.

A, seen from above ; B, seen from the left side, a, anterior-gut diverti-
cula ; g, gut ; ?i, notochord ; nc, nerve-cord ; s, mesodermal somites.

region of the body gives rise to the gonocoels, the walls of which
(gonotomes) give rise to the gonads.

It is important to notice that the whole of the mesoderm in
Amphioxus is segmented, and that this segmentation is
retained everywhere except in the region of the splanchnoccel.

The Gut. — At the stage when there are two pairs of somites
nipped off, the embryo hatches and emerges from the vitelline
membrane as a larva. The gut is still a closed sac which
communicates only with the nerve-tube, through the neuren-
teric canal. The mouth forms on the left side by a perforation
between the ectoderm and the endoderm immediately under-
lying it. It is very asymmetrical and soon becomes a large
opening bordered with cilia. In a similar way, the anus

1 68 EMBRYOLOGICAL TYPES

forms as a perforation just beneath the neurenteric canal,
which becomes closed and obliterated. Behind and dorsal to
the anus the tail begins to grow back.

The cells lining the cavity of the gut become ciliated, and
the splanchnic layer of coelomic epithelium surrounding them
gives rise to smooth muscle-fibres. The liver grows out as a
diverticulum from the gut on the right side.

The origin of the structures of the pharynx is peculiar and
complicated by the extraordinary asymmetry which the larva
shows. A structure is formed by the downgrowth of a groove
from the front of the floor of the gut, and is converted into a
tube which eventually opens into the gut on the right side, and
to the exterior a little to the left of the midventral line. This
is the so-called club-shaped gland, which is regarded as the
first gill-slit of the right side. The first gill-slit of the left side
arises ventrally by a perforation between the gut and the
ectoderm, and it moves up the right side of the body, opposite
the mouth. Behind this slit, about a dozen more are formed
ventrally, and likewise move up the right side, although they
are destined to become the left gill-slits eventually. This
series is known as the primary gill-slits. These slits corre-
spond with the segmentation of the body at this stage ; but
this correspondence is lost later on.

The definitive gill-slits of the right side, or secondary
gill-slits, arise later than the primary, and above them on the
right side to the number of eight. The most anterior secondary
slit corresponds to the second primary slit, which is what would
be expected if the club-shaped gland is really the first right
gill-slit, corresponding to the first primary gill-slit.

In front of the club-shaped gland, there arises a thickening
of the wall of the gut consisting of a strip of ciliated and
glandular cells. This is the rudiment of the endostyle. It
becomes V-shaped with the apex pointing backwards, and this
apex grows backwards as a double strip along the wall of the
pharynx on the right side above the primary slits and below
the secondaries. It is as if the morphologically midventral
line of the larva in the region of the pharynx were displaced
up on to the right side. Soon the primary slits move round to

DEVELOPMENT OF AMPHIOXUS

169

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170 EMBRY0L0GICAL TYPES

the left side, the endostyle assumes a midventral position, and
the secondary slits on the right side correspond more or less
symmetrically with the primaries on the left. The first
primary (left) gill-slit, and the club-shaped gland disappear,
and the number of slits on each side is regulated to eight by
the disappearance of the posterior primaries. After this stage,
more and more gill-slits are formed symmetrically on both
sides, and the segmental correspondence is lost.

All the gill-slits except the anterior pair become sub-
divided into two by the downgrowth of the secondary or
tongue-bars. The perforation of the gill-slits naturally
obliterates the coelomic cavity at the place of perforation ; the
ccelomic cavity is therefore restricted to the primary bars
between the gill-slits, and to the dorsal coelomic canals above
and the subendostylar coelom below. The tongue-bars have
no coelomic cavity, being downgrowths across the openings
of the gill-slits. It is because of this difference in method of
formation between the primary bars and the tongue-bars, that
in the adult the former contain a portion of coelomic cavity
and the latter do not.

During the rearrangement of the gill-slits, the mouth
moves round to the anterior end. Its aperture decreases in
size as its margin grows-in all round to form the velum.

Folds of the skin give rise to the oral hood, in the roof of
which the preoral pit finds itself. The latter flattens out, and
its cells give rise to the wheel-organ, or ciliated organ of
Miiller.

The Atrium. — The atrium arises as a pair of ventral longi-
tudinal folds, the metapleurs. These folds pass on each side
of the region of the gill-slits, which come to be situated between
them. From each fold, a median shelf or epipleur extends
and meets its fellow from the opposite side, thus enclosing a
part of the outside world as the cavity of the atrium. The
cavity is completely closed in front ; behind it remains in
communication with the exterior by the atriopore. The
atrium is lined throughout by ectoderm.

The nephridia arise as little blind sacs eventually connect-
ing with the exterior, at the top of each gill-slit (before the

DEVELOPMENT OF AMPHIOXUS

171

formation of the tongue-bars, so that in the adult there is a
nephridium to every two gill-slits). Hatschek's nephridium

A

B

Fig. 72. — Amphioxus : transverse sections through larvae showing the
development of the atrium.
A, early stage in which the coelom (c) is still large, and the atrial cavity
(a) is small. B, later stage ; ep, epipleural folds ; fr, fin-ray box ; g, gut ;
m, myotome ; ?np, rnetapleural fold ; n, notochord ; nc, nerve-cord.

arises as a small tube near the preoral pit, but in the adult its
opening leads into the pharynx just behind the mouth.

Primitive features in the development of Amphioxus :
Cleavage total ;

Gastrulation with invagination ;
All mesoderm segmented ;
Enteroccelic pouches.

Literature

Kellicott, W. E. Chordate Development. Henry Holt, New York.

I9I3-
MacBride, E. \V. Text-Book of Embryology, Vol. I. Macmillan,

London, 1914.

CHAPTER XII

DEVELOPMENT OF RANA (THE FROG)

Fertilisation. — The egg contains a large quantity of yolk, which
is aggregated at the vegetative pole. This pole is light in
colour when seen from outside, whereas the opposite animal
pole, and indeed the whole animal hemisphere, is darkly
pigmented. The nucleus is near the animal pole, which is
determined in the ovary, probably by the relation of the
developing egg to the little arteries and veins.

The egg is surrounded by three membranes. The inner
vitelline membrane is secreted by the egg itself. Outside this
is a tough membrane formed by the follicle-cells which surround
the egg in the ovary. Outside this again is a coating layer of
jelly which is secreted by the glands of the wall of the oviduct,
as the egg passes down the latter on its way to the exterior.

At the time of spawning, the males climb on to the backs of
the females, and as the latter extrude the eggs from their
cloacal apertures, the former shed the sperm over them.
Fertilisation thus takes place in the water outside the bodies of
the animals. One polar body has been extruded before the
egg is laid, the second polar body is pushed out after penetra-
tion of the sperm, and the egg- and sperm-pronuclei then fuse.

The jelly swells out on contact with the water, and after
fertilisation the vitelline membrane becomes lifted off from the
surface of the egg. The egg is then able to rotate, and comes
to rest with the axis vertical, i.e. the vegetative pole with the
heavy yolk is turned downwards.

The point of entrance of the sperm determines the median
plane of symmetry of the future embryo, and, soon after fertili-
sation, this is indicated by the formation of the grey crescent

172

DEVELOPMENT OF RANA

173

(due to the retreat of pigment into the egg) at the point dia-
metrically opposite to that at which the sperm entered. The
egg can now be orientated with regard to the axes of the future
embryo. The animal pole will become the head, and the
vegetative pole the tail ; the grey crescent marks the future

A

B

C D

Fig. 73. — Egg of Rana temporaria (common frog) before and after fertilisa-
tion, showing the formation of the grey crescent. (From Jenkinson.)

A and B seen from the side ; C and D seen from below ; A and C
before and B and D after fertilisation. The animal hemisphere is pig-
mented, the vegetative hemisphere is light in colour.

dorsal side, and the opposite side (where the sperm entered)
will be ventral.

Cleavage. — Cleavage in the frog's egg is total, but the size
of the various blastomeres is very unequal, owing to the large
quantity of yolk. The cells at the vegetative pole are much
larger (and fewer in number) than those at the animal pole.
The blastoccel is small, and situated nearer to the animal than
to the vegetative pole. The bias tula is now a hollow ball,
but the hollow is small and its walls are several layers thick.

Gastrulation. — The cells of the animal hemisphere (which

174

EMBRYOLOGICAL TYPES

are darkly pigmented) are relatively free from yolk, and there-
fore divide faster than the larger light-coloured yolk-laden

E F

Fig. 74. — Formation and closure of the blastopore during gastrulation in
Rana, seen from below. (From Jenkinson.)

In A the dorsal lip of the blastopore has just appeared ; in B the lateral
lips have extended, and they almost meet in C ; in D the ventral lip of the
blastopore (which is now a complete circle) has been formed ; and the
diameter of the blastopore decreases in E and F.

cells of the vegetative hemisphere. One result of this is that
the animal-pole cells begin to grow down over the lighter-
coloured cells. This process starts by the formation of a lip

__ _ -ap.

ap.^_

Fig. 75. — The process of gastrulation in Rana as shown by sagittal sections.

In A the dorsal lip of the blastopore bias just appeared, and it is accen-
tuated in B ; in C a definite ingrowth is visible resulting in the formation of
the archenteron : in D the ventral lip of the blastopore has appeared, and
the yolk-containing cells of the vegetative hemisphere project through the
now circular blastopore as the yolk-plug ; in E the archenteron has extended
greatly at the expense of the blastocoel, which in F is almost obliterated.
During gastrulation the yolk-cells are heaped up on the ventral side of the
archenteron, as a result of which the egg rotates until its original axis is more
or less horizontal.

a, archenteron ; apt animal pole ; b, blastoccel ; br, brain ; dl, dorsal
lip of blastopore ; in, mesoderm ; ?i, notochord ; nee, neurenteric canal ;
np, neuropore ; sc, spinal cord : vl, ventral lip of blastopore ; yc, yolk-cells;
yp, yolk-plug.

176

EMBRYOLOGICAL TYPES

of overgrowth in the centre of the grey crescent, forming the
dorsal lip of the blastopore. Underneath this lip is a groove
formed by the cells tucking in. The lips of the blastopore
extend right and left from the site of its first appearance. At
the same time the edge of overgrowth moves down towards the
vegetative pole, and more and more of the lighter-coloured
cells become covered over by the overgrowing darker ones.

Fig. 76. — Transverse sections through the closed blastopore of Rana (A)
and the primitive streak of Gallus (B).

The groove between the fused lips of the blastopore of Rana is the rem-
nant of the blastopore, and corresponds to the primitive groove (ps) of Gallus.
ec, ectoderm ; e?i, endoderm ; m, mesoderm ; all of which are continuous
with one another at the rim of the blastopore or primitive streak.

Eventually the two horns of the lip of the blastopore meet on
the ventral side, and the blastopore is then a closed ring, formed
by overgrowing dark cells, and beneath which the tucking-in
takes place. This tucking-in is most active on the dorsal side.
The groove sinks deeper and deeper into the embryo, as the
ingrowing cells push farther and farther fonvards beneath
the superficial layer. The groove represents the cavity of the

DEVELOPMENT OF RANA 177

archenteron, largely rilled up by the yolk-cells of the vegetative
pole, which are visible inside the rim of the blastopore. The
cavity of the blastoccel becomes reduced and obliterated as
the cavity of the archenteron increases and gastrulation
proceeds ; and the yolk-laden cells of the vegetative pole
come to lie on the ventral side of the archenteron.

As the blastopore approaches the vegetative pole its diameter
decreases, until when it reaches it, it is a small spherical hole
with yolk-cells showing through as the so-called yolk-plug.

The processes of gastrulation therefore entail overgrowth
or epiboly, and invagination ; but the invagination cannot
take place simply as in Amphioxus owing to the large quantity
of yolk present, and it is more in the nature of an ingrowth.
At all events, the result of gastrulation is the conversion of the
single-layered hollow ball (blastula) into a double-layered sac
(gastrula) ; the outer layer (ectoderm) is formed of the cells
of the animal hemisphere and those which have grown over,
the inner layer (future endoderm and mesoderm) is formed of
the cells which have grown in, and of the yolk-laden cells of
the vegetative hemisphere. The latter form most of the ventral
and the former most of the dorsal wall of the archenteron.
The heaping up of the heavy yolk-cells at the ventral side
causes the gastrula to rotate within its membranes, so that the
former egg-axis lies more or less horizontal instead of vertical ;
the ventral side now points downwards and the dorsal side
upwards.

Mesoderm and Notochord. — The wall of the archenteron
contains the cells which are destined to give rise to the noto-
chord and to the mesoderm. A strip of cells running along the
middle line of the roof of the archenteron is the rudiment of
the notochord, and the mesoderm arises as a splitting off (or
delamination) of a layer of cells from the remainder of the wall
of the archenteron. This layer of mesoderm now separates
the ectoderm from the endoderm in most parts of the embryo.
Soon, a split arises in the mesoderm layer itself, dividing it into
an inner splanchnic layer and an outer parietal or somatic
layer. This split is of course the ccelomic cavity.

The notochord splits off as a solid rod from the surface of

N

178 EMBRYOLOGICAL TYPES

the roof of the archenteron, which still forms a complete

3,

Fig. 77. — Transverse section through an embryo of Rana showing the
separation of the mesoderm (m) from the endodermal wall (en) of the
gut (g).
ec, ectoderm ; y, yolk-cells. Dorsally the ectoderm is thickening to

form the neural folds (nf).

Fig. 78. — Transverse section through an embryo of Rana slightly older than
the previous, showing the origin of the notochord (n) from the middle
line of the roof of the gut. The neural folds (nf) have risen up and
enclose a groove between them.

covering to the archenteric cavity. This delamination of the

DEVELOPMENT OF RANA 179

mesoderm and notochord from the wall of the archenteron
begins in the anterior region of the embryo ; farther back they
are not distinct, and merge into the rim of the blastopore.
The rim of the blastopore may indeed be defined as the region
where the ectoderm, mesoderm, endoderm and notochord are
all in contact with a zone of actively growing cells, which
contributes new tissue to each layer.

The new mesoderm formed from the blastopore-rim is

Fig. 79. — Transverse section through an embryo of Rana slightly older than
the previous, showing the complete separation of the notochord («)
from the endodermal wall (en) of the gut (g).

The coelom (c) has arisen as a split in the mesoderm (m), which forms a
somite (ms) on each side of the notochord. The neural folds («/) have closed
over the groove converting it into the nerve-tube (nc)> on each side of which
are the neural crests (ncr). ec, ectoderm ; y, yolk cells.

peristomial, that split off from the wall of the archenteron
farther forward, is called gastral mesoderm. Apart from their
method of formation, there is no difference between these two
kinds of mesodermal tissue.

This activity of the lip or rim of the blastopore is a continua-
tion of the process of epiboly, and its result is to produce an
elongation of the embryo. Eventually the blastopore becomes
oval and slit-like by the apposition to one another of its lateral

i8o

EMBRYOLOGICAL TYPES

lips, and its former aperture is represented only by a short
groove on the outer surface between these lips. This fact is
of importance in connexion with the interpretation of develop-
ment in higher forms. The activity of the cells of the
blastopore-rim continues after the blastopore is closed, and
leads to the outgrowth of the tail.

Nerve-tube. — The ectoderm along the middle line of the
dorsal side thickens forming the neural plate. A pair of
longitudinal ridges arise on each side of it, known as the neural

Fig. 80. — Sagittal section through an embryo of Rana.

a, anus ; b, brain ; g, gut ; h, hypophysis ; ht, heart ; /, liver
notochord ; sc> nerve- (spinal) cord ; y, yolk-cells.

folds, and they enclose a groove between them. This groove
is wider in front, where the brain will be, than behind, in the
region of the future spinal cord. Posteriorly the neural folds
embrace the blastopore. As the folds rise up they arch over
the groove, which becomes converted into the nerve-tube.
The nerve-tube communicates with the cavity of the gut
posteriorly through the neurenteric canal and blastopore, which
is still open at this stage.

The lateral part of the thickening which gave rise to the

DEVELOPMENT OF RANA 181

neural plate does not get folded into the nerve-tube when the
neural folds meet. It lies just to the side of the point of fusion
of the neural folds, and forms the neural crest. The cells of
the neural crest are destined to give rise to the afferent sensory
nerve-cells, whose cell-bodies form the ganglia on the dorsal
roots of the nerves, and to the sheaths of the nerves.

Segmentation. — The mesoderm on each side of the nerve-
tube and notochord becomes thickened and divided into blocks,
which are the somites from which the myotomes develop ;
they are metamerically segmented. This segmentation begins
anteriorly and proceeds backwards ; but it does not affect
the more ventrally-situated mesoderm. Whereas the dorsal
portion of the ccelomic cavity (on a level with the myotomes)
is interrupted by transverse septa separating the mesodermal
somites from the somites in front and behind, and consists of
a number of myoccels equal to the number of somites, the
ventral portion of the ccelomic cavity is continuous and un-
interrupted by septa.

The segmented region of the mesoderm is called the verte-
bral plate, the unsegmented portion is the lateral plate.
Between each somite and the lateral plate immediately below
it is a small region of segmented mesoderm known as the inter-
mediate cell-mass, or nephrotome. From these structures
the tubules of the kidneys will arise, and they are therefore also
segmental. Eventually, the vertebral plate separates completely
from the lateral plate, and the myotomes grow down in the body-
wall lateral to the splanchnoccel to give rise to the muscles of
the ventral surface, of the limbs, and the hypoglossal muscula-
ture beneath the mouth.

Muscles formed from myotomes are always innervated by
ventral nerve-roots, and as the myotomes are segmental, the
ventral nerve-roots which grow freely out from the nerve-tube
are segmental also. Further, the neural crest becomes sub-
divided into pieces corresponding to the myotomes, these are
the rudiments of the dorsal-root ganglia. The cells in these
ganglia develop one process which grows into the nerve- tube,
and another which pushes out to its destination in the body.
These dorsal nerve-roots are therefore segmental also.

l82

EMBRYOLOGICAL TYPES

Median to the myotomes, cells are proliferated by the
somites to form clouds of mesenchyme surrounding the nerve-
tube and notochord. These cells are the sclerotomes (likewise
segmental) from which later on the vertebrae are developed.

Another instance of segmentation will be seen in connexion

Fig. 8 i . — Transverse section through a
young tadpole larva of Rana showing
the origin of the kidneys.

Fig. 82. — Transverse section through a
tadpole larva of Rana older than the
previous, showing the formation of
the kidneys and the lungs.

The section'passes through the transverse septum across which the duc-
tus Cuvieri lead from the cardinal veins to the heart, c, ccelom dorsal to
the transverse septum ; cv, cardinal vein ; dC, ductus Cuvieri ; g, gut ; gl,
glottis ; icm, intermediate cell-mass or nephrotome ; /, lung ; Ida, lateral
dorsal aorta ; Ir, liver ; m, mesoderm ; my, myotome ; n, notochord ; nc,
nerve-cord ; tier, neural crest ; nl, nephrocoel ; pt, pronephric tubules ;
sp, splanchnoccel ; y, yolk-cells.

with blood-vessels, which run transversely in the septa between
adjacent segments. Although in the adult animal much of
this segmentation is obscured and modified, it is important
to note that in development, metameric segmentation is as
well marked as in Amphioxus, except for the splanchnoccel.

DEVELOPMENT OF RANA 183

As in invertebrates, segmentation begins with the mesoderm
and extends to the other tissues.

The Gut. — The gut is a cavity with an accumulation of yolk-
cells in the hinder part of its floor. This posterior region will
become the intestine, and in front of it will develop the pharynx,
oesophagus, and stomach. After the blastopore has closed,
the anus breaks through near the same spot, as a result of the
sinking in of an ectodermal pit (the proctodeum) till it meets
the endoderm, and perforation ensuing. In a similar way, the
mouth perforates in front, at the bottom of an ectodermal
pit (the stomodaeum).

Behind the mouth, in the region of what will be the pharynx,
five pouches grow out on each side from the endoderm to the
ectoderm. These are the rudiments of the visceral clefts.
The first pair corresponds to the spiracles of the dogfish, but
here they do not become perforated to the exterior. Their
cavities persist as the Eustachian tubes. The remaining four
pairs of pouches become the gill-slits, through which the pharynx
communicates with the exterior.

Alternating with the visceral clefts are the visceral arches.
The 1 st or mandibular arch separates the mouth from the
Eustachian tube (or hyomandibular cleft) ; the 2nd (or hyoid
arch) is between the latter and the 1st gill-slit. The 6th
visceral arch is behind the 4th gill-slit.

From the upper part of the 3rd, 4th, and 5th visceral arches,
tufts grow out on each side which will become the external
gills ; blood-vessels enter them, and they serve as the first
respiratory organs. The dorsal part of the 1st gill-pouch on
each side proliferates to form a body which is the rudiment
of the thymus gland.

In the floor of the pharynx between the 2nd gill-slits, a
downgrowth is formed, which ultimately loses its connexion
with the pharynx and forms the thyroid gland. Close to the
point of origin of the thyroid gland is an elevation which will
eventually give rise to the tongue. A little farther back, also
in the middle line of the floor of the pharynx, the rudiment of
the larynx appears as a groove. This deepens into a tube
remaining in connexion with the pharynx through the glottis.

184

EMBRYOLOGICAL TYPES

From the posterior end of the larynx, the lungs develop as
sacs stretching back parallel to the oesophagus on each side.

-ti3.

VC.3.

Fig. 83. — Horizontal section through the head of a tadpole of Rana, showing
the formation of the visceral clefts (gill-slits).

b 1, b 3, b 4, b 5, b 6, blood-vessels running in the first, third, fourth, fifth,
and sixth visceral arches (the vessels in the third and fourth arches will
become the carotid and systemic arches respectively) ; eg, external gills ;
1, infundibulum (floor of the forebrain) ; ic, internal carotid artery ; n, nasal
sac ; oe, oesophagus ; pd, pronephric duct ; pt, pronephric funnel ; sp,
splanchnoccel ; vc, 1 to 5, first to fifth visceral cleft (the first will give rise
to the Eustachian tube) ; V, VII, IX, branches of the trigeminal, facial, and
glossopharyngeal nerve, running in the first, second, and third visceral arches
respectively.

The liver arises as a ventral outgrowth of the floor of the
gut, just in front of the mass of yolk-cells, and extending back

DEVELOPMENT OF RANA 185

beneath them. Part of the cavity of this diverticulum becomes
the gall-bladder, and the open connexion with the rest of the
gut persists as the bile-duct. Close to this point, the pancreas
arises as a number (three) of outgrowths, which remain con-
nected with the gut by the pancreatic duct.

The cavity of the intestine is still small owing to the
presence of the yolk-cells. After hatching, this yolk becomes
absorbed and the intestine elongates very much, becoming
coiled like a watchspring. Behind the intestine is the region
of the gut which will become the rectum and cloaca. A
downgrowth from the latter gives rise to the urinary bladder.

During this time, the right and left splanchnoccelic cavities
have applied their outer for somatic) layer to the body- wall,
and their inner (or splanchnic) layer to the endoderm of the
gut and all its derivatives. Ventrally, most of the membranes
forming the separation between the right and left splanchno-
ccelic cavities break down ; but dorsally these walls persist
forming the dorsal mesentery. This mesentery is composed
of two closely apposed layers of ccelomic epithelium spreading
round the gut and suspending it. It may be noticed, therefore,
that the gut is not strictly in the ccelomic cavity at all ; it merely
hangs in a fold of ccelomic epithelium which bulges into the
ccelomic cavity. From the cells of this splanchnic layer are
developed the smooth muscles of the stomach, intestine, and
bladder.

Blood-vessels. — Beneath the floor of the gut, and between
it and the underlying splanchnic layer of ccelomic epithelium,
there are some scattered mesoderm-cells which become arranged
in the form of a tube, or subintestinal vessel. In the region
of the pharynx, this tube forms the endothelial lining of the
heart. The ccelomic epithelium (splanchnic layer) surrounds
this tube and suspends it as it were in a little mesentery of its
own from the floor of the pharynx (the dorsal mesocardium).
The musculature of the wall of the heart is derived from this
layer of ccelomic epithelium, and that part of the splanchnocoel
in which the heart finds itself is now called the pericardium.
Later on, the various parts of the heart are differentiated.
Posteriorly, the heart is continuous with two tubes, the vitelline

i86

EMBRYOLOGICAL TYPES

veins, which run from the yolk-cells and the rudiment of the
liver.

The dorsal aorta arises as a pair of longitudinal vessels,

Fig. 84. — Transverse sections through embryos of Rana, showing the origin

of the heart.
Only the ventral portion of the body is shown. A, early stage ; the cells
which will give rise to the endothelial lining of the heart (e) are still scattered ;
they lie between the endodermal floor of the gut (ef) and the mesoderm (m) ;
the mesoderm contains right and left ccelomic cavities (p) still separated
by a septum ; ec, ectoderm. B, later stage, the endothelial cells are begin-
ning to arrange themselves, and the ccelomic epithelium underlying them
becomes thickened and depressed. C, the endothelial lining of the heart
is now a closed tube, and the ccelomic epithelium has folded round it forming
its muscular wall (mzv) ; it remains connected with the ordinary ccelomic
epithelium above by the dorsal mesocardium (dtti) ; beneath the heart, the
septum between the right and left ccelomic cavities has disappeared so that
the ccelom is continuous and is now known as the pericardium (p, pc).

close beneath the notochord. The two remain separate
anteriorly, as the lateral dorsal aortas and their prolongations

DF.VELOPMENT OF RANA 187

into the head, the internal carotids. Behind, they join and
fuse together along the whole of the rest of the body, forming
the single dorsal aorta.

Beneath the pharynx, the heart communicates forwards
with the ventral aorta. In each of the 3rd to 6th visceral
arches, between the gill-slits, a vessel appears which com-
municates below with the ventral aorta and above with the
lateral dorsal aorta of its own side. In this way the series of
pairs of aortic arches arise, alternating with the gill-slits. When
the capillaries of the gills arise, they connect with the aortic
arches which become interrupted. There are now afferent
branchial arteries carrying blood from the ventral aorta to the
gills, and efferent branchial arteries connecting the gills with
the lateral dorsal aorta. Rudiments of aortic arches appear
in the mandibular and hyoid arches.

The dorsal aorta sends arteries to the gut, which they reach
by passing down between the two layers which form the dorsal
mesentery.

The arteries become surrounded by coats of smooth muscle.
Of the veins, the posterior cardinals arise near and parallel
to the dorsal aorta. Their anterior prolongations are the
anterior cardinal veins which run one on each side of the brain,
and which, later on, contribute to the formation of the internal
jugulars. At this period, the pericardial cavity is open
posteriorly and communicates with the general perivisceral
splanchnoccel. In the region of the heart, the splanchnic
and somatic layers of the ccelomic epithelium approach one
another and fuse, forming the lateral mesocardia which connect
the gut- wall with the body- wall. This connexion of course
interrupts the coelomic cavity, and soon the pericardial cavity
is completely shut off from the perivisceral cavity behind it.
The partition formed by the lateral mesocardia is the transverse
septum, and it is important in that it enables the cardinal veins,
which are in the body-wall, to communicate via the ductus
Cuvieri (or superior venae cava?) with the heart, which is of
course situated in the gut- wall.

A third connexion between the heart and the veins of the
body-wall is established by the formation of the inferior vena

1 88 EMBRYOLOGICAL TYPES

cava, which runs down in the mesentery from the hinder
region of the body.

As the liver develops, the vitelline veins (which are really
that part of the subintestinal vessel which is behind the heart)
undergo some modification. The hinder portion connects
the intestine behind with the liver in front, forming the hepatic
portal vein. The anterior portion connects the liver with the
heart, and gives rise to the hepatic veins. The formation of
the renal portal veins will be described in connexion with the
kidneys.

The blood itself, or rather its red corpuscles, arise from
structures known as blood-islands. These are formed from
the layer of mesoderm which was split off from the floor of the
original archenteric wall just beneath the mass of yolk-cells,
behind the rudiment of the liver ; they are regions of rapid cell-
proliferation. From here, the corpuscles enter the blood-
vessels (which were previously empty) through the vitelline
veins.

The Kidneys. — The kidneys are formed from the meso-
dermal tissue situated at the junction between the myoccels and
the splanchnoccel and which is known as the nephrotome or
intermediate cell-mass. On each side a thickening appears
in the region of the 2nd to 4th segments of the trunk of the
embryo. This thickening extends back on each side as a rod
of cells, between the outer layer of the splanchnocoel and the
skin, to the cloaca. The thickening hollows out and a cavity
appears which connects with the splanchnoccel by three
openings surrounded with cilia. These are the ccelomic
funnels. The rod of cells also becomes hollow and opens into
the cloaca behind and connects with the cavity in front into
which the ccelomic funnels open. There is now therefore a
direct communication on each side between the ccelom and the
cloaca. The three ccelomostomes and the little tubes or
tubules into which they lead, together form the pronephros,
which is the first and most anterior portion of the kidney to
develop. The tube connecting it with the cloaca is the
pronephric duct.

The tubules elongate and coil about, and as the posterior

DEVELOPMENT OF RANA 189

cardinal veins develop just in this region, the tubules are as
it were bathed in the venous spaces. At the same time,
capillaries grow out from the dorsal aorta forming the glomus,
which projects laterally towards the openings of the ccelo-
mostomes from the mesentery, on each side.

The pronephros is the functional kidney of the embryo
and early larva. Later on, however, it degenerates, and its
function is taken over by another set of coelomic funnels and
tubules, which together form the mesonephros.

The mesonephros is developed from the nephrotom.es of
half a dozen segments, some little distance behind the pro-
nephros. Cavities hollow out in the nephro tomes, and these
connect with the splanchnoccel by coelomic ciliated funnels,
and by coiled tubules with the pronephric duct. The latter
loses connexion with the degenerating pronephros, and, after
being tapped so to speak by the mesonephric tubules, it is
known as the mesonephric or Wolffian duct.

The tubules multiply by branching, and form little chambers
or Bowman's capsules which lose their connexion with the
coelomic funnels. Arterioles from the dorsal aorta and
venules from the posterior cardinal veins form little bunches
of capillaries which project into the capsules forming glomeruli.
Capsule and glomerulus together form a Malpighian corpuscle.
That portion of the posterior cardinal veins which lies behind
the mesonephros becomes the renal portal vein, which brings
blood from the posterior regions of the body to the kidneys.
The mesonephros is the functional kidney of the adult. It
extracts excretory matter from the blood stream and passes
it down the Wolffian duct to the cloaca, which develops a
ventral outpushing, the urinary bladder.

Reproductive Organs. — The gonads arise as ridges which
project into the splanchnoccel on each side of the dorsal
mesentery. The germ-cells which they contain are derived
partly from the coelomic epithelium in situ, and partly from
cells which have migrated up in the mesentery from the yolk-
mass. For a long time the sexes are indistinguishable.
Strings of germ- cells grow in, away from the surface of the
gonads, forming the genital strands. In embryos which are

i9o EMBRYOLOGICAL TYPES

going to be males, these hollow out forming the seminiferous
tubules which become connected with the cavities of the tubules
of the mesonephros. In this way the vasa efferentia are formed,
and they may be regarded as persistent coelomic funnels,
placing the testis in communication with the exterior (via the
cloaca). The sperms therefore make their way through the
tubules of the mesonephros, down the Wolffian duct or vas
deferens as it can also be called, to the exterior.

The Mullerian ducts develop as grooves in the roof of the
splanchnocoel at the side of the gonads. The sides of the
groove grow over, and convert it into a tube which opens into
the coelomic cavity in front (near the place where the pronephric
funnels were), and grows back to open into the cloaca behind.
In males the Mullerian ducts disappear.

The kidneys and gonoducts are mesodermal all the way,
and are really ccelomoducts, whose primitive function is
probably to connect the coelomic cavity with the exterior and
so allow the germ-cells to escape. They take on the function
of excretion as a result of the proximity of the tubules to the
blood-vessels.

On the other hand, the nephridia have excretion as their
primitive function ; they do not occur in Chordate animals
other than Amphioxus.

Paired Sense-organs and Brain. — The eyes make their
appearance as outpushings from the sides of the brain, forming
the optic vesicles. Each of these vesicles grows towards the
overlying ectoderm, and becomes an optic cup, with the
concave side turned outwards. The lens is formed from the
ectoderm overlying the optic cup, as a little vesicle which soon
becomes nipped off, and sinks into place at the mouth of the
cup. While the cup is really part of the brain, the lens is
part of the epidermis, but both are ectodermal. The outer
lining of the cup forms the pigment or tapetum layer, the
inner lining of the cup differentiates to form the sensitive
retina, and it is inverted since the nerve-fibres run between
the sensitive cells and the seen object (see p. 24). Outside
the tapetum, mesodermal tissue gives rise to the choroid and
sclerotic (including the transparent cornea) layers, just as round

DEVELOPMENT OF RANA

191

the brain it forms the pia mater (vascular) and dura mater
(protective). The eye-muscles arise from mesodermal tissue
which represents the three first somites of the head.

The ears arise as a pair of ingrowths from the ectoderm
behind the eyes, forming the auditory vesicles. Their
connexion with the ectoderm becomes severed and the remains

Fig. 85. — Transverse sections through the head of embryos of Rana showing
the development of the eyes.

A, early stage, in which the optic vesicles (ov) have been pushed out on
each side from the forebrain (fb). B, the outer walls of the optic vesicles
have been pushed in, converting them into optic cups (oc) ; the lens (/)
arises opposite the mouth of the optic cup from the ectoderm (ec). C, late
stage ; the cavity of the optic vesicle has been almost obliterated, the lateral
layer of the optic cup is the retina (r) and the median layer is the pigment
layer (pi), the stalk attaching the optic cup to the forebrain is the optic
nerve (on), the lens has become detached from the ectoderm.

of the connecting stalk is the ductus endolymphaticus. Each
vesicle now forms a closed sac at the side of the hinder part of
the brain, and above the tympanic cavity, which develops as
an expansion of the hyomandibular visceral pouch (Eustachian
tube). From the dorsal portion of each vesicle three shelf- like
projections are formed. The centre of each shelf becomes

192

EMBRYOLOGICAL TYPES

perforated, converting the shelf into a half- ring. In this way
the semicircular canals are formed. The cavity of the auditory
sac contains endolymph. Between the wall of the sac and
the capsule of connective tissue which surrounds it, is the
perilymph. The capsule eventually becomes cartilaginous,
and later on, bony ; but certain apertures are left. One of

i p. /

vs.

Fig. 86. — Transverse section through an embryo of Rana showing the
formation of the ears.

an, auditory nerve ; av, auditory vesicle ; bv, blood-vessels running in
the visceral arches ; eg, external gills ; g, gut ; h, heart ; hb, hindbrain ; n,
notochord ; p, pericardium ; ta, truncus arteriosus ; vs, ventral sucker.

these is the fenestra rotunda, and another is the fenestra ovalis
on to which the base of the columella auris fits. The outer
end of the columella auris is applied to the thin lateral wall of
the tympanic cavity which forms the tympanic membrane.

It may be mentioned here that, remarkable as it may seem,
the ears are responsible for the formation of the so-called calci-
gerous glands, or glands of Schwammerdamm. These glands

DEVELOPMENT OF RANA 193

are conspicuous objects in the trunk of the frog, lying on each
side of the vertebrae, close to the points of exit of the spinal
nerves. Diverticula from the auditory vesicles grow into the
brain-case, and back down the canal formed by the vertebrae
and which contains the spinal cord. From here, the diverticula
of the auditory vesicle emerge through the foramina for the
spinal nerves and give rise to the glands of Schwammerdamm
(function unknown).

The olfactory organs arise as a pair of thickenings of the
ectoderm, which sink in to form pits just above the mouth.
The cells lining these pits will give rise to the olfactory epithe-
lium. Behind, the pits reach the roof of the mouth and break
through forming the internal nostrils.

The various regions of the brain are roughly marked out
even before the neural folds have closed over. The definitive
form of the brain is soon reached by means of foldings and
thickenings of its walls in certain places.

A median ectodermal inpushing arises from the epidermis
of the front of the head, just above the mouth. This is the
hypophysis which grows back beneath the floor of the fore-
brain until it meets and fuses with the infundibular downgrowth
from the brain. Hypophysis and infundibulum together form
the pituitary body.

Placodes and Lateral-line Organs. — The dorsal nerves and
ganglia in the region of the trunk consist of nerve- cells which
have been derived entirely from the neural crests. In the
region of the head, the dorsal nerve-ganglia are derived not
only from the neural crest, but also from thickenings of the
ectoderm at the sides of the head called placodes. Placodes
are proliferations of the deeper layers of the epidermis which
contribute cells to the underlying ganglia. The profundus,
trigeminal, facial, glossopharyngeal and vagus ganglia all
derive cells from the epidermis in this way, and the auditory
nerve is formed from the placode which invaginates with
the auditory sac. Indeed, the thickenings of the epidermis
which later become pushed in to form the olfactory sacs, the
lens, and the auditory sacs, may themselves be regarded as
placodes.

194 EMBRYOLOGICAL TYPES

There are two kinds of placodes : an upper row of dorso-
lateral placodes which give rise to the lateral-line sense-
organs and to the nerve-cells whose fibres innervate them ;
a lower row of epibranchial placodes situated at the dorsal
ends of the visceral slits, and which give rise to the nerve-
cells whose fibres innervate the sense-organs of taste.

Sympathetic System and Adrenals. — The dorsal nerve-
root, formed by fibres which have grown out from cells in the
dorsal-root ganglion, and the ventral nerve-root which has
grown out from the spinal cord, join to form a mixed nerve.
Certain cells migrate out from the spinal cord, and, leaving the
mixed nerve, make for the side of the dorsal aorta where they
form the sympathetic ganglia. These ganglia remain con-
nected with the mixed nerve by the rami communicantes. The
sympathetic ganglia are, like the mixed spinal nerves, seg-
mentary arranged. They soon become connected by fibres
running to the (sympathetic) ganglia in front and behind them
forming the sympathetic trunks. From the sympathetic
ganglia, " postganglionic " fibres are distributed to the smooth
muscles of the gut, oviducts, and blood-vessels. Other cells
migrate out from the sympathetic ganglia, and give rise to the
medulla of the adrenal bodies. The cortex of these bodies
is derived from the ccelomic epithelium in the region between
the mesonephric kidneys.

It may be mentioned that cells migrate out from the hind-
brain along the vagus and eventually come to lie on the surface
of the heart and gut, forming part of the parasympathetic
system.

Skeleton. — The vertebral column arises in the form of
paired cartilages beside the notochord, derived from the
sclerotomes. Each vertebra arises opposite the septum
separating two segments ; the vertebrae are therefore inter-
segmental in position.

In the skull, paired trabecular arise as struts underlying the
forebrain, and, behind them, paired parachordals flank the
notochord. The ptery go- quadrate or skeleton of the upper
jaw arises early, and fuses on to the remainder of the skull by
its ascending process. The auditory sac becomes surrounded

DEVELOPMENT OF RANA 195

by a cartilaginous capsule which gets attached to the para-
chordals on each side. Similarly, nasal capsules surround
the olfactory sacs and become attached to the front of the
trabecular. The floor of the skull is established in this way,
and the sides and roof develop later.

In each of the visceral arches separating the gill-slits,
cartilaginous struts develop. In the mandibular arch, these
are the pterygo-quadrate, and Meckel's cartilage which forms
the lower jaw. The dorsal portion of the skeleton of the 2nd
or hyoid arch forms the columella auris. The cartilages of
the remainder of the arches eventually form a plate beneath
the floor of the mouth and pharynx, and which by raising and
lowering this floor assists in the process of respiration. The
skeleton of the limbs and girdles does not appear until a late
stage of development.

This cartilaginous skeleton is later on partly replaced by
cartilage-bones, and in addition, membrane-bones are
developed.

Teeth arise late. In their formation, an ingrowth of ecto-
derm takes place inside the margin of the mouth, forming the
enamel-organs of the teeth. These secrete a cap of enamel
beneath which the mesodermal cells produce the body of the
tooth which is composed of dentine. Eventually the tooth is
pushed up through the surface of the mouth and its base is
attached to the bone of the jaw.

Externals. — By the time that differentiation and the forma-
tion of organs have proceeded as far as has just been described,
the embryo emerges from its membranes and hatches into a
free-swimming larva which is familiarly known as the tadpole.
Its ectoderm is ciliated, and just beneath the mouth it has a
V-shaped sucker by means of which it can attach itself to
objects. Its tail elongates and develops dorsal and ventral
extensions or fins, which make it a very efficient organ for
swimming. Its food consists of vegetable matter, its stock
of yolk being by now used up. Food is seized by the edges of
the mouth or lips which are assisted by horny epidermal teeth,
which have of course nothing to do with the true teeth.

From the sides of the head, folds grow back which cover

196 EMBRYOLOGICAL TYPES

over the gill-slits. The external gills disappear, and so-called
internal gills develop in the walls of the gill-slits and subserve
the function of respiration. The folds just mentioned form
the operculum, which leaves only a small hole on the left side
through which the water which passes through the gill-slits
may escape.

The organisation of the larva is just like that of a fish, and
there is little indication of the frog into which it will develop.
The changes which take place in the conversion of the tadpole
into the frog are known as metamorphosis.

Metamorphosis. — The chief differences between the
organisation of the tadpole and that of the frog concerns the
limbs, lungs and pulmonary respiration, intestine, tongue, and
tail.

The limbs arise as buds in tadpoles about half an inch
long, and muscles grow into them from the myotomes. The
buds of the forelimbs are, however, concealed beneath the
operculum, and are therefore invisible. Those of the hind-
limbs are situated at the base of the tail, on each side of the
cloaca. In time, the fore limbs grow out through the operculum,
making use of the opening on the left side and making a new
one on the right. Soon the limbs become visibly jointed and
the toes appear.

Meanwhile, the lungs are developing, and to each of them
there runs a blood-vessel which is formed as a branch from the
efferent artery of the last or 6th arch. This vessel is the
rudiment of the pulmonary artery. From time to time, the
tadpole takes in a gulp of air at the surface of the water and
fills its lungs. A certain amount of oxygenation of the blood
now begins to take place in the lungs, and the gill-circulation
becomes reduced by the establishment of direct connexions
between the afferent and efferent branchial arteries. The gills
therefore become " short-circuited," and left out of the
circulation gradually as more and more of the blood goes to
the lungs to be oxygenated, and returns to the heart by the
pulmonary veins. The now continuous vessel in the 3rd
visceral arch becomes the carotid, that in the 4th becomes
the systemic arch, that in the 5th disappears, and the 6th as

DEVELOPMENT OF RANA 197

already seen becomes the pulmonary. The lateral dorsal
aorta between the dorsal ends of the carotid and systemic
arches (the ductus caroticus) disappears, as also does the
connexion between the pulmonary artery and the lateral dorsal
aorta (ductus arteriosus, or Botalli). After this change, the
organism is perfectly adapted to breathe in air after the manner
of land-animals.

The gills disappear ; the gill-slits close up ; the animal
ceases feeding, and the horny teeth drop off. The mouth
becomes wider and its angle moves farther back. The tongue
develops, and the eyes become more prominent and bulge
out from the top of the head. The lateral-line organs disappear
and the skin is shed. Glands appear which will keep it moist
on land. Internally, great changes take place in the intestine,
which loses its watchspring-like coils, and becomes relatively
much shorter. This is an adaptation to the carnivorous habits
of the frog, for less surface is required for the digestion of a
meal of animal food. Lastly, the tail becomes reduced and
finally completely absorbed, its debris being ingested by
wandering white blood-corpuscles, or phagocytes.

This astonishing and comparatively rapid change is brought
about by the secretion of the thyroid gland, which has been
increasing until it reaches a size sufficient to " pull the trigger "
of metamorphosis. During the process of change the weight
of the body actually decreases, but after coming out on land
and recommencing to feed, the size of the young frog increases.

Literature

Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon
Press, 191 3.

Kellicott, W. E. Chordate Development. Henry Holt, New York,

1913.
Morgan, T. H. The Development of the Frog's Egg. Macmillan, New

York, 1897.

CHAPTER XIII

DEVELOPMENT OF GALLUS (THE CHICK)

Fertilisation. — The true egg of the hen is all that is contained
within the membrane that just surrounds the yolk. It is
therefore of relatively enormous size for a single cell, and this
is due to the very large quantity of yolk which it contains.
The pure protoplasm, of which there is comparatively little,
is situated at the animal pole, which is the point at which the
follicle-stalk is attached to the ovary. The egg is surrounded
by the vitelline membrane which it has secreted and which
thickens to form the zona radiata, perforated by numerous
holes through which nutriment is passed to the egg from the
surrounding follicle-cells. This is a primary membrane. The
egg bursts out of its follicle into the ccelom, and the follicle
is left behind. There is therefore no secondary membrane.
The nucleus has grown to a very large size, and the first polar
body is formed inside the mouth of the oviduct, which as it
were grasps the follicle containing the egg before the latter has
left the ovary (or been " ovulated ").

Sperms are introduced into the cloaca of the female during
copulation, and they make their way up to the top of the oviduct
where several of them penetrate an egg. After the second
polar body has been formed, one of these sperm-nuclei fuses
with that of the egg, and the other sperms degenerate.

The fertilised egg then begins to develop, and passes
down the oviduct. The walls of the latter secrete the tertiary
membranes round it in the form of a layer of albumen, an inner
and an outer shell-membrane, a hard shell formed by depositing
lime salts, and this in many birds is coated with a layer of

198

DEVELOPMENT OF GALLUS 199

pigment which gives the " egg " its characteristic colour and
pattern.

The egg goes down the oviduct with its axis transverse
to the long axis of the oviduct, and it is rotated as it descends,
with the result that the denser albumen at the two ends, in the
axis of rotation, is spirally wound and forms the chalazse.

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Fig. 87. — Gallus : view of the blastoderm of a hen's egg. (After Jenkinson.)
A after 12 hours', B after 18 hours' incubation, as seen by transmitted
light, n, notochord ; pa, proamnion ; ps, primitive streak.

At the blunt end of the egg, the two shell-membranes are
separated by a space full of air, the air-chamber.

Cleavage. — Soon after fertilisation, the process of cleavage
begins ; its early stages therefore take place while the egg
is descending the oviduct, and it has proceeded some way
when the egg is laid. The amount of yolk compared with
that of protoplasm is so big that cleavage is incomplete, or
meroblastic. While the small quantity of protoplasm at the
animal pole divides into a number of cells arranged like a
small disc or blastoderm on the top of the yolk, the latter is

200

EMBRY0L0GICAL TYPES

undivided. The margin of the blastoderm merges with the
yolk round it, forming the periblast ; and beneath the blastoderm
is a cavity, the blastoccel, which separates it from the underlying
yolk. This stage, when the blastoderm is but a single layer
(though of many cells), represents the blastula of Amphioxus
and the frog.

Gastrulation. — A layer of cells becomes split off from the
under side of the blastoderm, between it and the underlying
yolk. This layer soon extends over the under surface of the
blastoderm and is known as the " lower layer," or secondary
endoderm. It is continuous with the upper layer all round

Fig. 88. — Gallus : transverse sections through the primitive streak of the
blastoderm of a hen's egg. (From Jenkinson.)

A after 10 hours', B, after 15 hours' incubation. //, lateral portion of
the primitive streak corresponding to the lateral lip of the blastopore (cf.
Fig. 76) ; mes, mesoderm ; pd, endoderm ; yp, primitive groove.

the margin, and, like it, merges into the periblast. The
blastoderm extends gradually over the yolk, and in so doing
it forms a margin of overgrowth. In this region, all round
the edge of the blastoderm (which is called the germ- wall),
the protoplasm is thicker than in the centre. When therefore
a blastoderm is looked at by transparency, two zones are
distinguishable. Centrally there is a relatively clear area
pellucida ; and round the edge is a denser area opaca. The egg
is usually at this stage when it is laid, some twenty-four
hours after fertilisation,

DEVELOPMENT OF CALLUS

201

A thickening of the
upper layer of the blasto-
derm appears in the
centre of the area pel-
lucida, in the form of a
straight band stretching
at right angles to the
line passing through the
pointed to the blunt end
of the " Qgg^ This is
the primitive streak, the
first differentiation of the
embryo, which will be
formed along its axis.
When an observer holds
an egg in front of him
with the blunt end to the
left, the axis of the embryo
will therefore run straight
in front of him, and the
embryo is so orientated
that its head is away from
the observer, and its tail
towards him.

Running along the
middle line of the primi-
tive streak is a shallow
groove on the surface,
the primitive groove,
which runs into a small
depression at the front
end of the primitive streak
known as the primitive
pit. Immediately in front
of the primitive pit is a
slight rise, forming the
so-called primitive knot.
Now the primitive knot

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202 EMBRY0L0GICAL TYPES

is the dorsal lip of the blastopore, and the primitive streak
represents the lateral lips of the blastopore, fused together
along their whole length. All that is left of the aperture of
the blastopore is the primitive pit and the primitive groove.
This is the condition of the blastopore of the frog after its

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Fig. 90. — Gallus : view of the blastoderm of a hen's egg after 20 hours'
incubation, as seen by transmitted light. (After Jenkinson.)

ao, area opaca ; ap, area pellucida ; av, area vasculosa ; hf, head-fold ;
n, notochord ; nf, neural fold ; pa, proamnion ; ps, primitive streak.

aperture has become slit-like and closed by the apposition of
its lateral lips. The chick therefore starts straight away from
the condition reached in the frog at the close of gastrulation.
The primitive streak, like the lip of a blastopore which it
is, is a region of cell-proliferation, and it gives rise to mesoderm-

DEVELOPMENT OF GALLUS

203

cells which grow out laterally between the upper and lower
layers. The primitive knot produces tissue which gets pushed
forwards underneath the upper layer in the middle line, and

pa

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FlG. 91. — Gallus : view of the blastoderm of a hen's egg after 24 hours'
incubation, as seen by transmitted light. (After Jenkinson.)

fg, foregut ; m, mesoderm ; ms, mesodermal somites ; tip, neuropore ;
vv, vitelline vein. Other letters as Fig. 90.

gives rise to the notochord. During development the primitive
streak moves back along the blastoderm, leaving in front of it
a trail of cells destined to become notochord, and on each side

204

EMBRY0L0GICAL TYPES

(peristomial) mesoderm. At the extreme front end of the
embryo, anterior to the region of the midbrain which is the
farthest spot reached by the proliferation of the primitive
pit and streak, a certain amount of mesoderm and the front
end of the notochord are formed by splitting off from the lower
layer. As in Amphioxus and the frog, therefore, the most
anterior part of the mesoderm (gastral) and of the notochord
are formed from the endoderm by delamination or splitting,

Fig. 92. — Gallus : transverse sections through the blastoderm of a hen's egg
after 24 hours' incubation. (From Jenkinson.)

A, through the posterior ; B, through the anterior region of the embryo.
en, endoderm ; m, mesoderm ; mg, neural groove ; mlp, unsegmented meso-
derm of the lateral plate ; mvp, segmented mesoderm (somites) of the
vertebral plate ; n, notochord. The posterior region of the embryo is
at a less advanced stage of development than the anterior region.

while farther back they are formed as a result of the activity
of the cells of the lips of the blastopore (primitive streak).

The upper layer of the blastoderm may now be called
ectoderm, the lower layer endoderm, and the mesoderm
extends out to the side between them. Gastrulation in the
chick therefore does not involve invagination. The endoderm
is formed precociously, probably so as to assist in digesting
the enormous quantity of yolk.

Head-fold. — A very important thing to notice is that the

DEVELOPMENT OF GALLUS

205

embryo in the chick will develop from only a part of the egg
and blastoderm. The remainder will give rise to the
membranes outside the embryo. At first, all that exists of
the embryo proper is the middle line of its back represented
by the primitive streak. Its sides are formed as this streak-

mb, m

Fig. 93. — Gallus : embryo chick after 30 hours' incubation seen by reflected
light A from the dorsal, B from the ventral side. (After Jenkinson.)

aip, anterior intestinal portal ; bi, blood-islands ; fb, forebrain ; hb,
hind brain ; ht, heart ; mb, midbrain. Other letters as Figs. 90 and 91.

region rises and becomes folded up from the surface of the
blastoderm around it. This process begins in front with the
formation of the head-fold ; by this means the ectoderm,
notochord, mesoderm, and endoderm are lifted off from the
surface of the underlying yolk, and a cavity appears between

206

EMBRYOLOGICAL TYPES

the latter and the endoderm which represents the foregut-
region of the enteron. As yet there is no floor to the gut,
nor is the ventral side of the embryo formed at all. The
mesoderm, lying on each side of the notochord becomes
segmented into somites. That part which is nearest to the
notochord will produce the myotomes ; farther laterally, a
split arises in the mesoderm which becomes the ccelomic
cavity, and which separates a somatic layer of mesoderm closely
applied to the ectoderm from a splanchnic layer which is
similarly applied to the endoderm. The ectoderm, mesoderm,
and endoderm extends to the side far beyond the limits of the

Fig. 94. — Gallus : longitudinal section through the head-region of an
embryo chick after 30 hours' incubation. (From Jenkinson.)

aip, anterior intestinal portal ; en, endoderm ; pc, pericardium ; pr,
proamnion ; spc, spinal cord ; st, stomodaeum. Other letters as Figs. 90,
91, and 93. The lines marked 1 to 5 indicate the planes of the transverse
sections shown in Figs. 95 and 96.

embryo, and so it comes about that the ccelomic cavity of the
embryo is perfectly continuous with the " extra-embryonic "
ccelom. As this extra-embryonic splanchnic mesoderm
spreads out, blood-islands develop between it and the endo-
derm. This is seen in blastoderms observed by transparency
as the spreading of an area vasculosa over the area pellucida.
Eventually this area vasculosa spreads over most of the
blastoderm up to the germ-wall, except for a region immediately
in front of the head-fold which is known as the proamnion.
The peripheral extent of the area vasculosa is marked by a
blood-vessel, the sinus terminalis.

Nerve-tube. — The neural plate develops as a thickening

DEVELOPMENT OF GALLUS

207

of the ectoderm along the axis of the embryo, in front of the
primitive knot. At its sides are the neural folds which rise
up and meet forming the neural tube. The first part of the
neural tube to form is the brain, which is clearly marked out

3

Fig. 95. — Transverse sections through an embryo chick after 30 hours'
incubation. (From Jenkinson.)
Taken in the planes shown by lines 1, 2, and 3 on Fig. 94. / is posterior.
For lettering see Fig. 96.

into the regions which will become the fore-, mid-, and hind-
brain. As the primitive knot and streak move back, the neural
folds follow them and cover up the spots which they formerly
occupied, and so the neural tube comes to be formed
immediately above the notochord. On each side of the neural

208

EMBRYOLOGICAL TYPES

tube, the neural crests come into existence, as in the frog.
Even at very early stages, the rudiments of the eyes may be
seen as outpushings to each side from the fore-brain. In

Fig. 96. — Transverse sections through an embryo chick after 30 hours'
incubation. (From Jenkinson.)

Taken in the planes shown by lines 4 and 5 on Fig. 94. 5 is anterior.
a, lateral dorsal aorta ; av, auditory vesicle ; c, coelom ; cv, cardinal vein ;
ec, ectoderm ; en, endoderm ; fg, foregut ; ht, heart endothelium ; htm, mus-
cular wall of heart ; kt, kidney tubule ; mg, neural groove ; ms, mesodermal
somite ; mt, nerve-tube ; n, notochord ; nc, neural crest ; nt, nephrotome ;
pct pericardium ; so, somatopleur ; spl, splanchnopleur ; vv, vitelline vein.

front, the neural tube remains open for a time at the neuropore,
which is situated at that part of the brain destined to become
the lamina terminalis.

Amnion. — In front of the head of the embryo, a fold rises

DEVELOPMENT OF GALLUS

209

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Fig. 97. — Gallus : embryo chick after 36 hours' incubation seen by trans-
mitted light, A from the dorsal, B from the ventral side. (After Jenkinson.)

The heart is beginning to bend to the right, and the amniotic folds are
covering over the head, af, amniotic fold ; as, auditory sac ; ov, optic
vesicle; va, vitelline artery. Other letters as Figs. 90, 91, and 93.

up from the extra-embryonic part of the blastoderm. This
fold extends backwards, and soon covers over the head. It
now continues growing backwards by the upgrowth of folds

p

210

EMBRY0L0GICAL TYPES

on each side of the embryo, and soon covers over the latter
completely, much in the same manner as the neural folds

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Fig. 98. — Gallus : embryo chick after 60 hours' incubation seen, A, from
the dorsal side by transmitted light, B, from the ventral side by reflected
light. (After Jenkinson.)
The head-end has turned and is lying on its left side ; the heart is twist-
ing ; the tail-fold has appeared, and the amniotic folds cover half the embryo,
a, auricular portion of the heart ; e, eye ; /, lens ; tf, tail-fold ; v, ventricular
portion of the heart ; vc, visceral cleft. Other letters as Figs. 91, 93, and 97.

previously covered over the neural tube. The folds join
above the embryo, which now finds itself in a sac, the amniotic

DEVELOPMENT OF GALLUS 211

cavity, covered over by two membranes of which the inner
is the amnion and the outer is the chorion. Both these
membranes are of course part of the extra- embryonic ectoderm,
and the fact that there are two of them is due to the amniotic
fold having two layers as it rises up. At the hind end, the
amnion and chorion remain in contact at their point of fusion,
forming the so-called sero-amniotic connexion. Extra-
embryonic mesoderm gets carried up with the ectoderm in
the amniotic fold, and forms a layer on the outer side of the
amnion and on the inner side of the chorion. The space
between the amnion and the chorion is therefore occupied
by extra- embryonic coelomic cavity. The amniotic cavity is
of course lined by ectoderm, and contains fluid. Although
laid on dry land, therefore, the chick embryo develops in a
fluid medium which may be said to be an artificial " pond,"
equivalent to the pond in which the (more aquatic) ancestors
developed, as the frog now does. The embryo is also pro-
tected by the amnion as by a water cushion from shocks and
knocks to which the shell may be subjected, and from too
sudden changes of temperature.

The Gut. — The base of the amnion grows in beneath the
embryo, thus accentuating the folding off of the latter from the
rest of the blastoderm. The head-fold has already been
noticed, and as the base of the amnion grows in beneath it and
backwards, a floor is formed for the most anterior region of the
gut. In the same way at the posterior end, a tail-fold develops,
and the base of the amnion growing in forms a floor for the
hindmost region of the gut. The middle portion of the
gut has as yet no floor, and is directly open to the surface of
the yolk underneath ; its sides are formed, however, and it is
known as the intestinal groove. The passage between the
intestinal groove and the foregut, over the edge of the floor
of the latter, is called the anterior intestinal portal. Similarly,
at the hinder end a posterior intestinal portal is formed. At
first the two intestinal portals are far apart, which is the same
thing as saying that the formation of the floor of the gut has
not yet proceeded very far back from the front end or forwards
from the hinder end of the embryo. As development goes

212

EMBRYOLOGICAL TYPES

on, however, more and more of the floor of the gut and ventral
wall of the embryo is formed, and the intestinal portals come
close together leaving only a narrow opening between the
cavity of the gut in the embryo and the yolk — the umbilicus.

eec.

Fig. 99. — Gallus : transverse sections through a chick embryo after three

days' incubation through A the hinder, B, the middle, and C, the

anterior regions of the trunk ; showing stages in the development of

the amnion.

a, amnion ; ac, amniotic cavity ; af, amniotic fold ; c, coelom ; ch,

chorion ; da, dorsal aorta ; ec, ectoderm ; eec, extra-embryonic coelom ;

en, endoderm ; m, dorsal mesentery ; my, myotome ; n, notochord ; nc,

nerve-cord ; pc, posterior cardinal vein ; sac, sero-amniotic connexion ;

som, somatopleur ; spm, splanchnopleur ; vv, vitelline vein ; Wd, Wolffian

duct.

Meanwhile, the extra-embryonic portion of the blastoderm
has been extending down over the surface of the yolk, and
eventually covers it completely except for a small aperture
which is left open, and through which the yolk is separated
from the albumen only by the vitelline membrane. The

DEVELOPMENT OF GALLUS 213

ectoderm of this region is continuous with and forms part of
the chorion ; the endoderm, continuous with the endoderm
of the embryo, now contains the yolk, and is known as the
yolk-sac. There is a layer of mesoderm on the inner side of
the ectoderm, and another on the outer side of the yolk-sac,
so that the extra-embryonic coelomic cavity extends down into
this region. The yolk-sac contains the store of nourishment
for the developing embryo, and the yolk digested is brought
into the embryo by the blood-vessels of the area vasculosa.
The yolk-sac represents the heap of yolk-cells in the hinder
part of the gut of the developing frog, but there is so much
yolk that it cannot be accommodated inside the cavity of the
gut as in that animal. Instead, it hangs in a sac beneath the
gut, and gets gradually drawn up as its contents diminish,
until, right at the end of development, it passes up through
the umbilicus into the intestine of the embryo. Just before
doing so, the albumen surrounding the chorion becomes con-
tained in a sac formed by folds of the latter. This sac com-
municates with the base of the yolk sac through the aperture
which was left open, and it communicates also with the
amniotic cavity by a reopening of the sero-amniotic connexion.
The amniotic fluid and albumen are therefore able to pass
into the yolk-sac and get absorbed.

Allantois. — Shortly after the formation of the amnion a
median ventral downgrowth is developed from the floor of the
hind gut. This endodermal sac, covered on the outside by
a layer of mesoderm, is the allantois and it represents the
bladder of the frog. The allantois grows out into the extra-
embryonic coelomic cavity, and its size increases as that of
the yolk-sac diminishes. It soon occupies almost the entire
space within the chorion which is not filled by amnion and
yolk-sac. Its outer wall becomes applied to the inner surface
of the chorion, and the extra-embryonic coelomic cavity between
them disappears by the fusion of the two layers of mesoderm
(on the inner side of the chorion and on the outer side of the
allantois). This fused layer of allantois and chorion now
lies close against the inner surface of the shell, only separated
from it by the shell-membranes ; it is also highly vascular

214

EMBRYOLOGICAL TYPES

being supplied by blood-vessels which run out from the embryo
along the allantoic stalk. As the shell is porous, the blood-
vessels of the allantois form a region where oxygen is taken

Fig. ioo. — Gallus : view of an embryo chick after four days' incubation,
from the right side.
aa, arterial arches ; ac, anterior cardinal ; al, allantois ; au, auricular
portion of the heart ; cf, choroid fissure of the optic cup ; da, dorsal aorta ;
dC, ductus Cuvieri ; dv, ductus venosus ; e, optic cup ; ic, internal carotid
artery ; /, lens ; mb, midbrain ; pc, posterior cardinal ; t, tail ; ua, umbilical
artery ; uv, umbilical vein ; v, ventricular portion of the heart ; va, vitelline
artery ; vv, vitelline vein.

into and carbon dioxide is given off from the blood. The
allantois therefore functions as a respiratory organ, and is of
the highest importance. The gill-slits do not function as

DEVELOPMENT OF GALLUS

215

respiratory organs, for they communicate with the amniotic
cavity in which the oxygen cannot be renewed.

f.ajn.

Fig. ioi. — Diagrams showing the formation and relations of the amnion,
chorion, yolk-sac and allantois in the chick. (From Jenkinson.)

1, transverse section ; the amniotic folds are rising up, but the gut is
not yet folded off from the yolk. 2, transverse section ; the amniotic folds
have met and fused ; the amniotic cavity is closed and the amnion is
separated off from the chorion ; the gut is beginning to be folded up from
the yolk. 3, longitudinal section ; the amniotic folds are about to fuse ; the
allantois is growing out from the hind gut of the embryo. 4, longitudinal
section ; the amniotic cavity is closed and the amnion is separated off from
the chorion except at the sero-amniotic connexion ; the yolk is now almost
enclosed in a yolk-sac which remains open beneath ; the allantois is greatly
enlarged, all, allantois ; am, amnion ; ante, amniotic cavity ; c, coelom ;
jam, chorion ; ham, head-fold of the amnion ; lam, lateral amniotic fold ;
sac, sero-amniotic connexion ; tarn, tail-fold of the amnion ; u, side wall
of the gut ; y, yolk.

Another function of the allantois is excretion, for the
Wolffian ducts run into the hind gut (cloaca) near its base, and
it acts as a reservoir for the excretory products accumulated

2l6

EMBRYOLOGICAL TYPES

during the embryonic life. At the end of this period the
allantois is not drawn up into the embryo, but nipped off at
the umbilicus and left behind.

The relations of the embryonic membranes are simple
to make out if it is remembered that mesoderm underlies
ectoderm, and mesoderm overlies endoderm. All the
boundaries of any particular cavity are continuous when traced

oat y c

Fig. i 02. — Gallus : diagram showing the final relations of the embryonic
membranes. (From Jenkinson, after Duval and Lillie.)

ach, air-chamber ; all, allantois the outer wall of which is closely-
pressed against the shell-membrane ; amc, amniotic cavity which com-
municates with the (as) albumen sac through the reopened (sac) sero-
amniotic connexion ; ast, allantoic stalk ; c, extra-embryonic ccelom ; shy
shell ; y, yolk in the yolk-sac which still communicates with the albumen-
sac ; x, point at which the yolk-sac will eventually close.

right round, and are formed from one germ-layer. They are
consistent in their arrangement, so that an endodermal cavity
(gut, yolk-sac or allantois) cannot communicate with a cavity
lined by mesoderm (ccelom), or with one lined by ectoderm
(amnion, or atrium of Amphioxus).

Vascular System. — At an early stage, the blood-islands of
the area vasculosa become connected up with a pair of anterior
vitelline veins which run towards the embryo from each side,

DEVELOPMENT OF GALLUS 217

and pass just in front of the anterior intestinal portal where
they fuse in the middle line. This vessel, lying immediately
beneath the floor of the fore gut, represents the subintestinal
vein of the frog, and in this region it becomes differentiated
into the heart. It is suspended from the floor of the fore gut
by a mesentery, the dorsal mesocardium, and that part of the
coelomic cavity into which it hangs will become the pericardium.
At early stages, the heart is still outside the embryo, and it is
not drawn into it until the base of the amnion has grown in

Fig. 103. — Gallus : view of the face of a chick after four days' incubation.
cf, choroid fissure of the optic cup ; e, optic cup ; fnp, fronto-nasal
process ; ha, hyoid arch ; /, lens ; Ing, lachrymo-nasal groove ; Inp, lateral
nasal process ; m, mouth ; ma, mandibular arch ; mp, maxillary process ;
ns, nasal sac.

beneath it. The anterior vitelline veins from the yolk-sac
soon become replaced by the larger pair of posterior vitelline
veins.

Running forwards from the heart, the aortic arches run up
round the fore gut (pharynx) on each side passing between the
gill-slits in the visceral arches. The hyomandibular and three
pairs of branchial pouches are developed as outgrowths from
the endoderm to the ectoderm. Of these, the hyomandibular
and the first two branchials actually become perforated for a

218 EMBRYOLOGICAL TYPES

time, and place the cavity of the fore gut in communication
with that of the amnion.

In the embryo, paired dorsal aortae develop, beneath the
notochord. Anteriorly they connect with the aortic arches,
and posteriorly at an early stage they simply spread out over
the yolk-sac on each side forming the vitelline arteries. Later,
the single median dorsal aorta arises by the fusion of the paired
vessels (as far forwards as the pharynx), and it extends back
behind the vitelline arteries into the tail as the latter is formed.

The aortic arch in the 4th visceral arch becomes the
systemic (that on the left disappears), that in the 6th arch
the pulmonary. The pulmonary arches also connect with
the dorsal aorta by the ductus arteriosus.

The cardinal veins also arise as paired vessels, on each side
of the aorta, and they communicate with the heart across a
transverse septum by means of the ductus Cuvieri. Beneath
the hind portion of the posterior cardinal veins, the subcardinal
veins arise, in the region of the mesonephros. The sub-
cardinal veins acquire connexion with the developing inferior
vena cava.

As the anterior intestinal portal moves farther back in the
embryo, the fusion of the two posterior vitelline, or omphale-
mesenteric veins, with one another becomes more extensive.
This combined vessel, which lies in the region of the developing
liver and behind the heart, is known as the ductus venosus.
At the hind end of the ductus venosus, the two posterior
vitelline veins are separate, but farther back still they fuse
together again twice : in one place dorsal to the gut, and in
another place behind again, ventral to the gut. The piece
on the left side between the dorsal point of fusion and the
ductus venosus disappears ; the piece on the right side between
the dorsal fusion and the ventral fusion behind it disappears
also. The net result of all these modifications is that the
vitelline veins run into the embryo on each side from the yolk-
sac and join beneath the gut. From this point a single vein
runs forwards and makes one complete twist round the gut
in the direction of the thread of a corkscrew, and runs into the
ductus venosus. Part of these posterior vitelline veins becomes

DEVELOPMENT OF GALLUS

219

the hepatic portal vein, and the ductus venosus becomes the
hepatic veins and the base of the inferior vena cava.

The allantois is supplied with blood by the umbilical
arteries (branches from the artery to the hind leg), and drained
by the umbilical veins. These run in the side wall of the
abdominal cavity and correspond to the lateral abdominal
veins of the dogfish. At first they run into the ductus Cuvieri
of their side, but later the right umbilical vein is reduced and

Fig. 104. — Longitudinal section through an embryo showing the develop-
ment of the metanephros.

(Actually, this section is of a mammal, not a bird, but the difference is
immaterial.) The ureter arises as an outgrowth from the Wolffian duct.
ce, ccelomic epithelium ; m, myotomes ; me, metanephrogenous tissue ;
mt, mesonephric tubules ; u, ureter ; Wd, Wolffian duct.

the left one runs into the ductus venosus. At all events, the
blood from the allantois, where it has been oxygenated, runs
into the right auricle of the heart when the latter becomes
subdivided by interauricular and interventricular septa. The
left auricle receives the pulmonary veins. The lungs are, how-
ever, not functional, and by the breaking down of the septum
between the auricles, part of the blood from the allantois is
able to get from the right to the left auricle, and so to the left

220 EMBRY0L0GICAL TYPES

ventricle and the systemic arch without going through the right
ventricle. The remainder of the blood passes through the right
ventricle to the pulmonary arches, and from them through the
ductus arteriosus to the dorsal aorta. Little if any goes to
the lungs. The interauricular septum is reconstituted after
hatching, when the lungs are open and functional, and
oxygenated blood reaches the left auricle from the lungs.
The interventricular septum is complete.

Kidneys. — With the exception of the urino-genital organs,
the remaining organs of the chick develop in a manner very
similar to that which obtains in the frog, and no useful purpose
would be served by going over them in detail. The limbs
may be mentioned, since they show in their development
certain characters which are more primitive than the definitive
adult condition ; and especially the urino-genital system is
worthy of note. Not only does it differ in some respects
radically from that of the frog, but in others it shows certain
features more clearly.

In some half a dozen segments in the anterior region of
the trunk, the intermediate cell-mass or nephrotome of each
segment pushes out a rod of cells which curves backwards
and meets and fuses with the similarly produced rod from the
segment next behind it. Each of these rods represents a
tubule of the pronephros ; they are solid instead of being
hollow and opening into the ccelom by ciliated funnels, because
the pronephros is degenerate in the chick, and does not even
function as an excretory organ. However, the rod of cells
formed by the pronephros on each side in this way grows back
to the cloaca, and later becoming hollow, forms the pronephric
duct.

In about a dozen segments, behind the pronephros, the
mesonephric tubules develop. Like the pronephros, they are
segmental in origin, and as they grow out from the intermediate
cell-mass, they find the pronephric duct so to speak ready-made
for them. They connect with it which now becomes known
as the mesonephric or Wolffian duct. The mesonephric
tubules do not as a rule open into the splanchnocoelic cavity ;
they have no ccelomic funnels. The tubules give rise by

DEVELOPMENT OF GALLUS

221

branching to a number of Bowman's capsules, and each be-
coming vascularised by a glomerulus becomes a Malpighian
corpuscle. The mesonephros is the functional kidney of the
embryo, and is therefore present in the early life of both sexes.
Later, it is preserved only in the male, for it establishes con-
nexion with the testis by means of the vasa efferentia, and the
Wolffian duct functions as the vas deferens. It disappears
in the female.

The kidney of the adult is the metanephros. It is formed
after the mesonephros, and at a time when the segmental

Fig. 105. — Longitudinal section through a metanephric kidney.

Showing the Malpighian corpuscles (Mc) in communication with the
ureter (w) by means of the collecting tubules (ct) ; ra, renal artery.

arrangement of the intermediate cell-mass has been lost. A
diverticulum develops from the base of the Wolffian duct,
near the cloaca ; this is the ureter. The ureter grows forwards
dorsal to the mesonephros, and branches repeatedly forming
a large number of collecting tubules. The metanephric
tubules, or Bowman's capsules, arise from an indistinct heap of
cells of the intermediate cell-masses belonging to one or two
segments of the hinder region of the body, and called simply
the metanephrogenous tissue. These capsules hollow out and
connect with the collecting tubules which were formed by the

222 EMBRYOLOGICAL TYPES

branching of the ureter. Each capsule is vascularised by a
glomerulus, and forms a Malpighian corpuscle. The meta-
nephric tubules never have ccelomic funnels.

So long as the mesonephros functions as an excretory organ,
there is a renal portal system formed by the hinder region of
the posterior cardinal veins. These veins filter through the
mesonephros, and are collected into the so-called subcardinal
veins, which contribute to the formation of the inferior vena
cava. The renal portal system disappears with the excretory
function of the mesonephros ; meanwhile the metanephros
has developed and renal veins connect its glomeruli to the
inferior vena cava. The posterior cardinals then run into these
renal veins.

The Mullerian ducts develop in both sexes as grooves in
the coelomic epithelium which become closed over to form
tubes, and these tubes grow back to the cloaca. In the male,
both these ducts disappear ; in the female the right duct is
lost and that on the left side persists as the definitive oviduct.

Limbs. — The limbs appear as buds at a stage relatively
earlier than that at which they arise in the frog. Their interest
lies in the fact that the cartilaginous skeleton of the limbs in
larvas reflects the primitive condition, before the modifications
of the wings and the mesotarsal joints arose. In the wrist,
the distal carpals and the metacarpals are at first separate ;
there is as yet no carpo-metacarpus. There are also vestiges
of the i st and 5th digits, so that the wing at this stage resembles
a more normal pentadactyl fore limb. Similarly in the hind
limb, the proximal tarsal cartilages are not yet fused on to the
tibia to form a tibio-tarsus, neither are the distal tarsals yet
joined on to the metatarsals to give rise to a tarso-metatarsus.

The early stages of the pelvic girdle are of great interest,
for the pubis when it first arises in cartilage points forwards
and downwards ; it is only later that it extends back beneath
the ischium.

Hatching. — Meanwhile the mouth and the anus have broken
through into the gut as the result of the sinking in of the
stomodaeum and proctodeum. As the chick grows during its
development, its position changes to accommodate it to the

DEVELOPMENT OF GALLUS

223

surrounding membranes. At an early stage it turns and lies on
its left side, and later, its body lies along the long axis of the egg
with its head near the blunt end of the shell, which is where
the air-chamber is situated. Its beak pierces the inner shell-

V 0---P

o-t.

3 w

Fig. 106. — Gallus : views of the (cartilaginous) skeleton of the limbs and
girdles of embryo chicks, A and B, after 5 days' C and D after 9 days'
incubation, as seen from the left side.

A and C, pectoral girdle and limb ; B and D, pelvic girdle and limb.
Note that in the earlier stage the pubis points forwards, c, coracoid ; ca,
carpals ; dt, distal tarsals (which will fuse with the metatarsals to form the
tarso-metatarsus) ; fe, femur ; fi, fibula ; ht humerus ; i, ilium ; is, ischium ;
mc, metacarpal ; mt, metatarsal ; p, phalanx ; pt, proximal tarsals (which
will fuse with the tibia to form the tibio-tarsus) ; pu, pubis ; r, radius ; s,
scapula ; t, tibia ; u, ulna.

membrane, and it begins to breathe the air in the air-chamber
into its lungs, often making the characteristic " peep peep "
sound. The connexions between the pulmonary arteries and
the dorsal aorta (ductus arteriosus) disappear, and more
and more blood passes through the lungs. The yolk-sac has

224 EMBRYOLOGICAL TYPES

been completely absorbed within the body. The beak of the
upper jaw bears a sharp projection, the egg-tooth, with which
the chick pierces the shell, and soon after, it emerges, having
severed its connexion with the allantois. The septum between
the auricles of the heart is reformed, and the chick now lives
in the same manner as the adult bird.

Feathers. — Feathers begin developing at about the seventh
day of incubation and the first sign of their appearance is in
the form of a thickening of the epidermis overlying a con-
densation of the dermis, and forming a papilla. The rudiment
of the feather soon becomes conical and eventually takes
the form of an elongated cylinder. The papilla at its base
becomes sunk beneath the general level of the skin forming
a follicle. The deepest layer of the epidermis differentiates
into a number of longitudinal thickened ridges, two of which
will become the rachis, and the remainder will give rise to the
barbs which come off from the rachis. At this stage they are
still rolled up inside the cylinder and covered by the outer
layer of epidermis forming the feather-sheath. The central
dermis is nutritive in function, and eventually degenerates.
The vane of the feather is formed by the shedding of the sheath,
the splitting of the cylinder on the side opposite the rachis,
and the flattening out of the barbs which have thus been
released, on each side of the rachis. The former cylindrical
nature of the feather is betrayed by the presence of a hole at
the base of the quill — the inferior umbilicus — and another
at the bottom of the vane (between it and the aftershaft), the
superior umbilicus. The aftershaft represents the thickenings
of deeper layers of the epidermis on the side of the cylinder
opposite the rachis, and below the lowermost barbs belonging
to the rachis.

The feathers of the adult bird, pennae, plumulae, and
filoplumes, are typically preceded by " nestling-down " in
the form of prepennae, preplumulae, and prefiloplumes
respectively. There are two generations of prepennae, but
in the majority of birds it is the first generation which forms the
nestling- down, and the second is reduced. Both generations
are present in the penguins.

DEVELOPMENT OF GALLUS 225

The nestling- down feathers are carried out on the tip of

Fig. 107. — Sections and diagrams showing the development of feathers.

A, section through an early papilla ; F, slightly later stage in which the
sides of the papilla are beginning to sink beneath the surface ; C, longitudinal
section through a young feather in which the epidermal thickenings are
present, but the feather-sheath has not yet disappeared ; D, transverse
section of C ; E, the feather-sheath has been shed from the end of the
feather and the barbs are freed ; F, diagram showing the relations of the
rachis, barbs, and aftershaft ; G, view of an adult feather, a, aftershaft ;
b, barbs ; c, calamus ; e, ectoderm ; fs, feather-sheath ; iu, inferior um-
bilicus ; m, mesoderm ; p, papilla ; r, rachis ; su, superior umbilicus ; t,
ectodermal thickenings (which will give rise to the barbs) ; v, vane.

the adult feathers, for the latter grow from the same papillae

Q

226 EMBRYOLOGICAL TYPES

as their predecessors, and when the adult feathers are properly
formed, the nestling-down is worn off.

Literature

Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon

Press, 1913.
Kellicott, W. E. Chordate Development. Henry Holt, New York,

1913.
Lillie, F. R. The Development of the Chick. Henry Holt, New York,

1919.

CHAPTER XIV

DEVELOPMENT OF LEPUS (THE RABBIT)

Fertilisation. — The egg is very small, and contains very little
yolk. It is surrounded by a vitelline membrane secreted by
itself, and by a secondary membrane formed from the follicle-
cells, the zona pellucida. The follicle-cells are several layers
thick surrounding each egg- cell, which however they do not
fit closely. There is a large space inside the follicle filled with
fluid and bathing the eggt which gives the characteristic
appearance of the Graafian follicle, typical of mammals. One
polar body is extruded in the ovary, the second is extruded
after fertilisation.

Ovulation is the process of escape of the egg from the
ovary. The follicle vacated by the tgg becomes filled by the
great increase in size of the follicular cells and by the ingrowth
of connective tissue and blood-vessels, and becomes a corpus
luteum. Should the egg just ovulated be fertilised, the
corpus luteum becomes an important structure, functioning as
a gland of internal secretion, and among its functions are the
following. It prevents ovulation of other eggs during the
period of pregnancy, it stimulates the uterus to hypertrophy
and so prepares for the reception and fixation of the embryo,
and it stimulates the mammary glands to secrete. The corpus
luteum disappears at the end of the period of gestation, but if
no pregnancy has ensued it disappears soon after ovulation.

Many mammals ovulate spontaneously during periods of
" heat," or oestrus, and the mouse is among them. Others, such
as the rabbit, only ovulate after copulation. During copulation
sperms are introduced into the vagina, and they make their
way up through the uteri to the oviducts, near the top of which

227

228

EMBRYOLOGICAL TYPES

they meet and fertilise the egg. Fertilisation is therefore
internal, as in the chick.

Cleavage. — Cleavage is total and gives rise to a ball of cells,
or morula. A cavity appears within it, and it soon becomes
differentiated into an outer layer and an inner mass of cells.

Fig. 108. — Lepus : early stages in the development of the rabbit. (After

Assheton.)
A, two-cell stage, enclosed by the zona pellucida (zr) ; B, morula (m)
stage ; C, blastocyst showing the differentiation into the trophoblast (t) and
the inner mass (im) ; D, the inner mass has become the embryonic plate and
is differentiated into ectoderm (ec) and endoderm {en) ; E, the trophoblast
overlying the embryonic plate — the cells of Rauber (cR) — disappear ; F,
after the disappearance of the trophoblast over the embryonic plate ; G,
transverse section through the primitive streak (ps). c, coelom ; me,
mesoderm.

The former is called the trophoblast, and the whole structure
is known as a blastocyst.

Implantation. — The lining of the uterus has on its meso-
metrial side (see p. 276) a pair of prominent folds, which pro-
ject into the cavity or lumen of the uterus. To these, the

DEVELOPMENT OF LEPUS

229

blastocyst becomes attached by means of its trophoblast.
This process is called implantation.

Formation of the Embryo. — The blastocyst enlarges and
expands in the cavity of the uterus. The cells of the inner
mass become arranged in the form of a flattened disc, imme-
diately beneath the trophoblast. This disc is known as the
embryonic plate. At the same time, the inner mass gives
rise to a layer of cells which grow as an epithelium lining the

lam-

Fig. 109. — Diagrams showing the formation of the amnion in the rabbit.
(From Jenkinson, after van Beneden.)

The earlier stage is on the left ; the later stage on the right. Since the
cells of Rauber have disappeared, the embryo is at the surface of the blasto-
cyst until the amnion has formed, all, allantois ; atr, region of the tropho-
blast where the allantoic placenta will be formed ; c, extra-embryonic
ccelom ; e, embryo ; ham, head amniotic fold ; otr, region of the trophoblast
where the omphaloidean placenta is formed ; st, sinus terminalis (blood-
vessel) of the area vasculosa ; tarn, tail amniotic fold, ys, yolk-sac.

inner surface of the trophoblast. This layer is endoderm
(also called the " lower layer "), and the cavity which it encloses
represents the yolk-sac of the chick. Here, however, there is
no yolk, and the yolk-sac is consequently empty.

The cells of the trophoblast immediately overlying the
embryonic plate (the cells of Rauber) disappear, and the
embryonic plate thus comes to the surface of the trophoblast.
A primitive streak is formed in the centre of the embryonic
plate, and, as in the chick, it proliferates mesodermal cells

230

EMBRY0L0GICAL TYPES

to each side, and forms the notochord in the middle line as
it retreats towards the hind end of the embryo. Neural folds
rise up and enclose the neural tube, and the embryo becomes

ram

Fig. i 10. — Diagram showing the relations of the embryonic membranes and
placenta of the rabbit, as seen in an idealised transverse section of the
uterus. (From Jenkinson, after Duval and van Beneden.)
all, allantois ; c, extra-embryonic ccelom ; ep, epithelium of the uterus ;

hi, cavity of the uterus ; m, mesometrium ; otr, omphaloidean trophoblast ;

pi, placenta (allantoic) ; pr am, proamnion ; ys, yolk-sac.

folded up from the surrounding tissue by the head-fold and
tail-fold. In this way, the gut begins to be formed, and,
as in the chick, anterior and posterior intestinal portals arise
(see p. 21 1).

DEVELOPMENT OF LEPUS 231

The amnion arises by the upgrowth of folds at the edge
of the embryonic plate. The hinder amniotic fold develops
faster than that in front, and when these folds meet, the embryo
is no longer at the surface of the trophoblast, but folded away
within it. The embryo is then enclosed in the amniotic
cavity, just as in the chick, and the trophoblast of the rabbit
corresponds to the chorion of the chick, the relations of which
are identical (see p. 211). (It may be mentioned that in some
other mammals such as the mouse, the amnion is not formed
quite in this way, but arises precociously, even before the
embryo (see p. 254). The rabbit has been chosen for de-
scription here because its development is so easily comparable
with that of the chick.)

The mesoderm splits into somatic and splanchnic layers
with the ccelomic cavity between them. The splanchnic
layer overlies the yolk-sac. The somatic layer grows up
round the amnion and separates the latter from the trophoblast.

An area vasculosa develops in the wall of the yolk-sac, and
the blood-vessels so formed extend as far as the sinus terminalis.
The lower wall of the yolk-sac is not vascularised. In some
mammals this lower wall of the yolk-sac with its overlying
trophoblast persist for some time, and absorb nourishment
from the walls of the uterus. In the rabbit, however, this
" omphaloidean " region of the trophoblast together with
the lower wall of the yolk-sac disappear, and the cavity of
the yolk-sac is then openly continuous with that of the lumen
of the uterus. This disappearing part of the blastocyst
contained neither blood-vessels nor mesoderm.

Placenta.— Meanwhile, the upper part of the trophoblast
which is in contact with the wall of the uterus on the mesometric
side becomes much thicker, forming a syncytium (or plasmodi-
trophoblast). The more basal part of the trophoblast, between
the syncytium and the mesoderm, retains its cell-boundaries
(and is called the cyto-trophoblast). The allantois grows out
from the region of the hind gut and brings with it a covering
layer of mesoderm and blood-vessels. The mesoderm
covering the allantois fuses with the mesoderm underlying
the cyto-trophoblast, and the allantoic blood-vessels make

232

EMBRYOLOGICAL TYPES

their way into the trophoblast. In this way the placenta is
formed, and since it is related to the allantois, it is called an
" allantoic placenta." The placenta is an organ which places
the mother and embryo in physiological communication, for
the interchange of substances. The epithelium of the wall
of the uterus disappears where the trophoblast touches it, with
the result that the trophoblast is in contact with the subepithelial
tissues and blood-vessels of the uterine wall. The blood from
these maternal vessels bathes the surface of the trophoblast.

Fig. hi

-Section through a part of the allantoic placenta of the rabbit.

The maternal tissue is on the right, the embryonic tissue on the left.
They can often be distinguished by the fact that the red blood-corpuscles
of the embryonic blood have not yet lost their nucleus, a, allantois ; ct,
cyto-trophoblast ; eb, embryonic blood-vessels ; eg, embryonic glycogenic
layer ; /, lacunas in the trophoblast and filled with maternal blood ; mb,
maternal blood-vessels ; mg, maternal glycogenic layer ; st, syncytium or
plasmodi-trophoblast ; uv, umbilical vein.

Further, the trophoblast, which in this region is now thick,
becomes hollowed out by a number of spaces or lacunae ; and
these lacunae become filled by maternal blood which oozes
out from the wall of the uterus.

The capillaries of the allantois branch in the substance of
the placenta, and the blood which they contain is separated
from the maternal blood only by the lining of the capillaries
and the surface of the trophoblast. (The blood of mother and
embryo are never in direct communication.) Across these,
substances are passed by diffusion. The maternal blood
supplies not only oxygen but food, and the embryonic blood

DEVELOPMENT OF LEPUS 233

brings carbon dioxide and excretory products which are passed
on into the maternal circulation. The placenta therefore
functions as a respiratory, nutritive, and excretory organ. At
the same time, a certain amount of nutriment is obtained from
the glands of the uterus, and is either ingested phagocytically
by the trophoblast or absorbed into the blood-vessels of the
yolk-sac (the cavity of which opens freely into that of the
uterus). But the functions of the placenta do not end there,
for it also serves as a store of food material for the developing
embryo. In particular, glycogen is accumulated in the
placenta at early stages before the embryo has a liver of its
own ; when the latter develops, the gycogen content of the
placenta decreases.

The vascular system of the embryo rabbit resembles that
of the chick, but the posterior cardinals persist as the azygos
and hemiazygos veins. The blood from the placenta arrives
in the umbilical veins, of which the right disappears and the
left runs into the ductus venosus and so to the right auricle.
As in the chick, the septum between the auricles in the heart is
perforated, and the oxygenated blood from the placenta can
get through to the left auricle, left ventricle, and so to the
carotids and brain, which requires the purest blood in the body.
The pulmonary artery connects with the aorta on the left side
by the ductus arteriosus, so that the remainder of the venous
blood in the right auricle passes through the right ventricle,
pulmonary artery, and ductus arteriosus to the aorta below the
place where the carotids come off, and does not have to go
through the lungs. The ductus arteriosus degenerates and
the perforation of the interauricular septum is closed at
birth when the lungs begin to function. The right systemic
arch disappears.

As in lower forms, the fore gut and the hind gut remain
blind for a long time. In these regions the endoderm becomes
apposed to the overlying ectoderm forming the oral plate and
cloacal plate respectively. Perforation of these give rise to
the mouth and cloaca, which latter is divided into anus and
urino-genital aperture. The bladder forms from the base
of the allantois.

234 EMBRYOLOGICAL TYPES

The urino-genital ducts develop much as in the chick,
except that the right oviduct persists, and the testis descends
into the scrotum.

From the fact that the perforation of the mouth does not
occur at the extreme front end, but in the centre of the oral
membrane, a small pocket is formed morphologically in front
of the mouth. This is the so-called preoral gut. In a similar
way, a post- anal gut is left after perforation of the anus.

In the region of the pharynx, the gill-pouches arise as
outpushings from the gut to the ectoderm. They do not,
however, become perforated.

Several structures enter into the formation of the diaphragm.
The transverse septum moves backwards a considerable
distance during development, and it is followed in its course
by the phrenic nerve. The transverse septum forms the
ventral portion of the diaphragm, and the wall which separates
the pericardium from that part of the perivisceral cavity into
which the lungs extend. The dorsal portion of the diaphragm
separates this pleural ccelom from the abdominal cavity behind,
and it is formed by the growth of the mesenteries associated
with the liver (which enlarges), kidneys, lungs, and gut.

As the placenta and the embryo increase in size, the uterus
becomes enlarged to accommodate them. This is effected
by a great increase in the size of the smooth muscle-cells of
which the wall of the uterus is composed, without any increase
in their number.

When the period of gestation is accomplished, the amnion
breaks and the embryo is expelled by the contractions of the
muscular walls of the uterus. The umbilical cord is torn.
The placenta also becomes detached from the wall of the uterus,
and, together with clots of blood and debris, is expelled as
the after-birth.

Hair. — The development of hair starts by a thickening
of the deeper layer of the epidermis, and its downgrowth into
the dermis forming a little cylinder. At its base a papilla is
formed, and just above this, the epidermal cells proliferate and
give rise to the shaft of the hair. This elongates as more
material is added to it from beneath, and it finally emerges

DEVELOPMENT OF LEPUS

235

from the follicle and grows freely out. The centre of the
hairshaft is composed of the medulla ; surrounding this is the
cortex, and round this again is the cuticle. The outer wall
of the follicle forms a sheath round the base of the hair, and
the following layers can be made out in it. In contact with
the cuticle of the hair is the cuticle of the sheath, and next
outside that are Huxley's layer, Henle's layer, and the main

Fig. 112. — Sections through the skin of mammalian embryos showing stages
in the development of the hairs.

A, early stage showing the ectodermal inpushing (ei) i nd the concentra-
tion of the mesoderm to form a papilla (mp) ; B, the ectoderm forms a
follicle (/) inside which the hair (h) is developing ; C, late stage after the
hair has erupted from the surface, apm, arrector pili muscle ; sg, sebaceous
gland.

epidermal layer of the sheath. Surrounding this again is the
dermal sheath of the follicle. The epidermis of the wall of the
follicle gives rise to little pouches which become the sebaceous
glands. Some mesenchyme cells outside the follicle become
differentiated into smooth muscle-fibres ; they gain attach-
ment to the wall of the follicle and become the arrector muscles
of the hair.

236 EMBRYOLOGICAL TYPES

Literature

Bonnet, R. Lehrbuch der Entwicklungsgeschichte. Parey, Berlin, 1920.
Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon

Press, 1 91 3.
Kellicott, W. E. Chordate Development. Henry Holt, New York, 1913.
Prentiss, C. W., and, Arey, L. B. A Laboratory Manual and Text-book

of Embryology. Saunders Co., Philadelphia and London, 1922.

PART III

COMPARATIVE ZOOLOGY OF
CHORDATES

Outline Classification of the main groups of Chordate animals,
showing the meaning and value of the comprehensive terms
employed. For complete classification, see p. 487.

(Most of the extinct groups have been omitted.)

Phylum.
Subphylum.

Subphylum.
Class.

Class.

Subphylum.

Grade.

Branch and Class.

Branch.

Chordata. Animals with gill-slits, notochord,
dorsal tubular nerve-cord, and
post-anal tail.
Hemichordata. Very lowly forms with a
small notochord only in the
anterior region of the body,
e.g. Balanoglossus.
Protochordata. Without a specialised head
or skull. .

Urochordata. Degenerate forms with a
notochord only in the tail of
the larva, e.g. Ascidia.

Cephalochordata. Primitive forms with
the notochord extending the
whole length of the body, e.g.
Amphioxus.
Craniata. With a specialised head and skull,
paired eyes, ears, and noses,
heart and coelomostomic
kidneys.
Anamnia. Without an amnion. Breath-
ing by gills at some stage of
life if not altogether.

Cyclostomata. With a round sucking
mouth, no jaws or paired fins,
e.g. Petromyzon (lamprey)
Myxine (hag).

Gnathostomata. With biting jaws,
stomach, paired fins or limbs,
Wolffian and Miillerian ducts.
237

238 COMPARATIVE ZOOLOGY OF CHORDATES

Class.
Grade.

Order.

Order.

Grade.

Subclass.

Subclass.

Class.

Order.

Order.

Order.

Order.

Grade.

Class.

Subdivision.

Order.

Order.

Order.

Order.

Order.

Subdivision.

Class.

Subclass.

Pisces (Fish). With paired fins.
Chondrichthyes. With cartilaginous
skeleton only.
Selachii. Gills uncovered,
hyostylic or amphistylic, e.g.
Scy Ilium, sharks and rays.

Holocephali. Pseud-autostylic,
e.g. Chimaera.
Osteichthyes. With bony skeleton,
lung or air-bladder.

Teleostomi. With air-bladder,
hyostylic, e.g. Gadus (cod).

Dipnoi. Lung used for breath-
ing, e.g. Ceratodus.
Amphibia. With an aquatic gill-
breathing larval stage followed
by a terrestrial air-breathing
adult.

Labyrinthodontia (or Stegoce-
phalia). Primitive extinct
forms with a complete roofing
to the skull, e.g. Eogyrinus.

Urodela. With a tail in the adult,
e.g. Triton (newt).

Anura. Without a tail in the
adult, e.g. Rana (frog).

Gymnophiona. Limbless, e.g.
Ichthyophis.
Amniota. Embryo develops on land
inside an amnion.
Reptilia. Body covered with horny
scales, cold-blooded.
Sauropsida. Reptiles related to the
birds.

Chelonia. Body enclosed in a
carapace, e.g. Testudo (tor-
toises and turtles).

Rhynchocephalia. Primitive

forms, e.g. Sphenodon.

T acertilia. Quadrate loose, e.g.
Lacerta (lizard).

Ophidia. Both halves of lower
jaw loose, e.g. Vipera (snakes).

Crocodilia. Heart 4-chambered,
e.g. Crocodilus.
Theropsida. Reptiles related to the
mammals.
Aves (Birds). With feathers, warm-
blooded.

Palaeognathae. With a large pre-
vomer, e.g. Struthio (Ostrich).

OUTLINE CLASSIFICATION

239

Subclass.

Neognathse. With a small pre-

vomer, e.g. Columba (pigeon),

Gallus (fowl).

Class,

Mammalia. Mammary glands, hair,

diaphragm, warm-blooded.

Grade and Subclass.

Monotremata. With a cloaca, e.g.

Ornithorhynchus (duck-billed-

platypus).

Grade.

Ditremata. Anus and urinogenital

apertures separate.

Subclass.

Marsupialia. With marsupial

pouch, e.g. Perameles.

Subclass.

Placentalia. Well-formed allan-

toic placenta, e.g. Lepus (rabbit).

Note. — Vertebrata may be used as roughly synonymous with
Craniata. Tetrapoda includes Amphibia and Amniota.

CHAPTER XV

THE BLASTOPORE

The blastopore is one of the most important structures in
development, for as a result of the processes which are entailed
in its formation the fundamental architecture of the future
embryo is laid down. Further, experimental investigations
have shown that the region of the dorsal lip of the blastopore
(which is the first part of the blastopore to develop) is
responsible for organising the embryo. That is to say that it
determines the place of formation of the various organs, and
is necessary for the start of the processes of differentiation.
The blastopore itself introduces the first differentiation (after
the establishment of the axis of the egg in the ovary, and of the
plane of bilateral symmetry by the point of entrance of the
sperm) in that it converts the single-layered hollow ball
(blastula) into the double-layered bowl (gastrula). In those
animals where the relation of the sperm's entrance point to
the blastopore is known (amphibia), it is found that the dorsal
lip of the blastopore arises opposite the sperm-entrance point,
and marks the dorsal side of the future embryo.

In the development of the dogfish, the egg contains so much
yolk that cleavage is incomplete or meroblastic, and a disc of
cells or blastoderm is formed lying on the top of the yolk.
Now the important point to notice is that all round the edge
of this blastoderm, cells are growing over the yolk and tucking-
in underneath the upper layer of the blastoderm to form endo-
derm. In fact, the edge of the blastoderm is the rim of the
blastopore, and mesoderm-cells are also proliferated from it.
The embryo forms in front of the posterior edge of the blasto-
derm, which is the dorsal lip of the blastopore, and does not

240

THE BLASTOPORE

241

wait for the blastopore to close. Indeed, this takes a long
time, for the anterior edge of the blastoderm has to grow a
long way down and back under the yolk before it comes up
underneath and opposite the dorsal lip to form the ventral
lip of the blastopore.

The edge of the blastoderm in the dogfish corresponds to

oe^v!.

Fig. 113. — Views of a developing embryo of a dogfish. (After Jenkinson.)

A from above ; B, C, and D from the left side. The lips of the blasto-
pore are formed from the edge of the blastoderm, ae, anterior edge of the
blastoderm ; dl, dorsal lip of the blastopore ; e, embryo ; vl, ventral lip of
the blastopore ; y, yolk.

the edge of the pigmented cells of the animal hemisphere in
the frog, and this is the place where the overgrowing lip which
is the rim of the blastopore arises in the frog also. All round
the rim of the blastopore, the ectoderm, mesoderm, and
endoderm are in contact. In the case of the frog, the blasto-
pore starts a little below the equator of the spherical embryo,

R

242 COMPARATIVE ZOOLOGY OF CHORDATES

and as it grows down to latitudes nearer the vegetative pole,
the diameter of the blastopore naturally decreases. Any given
point on the rim of the blastopore grows straight down along a
meridional line towards the vegetative pole ; but as the
diameter of the blastopore decreases, any two given points
on the rim at the start will find themselves closer together at
the finish of gastrulation. This process is called confluence.
In the case of the dogfish, the diameter of the blastopore (edge
of the blastoderm) has to increase considerably until it has
grown down and passed the equator of the yolk, whereupon
it decreases again.

It is characteristic of these lower vertebrates (fish, frog, and
newts), that the rim of the blastopore arises along the margin

yj> c

A " 3>

Fig. 114. — Views of the blastoderm of Hypogeophis, one of the Gymno-
phiona showing the origin and closure of the blastopore. (From
Jenkinson, after the brothers Sarasin.)

The anterior edge of the blastoderm here does not become the ventral
lip of the blastopore, yp, yolk-cells seen through the blastopore.

separating the protoplasmically-rich cells of the animal hemi-
sphere from the cells rich in yolk (or the undivided yolk) of
the vegetative hemisphere. Also, that in the closure of the
blastopore, the yolk should be enclosed by the growth of the
anterior part of this margin which becomes the ventral lip of
the blastopore.

This is, however, not the case in the higher forms (reptiles,
birds, and mammals), in which there is a primitive streak.
In order to understand the evolution of the primitive streak
from the simple blastopore of the lower vertebrates, it is
necessary to consider the condition in the Gymnophiona,
which is more or less intermediate. The quantity of yolk in
the Gymnophionean egg brings about the formation of a

THE BLASTOPORE 243

blastoderm. The posterior edge of this blastoderm grows
back over the yolk and tucks cells in beneath itself, like the
typical dorsal lip of the blastopore which it is. Overgrowth
also takes place at each side of the dorsal lip, and the blastopore
becomes crescentic. Eventually the two horns of the crescent
meet and the blastopore is then a closed circle. But the
anterior edge of the blastoderm has not moved, it has not grown
round underneath the yolk, and it takes no share whatever in
the formation of the blastopore. At the same time it is to be
noticed that the blastopore is a real aperture, through which
the yolk can be seen from the outside. The cavity which
communicates with the exterior through the blastopore is,
of course, the archenteron, and the lining of this cavity is the
endoderm, formed by the activity of the edge of the blastopore.
In the reptiles, yolk is abundant, and cleavage leads to the
formation of a blastoderm. At a place which marks the
posterior end of the future embryo, cells are proliferated under
the blastoderm, forming a lower layer between the blastoderm
and the yolk. This lower layer is really the endoderm, which
has been formed precociously, probably serving the function
of digesting the large quantity of yolk. A dorsal lip of a
blastopore arises (not at the extreme hind edge of the blasto-
derm, but well within its margin) as a rim beneath which cells
become tucked in and passed forwards beneath the blastoderm
and above the lower layer. The rim of the blastopore extends
to the sides, and so the lateral lips come into being. Eventu-
ally the lateral lips extend backwards, and lie parallel to one
another. The blastopore is now slit-like, and resembles a
primitive streak. The lateral lips of the blastopore join
posteriorly, and the blastopore is then closed. The cells
which get tucked in by the lips of the blastopore line a cavity
which is the archenteron, so that here as in fish, frogs, newts,
and Gymnophiona, the blastopore is a real aperture. The
archenteron extends far forwards as the result of invagination,
and its roof in the middle line becomes the notochord ; on
each side the roof becomes mesoderm. The floor of the
archenteron fuses with the underlying lower layer and then
disappears, so that the blastopore leads right down through

244 COMPARATIVE ZOOLOGY OF CHORDATES

the archenteron to the surface of the yolk. The walls of the
definitive alimentary canal are formed from the lower layer,

c?.*

t,rJk.

Ct.fe.Jk.

c?J.

Fig. 115. — Longitudinal sections through the blastoderm of a reptile,
showing the origin of the blastopore. (From Jenkinson, after Will.)
A-E, successive stages. A, precocious origin of the endoderm or lower
layer (pd), which is in contact with the upper layer of the blastoderm at
pp ; B, origin of the blastopore, dl, dorsal lip of the blastopore (cf. Fig. 89) ;
C, invagination at the blastopore to form the archenteron ; the lower layer
forms a continuous membrane separated from the yolk by the subgerminal
cavity (sgc) ; mesv, mesoderm formed from the lip of the blastopore ; D,
the archenteron (arch) extends a long way forwards beneath the upper layer
of the blastoderm ; yp, yolk-plug (cf. Fig. 75) ; E, fusion of the floor of
the archenteron with the underlying region of the lower layer, and their
subsequent disappearance, so that the archenteron communicates with the
subgerminal cavity.

which is endoderm formed really before the blastopore proper
can be said to exist.

THE BLASTOPORE 245

The conditions in the reptile lead on easily to those which
obtain in birds. Here, again, the endoderm is formed pre-
cociously as a lower layer split off from the underside of the
superficial layer of the blastoderm. The blastopore, however,
never is a real aperture, because its lateral lips are fused
together all along their length forming the primitive streak.
The dorsal lip is the primitive knot beneath which a solid
strand of cells is tucked in to form the notochord. As the
primitive streak moves backwards over the blastoderm, it
pays in a stream of cells into the hinder end of the notochord.
The primitive streak gives off mesoderm to each side. In the
bird, therefore, the blastopore is closed from the start, and its
aperture is represented only by the depression of the primitive
pit just behind the primitive knot, and by the primitive groove
which runs along the centre of the primitive streak. The
bird's blastopore begins where that of the frog leaves off, for
in the latter it will be remembered that the blastopore which
was spherical becomes oval, and its lateral lips become apposed
to one another, forming what is in fact a short primitive streak.
In the bird, there is no invagination, and no archenteron, and
the walls of the alimentary canal are derived (as in reptiles)
from the lower layer.

In mammals, the embryo develops from a primitive streak.
In some cases the blastopore is a real aperture, or in other
words, the primitive pit sinks down and opens into an
archenteron beneath the superficial layer of the floor of the
amniotic cavity (corresponding to the blastoderm). In others,
the blastopore is reduced. The primitive streak and archen-
teron give rise to the notochord and mesoderm, and the
endoderm is formed from the lower layer.

In reptiles, birds, and mammals, therefore, the blastopore
either closes or arises already closed without the yolk becoming
enclosed. For the anterior edge of the blastoderm does not
grow down under the yolk to form a true ventral lip of the
blastopore. , J

In Amphioxus where there is little yolk, the rim of the
blastopore is formed as the result of simple invagination of the
vegetative hemisphere. Thereafter the rim of the blastopore

246 COMPARATIVE ZOOLOGY OF CHORDATES

grows backwards and the embryo increases in length. In
Craniates, the quantity of yolk present prevents simple in-
vagination, and the rim of the blastopore arises as the result
of overgrowth (epiboly) accompanied by invagination or some
form of ingrowth. There is an increasing tendency for the
invagination to become reduced as the quantity of yolk in-
creases, and the yolk ceases to become encircled in the process
of closure of the blastopore. At the same time, the endoderm
appears early (one might say out of its turn), and the aperture
of the blastopore becomes virtual.

Amphioxus.

Frog.

Reptile.

Bird.

Yolk

Very little.

Little.

Much.

Very much.

Yolk

Enclosed.

Enclosed.

Not enclosed.

Not enclosed.

Gastrulatlon

Invagination.

Overgrowth

Primitive

Primitive

and invagi-

streak and

streak, no

nation.

invagination.

invagination.

Blastopore

Open.

Open.

Open.

Closed from
start.

Gut formed

Wall of

Wall of

Lower layer.

Lower layer.

from

archenteron.

archenteron.

Literature

Assheton, R. Growth in Length. Cambridge University Press, 191 6.
Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon
Press, 1 91 3.

CHAPTER XVI

THE EMBRYONIC MEMBRANES

The Yolk-sac. — In those forms in which the quantity of yolk
contained in the egg is large, the embryo is formed from a
blastoderm on the surface of the yolk and does not wait for
the latter to be enclosed. So it comes about that the yolk is
not situated within the embryo, as, for example, it is in the frog ;
indeed, in the chick it would be manifestly impossible. In
the heavily-yolked forms, then, the yolk is outside the embryo,
and it becomes surrounded by a layer of cells which are
endodermal and continuous with those of the gut-wall inside
the embryo. The yolk then finds itself inside a " yolk-sac,"
which may be regarded as temporarily extra-embryonic gut.
This sac carries a layer of mesoderm outside its (endodermal)
wall, and blood-vessels passing between the mesoderm and
endoderm absorb the yolk (which has been digested) and
convey it into the embryo. Chief among these vessels are the
vitelline arteries and veins. Indeed, in most groups of verte-
brates, the wall of the yolk-sac is the site of origin of the blood
in the form of blood-islands.

In the fish, the function of the yolk-sac circulation is not
only to convey digested yolk, but also to oxygenate the blood in
its many capillaries, at the early stages of development before
the gills have become functional.

The yolk-sac reaches the height of its development in
reptiles and birds ; and in the Monotremes which, although
mammals are oviparous, yolk is present and the yolk-sac is
large. What yolk there is in the egg of the Marsupials is

247

248 COMPARATIVE ZOOLOGY OF CHORDATES

extruded, and the egg of the Placental mammals contains no
yolk. Nevertheless, in both the last-mentioned groups a
yolk-sac is present although it contains no yolk.

In several groups of vertebrates, the yolk-sac may come to
bear interesting relations to the wall of the oviduct, with which
it is in contact if the egg is not laid but undergoes development
within the body of the mother. The blood-vessels of the
yolk-sac are in these cases able to absorb substances from the
circulation of the mother (by diffusion), and such an organ of
physiological communication between mother and embryo is
a placenta. It is necessary to specify the organ which forms
the placenta, and a placenta derived from the yolk-sac is called
an omphaloidean placenta, to distinguish it from the allantoic
placenta which is formed by the allantois.

An omphaloidean placenta is present in the dogfish
Mustelus, and in the reptile Chalcides ; it is the chief nutritive
organ in the embryonic development of the Marsupials (in
fact, it is the only form of placenta in all except Perameles
which in addition has an allantoic placenta), and in the
" Placental " mammals it arises early and disappears later.

As development proceeds, and the quantity of yolk is
reduced, the size of the yolk-sac decreases and finally it is
withdrawn into the body through the umbilical stalk.

The Allantois. — The allantois occurs in reptiles, birds, and
mammals, and attains its greatest development in the latter.
It develops as an outgrowth from the hind part of the gut, and
is an endodermal sac covered with mesoderm in which blood-
vessels run. In amphibia it is represented by the (allantoic)
bladder. In reptiles, birds, and Monotremes, the allantois
functions as a respiratory and excretory organ, for which it
is well fitted since the excretory ducts open into its base, and
its distal portion is spread out close beneath the (porous) shell.
In the reptile Chalcides, the Marsupial Perameles, and the
Placental mammals, the allantois enters into relations with the
wall of the oviduct (or uterus) and forms the allantoic placenta.
Its function is then nutritive as well as respiratory and
excretory. It is easy to see how this may have occurred in
evolution by the retention of the egg within the oviduct and

THE EMBRYONIC MEMBRANES 249

the disappearance of the shell. It is necessary to mention this
last proviso because in some forms the egg is not laid ; it
undergoes development in the oviduct but does not lose the
shell. This condition, which occurs in the viper, is called
ovo-viviparous.

In the Placental mammals, the allantois relieves the yolk-sac
in the formation of the placenta, and the higher the order of

Fig. 116. — Diagram of the relations of the embryonic membranes in the
human embryo. (From Jenkinson, after Graf Spee.)

The embryo, developed in the floor of its amniotic cavity (amc) is
attached to the trophoblast by the mesodermal body-stalk (bst), into which
the allantois is beginning to grow ; bv, blood-vessels round the wall of the
yolk-sac (ys) B, transverse section through the body-stalk in the plane
indicated.

mammals the earlier does this happen. Indeed, in the highest
of all, the Primates (including man), the mesoderm of the
allantoic stalk appears from the beginning (the " body-stalk "),
and the endodermal allantois grows into it later. In these
animals the allantoic blood-vessels (the umbilical arteries and
veins) are ready at a very early stage to transport to and from
the embryo, which increases the efficiency of the placenta.
The blood-vessels of the allantois are usually covered by

250 COMPARATIVE ZOOLOGY OF CHORDATES

the outermost layer of extra-embryonic ectoderm, known as
the chorion in reptiles and birds, and the trophoblast in
mammals.

The Allantoic Placenta. — The blood of the mother and that
of the embryo are never in direct communication. The
passage of foodstuffs, excretory and respiratory substances
must therefore take place by diffusion through the membranes.
The efficiency of the placenta is conditioned by the area of
mutual contact between the maternal and embryonic circula-

+ U&&™

em.

Fig. 117. — Section through a part of the allantoic placenta of Perameles :
embryonic tissue on the left, maternal on the right. (After Hill.)

al, allantois ; eb, embryonic blood-vessels ; et)i, embryonic mesoderm ;
mb, maternal blood-vessels ; mc, maternal connective tissue ; ue, uterine
epithelium (which has become syncytial).

tions, and by the thickness and number of the intervening
membranes. The area of contact can be increased by throwing
the surfaces of the maternal and embryonic tissues into folds ;
and the intervening membranes can be decreased by removal
or erosion of certain of the layers of the uterus. Four grades
of structure and corresponding efficiency can be seen in the
mammals, which will now be taken in order.

(i) The embryonic and maternal surfaces are flat and un-
folded ; the area of contact is therefore small. However, the

THE EMBRYONIC MEMBRANES

251

trophoblast (embryonic outermost layer) disappears, and the
uterine epithelium (innermost maternal layer) becomes
syncytial and contains blood-vessels. Substances therefore
have to pass through the wall of the maternal capillaries,
through the uterine epithelium, across the intervening space
and through the wall of the embryonic capillaries. This type
of placenta occurs in Perameles, the only Marsupial to possess
an allantoic placenta at all. It was probably present in the
ancestors of the Marsupials, and has been lost in the other
living Marsupials.

Fig. 118. — Section through a part of the allantoic placenta of the cow;
embryonic tissue to the left, maternal to the right.
The trophoblast (t) is produced into villi (v), which fit loosely into
crypts (c) in the uterine wall, the epithelium of which (ue) persists ; al,
allantois ; eb, embryonic blood-vessel ; ec, embryonic connective tissue ;
mb, maternal blood-vessel ; mc, maternal connective tissue ; ug, glands in
the wall of the uterus.

(ii) The embryonic and maternal tissues are thrown into
folds ; embryonic " fingers " or villi fitting into corresponding
crypts in the uterine wall. In the pig, villi are distributed all
over the trophoblast, but in the cow they are grouped together
in clumps forming cotyledons. The uterine epithelium per-
sists. Substances therefore must diffuse through the wall of
the maternal capillaries, connective tissue (uterine epithelium),
trophoblast, and the wall of the embryonic capillaries.

(iii) The epithelium of the uterus disappears, and the

252

COMPARATIVE ZOOLOGY OF CHORDATES

underlying maternal connective tissue is invaded by the
developing villi of the trophoblast, so that the latter comes
into contact with the walls of the maternal capillaries. Sub-
stances have only to pass through the wall of the maternal
capillaries, the trophoblast, and the wall of the embryonic
capillaries in order to diffuse through. This type of placenta
occurs in carnivora (cat and dog), and is restricted to a zone of
the trophoblast, whence its name zonary.

(iv) The epithelium of the uterus is removed, but the
underlying connective tissue is not invaded as in the carnivores ;
instead the trophoblast is very much thickened and then

°J

eh njc +• f- mb. ef

Fig. 119. — Section through a part of the allantoic placenta of the cat ;
embryonic tissue to the left, maternal to the right.

Letters as Fig. 118.

hollowed out here and there to form lacunae. The remaining
projections from the trophoblast are called pseudovilli to
distinguish them from the true villi which are definite out-
growths. The maternal blood-vessels are " tapped " by the
very thorough erosion of the uterine wall, and the blood
flows out of them and into the lacunae in the trophoblast.
The pseudovilli are therefore bathed in the blood of the
mother, and the substances have only to pass through the
trophoblast and the wall of the embryonic capillaries to enter
into the embryonic circulation. This is the highest type of
placenta, and it is found in the rabbit, mouse, bat, shrew,
hedgehog, mole, Tarsius, monkey, and man. It is interesting

THE EMBRYONIC MEMBRANES 253

to note that these mammals are more closely related to one
another than to other mammals. This type of placenta
occupies a disc-shaped region of the trophoblast, whence its
name discoidal.

At birth, the allantois and placenta are nipped off from the
embryo ; and the placenta separates from the uterus, and is
expelled as the " after-birth." In the carnivores (type iii)
this entails a certain amount of loss of maternal tissue ; in
the others the mother only loses blood. In Perameles, on the
other hand, the placenta is absorbed by the uterus.

It must be remembered that as well as being an organ of
exchange between mother and embryo, the placenta functions
during early stages of development as a regulator of meta-
bolism of substances such as glycogen. Later on, this function
is taken on by the liver of the embryo.

The Amnion. — The amnion is found only in reptiles, birds,
and mammals. All these animals differ from the fish and
amphibia in that the eggs are laid on dry land and not in water.
The amnion is formed by folds of the extra-embryonic ectoderm
and underlying mesoderm which rise up on all sides of the
embryo and meet above it. The inner layer so formed en-
closing the amniotic cavity is the amnion proper ; the outer
layer is the chorion. In the reptiles and the Monotremes, the
fusion of the folds above the embryo is not complete, so that
the amniotic cavity is not quite closed. In the birds, the
amniotic cavity is closed, but it opens again later (at the sero-
amniotic connexion).

In the mammals, there are two principal types of amnion-
formation. In the one type, of which the rabbit is character-
istic, the embryonic plate comes to the surface of the blastocyst
by the disappearance of the overlying trophoblast (cells of
Rauber), and the amniotic folds rise up on each side of the
embryo from the edge of the embryonic plate. This method
of formation of the amnion is very similar to that which holds
in birds ; the chorion of the latter corresponds to the tropho-
blast of the mammals. The only difference is the fact that
the trophoblast in the mammal forms a complete investment
from the earliest stage.

254 COMPARATIVE ZOOLOGY OF CHORDATES

In the other type, of which the mouse is an example, the
amnion arises as a cavity hollowed out in the inner mass of cells,
within the trophoblast. This method is called amnion-
formation with entypy of the germ. In this case, there are
no amniotic folds, and the trophoblast (which forms a complete
investment, as in the rabbit) does not become interrupted by
any disappearance of Rauber's cells. When the amniotic
cavity is formed in the mouse, the embryo becomes differ-
entiated on its floor. The mouse therefore starts from a
condition which the reptiles do not reach until a fairly late
stage of development, when the amniotic folds have been
formed.

The amniotic cavity contains fluid, and this enables the
embryo to develop in a fluid medium, although its egg was not
laid in water.

It is interesting to note how in the higher vertebrates
certain processes take place as if they were abbreviations of the
conditions which prevail in lower vertebrates. To start with,
the primitive streak which is the beginning in higher verte-
brates, represents a stage which the lower vertebrates only
reach after the blastopore has formed, and become closed by
the apposition of its lateral lips. Similarly, the mammal with
a hollowed-out amniotic cavity within the trophoblast starts
from a condition which the reptiles and birds reach after the
upgrowth and fusion of the amniotic folds. Not only this,
but in such mammals the amniotic cavity arises first and the
embryo forms in its floor ; whereas in the reptiles and birds the
embryo forms first and the amniotic folds arise afterwards.
This process of " short-circuiting " and telescoping of develop-
mental processes reaches its climax in the Primates, where the
body-stalk develops first and the allantois grows into it later.
In lower mammals as well as in reptiles and birds, the allantois
grows out from the hind gut at a fairly late stage. In the
Primates, the conditions are as if everything were first got
ready for the embryo, after which it makes its appearance.
This is not without interest in connexion with the superior
organisation and differentiation of the highest mammals, for
this superiority in construction is dependent on a prolonged

THE EMBRYONIC MEMBRANES 255

and intense period of embryonic development, when the
efficiency of the embryonic membranes and placenta is of the
utmost importance.

Literature

Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon
Press, 191 3.

CHAPTER XVII

THE SKIN, AND ITS DERIVATIVES

The skin forms the outermost layer of the body, and its
functions are protective, excretory, and sensory ; for all
information which the animal receives concerning the outer
world must come through the skin. Correlated with these
functions, it is found that the constituents of the skin may
undergo various modifications.

The skin is formed of an outer ectodermal layer, the
epidermis, and an inner mesodermal layer, the dermis. In
Amphioxus the epidermis is only one-cell thick (as in most
invertebrates) ; while in all Craniates it is several layers of
cells in thickness. Of these, the innermost form the stratum
germinativum (or stratum Malpighi) which constantly pro-
duces new cells, while the outermost layers tend to become
horny forming the stratum corneum. As the cells become
horny the protoplasm within them dies, and they become worn
away by friction with the environment and replaced from the
stratum germinativum. In many reptiles and amphibia, it
is common for the superficial layer of the epidermis (overlying
the horny scales) to be sloughed off all at once and replaced.

The epidermis may be ciliated in early stages of develop-
ment in the lower forms, such as Amphioxus and the frog
tadpole.

The epidermis covering the eye becomes very thin and
transparent forming the conjunctiva. Sensory cells are present
in the stratum germinativum, and it will be remembered that
the sensory epithelium of the nose, of the eye, the ear, the lens
and the placodes which contribute nerve-cells to the cranial
ganglia, are all formed from the epidermis.

256

THE SKIN, AND ITS DERIVATIVES 257

The skin excretes by means of glands which may be com-
posed of single cells or many cells. Examples of the latter
are to be found in the mammary, sebaceous, and sweat-glands
of the mammals. These glands arise in the epidermis and
project inwards into the underlying dermis. In some animals
the glands may be modified into poison-glands ; and in deep-
sea fish they may produce a luminous secretion.

The epidermis may be modified into a variety of structures
such as horny scales (corneoscutes) which are present in reptiles,
birds (chiefly on the feet) and mammals (all over the body
of the Pangolin, at the base of the tail of the rat). The
epidermis also gives rise to feathers which are characteristic
of birds (see p. 224) ; hairs which are characteristic of mammals
(see p. 234) ; the termination of the digits which may take
the form of claws, nails, or hoofs ; and the horny covering of
the beak in tortoises and birds. The " horns " of cattle are
formed of a layer of epidermal horn overlying a central dermal
bony core. The horn of the rhinoceros is made of fused hair.
Special epidermal structures on the edge of the mouth of
Petromyzon, frog tadpoles, and Ornithorhynchus, give rise to
the so-called " horny teeth," which have nothing to do with
true teeth. Lastly, the epidermis produces the cap of enamel
which forms a covering to the dentine of denticles and true
teeth.

Hairs and feathers are commonly moulted at intervals and
replaced.

The dermis forms the leathery layer of the skin. It con-
tains blood-vessels which serve to supply the cells of the
epidermis as well as those of the dermis, and especially the
papillae at the bases of hairs and feathers, and the glands. In
amphibia this dermal circulation also serves respiratory pur-
poses, and in the mammals it forms part of the mechanism for
regulating the heat of the body. In amphibia the dermis is
separated from the underlying muscles by lymph-spaces, but
in higher forms the skin is firmly attached to the muscles by
connective tissue. In higher forms, special muscles arise in
connexion with the dermis. Some of them are attached
to scales, feathers, or hair-follicles, which they move. It is

s

258 COMPARATIVE ZOOLOGY OF CHORDATES

by the contraction of these (smooth) muscles in mammals
that hair is made to " stand on end," and the puckering of
the skin round the hair-follicles gives rise to the condition
known as " chicken-skin. "

In addition to these dermal muscles, there are in the higher
forms, and especially in the mammals, sets of muscles beneath
the skin and which move the skin as a whole. The panniculus
carnosus muscles are in the region of the trunk and they serve
to shake the skin. (They are of somatic origin.) In the head
and neck regions the platysma muscles (of visceral origin)
move certain parts of the skin such as the lips, eyebrows, and
ears. In man, these are the muscles of expression. The
smooth dermal muscles are innervated by sympathetic fibres,
the panniculus carnosus by ventral nerve-roots, and the
platysma by the facial nerve.

Just as the cells of the epidermis seem to be prone to the
production of horn and horn-like structures, so the cells of the
dermis seem to run to the formation of bone and dentine.
Dentine is the substance of which denticles and teeth are
formed, under the epidermal cap of enamel. The bone pro-
duced in the dermis takes the form of dermal or membrane-
bone, bony scales, or fin-rays (lepidotrichia). In Selachii the
dermis forms dentine but no bone.

Dermal bones are widely distributed over the body in forms
above the Selachii. They play an important part in the
formation of the skull, and of the pectoral girdle. In some
animals, the body may be entirely covered by an armour of
bony plates, as in the Labyrinthodonts, or the armadillos.
These bony plates are osteoscutes, and remnants of them are
to be found in the carapace and the ventral shield (or plastron)
of the tortoise, and in the so-called abdominal ribs or gastralia
of Sphenodon, crocodile, Plesiosaurs, Ichthyosaurs, Pterosaurs,
and Archaeopteryx (see Fig. 160). Osteoscutes are also present
in Gymnophiona, lizards, and crocodiles.

In the fish, the dermal bones come into relation with the
overlying denticles, forming complex scales. In the
Osteolepidoti and primitive (extinct) Dipnoi, the denticles
have fused together forming a layer of " cosmin," and this

Siojtfcruaitnj

THE SKIN, AND ITS DERIVATIVES

259

is attached to the underlying bony plate, which forms the
so-called " isopedin " layer. This is the " cosmoid " scale.
In the primitive (extinct) sturgeons (the Palaeoniscoidea)
and in Polypterus, the layer of cosmin is not only covered by
bone underneath (the isopedin), but also on top, the superficial

/ C °- d'

PC _^- ■*=

Fig. 120. — Sections through the skin of Scyllium embryos, showing the
mode of development of the placoid scales or denticles.

d, dentine ; e, ectoderm ; ec, modified ectoderm cells which produce the
enamel ; en, enamel ; m, mesoderm ; 0, odontoblasts, mesoderm cells
which produce the dentine ; pc, pulp-cavity.

layer of bone being called the ganoin. This type of scale is
called palaeoniscoid. In Lepidosteus the structure of the scale
is similar, but the layer of cosmin has disappeared, and the
scale consists simply of a layer of ganoin overlying a layer of
isopedin. This is the lepidosteoid type of scale. The
palaeoniscoid and lepidosteoid scales are of course beneath

260 COMPARATIVE ZOOLOGY OF CHORDATES

the epidermis since the layer of ganoin (bone) is a mesodermal
structure. The epidermis overlying these scales may possess
true denticles. It is also worth noticing that the structure
of the dermal bones and of the dermal fin-rays (lepidotrichia)
in a given animal tends to be identical with that of the scales.

In the higher bony fish or Teleosts, the scales lose the layer
of ganoin. The scales form in the dermis, but the bone
cells become lost and the scales are very thin. It is obvious
that these dermal scales together with the dermal scales of
Gymnophiona and lizards (osteoscutes) must not be regarded
as having anything in common with the epidermal scales
(corneoscutes) of higher forms. Dermal scales are retained
throughout life ; epidermal scales and denticles are shed.

Other examples of dermal ossifications are to be found in
the bone (os corneum) which forms the core of the " horn "
of cattle, and which becomes attached to the frontal bone of
the skull. Similar little bones form the knobs on the head of
the giraffe, while large bony structures in this position give
rise to the antlers of deer. Antlers are restricted to the males,
they may be forked, and they are shed every year. The size
of the antler often bears an interesting relation to the size
of the body (see p. 482). Horns, on the other hand, may be
present in both sexes, and, except in Antilocapra (the
American prong-buck), they are neither forked nor shed.

Lastly, when dealing with the skin, mention must be made
of colour. Pigment-cells may occur in the epidermal and the
dermal layers of the skin. In some cases, the pigment-cells
are capable of altering the distribution of their pigment, with
the result that the animal may change colour (as, for example,
the frog, or the chamaeleon). Pigment may also be present
in feathers and in hair, but in these structures the texture of
the surface may also produce effects of colour without any
pigment being there.

Literature

Goodrich, E. S. Vertebrate Craniata, Cyclostomes and Fishes. Black,
London, 1909.

CHAPTER XVIII

THE TEETH

Teeth and the denticles (or placoid scales) of the dogfish are
identical in that they consist essentially of a hollow cone of
dentine, inside which is a pulp-cavity, and outside which is a
layer of enamel. The dentine is formed from the mesoderm
of the skin, and the enamel is produced from the overlying
ectoderm. The denticle or tooth is formed below the surface
of the skin, and is subsequently erupted through it. In the
dogfish the denticles are not restricted to the borders of the
mouth, but occur all over the surface of the body. In a few
bony fish such as Polypterus, Lepidosteus, and catfish, denticles
also occur over the surface of the body ; but in the remainder,
and all higher vertebrates, teeth are restricted to the mouth.
In addition to those on the premaxilla, maxilla, and dentary,
teeth may be carried by the prevomer, parasphenoid, palatine,
and pterygoid bones in lower vertebrates ; in the bony fish
teeth may even be carried on the branchial arches. In
Selachians, the teeth are loosely attached to the underlying
skeleton by connective tissue. In bony fish, they are firmly
fixed on to the underlying bone by " cement,'' a modified
form of bone, which is absorbed when the tooth is shed. In
some cases the teeth may be hinged. In higher forms the
bone grows round the base of the teeth, which thus come to
lie in grooves (pleurodont) or sockets (thecodont). In
Sphenodon and Chamaeleo the teeth are fused on to the edge
of the bone (acrodont condition) and are not replaced.

The teeth of Osteolepidoti and of the earliest amphibia are
peculiar in that their walls are thrown into folds, giving a
characteristic appearance when seen in section, and which is

261

262 COMPARATIVE ZOOLOGY OF CHORDATES

Fig. 121. — Transverse sections through the lower jaw of mammalian
embryos showing the development of the teeth.

A, early stage, the dental lamina (dl) has grown in from the ectoderm ;
B, a tooth-germ has been formed on the dental lamina, the cells of which
(ectodermal) produce the enamel (e) ; beneath the enamel the mesodermal
cells produce dentine (dn). C, the development of the first or milk-tooth
imt) is nearing completion : beneath it the dental lamina has formed
another tooth-germ which will produce a permanent tooth (pt) ; the
dentary bone (d) encloses the teeth in a socket, ftn, floor of the mouth ; /,
lip ; Mcy Meckel's cartilage ; mp, mesodermal papilla ; pc, pulp-cavity ;
t, tongue. (C/. Method of development of the denticle, Fig. 120.)

THE TEETH

263

responsible for the term Labyrinthodontia which is applied
to the earliest amphibia. In snakes the teeth may be grooved
or even hollow and converted into poison-fangs. The
poisonous secretion passes in the groove or tube and is inserted
as with a hypodermic needle into the tissues of the prey.

Living Chelonia have no teeth, but they were present in
the primitive fossil Triassochelys. The same applies to birds,
which are toothless to-day, but which originally possessed
teeth, as is shown by the fossil Archaeopteryx and others.

The teeth of mammals and of those extinct reptiles which
were on the mammalian line of descent differ from those of
other vertebrates in that they are not all similar, but differ in

Fig. 122. — The origin of teeth in the dogfish.
A, inner side of one half of the upper jaw, showing the rows of reserve
teeth ; B, section through the lower jaw ; the smallest teeth are the most
recently formed. Mc, Meckel's cartilage.

shape in the various regions of the mouth. This condition is
called heterodont, as opposed to the homodont condition when
the teeth are all similar.

The most anterior teeth are the incisors, and (except in
some Marsupials) they are never more than three in number
on each side in each jaw. In the upper jaw they are carried
on the premaxilla. Next come the canines, the premolars,
and the molars. The molars differ from the premolars in
that there is only one set of them, whereas the premolars are
represented by a lacteal or " milk " dentition followed by a
permanent set which replaces them. In a few mammals, such
as the toothed whales, the teeth are all similar, but this is a
secondary and degenerate condition.

264 COMPARATIVE ZOOLOGY OF CHORDATES

Another difference between the teeth of mammals and
those of other vertebrates lies in the fact that they arise in two
sets, or, in other words, they are replaced once only (except for

A

- 4.-:""^

<T

Fig. 123.

-Diagrams showing the^relation of the mammalian to other
modes of tooth-succession. (After Bolk.)

A, diagrammatic view of the outer side of the dental lamina (dl) of the
lower jaw, showing the alternation between tooth-germs at the side (s) and
at the base (b) of the dental lamina. B, diagrammatic representation of
tooth-replacement in reptiles ; the teeth formed from the tooth-germs at
the side of the dental lamina are shaded : those formed from the tooth-
germs at the base of the dental lamina are white ; the tooth-germs produce
several teeth, which replace other teeth formed originally from the same
tooth-germ as themselves. C, diagrammatic representation of tooth-
replacement in mammals ; each tooth-germ produces one tooth only, and
the teeth formed from the tooth-germs at the base of the dental lamina
replace those formed from the tooth germs at the side of the dental lamina.
e, ectoderm.

the molars which are not replaced at all). Other vertebrates
have perpetual replacement of teeth as and when the exist-
ing ones wear out. The mammalian condition is called

THE TEETH 265

diphyodont, that of other vertebrates polyphyodont. However,
it is probable that the two sets of teeth of the mammal are not
to be regarded as simply an abridgement and reduction of the
many sets of teeth of, say, the crocodile, for the following
reason. The ectoderm, which sinks down beneath the
surface of the skin of the mouth to produce the enamel, forms
a long band extending parallel to the edge of the jaw, known
as the dental lamina. The rudiments of the teeth appear on
the outer side of the dental lamina in two families ; one from
the middle of the side of the lamina, and the other from its
base. The teeth formed by one family of rudiments grow up
and are intercalated between the teeth formed by the other
family. When, in the crocodile, for example, a tooth has been
formed, another tooth arises beneath it from the same rudiment,
and this second tooth will eventually push out and replace the
first. But any given tooth is only replaced by a tooth belong-
ing to its own family, and which has arisen from the same part
of the dental lamina. In the mammal there are the same two
families of tooth- rudiments, but each rudiment gives rise to
one tooth only. Further, owing to the reduction in size of
the jaw, there is not room for both families of teeth at the same
time. One family appears first, and gives rise to the lacteal
or " milk " dentition. Later on, the other family appears
and forms the permanent dentition which pushes out and
replaces the lacteal teeth. In the mammal, therefore, replace-
ment is effected by a tooth of one family displacing a tooth of
the other family ; in the other vertebrates replacement is
brought about by the displacement of a tooth by another
tooth belonging to the same family. It is probable that the
molars belong to the permanent family, the corresponding
lacteal teeth having been suppressed.

The Marsupials have a peculiar mode of reproduction in
that the young are born very early and in a very undeveloped
condition. They are attached to the nipples of the mother
and continue their development in her pouch, or marsupium.
During this period no teeth are required, and it is found that
in Marsupials the lacteal dentition is reduced ; in fact only one
tooth (the third premolar) is replaced. That this is a secondary

266 COMPARATIVE ZOOLOGY OF CHORDATES

Fig. 124. — Types of teeth (the different teeth are not drawn to the same

scale).

A, longitudinal section through a human molar, showing : c, cement
(here restricted to the base of the tooth) ; d, dentine ; e, enamel ; pc, pulp-
cavity ; r, roots or fangs ; such a tooth is short and low in the crown, and
conforms to the type called brachyodont. B, longitudinal section through a
premolar of the horse ; cement, dentine and enamel all enter into the com-
position of the crown of the tooth, and as the hardness of these substances
differs, they are worn away to different extents ; such a tooth is long and
high in the crown, and conforms to the type called hypsodont. C, longi-
tudinal section through the incisor of a rabbit, showing the open (" root-
less ") pulp-cavity or persistent pulp (pp). D, view of the crown of an
upper molar of a pig, showing the separate cusps characteristic of bunodont
teeth. E, crown of an unworn, F, crown of a worn lower molar of a
camel, showing the crescent-shaped ridges joining the cusps, characteristic

THE TEETH 267

reduction is proved by the fact that in extinct forms replace-
ment took place in more of the teeth. There is another point
of interest in the teeth of the Marsupials, which refers to the
fact that they are the only mammals in which more than three
incisors are found on each side. The probable explanation
is that in this region of the mouth, the teeth of one family are
not replaced by the teeth of the other, but that both families
of teeth are erupted together, the members of the two families
intercalated as in the crocodile. Behind the canine, however,
the families of teeth replace one another as in other mammals.
The marsupials, then, are intermediate between the reptiles
(simultaneous presence of teeth of both families all over the
jaw with complete intercalation) and the higher mammals (no
intercalation of teeth of the two families).

The primitive shape of the molar teeth in the mammal is
three-cusped or tritubercular in the upper jaw, while those of
the lower jaw have three cusps and a posterior " heel " or
talonid, and are called tuberculo-sectorial. The three cusps
of the upper teeth form a triangle or " trigon," with the apex
pointing inwards ; the three cusps of the lower teeth form a
" trigonid," with the apex pointing outwards. They are so
arranged by this means that the teeth of the upper and those
of the lower jaw fit into and work against one another. This
type of molar was evolved from the primitive reptilian type
in which each tooth had but one cusp. The original cusp
is represented by the outer cusp of the trigonid in the lower
molars, while in the upper molars the original cusp has been
split into two and is represented by the two lateral cusps of
the trigon. The remaining cusps and the talonid were
subsequently developed in relation to the " fit " of the teeth
on one another. The number and arrangement of the cusps
may be much modified in the different groups, but the primitive

of selenodont teeth. G, crown of an unworn, H, crown of a worn lower
molar of a tapir, showing the transverse ridges joining the cusps, character-
istic of lophodont teeth. I, inner side view of a tritubercular (upper)
molar, in relation to J, a tuberculo-sectorial (lower) molar ; t, talonid. K,
diagram of the relative positions of the cusps of tritubercular molars (dotted
lines) of the upper jaw, and tuberculo-sectorial molars (full lines) of the
lower jaw ; an, anterior side ; ou, outer side. L, simple peg-like tooth of
a reptile.

268 COMPARATIVE ZOOLOGY OF CHORDATES

forms of most groups of mammals have molars of this tri-
tubercular and tuberculo-sectorial type.

When the cusps remain separate as in the pig, the tooth is
called bunodont. In other forms, the cusps may be joined
to one another by ridges running at right angles to the length
of the jaw, as in the tapir (lophodont condition). In others,
again, the cusps are splayed out to form crescents running in
the line of the length of the jaw, as in the camels (selenodont
condition).

It is characteristic of mammalian molars to have divided
roots or " fangs."

Normally, a tooth grows to a certain size (not very big), and
after that the pulp-cavity becomes almost closed at the base.
Such a tooth may have one or more " roots " or fangs, and when
these have formed, the tooth ceases growing. This is the
brachyodont type, the name being derived from the fact that
the teeth are comparatively short, and as a rule their possessors
do not make use of them for grinding hard materials. Where
the diet consists of resistant material which requires grinding,
and in other cases where the teeth are subjected to hard wear,
the pulp-cavity remains widely open at the base, and the teeth
are capable of continuous growth. These teeth are described
as being " rootless," or possessing persistent pulps, and from
the fact that they are usually long, this condition is known as
hypsodont. Examples of hypsodont teeth are to be found in
the premolars and molars of the horse, the incisors of the
rabbit, the incisors (tusks) of the elephant, and the canines of
the boar, to mention only a few.

In the carnivores (cats and dogs) one tooth in each jaw on
each side becomes enlarged and modified for tearing flesh,
forming the so-called carnassial tooth. It is the last premolar
(4th) in the upper jaw and the first molar in the lower jaw.
Other carnivores (bears and seals) do not have the carnassial
tooth well developed.

In addition to dentine and enamel, it is common for the
teeth of mammals to have a complete or partial covering of
bone which is called " cement." This may be restricted to the
roots of the teeth, as in man, or it may form a complete covering

THE TEETH 269

over the crown before the tooth is erupted, as in ungulates.
After the tooth has been erupted and projects above the gums,
it is subjected to wear, and its different constituents become
worn away according to their softness. The hardest substance
is the enamel, and next comes the dentine, and lastly the
cement which is the softest. The result of the unequal wear
in teeth like those of the elephant or of some rodents is that the
crown is not smooth but becomes ridged like a file, and such
teeth are as efficient as mill-stones grinding against one another.
In some of the Edentates (sloths and armadilloes) the teeth
have no enamel, while in others (ant-eaters) and in some whales
there are no teeth at all.

There is no difficulty in tracing the teeth of vertebrates
back to the denticles of the Selachians, and of some of the
Ostracoderms. It has been suggested that denticles also gave
rise to dermal bone by fusing together. This is very im-
probable. Denticles are composed not of bone but of dentine,
which differs from bone in that the cells which secrete it do
not remain in it but migrate out. Denticles are often found
attached to true scales or dermal bones, but these are developed
independently from the denticles.

It can be said that the dermal bones and scales develop in
relation to the denticles, but not from them.

The so-called teeth of Petromyzon, of Ornithorhynchus, and
of the tadpole of the frog are epidermal horny structures, and
have nothing whatever in common with the true teeth.

Literature

Bolk, L. Odontological Essays. Journal of Anatomy. Vol. 55, 1921 ;

Vol. 56, 1922 ; and Vol. 57, 1922.
Mummery, J. H. The Microscopic Anatomy of the Teeth. Henry

Frowde and Hodder & Stoughton, London, 191 9.

Tomes, C. S. A Manual of Dental Anatomy, Human and Comparative,
Churchill, London, 1808.

CHAPTER XIX

THE CCELOM AND MESODERM

In Amphioxus and all Craniates the most dorsal mesoderm is
segmented into somites. These each contain a portion of
coelomic cavity called myoccel, which persists in Amphioxus,
but becomes obliterated in higher forms. The median wall
of the myocoel is thickened and produces the myotome : a
plate of muscle with striated fibres, innervated by somatic
efferent fibres (voluntary) through the ventral nerve-roots.
The outer layer of coelomic epithelium lateral to the myocoel
gives rise to the dermatome or cutis-layer, beneath the skin.
On the median side, the myotome also produces the sclerotome.
In Amphioxus this is in the form of a hollow outgrowth, but
in higher forms it is composed of mesenchyme. It gives rise
in Craniates to the basidorsal and basiventral elements which
go to make up the vertebral column.

The dorsal segmented portion of the mesoderm is known
as the vertebral plate. The more ventral portion of the
mesoderm arises segmentally in Amphioxus, each segment
separated from the ones in front and behind by septa. These
septa, however, break down, and the ventral coelomic cavity or
splanchnocoel is continuous from end to end of the animal.
This condition arises from the first in the Craniates, where the
mesoderm in this region, known as the lateral plate, is not
segmented. The outer wall of the splanchnocoel becomes
applied to the body- wall, and the inner wall covers the gut-
wall. The separation between right and left splanchnocoel
usually breaks down ventrally, but persists dorsally as the
mesentery which suspends the gut. The muscles which the
coelomic epithelium of the splanchnocoel produces are smooth,

270

THE CGELOM AND MESODERM 271

involuntary, and innervated by the autonomic nervous system,
except for those which are situated in the anterior region of
the body, in connexion with the gill-slits. The gill-slits
pierce through from the gut to the outside in the region of the
lateral plate ; between the gill-slits, in the visceral arches, the
lateral-plate mesoderm gives rise to the muscles which move
the arches, including the jaws. These muscles are striated and
voluntary, but they are not myotomic, and they are innervated
by visceral efferent fibres through the dorsal roots of the
cranial nerves.

Between the myoccels and the splanchnocoels there are
typically little hollow stalks, through which at early stages the
cavities of the latter can communicate with those of the former.
They are segmental in arrangement. In Amphioxus, these
regions of the coelom represent the future gonads, and are
called the gonotomes with their cavities the gonocoels. In the
Craniates, they are called the nephrotomes (or intermediate
cell-masses) ; the cavities (communications between the
myocoels and the splanchnocoel) are the nephrocoels, and they
give rise to the tubules of the kidneys and associated structures,
eventually losing connexion both with myocoels and splanch-
nocoel.

In Amphioxus the splanchnocoel is continuous from end
to end of the body as in the Ammocoete, for the transverse
septum in which the ductus Cuvieri crosses over from the
body- wall to the gut- wall, is not large. In Selachians, the
transverse septum separates an anterior pericardial cavity from
a posterior peritoneal or perivisceral cavity, leaving only very
small communications between them in the form of the
pericardio-peritoneal canals. In higher forms the separation
between pericardial and perivisceral cavities is complete.
Beginning in the Dipnoi, the pericardium becomes thin- walled
and projects backwards into the perivisceral cavity.

All viscera are morphologically outside the coelomic cavity
and only suspended in it by a bag of coelomic epithelium which
forms a double membrane or mesentery. So the gut is
suspended by the dorsal mesentery from the roof of the
perivisceral cavity, and between the two membranes composing

272 COMPARATIVE ZOOLOGY OF CHORDATES

it there pass the arteries from the dorsal aorta to the gut. The
gut and liver are connected by the lesser omentum, through
which the bile-duct runs from the liver to the anterior portion
of the intestine. The lungs in amphibia are of course covered

Fig. 125. — Transverse section through the trunk of an embryo of Lacerta,
showing the relations of the coelom.

da, dorsal aorta ; dm, dorsal mesentery ; fl,
I, liver ; In, lung ; lo, lesser omentum ; nt
phi, pulmo-hepatic ligament ; phr, pulmo-

am, accessory mesentery ;
falciform ligament ; g, gut ;
notochord ; nc, nerve-cord ;
hepatic recess.

over by coelomic epithelium (pleura) which is continuous with
the ordinary lining of the perivisceral cavity round the stalk of
the lungs. In some reptiles, the coelomic epithelium covering
the lung is also attached to the roof of the perivisceral cavity
forming the accessory mesentery, and attached to the liver

THE CCELOM AND MESODERM

273

below by the pulmo-hepatic ligaments. On each side of the
dorsal mesentery therefore there is a recess, the pulmo-hepatic
recess, bounded on the median side by the dorsal mesentery
and stomach, laterally by the accessory mesentery and pulmo-

Fig. 126. — Transverse section through the trunk of a bird showing the
relations of the ccelom.
as, air-sac ; os, oblique septum ; pc, pleural cavity ; v, vertebra ; other
letters as Fig. 125.

hepatic ligament, and below by the liver. Owing to the
curvature of the stomach and the return of the anterior portion
of the intestine to form the loop of the duodenum, the pulmo-
hepatic recess of the right side comes to form a pocket, the

T

274 COMPARATIVE ZOOLOGY OF CHORDATES

omental cavity. This pocket communicates with the general
perivisceral cavity by an opening the front edge of which is
formed by the hind border of the accessory mesentery and
pulmo-hepatic ligament. Along this edge runs the inferior
vena cava. The hind edge of the opening is formed from the
dorsal mesentery and lesser omentum and along the latter run
the bile-duct, the portal vein and the hepatic artery. The
opening is the primitive foramen of Winslow.

In the birds, the conditions start similarly with regard to
the accessory mesenteries and the pulmo-hepatic ligaments,
but the latter in addition are connected to the side wall of the
perivisceral cavity. In this manner the oblique septa are
formed, which separate a pair of dorso-lateral pleural cavities
(into which the lungs project) from the perivisceral cavity.
The latter is further obstructed by the post-hepatic septum
which connects the gizzard to the floor of the cavity.

The mammals are characterised by the presence of the
diaphragm. This is formed partly from the transverse
septum which separates the pericardial cavity from the rest,
and partly from ccelomic epithelium in connexion with the
mesentery and the folds in which the kidneys hang down from
the roof of the perivisceral cavity. By this means the pleural
cavities (already separated from the pericardial by the trans-
verse septum) are separated from the remainder of the peri-
visceral cavity. But it is important to note that the pleural
cavities of the mammal are formed in an altogether different
manner from those of birds.

Anterior to the diaphragm in mammals therefore there are
three ccelomic spaces : the pericardium and the two pleural
cavities. The diaphragm contains striped myotomic muscles
innervated by the phrenic nerves. Originally the heart and
the transverse septum were far forward in the body in the region
of the neck, from the spinal nerves of which the phrenic nerve
arises. Later in development the heart and transverse septum
become shifted backwards, with the result that the phrenic
nerves have long courses to run from their origin in the neck
to the diaphragm. The diaphragm is pierced by the gut,
aorta, and the inferior vena cava.

THE CQSLOM AND MESODERM

275

In many mammals, the dorsal mesentery supporting the
stomach from the roof of the perivisceral cavity, becomes

Fig. 127. — Longitudinal section through the trunk of a mammalian embryo,
showing the relations of the ccelom and viscera.

a, anus ; au, auricle of heart ; b, bladder (continuous with allantois) ;
bd, bile-duct ; d, diaphragm ; e, liver ; gb, gall-bladder ; i, intestine ; In,
lung ; m, metanephric kidney ; oe, oesophagus ; p, pancreas ; pi, pleural
ccelomic cavity ; pn, penis ; pr, pericardial ccelomic cavity ; r, rectum ; sc,
spinal cord ; s?n, sternum ; sp, perivisceral splanchnoccel ; st, stomach ;
t, tail ; ta, truncus arteriosus ; th, thyroid ; tm, thymus ; tr, trachea ; ts,
testis ; u, ureter ; uo, opening of urethra ; v, ventricle of heart ; vc, verte-
bral column ; vd, vas deferens.

drawn out into a double sheet of ccelomic epithelium which
overlaps the transverse colon of the large intestine on the
ventral side. Eventually this sheet may fuse with the

276 COMPARATIVE ZOOLOGY OF CHORDATES

mesentery suspending the large intestine (mesocolon). This
extension, which is called the great omentum, brings about an
increase in size of the omental sac, on the wall of which fat
is often deposited.

The Mullerian or oviducts and the uterus are suspended by
mesenteries, called mesometria, and which are of interest in
determining the relation of the implanted blastocyst to the
walls of the uterus. The mesentery supporting the testis is
called the mesorchium, that supporting the ovary the meso-
varium.

Ccelomic cavities are always lined by mesodermal tissue.
In Amphioxus, the coelomic cavities of the somites, when they
arise, are in open communication with the gut, and are hence
known as enteroccels. In higher forms, the coelomic cavities
appear as splits in the mesoderm, without communicating with
the gut. These cavities are known as schizoccels. The method
of origin is not of much importance, but it is important to
realise that all cavities which arise, either as subdivisions of,
or outgrowths from, enteroccels and schizoccels, are ccelomic.
So the cavities of the pericardium, of the kidney- tubules, of
the Wolffian and Mullerian ducts, of the gonads in Amphioxus,
are ccelomic. On the other hand, no cavity is coelomic which
does not arise in this way. The cavities of the blood-vessels
are not coelomic, although their walls are composed of meso-
dermal tissue. Cavities lined by tissue other than mesoderm,
such as those of the atrium of Amphioxus, nerve- tube,
nephridia, amnion, gut, or blastocoel, are, of course, not coelomic.

The coelomic cavities originally probably opened to the
outside in each segment for the purpose of freeing the germ-
cells. Something like this happens in Amphioxus, where also
the left anterior head-cavity opens into the preoral pit. Rarely,
in higher forms, the cavities of the premandibular somites
may open into the hypophysis. Comparable " proboscis-
pores " (see p. 360) occur in Balanoglossus and its allies, and
in the Echinodermata. In the Craniata, the splanchnocoel
may communicate with the outside, through the genital pores
via the cloaca as in Petromyzon, through the abdominal pores
as in Scyllium, or through the Mullerian ducts.

THE CCELOM AND MESODERM 277

Mention must be made of the fact that in some cases the
electric difference of potential which always accompanies
muscular activity has been specially increased, with the result
that some muscles have been converted into " electric organs."
It is interesting to notice that while in Raia it is the somatic
(myotomic) muscles in the region of the tail that have become
thus modified, in Torpedo it is the visceral muscles derived
from the visceral arches.

The first three pairs of somites (in the Craniates) are small
and give rise to the extrinsic eye-muscles. From the fact that
they are situated in front of the ear, they are known as prootic
somites, and their development is described in Chapter XXVIII.
The myotomes which are produced from the next posterior
(or metotic) somites are divided by the gill-slits into dorsal
and ventral portions, the latter portion forming the hypoglossal
muscles.

In Amphioxus, each myotome is a plate of muscle extending
from near the middorsal to the midventral line, on one side of
the body. When seen from the side, each myotome is bent
into the shape of a V with the apex pointing forwards. In
Petromyzon, the myotomes behind the region of the gill-slits
are like those of Amphioxus ; only the septa are slightly more
bent so that each myotome seen from the side is in the form of
a W. In fish and all higher forms, however, each myotome
behind the gill-slits is divided into two by a horizontal parti-
tion or septum. It is in this septum that the " true "
or dorsal ribs are formed. The myotomes are then repre-
sented by dorsal or epaxonic, and by ventral or hypaxonic
muscles.

The muscles of the fins in Craniates are formed from
" muscle-buds," which are nipped off from the myotomes.

In the Tetrapods, the epaxonic muscles are much reduced,
while the hypaxonic muscles assume greater importance. The
muscles of the paired fins and limbs are derived from the
hypaxonic portions of the myotomes. Apart from the latter,
the great development of which in Tetrapods is connected
with the greater strength necessary for locomotion on dry land,
the hypaxonic portions of the myotomes also give rise to the

278 COMPARATIVE ZOOLOGY OF CHORDATES

intercostal muscles in the thorax, and the muscles of the
abdominal wall.

The simple segmental arrangement of the myotomes which
is so characteristic of Amphioxus and lower Craniates, tends
to be obscured and lost in the Tetrapods.

With regard to the dermal musculature, the muscles
attached to the hair-follicles (arrectores pili) are smooth and
innervated by sympathetic fibres. The panni cuius carnosus
muscles are derived from the striped muscles of the trunk and
are therefore innervated by ventral nerves. The platysma
muscles, and the muscles of expression are derived from the
striped muscles of the 2nd visceral arch, and consequently
are innervated by the facial nerve.

Literature

Maurer, F. Die Entwicklung des Muskelsystems und der elektrischen
Organe. Hertwig's Handbuch der verg. und exp. Entwicklungslehre
der Wirbeltiere. Part 3. Fischer, Jena, 1906.

VAN Wijhe, J. W. Ueber die Mesodermsegmente und die Entwicklung der
Nerven des Selachierkopfes. de Waal, Groningen, 1915.

CHAPTER XX

THE SKULL

The skull consists of the protective case round the brain
(neurocranium) and of the skeletal supports of the jaws
(splanchnocranium). It is formed in all chordates from
Petromyzon upwards (whence the name Craniate) and is always
cartilaginous at first. In Cyclostomes and Selachians the
skull remains cartilaginous throughout life, but in other forms
this cartilaginous chondrocranium becomes more or less
thoroughly replaced by cartilage-bone, and membrane-bones
are added to it. The chondrocrania of the various vertebrates
may be compared with one another on the one hand, and on
the other, the bony skulls may similarly be compared.

Cartilaginous Skull. — The typical structure of the chondro-
cranium may now be considered. The floor of the neuro-
cranium is formed of paired trabecular in front (enclosing the
hypophysial fenestra between them) and of paired parachordals
(on each side of the notochord) behind. The auditory capsules
are firmly anchored on to the parachordals on each side.
Behind the auditory capsules the paired occipital arches rise
up from the parachordals, and become attached to the hind
part of the auditory capsule. In so doing they enclose a
fenestra (metotica) through which the glossopharyngeal and
vagus nerves and the internal jugular vein pass. In front of
the auditory capsule paired pillars rise up from the para-
chordals and join on to the orbital cartilages. The latter
form the sides of the brain-case in front of the auditory capsules,
and the pillars just mentioned are the pilae antoticae. The pila
antotica joins the front part of the auditory capsule of its own
side, and in so doing encloses the trigeminal, facial, and

279

28o COMPARATIVE ZOOLOGY OF CHORDATES

abducens nerves in a fenestra prootica. In front of the pila
antotica the optic, oculomotor, and trochlear nerves, and the
pituitary vein pass.

The olfactory or nasal capsules are formed at the front of
the skull. They are separated from one another by the inter-
nasal septum formed from the trabecular which anteriorly

THE SKULL

281

join together in the middle line. The nasal capsule is separated
from the orbit of its side by the lamina orbito-nasalis, which
reaches from the trabecula to the orbital cartilage. The roof
is often very incomplete, and may be formed only in front and
behind. That part of the roof which connects the two
auditory capsules is called the tectum synoticum.

The relations of the trabecular are of importance, for the
hypophysial fenestra which they enclose between them also

PQ,

Fig. 129. — Diagram of a schematic chondrocranium seen from the left side,
and showing the relations of the cartilages to the principal nerves and
blood-vessels.

This diagram does not represent any particular form, but shows the type
on which nearly all skulls are built, abn, abducens nerve ; ac, auditory
capsule ; at, ala temporalis ; btp, basal process ; hf, hyomandibular facial
nerve ; ic, internal carotid artery ; jv, jugular vein ; Ion, lamina orbito-
nasalis ; 0, occipital arch ; oa, orbital artery ; oc, olfactory capsule ; ocn,
oculomotor nerve ; oln, olfactory nerve ; on, optic nerve ; op, otic process ;
pa, ascending process ; pf, palatine facial nerve ; pp, pila antotica ; pq,
ptery go-quadrate ; pv, pituitary vein ; rop, profundus ophthalmicus nerve ;
tc, trabecula ; vn, vagus nerve.

serves for the admission of the internal carotid arteries to the
brain-case. In those cases where the trabecular are wide
apart from one another, as in the frog, the skull is said to be
platytrabic (or platybasic) ; in others, such as the trout, the
trabecular are close to one another and fuse in the middle line,
and this condition is called tropitrabic (or tropibasic).

The splanchnocranium consists of the pterygo- quadrate
of the upper jaw, Meckel's cartilage of the lower jaw, the

282 COMPARATIVE ZOOLOGY OF CHORDATES

hyomandibula and ceratohyal in the hyoid arch, and the
cartilages of the branchial arches.

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One of the most important features of a skull is the method
by which the splanchnocranium is attached to the neuro-
cranium. The hyomandibula is always firmly attached to the

THE SKULL 283

auditory capsule, but with regard to the jaws, there are three
types of attachment :

Ampkistylic, as in the dogfish Hexanchus, and in Cladose-
lache. Here the upper jaw has an otic process which abuts
against the auditory capsule, and in addition the hyomandibula
serves to sling the upper jaw from the neurocranium.

Hyostylic, as in Scyllium. The upper jaw nowhere touches
the auditory capsule, and is suspended by the hyomandibula,
and ligaments.

Autostylic, as in Ceratodus and higher vertebrates. The
hyomandibula takes no share in the suspension of the upper
jaw, which is attached to the neurocranium by its own
processes.

The processes of attachment of the upper jaw in autostylic
skulls are typically three in number. The otic process abuts
against the auditory capsule, and lies in front of the main
branch of the facial nerve ; the basal process abuts against the
floor of the neurocranium, and lies above and in front of the
palatine nerve (facial) ; the ascending process rises up on the
outside of the pila antotica with which it may or may not join,
and lies between the ophthalmic (Vi) and the maxillary (V2)
branches of the trigeminal nerves.

The autostylic vertebrates above Ceratodus are terrestrial
animals which no longer use the gill-slits for respiratory
purposes in the adult. So the spiracular cleft gives rise to the
tympanic cavity and Eustachian tube, and the hyomandibula
becomes the columella auris.

This description of the typical chondrocranium can be
applied to most groups of vertebrates. In the mammals an
important modification occurs in that the ascending process
comes to lie between the maxillary (V2) and mandibular (V3)
branches of the trigeminal nerve, and it is usually known as the
ala temporalis.

Bony Skull. — Attention may now be turned to the bony
skull. The replacing, or cartilage-bones, are fairly constant
throughout the vertebrate series. In the neurocranium they
surround the brain, the olfactory and auditory capsules ;
while in the splanchnocranium they form the main skeletal

284 COMPARATIVE ZOOLOGY OF CHORDATES

supports. The dermal or membrane-bones form a covering
just beneath the skin, and in certain regions they line the
mouth-cavity. The external covering of membrane-bones is
primitively complete, as in Osteolepid fish, and several of them

ppCU

Fig. 131. — Osteolepis : dorsal view of a skull (in the collection of Prof.
D. M. S. Watson, F.R.S.), showing the course of the lateral-line canals.
Explanation of lettering for Figs. 131 to 149 :

al, alisphenoid ; art, articular ; bo, basioccipital ; bpp, basipterygoid
process ; bs, basisphenoid ; c, canine ; d, dentary ; en, external nostril ;
eo, exoccipital ; ep, epipterygoid ; /, frontal ; fm, foramen magnum ;
i, incisor ; it, intertemporal ; /, jugal ; /, lachrymal ; Is, laterosphenoid ;
m, maxilla ; mp, mastoid process ; n, nasal ; 0, orbit ; oc, occipital con-
dyle ; on, otic notch ; 00, opisthotic ; op, opercular ; os, orbitosphenoid ;
p\, fourth premolar ; pa, parietal ; pe, periotic ; pf, postfrontal ; pi, pineal
foramen ; pi, palatine ; pm, premaxilla ; po, postorbital ; pop, preopercular ;
pp, paroccipital process; ppa, postparietal ; pr, prootic ; prf, prefrontal;
ps, parasphenoid ; psp, presphenoid ; pt, pterygoid ; pv, prevomer ;
q, quadrate ; qj, quadratojugal ; s, squamosal ; sm, septomaxilla ; sn,
spiracular notch ; so, supraoccipital ; st, supratemporal ; t, tabular ;
tb, tympanic bulla ; tp, transpalatine ; v, vomer.

are traversed by the canals of the lateral-line system. In these
forms, the only openings in the roof of the skull are the orbits,
the chinks through which the spiracles opened, and the median
hole for the pineal eye which in fish is situated between the
frontal bones. In the higher bony fish or Teleostei, it is

THE SKULL

285

common to find that some of the rectus eye-muscles pass back
into a tunnel beneath the brain-case ; the so-called eye-
muscle-canal or myodome.

In the most primitive amphibia, the membrane-bones also
make a complete covering to the skull, for which reason these
animals are called Stegocephalia. Many of these bones can
be identified with those of Osteolepid fish because they are

*pm.

pc.^

Fig.

132. — Stegocephalian (Loxomma) : dorsal view of a skull, showing
the course of the lateral-line canals. (Drawn from a cast.)

grooved by the lateral- line system. The only openings in the
roof of the skull in the Stegocephalia are the nostrils, the orbits,
and the median pineal foramen which in these animals lies
between the parietal bones. The spiracles are, of course,
closed in land-vertebrates, but the position of their former
openings is indicated by a notch in the hind border of the roof
on each side.

In land-vertebrates the skull and vertebral column are

286 COMPARATIVE ZOOLOGY OF CHORDATES

separated by a joint, which allows the head to move. The
articular facets belonging to the skull which take part in this
joint are the condyles. In Stegocephalia there are three such
condyles, formed by the two exoccipitals and the basioccipital.
In higher forms, as will be seen, the number of condyles may
be reduced to one or to two, according as to whether the
exoccipitals or the basioccipital (respectively) drop out of
sharing in the joint.

In the most primitive reptiles such as Seymouria, the
covering of dermal bones is complete, and differs from the

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condition of the Stegocephalia only in that there are no grooves
for lateral-line canals. As the nature of the roof of the skull
is of the greatest importance in regard to classification in the
reptiles, it is necessary to consider a few of the relations which
the membrane-bones bear to underlying structures. The
more median membrane-bones, such as the nasals, frontals, and
parietals, overlie the brain-case directly, and form its roof.
But the more lateral membrane-bones of the skull-roof, such
as the postorbital, supratemporal, and squamosal lie over the
auditory capsules. The auditory capsule, formed by the

THE SKULL

287

prootic, and opisthotic
(cartilage-) bones lies
deep beneath the sur-
face of the skull, with
the result that between
it and the overlying
membrane-bones of the
skull-roof there is a
space. This space is
the temporal cavity ;
it is continuous in front
with the orbit or eye-
ball-space, and pos-
teriorly the temporal
cavity opens on the
hind face of the skull
by the post-temporal
fossa. It must be
remembered that the
word " cavity " is here
used to denote a space
which is not occupied
by bone ; it is, how-
ever, not hollow, but "*
filled by the muscles of
mastication which b

q:

actuate the lower jaw.
Below, the temporal
cavity opens on to the
palatine surface of the
skull, in front of the
auditory capsule, and
through this opening
the above-mentioned
muscles pass. The
roof of the temporal
region typically has
three borders : an

288 COMPARATIVE ZOOLOGY OF CHORDATES

anterior border which is also the hind border of the orbit ;
a lower border, reaching from the maxilla to the quadrate ;
and a posterior border which is also the upper border of the
post-temporal fossa.

The most primitive reptiles or Cotylosaurs of which
Seymouria is an example, are characterised by skulls of this
type, in which the temporal cavity is completely roofed over ;
a condition inherited from the Stegocephalian ancestors.

In the Chelonia probably the skull was primitively of this
kind also, and Chelone is a good example of a skull with a
temporal cavity completely roofed over, opening behind by a
post-temporal fossa.* In other forms of tortoises and turtles,
however, the roof over the temporal cavity becomes reduced
by a process known as emargination. The skull-roof becomes
as it were eaten away from the edge, and this reduction may
affect the hind border or the lower border of the roof of the
temporal region, or both. When reduction by emargination
has taken place, the prootic and opisthotic bones of the auditory
capsule become visible from the dorsal side of the skull. It is
important to notice that in emargination there is no perforation
of the skull-roof.

It is common to find the Cotylosaurs and the Chelonia
grouped together as Anapsida, since they have skulls completely
roofed-over or sometimes emarginated, but never perforated
as regards the roof by apertures other than the orbits and
nostrils. These forms usually have three condyles.

The remaining vertebrates are characterised by the fact
that the roof of the skull in the temporal region has been
perforated, with the result that windows are formed, completely
surrounded by bone, and opening into the temporal cavity.
A window of this kind is called a temporal fossa or vacuity, and
it enables the muscles of mastication to become enlarged.
Through the window the auditory capsule is visible. It must
be clearly understood that a temporal fossa is only a perforation

* It should be mentioned that some authorities prefer to regard the
complete roofing of Chelone as secondarily developed. This is immaterial
for the present purpose, which aims only at pointing out the typical
relations of the temporal region of the skull.

THE SKULL

289

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29o COMPARATIVE ZOOLOGY OF CHORDATES

in the roof of the temporal cavity, it is not an opening into the
brain-case.

Some reptiles have a single temporal fossa on each side.
Others have a pair on each side, for which reason they are
called the Diapsida. The Diapsida have a superior and an
inferior temporal fossa, and these fossae are separated from the
orbit by the post-orbital bar (usually formed by the post-
frontal and post-orbital bones) ; they are separated from the
post-temporal fossa by the post-temporal bar (supratemporal
and squamosal bones) ; and they are separated from one
another by the superior temporal bar (post-orbital and squa-
mosal bones. The superior temporal fossa is bordered above
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below by the inferior temporal bar (jugal, quadrato-jugal,
and squamosal bones).

The Diapsida include the Rhynchocephalia of which
Sphenodon is an example, the Crocodilia, the Dinosauria, the
Pterosauria, and the birds. In the latter, however, the post-
orbital and temporal bars have been broken, with the result
that the temporal fossae can no longer be clearly recognised.
It can nevertheless be seen that the bird's skull must have
been derived from a Diapsid type which had the typical two
temporal fossae. In the primitive crocodiles, in the Pterosaurs,
Dinosaurs, and birds, there is also a prelachrymal fossa on each
side, between the orbit and the nostril. The condyle is usually
single in the Diapsida.

THE SKULL

291

The remaining reptiles
side, but whereas in some
temporal fossa, in others
fossa of Diapsida.

have a single temporal fossa on each
this would seem to be the superior
it represents the inferior temporal

Forms with a single inferior temporal fossa on each side
are called Synapsida, including the Theromorph reptiles and
the mammals. The inferior temporal fossa is primitively
bounded above by the post-orbital and squamosal bones. In

292 COMPARATIVE ZOOLOGY OF CHORDATES

the higher forms, however, it often happens that the post-
orbital and squamosal bones no longer touch one another.
The result of this is that the inferior temporal fossa now
extends up between them and is bordered above by the parietal
bone. From the mere fact that it touches the parietal it must
not be mistaken for a superior temporal fossa. This enlarged
type of inferior temporal fossa is present in the higher Thero-
morph reptiles, and in the mammals. A fossa of this type is also
found in the Sauropterygia, or Plesiosaurs. Here again,
although the fossa is bordered by the parietal, it is probably
an inferior temporal fossa which has extended in the manner
just described. For this reason, the Sauropterygia are usually
classed as Synaptosauria, close to the Synapsida. Synapsida
usually have two condyles.

The Parapsida have a single superior temporal fossa on
each side, lying above the post-orbital and squamosal bones,
and the supra-temporal bone appears to have been retained.
To this group belong the Ichthyosaurs and the Squamata,
which latter consist of the Lacertilia and Ophidia. In the
Lacertilia, the bar beneath the lateral temporal fossa has been
much reduced by emargination from below.* The result of
this is that there is very little roofing left over the temporal
region, and the quadrate, which still retains the otic process
abutting against the paroccipital process of the auditory capsule
(see p. 104), becomes uncovered and loose. The quadrate is
therefore capable of movement relatively to the squamosal
and to the brain-case. This condition, which is called strepto-
stylic, is associated with the fact that the upper jaw can move
relatively to the brain-case, which arrangement enables the
animal to open its mouth with a gape wider than would
otherwise be possible.

An extreme case of the streptostylic condition is found in
the Ophidia or snakes. Here the postorbital bar and the
temporal bar are completely broken down, so that the temporal
region is uncovered. The quadrate has lost its connexion
with the auditory capsule, and is only indirectly articulated

* Some authorities prefer to regard the Lacertilia as derived from
Diapsida which have lost the inferior temporal bar.

THE SKULL

293

with it by the intermediary of the squamosal. When a snake
opens its mouth the lower jaw drops and the quadrate moves
forward. This movement of the quadrate is imparted to the
pterygoid and transpalatine bones, which, moving forward in
their turn, cause the maxilla and associated bones to rotate

n pjf. f. R^

Fig. 145. — Left side view of the skull of a snake (puff adder) showing the
streptostylic condition of the jaws.. A, with the mouth closed ; B,
with the mouth open.

upwards. In some snakes such as the viper, this process of
rotation of the maxilla is especially interesting, for the maxilla
carries the long teeth which are modified into poison fangs.
When the mouth is open these poison-fangs are made to
project forwards out of the mouth ready for " striking " ;
whereas in the normal position of rest with the jaws closed,

294 COMPARATIVE ZOOLOGY OF CHORDATES

the fangs extend back parallel to and beneath the roof of the
mouth.

Strep tostylic skulls are also found in the birds, and especially
in the parrots. Here the upper beak is freely movable relatively
to the brain-case. Some of the Dinosaurs also had strep to-
stylic skulls.

When the quadrate is fixed, and the upper jaw is incapable
of separate movement (as in Sphenodon, crocodiles, and
mammals, for instance), the skull is described as monimostylic.

In many Theromorph reptiles, as in mammals, it is common
for the post-orbital bar to disappear, and the temporal fossa

Fig. 146. — Hind view of the skull
of Chelone, showing the rela-
tions of the post-temporal fossa
(through which an arrow is
passed into the orbit).

Fig. 147. — Hind view of the skull
of Ornithorhynchus, showing the
small post-temporal fossa, indi-
cated by an arrow.

then becomes confluent with the orbit. In Ornithorhynchus,
for example, the temporal fossa has extended upwards in the
manner described above in other Synapsida, and its upper
border is formed by the parietal. It is bounded behind by
the squamosal, below by the squamosal and jugal (forming the
zygomatic arch), and in front it has no border since it is
confluent with the orbit. Behind, the temporal fossa of
Ornithorhynchus communicates with a small post-temporal
fossa between the squamosal and the auditory capsule (periotic
bone) and which opens on the hind face of the skull. This is
the last appearance of the post-temporal fossa, for in higher
mammals it is obliterated, as, for instance, in the dog.

THE SKULL

295

In the higher Primates including man, the post-orbital bar
not only persists but actually extends inwards, forming a
complete partition between the orbit and the temporal fossa.
It may be mentioned that the alisphenoid bone of the mammal
is an ossification of the ala temporalis, which corresponds
roughly to the ascending process of the ptery go- quadrate of
the reptile. The mammalian alisphenoid therefore represents

Fig. 148.

-Palatal view of a skull
of Varanus.

Fig. 149. — Palatal view of a skull of

a dog, showing the false palate.

An arrow is passed through the
nasal passage.

the reptilian epipterygoid,both being really parts of the splanch-
nocranium. It follows that the so-called " alisphenoids " of
fish, reptiles, and birds, which are ossifications of the primitive
wall of the brain-case, have nothing in common with the
mammalian alisphenoid. Their proper name is laterosphenoid.
In birds as in mammals, the brain has undergone great develop-
ment and enlargement, and so it happens that in the bony

296 COMPARATIVE ZOOLOGY OF CHORDATES

skull certain bones come to form part of the wall of the brain-
case although primitively they had nothing to do with it.
This applies to the mammalian alisphenoid, and to the
squamosal in birds and mammals.

Primitive forms have a large number of bones on the
palatal surface of the skull. The pterygoids, of which the fish
have three on each side, become more and more reduced in the
higher forms. The transpalatine of amphibia and reptiles
corresponds to the ectopterygoid of the fish, and it disappears
in birds and mammals. In the Tetrapods the pterygoid is a
membrane-bone, underlying the pterygo-quadrate cartilage.

The articulation of the pterygoid with the basipterygoid

Pp.*

Fig. 150. — Side views of the lower jaws of A, Varanus ; B, a dog.
The mammalian lower jaw contains only one bone ; the dentary. an,
angular ; ar, articular ; c, coronoid ; d, dentary ; ml, first molar ; par,
supra-angular.

process of the basisphenoid, as for instance in Varanus, corre-
sponds roughly to the connexion between the pterygo-
quadrate cartilage and the brain-case by the basal process.

In crocodiles, some Theromorph reptiles and in mammals,
the maxillae and palatines have shelf-like extensions which
meet in the middle line beneath the original roof of the mouth.
These shelves are the false palate, and between it and the
original roof of the mouth (formed by the vomer and meseth-
moid bones) is the nasal passage leading from the external
nostrils to the secondary choanae. The prevomers of the
lower vertebrates are represented by the " dumb-bell-shaped

THE SKULL 297

bone " of Ornithorhynchus. The mammalian vomer repre-
sents the anterior part of the parasphenoid of lower forms.

In the lower jaw, Meckel's cartilage ossifies as the articular,
and dermal bones are formed round it. In the lower verte-
brates these dermal bones are numerous, consisting in the
Stegocephalia, for instance, of the dentary, angular, supra-
angular, splenial, and three coronoid bones. The number of
these bones becomes reduced in higher forms. The two
halves of the lower jaw in snakes are separate, and their front
ends can be moved wide apart. This allows the mouth to be
opened very wide indeed, so that the snake is capable of
swallowing relatively enormous prey. In some Lacertilia such
as Varanus and in the extinct Mosasauria, there is a joint on
each side of the lower jaw. These joints enable the space
between the two halves of the lower jaw to be widened, and
large prey to be swallowed.

In crocodiles, and in the fossil bird Archaeopteryx, the
lower jaw is characterised by being pierced by a foramen on
each side. Among the Dinosauria, the Predentata are peculiar
in possessing a predentary bone, the most anterior in the lower
jaw. The lower jaw of the Marsupials is characterised by the
fact that the lower edge of the hindmost region of each half is
bent inwards, forming the " inflected angles."

In all mammals the lower jaw is peculiar in consisting of a
single bone : the dentary. Very interesting stages in the
reduction in number of bones are found in the Theromorph
reptiles. Cynognathus has a large dentary, while the articular,
angular, supra- angular, prearticular, coronoid, and splenial are
small. The dentary develops an uprising coronoid process
which touches the squamosal, and so takes on the function of
articulating the lower on to the upper jaw. At the same time
the original quadrate-articular articulation (which is present
in all lower forms) falls into disuse, and the quadrate becomes
small and loose. The next stage is that of the mammals, of
which the dog may be taken as an example ; and since here the
lower jaw consists of the dentary alone, the question arises as
to what has happened to the other bones. The quadrate and
articular have been intercalated between the columella auris

298 COMPARATIVE ZOOLOGY OF CHORDATES

and the tympanic membrane, thus forming part of the chain
of three auditory ossicles which is characteristic of mammals.
The columella auris (or hyomandibula), pierced by the
stapedial artery, comes to look like a stirrup and is hence called
the stapes ; the quadrate is now called the incus, and the
articular becomes known as the malleus. At the same time
the angular becomes converted into the tympanic bulla (also
peculiar to mammals) and the supra-angular is represented by
the processus Folii ; the remaining bones of the Theromorph
reptiles have disappeared. It is a striking fact that the

Fig. 151. — Diagrammatic views showing the transition from the reptilian
to the mammalian method of articulation of the lower jaw : A, reptile ;
B, mammal.

a, angular ; art, articular ; ca, columella auris ; d, dentary ; t, incus
(quadrate) ; m, malleus (articular) ; Mc, Meckel's cartilage ; pt, processus
Folii (supra-angular) ; q, quadrate ; s, squamosal ; sa, supra-angular ; st,
stapes (columella auris) ; /, tympanic (angular).

mammalian ear is associated with bones which in the ancestors
served to form the articulation between the upper and lower
jaws. The remarkable change of function which these bones
have undergone is, however, less remarkable than would
appear at first sight, for their essential feature is that they
remain articulated to one another, and so are able to transmit
the vibrations of sound. The columella auris is pierced by
an artery and resembles the stapes in certain lizards and
Gymnophiona, and in the latter group of animals it may be
connected with the quadrate. There is therefore no radical
innovation in the fact that the incus articulates with the stapes.

THE SKULL 299

The most remarkable feature of this change is the fact that it
was effected without functional discontinuity. There was
never a time when, the quadrate and articular having been
drawn away into the service of the ear, the lower jaw had no
articulation with the upper, because, before this happened, the
dentary had already established connexion with the squamosal.
This new squamoso-dentary method of articulation, which is
peculiar to mammals, was in working order before the quadrate
and articular underwent their modification.

Literature

Gaupp, E. Die Entwicklung des Kopfskelettes. Hertwig's Handbuch
der Verg. und Exp. Entwickelungslehre der Wirbeltiere. Part 3.
Fischer, Jena, 1906.

Goodrich, E. S. Vertebrata Craniata. Cyclostomes and Fishes. Black,
London, 1909.

Gregory, W. K. Present Status of the Problem of the Origin of the

Tetrapoda. Annals of the New York Academy of Sciences. Vol. 26.

1915.
Huxley, T. H. Contributions to Morphology. Ichthyopsida. On

Ceratodus forsteri. Proceedings of the Zoological Society of London,

1876.

Williston, S. R. The Osteology of the Reptiles. Harvard University
Press, 1925.

3oo COMPARATIVE ZOOLOGY OF CHORDATES

TABLE OF VERTEBRATE BONES

Cartilage from

Part of Skeleton.

Membrane-bones.

Cartilage- (and
tendon-) bones.

Bones of
mixed
origin.

which the car-
tilage-bone, or
part of the
mixed bone os-
sifies.

Skull

Nasal
Frontal

Basioccipital

Basal plate
(Parachordal)

Parietal

Basisphenoid

Basal plate

Postfrontal=-part of

(Parachordal)

Sphenotic

Presphenoid

Trabecular plate

Lachrymal

Exoccipital

Occipital arch

Suborbitals

Supraoccipital

Tectum synoticum

Postorbital

Intertemporal = part

Prootic ) Peri-
Opisthotic 3 otic

Auditory capsule

of pterotic ?

Auditory capsule

Supratemporal

Epiotic

Post-temporal

Neuro- /

Squamosal

tendon

cranium \

Postparietal

Tabulare

Prevomer

Mesethmoid

Trabecular inter-
nasal septum,
and nasal capsule

Parasphenoid

Laterosphenoid

Pila antotica

Vomer = part of

Orbitosphenoid

Orbital

Parasphenoid

Turbinals

Turbinals

Basitemporal = part of

Parasphenoid

Prefontal

Lamina orbito-

i

Septomaxillary

Sphenotic
Pterotic

nasal is
Auditory capsule

Auditory capsule

Premaxilla

Quadrate = Incus

Pterygo-quadrate

Maxilla

Metapterygoid

Pterygo-quadrate

Jugal

Epipterygoid

Processus ascen-

Quadrato-jugal

dens

Ectopterygoid = trans-

Alisphenoid

Ala temporalis

palatine

Articular = Malleus

Meckel's

Endopterygoid

Hyomandibula

Pterygoid == Endo-

= Columella auris

pterygoid ?

= Stapes

Dentary

Symplectic

Splanchno- ,

Predentary

Epihyal ;

Hyoid arch

cranium

Angular = Tympanic
Supra-angular = Pro-
cessus Folii
Coronoids
j Splenial

Ceratohyal

Hypohyal

Interhyal

Basihyal /

Pharyngobranchial j

Epibranchial

Ceratobranchial

1 Branchiostegal rays

! Preopercular

Branchial arch

; Opercular

Hypobranchial

! Subopercular

Basibranchial /

i Interopercular

|

\

Palatine

Pterygo-quadrate

TABLE OF BONES

301

Part of Skeleton.

Membrane-bones.

Cartilage- (and tendon-)
bones.

Cartilage from which the
Cartilage-bones ossify.

Axial skeleton

Neural arch

Basidorsal

(Vertebral Column,

Haemal arch

Basiventral

etc.)

Hypocentrum

Basi ventral

Pleurocentrum

Interdorsal and inter-
ventral

Radials

Radials

Ribs

Costals

Appendicular skele-

ton (Limbs, Girdles,

and Sternum)

Post-temporal

Scapula |

Supracleithrum

Coracoid >

Scapulo-coracoid

Postcleithrum

Precoracoidl

Cleithrum

Humerus

Humerus

Clavicle

Radius

Radius

Ulna

Ulna

Radi ale = Scaphoid \

Intermedium = Lunar

Ulnar e = Cuneiform

Centrale \
Trapezium /

Carpals

Trapezoid

Magnum

Unciform /

Metacarpals

Metacarpals

Phalanges

Phalanges

Ilium

Bium

Ischium

Ischium

Pubis

Pubis

Epipubic =* Marsupial

Epipubic

Femur

Femur

Tibia

Tibia

Fibula

Fibula

Tibiale )=Astra-\
Intermedium/ galus ?

Fibulare = Calcaneum

Centrale = Navicular s

Tarsals

Endocuneiform 1

Mesocuneiform

Ectocuneiform

Cuboid )

Metatarsals

Metatarsals

Radials

Radials

Sternum

Sternum

Sesamoids and patella

Tendons

Dermal skeleton

Lepidotrichia
Osteoscutes

-

Gastralia

CHAPTER XXI

THE VERTEBRAL COLUMN, RIBS, AND STERNUM

The primitive skeletal stiffening of the body is the notochord.
In Amphioxus this extends to the extreme anterior end of the
body ; in Petromyzon it does not reach further forwards than
the region of the infundibulum, but in this position it persists
throughout life. In the remaining vertebrates, the notochord
usually disappears in the skull region.

Surrounding the notochord are two sheaths, the so-called
elastica interna and the elastica externa. These are of import-
ance in some forms in connexion with the formation of the
vertebral column.

Amphioxus has no structures comparable to vertebrae,
but they appear first in Petromyzon in the form of little paired
pegs or struts on each side of the nerve-cord, rising up from the
notochord. There are two pairs of these pegs to each segment
as a rule. The notochord in Petromyzon is continuous and
unconstricted, a primitive feature.

A properly formed vertebral column appears first in the
Selachii. Each vertebra is composed of a neural arch formed
from a pair of basidorsals, and a pair of basiventrals which in
the region of the tail form a haemal arch. Between them, the
basidorsals and basiventrals form the body of the vertebra or
centrum, which constricts the notochord, and usually obliterates
it altogether except between one centrum and the next.
Alternating with the basidorsals are the interdorsals, and in
some, interventrals are present. The basidorsals and basi-
ventrals perforate the elastica externa and cartilage-cells invade
the notochordal sheath. Such centra are called chordal, and
they occur in the Selachii, in the sturgeons and in the

302

VERTEBRAL COLUMN, RIBS, AND STERNUM 303

Dipnoi. In all other forms the vertebrae arise outside the
notochord and do not invade its sheath. These are called
perichordal vertebrae.

The vertebrae of the higher bony fish are compact bony
structures obliterating the notochord. Amia is interesting
with regard to its vertebral column, for in the region of the
tail there are what look like vertebrae with neural and haemal
arches alternating with vertebrae without. Those vertebrae
with the neural and haemal arches are the basidorsals and basi-

FiG. 152. — Transverse sections through the developing vertebral column of

Scy Ilium embryos.

A, early stage ; B, late stage (in the region of the tail), b d, basidorsal
bv, basiventral ; ca, caudal artery ; cvy caudal vein ; ee, elastica externa ;
ha, haemal arch ; hs, haemal spine ; n, notochord ; na, neural arch ; nc,
nerve-cord ; ns, neural spine.

ventrals ; those without are the interdorsals and interventrals.
The neural arches are always formed from basidorsals and the
haemal arches from basiventrals.

The vertebrae are formed from the sclerotome, which is
segmented. The anterior part of the sclerotome in each seg-
ment gives rise to the interdorsals and interventrals, while
the posterior part produces the basidorsals and basiventrals.
Later it is found that the basidorsals and basiventrals of one
segment fuse on to the interdorsals and interventrals of the

304 COMPARATIVE ZOOLOGY OF CHORDATES

next posterior segment. The vertebrae are therefore inter-
segmental in position, which enables the myotomes, which of
course are intrasegmental, to be attached to two vertebrae.

The most posterior haemal arches are enlarged to form the
hypurals which support the ventral lobe of the tail-fin.

In the most primitive amphibia, the Embolomeri of the
Labyrinthodonts, the vertebrae have neural arches and two

id n<k

Gfi K/"

he. pc. D D

Fig. 153. — Origin of the vertebral column, A, in Scyllium ; B, diagram
showing the relations of the vertebral elements to the nerves ; C, the
vertebral column in the tail-region of Amia ; D, the vertebral column
in the Embolomerous Stegocephalia.

dr, dorsal nerve-root ; he, hypocentrum ; id, interdorsal ; iv, inter-
ventral ; m, myotome ; pc, pleurocentrum ; vr, ventral nerve-root. Other
letters as Fig. 152.

centra. The anterior centrum of each vertebra is formed from
the basiventrals and is called the hypocentrum ; the posterior
centrum is formed from the interdorsals and interventrals,
and is called the pleurocentrum. While in the later amphibia
the hypocentrum has been enlarged and the pleurocentrum
reduced, in the reptiles, birds, and mammals the opposite

VERTEBRAL COLUMN, RIBS, AND STERNUM 305

has occurred, and the vertebras of these animals have centra
which correspond to pleurocentra. In consequence of this,

>M

Fig. 154. — Vertebral columns of A, a bony fish (tunny) ; B, crocodile (in
the tail-region) ; C, Sphenodon (trunk region).

c, centrum ; cb, chevron-bone ; ha, hasmal arch ; he, hypocentrum ;
nay neural arch ; pc, pleurocentrum ; pz, prezygapophysis ; r, rib ; tpy
transverse process.

the haemal arches in amphibia are always attached to the centra
themselves ; whereas in the amniota, when they occur (mostly
in the tail-region), the haemal arches are attached to separate

306 COMPARATIVE ZOOLOGY OF CHORDATES

little elements called intercentra which represent the hypo-
centra. These intercentra and haemal arches in the amniotes
are called " chevron-bones " ; they are never found in the
amphibia. Primitive reptiles like Seymouria and Sphenodon
have a complete set of intercentra all the way along the vertebral
column, and some primitive mammals (hedgehog ; mole)
have intercentra in the lumbar region.

The transverse processes are lateral extensions of the

Fig. 155. — View of the anterior region of the vertebral column of the croco-
dile seen from the left side.

C2, centrum of the 2nd or axis vertebra ; hi, hypocentrum of the 1st or
atlas vertebra ; mi, neural arch of the 1st or atlas vertebra ; M2, neural arch
of the 2nd or axis vertebra ; op, odontoid peg, or pleurocentrum of the atlas
vertebra which has become attached to the axis vertebra ; pa, proatlas ;
n, 2, 3, ribs of the 1st to 3rd vertebrae ; V3, 4, 3rd and 4th vertebrae.

vertebrae from the base of the neural arches. The dorsal (or
tubercular) head of the ribs is attached to the transverse process,
and in all land-vertebrates except the most primitive Stego-
cephalian amphibia (such as Eogyrinus),the transverse processes
of at least one vertebra are attached to the ilia of the pelvic
girdle forming the sacrum. The sacrum is of course not formed
in animals which do not possess hind limbs.

The first vertebra in amphibia is modified to carry the head,
and the vertebral column (which in fish is divisible only into

VERTEBRAL COLUMN, RIBS, AND STERNUM 307

trunk- and tail-regions) is now divisible into regions corre-
sponding to neck, thorax, sacrum, and tail. In the amniotes
the first vertebra (the atlas) becomes detached from its centrum,
which becomes attached to the second vertebra or axis, and
forms its odontoid peg. There are therefore two vertebrae
specially modified in connexion with the neck, and a variable
number of normal cervical vertebrae which differ from the
thoracic in that their ribs are short and do not reach the
sternum. The vertebrae between the thoracic (whose ribs
reach the sternum) and the sacral are the lumbar. In primitive
forms the sacrum affects only one vertebra, to the ribs or
transverse processes of which the ilia are attached. In higher
forms, and especially in birds, there are several sacral vertebrae.

The vertebrae of land-animals bear facets by means of which
they articulate with one another, and so enable the vertebral
column to bend with considerable flexibility without
diminishing its strength. These facets are the pre- and
postzygapophyses. In some groups such as the lizard and
snakes, additional facets may be developed. The faces of the
centra are either flat or slightly concave or convex, but in the
birds a special saddle-like shape has been developed, which
allows of very great flexibility.

In the mammals, the number of cervical vertebrae is seven
in all species with only three exceptions. These are the
Edentates Bradypus which has nine, and a species of Choloepus
which has six or seven, and the Sirenian Manatus which has
six.

Ribs are extensions of the basiventrals, and they may be of
two kinds. Those which pass just on the outside of the
splanchnocoelic cavity are pleural or ventral ribs, and they
occur in Dipnoi. " True " or dorsal ribs pass in the horizontal
septum which separates the myotomes into dorsal and ventral
portions, and they occur in Selachii and in the Tetrapods.
Both kinds of ribs are present in Polypterus and some Teleosts.

In the land- vertebrates, the ribs primitively articulate with
the vertebrae by a broad head which touches the centrum and
the neural arch. These holocephalous ribs as they are termed
are present in Labyrinthodonts, Cotylosauria, and Sphenodon.

3o8 COMPARATIVE ZOOLOGY OF CHORDATES

Later, that portion of the head which touches the centrum
(capitulum) became distinct from that which abuts against
the transverse process (tuberculum), by the reduction of the
intervening part of the head. In this way, the typical double-
headed or dichocephalous ribs arose. Between the two heads
of the rib and the vertebra there is a canal through which
the vertebral artery passes ; and this vertebrarterial canal is
conspicuous in the cervical vertebrae on to which the cervical
ribs are usually fused.

In many forms, the articular heads of the ribs are degenerate
and either the capitulum or the tuberculum may be lost. This

dr.-

Fig. 156. — Diagram showing the relations of dorsal (" true ") and ventral
ribs, as seen in transverse section.

c, centrum of vertebra ; co, coelom ; dr, dorsal rib ; e, epaxonic muscles ;
g, gut ; h, hypaxonic muscles ; ns, neural spine ; vr, ventral rib.

secondary single-headed condition must be distinguished from
the primitive holocephalous type.

In the Chelonia, the ribs are expanded into broad flat
plates which touch one another and are fused to the dermal
bones (osteoscutes) to form the carapace. In Sphenodon,
crocodiles and in birds the ribs bear uncinate processes, which
extend backwards and overlap the next posterior rib. In
many cases, the ribs are in two portions : a dorsal or vertebral,
and a ventral or sternal portion. The hindmost ribs do not
usually reach the sternum, and they are known as floating
ribs. Primitively, all the vertebrae as far back as the middle of

VERTEBRAL COLUMN, RIBS, AND STERNUM 309

the tail bore ribs. In higher forms they do not extend behind
the sacrum.

The sternum first appears in Amphibia. It arises from
paired rudiments of cartilage which may become replaced by
cartilage-bone. In the Amphibia which are alive to-day, the
sternum has no connexion with the ribs. The sternum in the
Amniotes is however connected with the ribs, and this was
probably the condition in the Stegocephalia also. The sternum
is also usually in contact with the coracoids and clavicles.
In the mammals, the sternum is often broken into a number of
pieces or sternebrae. In the birds (with the exception of the
Struthiones : ostrich and its allies) the sternum bears a median
projection forming the " keel " or carina to which the flight
muscles are attached. Analogous but not homologous keels
are developed on the sterna of Pterosaurs and bats.

Overlying the sternum on the ventral side there is in many
forms a dermal bone, the interclavicle. It is present in the
Stegocephalia but has been lost in the living amphibia. Among
the reptiles, it is present in all except the snakes. In birds
it is apparently absent, unless it is represented by the keel of
the sternum. Only the Monotremes preserve the interclavicle
among the mammals.

CHAPTER XXII

FINS AND LIMBS

The most primitive chordates relied for their locomotion on
the myotomes of the body-wall, which, by contraction on one
side and relaxation on the opposite side of the body, can produce
the sinuous bendings which pass like waves down the length of
the body and propel the organism along. Amphioxus is in
this condition.

Improvement of methods of locomotion is connected with
the formation of extensions of the body in the shape of fins.
The earliest of these to arise were apparently those which lie
in the middle line of the dorsal and ventral surfaces : the
so-called median fins. In Amphioxus they are foreshadowed,
but in Petromyzon well-developed median fins are present,
supported by cartilaginous radials provided with radial muscles
at their base on each side. Fish likewise have median fins,
and these show an advance over the conditions in Petromyzon
in that the web of the fin is supported by dermal fin-rays in
addition to the cartilaginous radials. These fin-rays are horny
(ceratotrichia) in the Selachians ; bony and jointed (lepido-
trichia) in the Teleostomes, and in the Dipnoi they are fibrous
and jointed (camptotrichia). The median fins of the amphibia
have neither cartilaginous radials nor dermal fin-rays at all,
and in some of them the fins develop and regress according
to the season and the breeding period.

In the fish, in addition to the median fins there appear the
two pairs of " paired " fins : a pectoral pair and a pelvic pair.

The method of origin of median fins and paired fins is very
similar. In each case a longitudinal fold of skin appears,

310

FINS AND LIMBS

3"

and into it little " muscle-buds " make their way, having been
formed from the myotomes and separated off from them.
Cartilaginous radials then appear, and on each side of these,
the dermal fin-rays. The fins contain structures derived from
several segments of the body, and this is reflected in the number
of radial muscles, cartilaginous radials, dermal fin-rays and
nerves which the fin contains.

In the most primitive forms, and in early stages of develop-
ment of other forms, it is common to find that the median
fins are continuous and form one fold which extends down the

Fig. 157. — The pectoral fin of Cladoselache, showing the radials (r) project-
ing parallel to one another and perpendicular to the side of the body (b).
(Drawn from a cast.)

dorsal side, round the tail and forwards again on the ventral
side. The presence of a number of separate and discontinuous
median fins in many fish is therefore probably due to the sub-
division of an originally continuous fin.

If the median fin was primitively continuous, it is possible
that the paired fins also were originally continuous folds on
each side of the body, and that they became subsequently
divided into pectoral and pelvic sections. The fact that in
some fish such as Scyllium, there is in early stages of develop-
ment a continuous series of muscle-buds given off from all
the segments of the trunk makes this possibility fairly probable.

312 COMPARATIVE ZOOLOGY OF CHORDATES

Later on during development the muscle-buds between the
positions of the pectoral and pelvic fins come to nothing.

The most primitive paired fins known are probably those
of the fossil Cladoselache, in which they are triangular flaps
with the apex pointing outwards and the broad base attached
to the side wall of the body. The radials are more or less
parallel to one another and stick out at right angles to the side
of the body. It is important to notice that the radials are
scarcely concentrated at all at their base ; in fact the base is
the broadest part of the fin. In the body- wall there are some
basal cartilages with which the bases of the radials articulate.

The next step in the evolution of the fins was probably
the concentration of the radials at the base of the fin. The
result of this was that the stalk attaching the fin to the side of
the body became narrow, and the fin became free to move in a
greater variety of manners. The arrangement of the radials
was now in the shape of a fan as in the Osteolepidoti, and the
fin itself was in the form of a blunt paddle. The centremost
radials formed what may be called the axis of the fin, but this
is not well marked in primitive forms in which the fin is short.

By a lengthening of the axis a pointed laurel-leaf-shaped
fin is arrived at, like that of Ceratodus (the so-called archi-
pterygium). This type of fin is also present in the fossil
Pleuracanthus, where it would seem to have evolved indepen-
dently from that of Ceratodus. The skeleton of the paired
fins of the primitive fossil Dipnoi resembles that of the
Osteolepidoti.

On the other hand, by a shortening of the axis and reduction
of the radials, the web of the fin comes to be supported mostly
by the dermal fin-rays, and this is the condition of the higher
bony fish.

The pectoral and pelvic girdles must have arisen in accord-
ance with the need for a firm point of attachment of the fins
in the wall of the body. The radials at the base of the fin
have fused together and grown inwards, and in so doing they
may enclose in foramina the nerves supplying the fin. In
the pectoral girdle it is usual to find a dorsal scapular and
a ventral coracoid element ; the pelvic girdle is not so well

FINS AND LIMBS

3i3

developed. These girdles lie in the body-wall and are not
primitively connected with any other part of skeleton.

In the bony fish, the scapula, coracoid and pelvis ossify
as cartilage-bones, and in addition, a number of dermal bones
arise in connexion with the pectoral girdle. In a primitive
bony fish like Polypterus, these dermal bones are the clavicle,
cleithrum, supra-clei thrum, and the post-temporal which is
attached to the hind part of the skull. This girdle, which is
composed of dermal bones, may be called the clavicular girdle,

co

e

Fig. 158. — Comparison|between the fin of Sauripterus, A, and a pentadactyl
limb, B. (A after Gregory.)

c, carpals ; co, coracoid ; h, humerus ; mc, metacarpals ; p, phalanges ;
r, radius ; ra, radials ; s, scapula ; u, ulna ; zo, web of the fin, supported by
lepidotrichia. The fin of Sauripterus is an example of the blunt paddle-
shaped type of fin.

to distinguish it from the other girdle, formed of cartilage or
cartilage-bones, to which the term scapular girdle may be
applied. The clavicle is present in the sturgeon (Acipenser), but
in all higher fish it is lost. It may seem curious that the pelvic
girdle never has any additions of dermal bones to the cartilage-
bones of which it is composed. The explanation is that the
dermal pectoral girdle originally had no connexion with the
pectoral fins. It provides a firm attachment for the muscles
of the body-wall just behind the gill-slits in those forms (the

314 COMPARATIVE ZOOLOGY OF CHORD ATES

bony fish) in which the gill-slits are highly developed. The
gill-slits occupy a region of perforation and weakness, and they
prevent the main mass of the lateral body-wall muscles from
becoming attached to the skull. The dermal pectoral girdle,
which itself is attached to the skull, gives these muscles some-
thing solid to work from. The joining of the scapular and
clavicular pectoral girdles is due to the fact that both are
situated close behind the gill-slits. Since there are no gill-

Fig. 159. — The forelimb of Sphenodon : an example of a typical penta-
dactyl limb with a primitive carpus.

The figures indicate the ordinal numbers of the digits, c, carpals of the
distal row, which are five in number ; ce, centralia ; h, humerus ; i, inter-
medium ; mc, metacarpal ; p, phalanges ; ps, pisiform ; r, radius ; ra,
radiale ; w, ulna : id, ulnare.

slits or other source of weakness near the pelvic girdle, the
latter has no dermal elements added to it.

From the nature of the water in which they live, the fins
of fish are necessarily more or less like paddles. But it is from
such paddles (or ichthyopterygia) that the five- digi ted or penta-
dactyl limb (cheiropterygium) of the Tetrapods or land-
vertebrates was evolved. It is interesting to inquire into the
question as to which type of fin most probably gave rise to the

FINS AND LIMBS

3*5

limb. The most convenient starting-point is the blunt lobate
fin of the Osteolepidoti (and primitive Dipnoi) with a single
large radial at its base, and an increasing number of radials
arranged fanwise running to the outer border of the fin. In
such a form as Sauripterus (one of the Osteolepidoti) the single
basal radial of the pectoral fin may perhaps be held to represent
the humerus, and the next two correspond to the radius and
ulna of the terrestrial fore limb. In a general way the next
radials represent the carpals and metacarpals.

The earliest limbs probably had more than five fingers,
and the number of rows of radials in the distal part of the fins

Fig. 160. — Ventral view of the abdominal ribs or gastralia, and pectoral
girdle of Sphenodon.
cl, clavicle ; co, coracoid ; g, gastralia ; gc, glenoid cavity ; ic, inter-
clavicle ; r, ribs (true) ; s, scapula.

of Sauripterus is greater than five. But if the pectoral fin of
Sauripterus be compared with the arm of a primitive amphibian
like Eryops, it is easy to see how the structure of the latter
could be derived from that of the former. The evolution of
the five-digited, or pentadactyl limb is an adaptation to
locomotion on land. During this transformation, the limb-
girdles must have become better developed, for an animal in
air is relatively heavier than in the water, and the limbs are
subjected to greater strains and stresses. At the same time,
the girdles of the earliest land-vertebrates closely resemble
those of their aquatic ancestors. So in Eogyrinus (fossil

3i6 COMPARATIVE ZOOLOGY OF CHORD ATES

Amphibian of the Carboniferous period), the clavicular pectoral
girdle is represented by the clavicle, cleithrum, supra-cleithrum
and post- temporal, which latter is attached to the hind end of
the skull, just as in bony fish. To these is added a median
ventral interclavicle. As in all Tetrapods, the scapula (cartilage-
bone of scapular girdle) rests on, but is not attached to, the
ribs. The pelvic girdle of Eogyrinus is interesting in that the
ilium rests on the ribs without fusing with them to form a
sacrum. In this respect, the pectoral and pelvic girdles are
similar, but in higher forms the ilium becomes firmly attached

Fig. 161. — Ventral view of the pectoral girdle of Ornithorhynchus.
ec, epicoracoid (or precoracoid). Other letters as Fig. 160.

to one or more sacral vertebrae. In addition to the ilium,
the pelvic girdle contains the pubis and ischium.

In the earliest land- vertebrates, the function of the limbs
was not to support the body of the animal but to row it along
while its ventral surface rested on the ground. Such move-
ment must have been slow, and improvement came in the
reptiles, in which the limbs lift the body off the ground. In
them, there was no friction to be overcome between the body
and the ground, and the higher the body was lifted, the longer
the limbs, the longer the stride and the faster was the pace.
In the reptiles the clavicular pectoral girdle is reduced to the
clavicle and interclavicle (the cleithrum persists only in some
primitive forms), while the scapular girdle usually consists

FINS AND LIMBS

3i7

of a dorsal scapula and a ventral coracoid. In the Theromorph
reptiles the scapular girdle may have two ventral elements,
the coracoid and precoracoid. In the pelvic girdle the ilium
becomes attached to the sacral vertebrae, and the ischio-pubic
foramen appears between the pubis and ischium. In some
Dinosaurs a post-pubis is present, extending back beneath the
ischium. In Chelonia, the pectoral and pelvic girdles have a
peculiar position in that they lie inside the ribs, instead of
outside them as in other forms. In birds, the pubis rotates

Fig. 162. — Diagrams illustrating the evolution of the limbs of Tetrapods.

A and B, views of the early stage when the limbs stick out laterally and
the ventral surface of the body rests on the ground. C and D, later stage,
when the body is lifted off the ground, the forearm and shank being vertical,
and the limbs projecting to the side. E and F, late stage, when the hind
limb is rotated forwards from the acetabulum, and the fore limb rotated
backwards from the glenoid cavity ; but the hand points forwards and the
radius and ulna are crossed.

backwards and comes to lie parallel to and beneath the ischium,
with which it may to a certain extent fuse.

In mammals, the coracoid, precoracoid, and interclavicle
are retained only in the Monotremes. The pelvic girdle of
Monotremes and of Marsupials is characterised by the presence
of a pair of epipubic bones, which support the pouch, or marsu-
pium. The clavicle is often missing in the higher mammals,
and especially those which use their limbs for fast running.
So the clavicle is absent in the horse, and it is much reduced

3i8 COMPARATIVE ZOOLOGY OF CHORD ATES

in the dog. In the more primitive forms, and those which are
specialised for tree-climbing and digging, the clavicle is usually
present.

The limbs themselves show interesting modifications. In
the earliest Tetrapods, the limbs stuck straight out at right
angles to the side of the body. When the ventral surface of
the body became lifted off the ground, the upper arm and thigh
stuck straight out laterally and horizontally ; at the elbow and
knee there was a right-angle bend, so that the forearm and
shank descended vertically to the ground. At the wrist and
ankle, there was another right-angle bend, so that the hand and
foot extended horizontally away from the body.

In the mammals, starting from the condition just described,
the limbs have undergone a rotation. The hind limbs have
been rotated forwards, so that the thigh runs forwards from the
hip-girdle, and parallel with the side of the body, the shank
runs downwards, and the foot points forwards again. In
the fore limb, however, the upper arm has been rotated back-
wards parallel with the side of the body, and the forearm runs
downwards. But the hand would point backwards if the fore
limb had undergone a simple rotation similar to that of the
hind limb (though in the opposite direction). As a matter of
fact, the hand points forwards, and this is brought about by a
rotation of the wrist through 1800 about a vertical axis which
coincides with the forearm. So it happens that the forearm
is twisted, and the radius runs from the outer side of the elbow
to the inner side of the wrist, passing in front of the ulna, which
runs from the inner side of the elbow to the outer side of the
wrist. This is the typical position (of pronation) in mammals ;
most Primates, including man, however, are able to uncross the
radius and ulna and so turn the palm of the hand upwards
(supination).

It is impossible to go into all the types of limb-structure,
but it is interesting to consider the adaptations of limbs to the
three great media, viz. to locomotion on land, in the air, and
in water.

The fingers and toes of land-living vertebrates above the
amphibia end in horny claws which may be modified into

FINS AND LIMBS

3*9

nails or hoofs. When the whole surface of the hand or foot
is applied to the ground, as in the human foot, the animal is
said to be plantigrade. Other animals, like the dog, rest only
the under surface of the fingers and toes on the ground, while
the palm of the hand and sole of the foot take no share in

Fig. 163. — Convergence in the adaptation of limbs for flight, A, in birds ;
B, Pterodactyls ; C, bats.

cm, carpo-metacarpus ; h, humerus ; mc, metacarpal ; p, phalanx ;
r, radius ; w, ulna.

bearing the animal's weight. This is the digitigrade condition.
Others again, such as the horse and cow, which rest only on the
end joints of the fingers and toes, are unguligrade. [The latter
form part of the order Ungulata.]

The limbs of the horse are specialised for rapid movement
on hard ground. Only the 3rd digit is retained, and its

32o COMPARATIVE ZOOLOGY OF CHORDATES

extremity is expanded and surrounded by the nail which gives
rise to the hoof. The other digits have disappeared, leaving
only small vestiges of the metacarpals and metatarsals (of the
2nd and 4th digits) in the form of " splint-bones.". The
fossil ancestors of the horse show different stages in this
process of reduction of the number of digits, and lead back
to normal pentadactyl animals. These odd- toed Ungulates
are called Perissodactyls. Curiously enough, a parallel process
of reduction in number of digits, and of formation of hoofs
consisting of a single digit, went on in a group of animals (all

°o o O O

FlG. 164. — Convergence in the adaptation of limbs for swimming, in, A,
Ichthyosaurs ; B, Plesiosaurs ; C, birds (penguin) ; D, mammals
(dolphin) .

now extinct) quite independently of the horses : the Thoatheria.
This is a very remarkable case of convergence in evolution.

In the " cloven-hoofed " Ungulates or Artiodactyls, the
hoof is formed from the end joints of digits 3 and 4, as in cattle,
where the metacarpals and metatarsals of the two digits
fuse.

Among mammals, limbs with a primitive type of structure
are those of the Primates, which preserve all the five digits.
In most Primates, the first digit (thumb or big toe) is capable
of touching any or all the remaining digits, i.e., is opposable.
This structure enables the limb to grasp objects firmly. Apes

FINS AND LIMBS 321

have this power in feet as well as hands, while man only
preserves the capacity to oppose the first digit in his hands.

Three separate and independent groups of vertebrates
have become adapted to life in the air, by the modification of
the fore limbs into wings. These are the extinct Pterosauria
(" flying reptiles "), the birds and the bats. In the Pterosauria,
the fourth digit of the hand was enormously elongated, and a
web of skin was stretched between it and the side of the body,
extending back to the hind limbs and tail. The bird's wing is
built on an altogether different principle, for the wing-surface
is made up of a number of feathers inserted on the hand and
forearm. The skeleton of the forelimb of the bird shows a
reduction in number of digits to three, and the claws at the
end of the digits have disappeared except in the young of some
birds, such as the ostrich and of the Hoatzin. The primitive
fossil bird Archaeopteryx had well-developed claws.

The wing of the bat is different, again, for in it the 2nd,
3rd, 4th, and 5th digits of the hand are much elongated, and
support a web of skin which stretches out from the side of the
body.

The three types of wings just described form another
interesting example of convergent evolution on the part of
unrelated animals, but the most striking example is that
furnished by the limbs of those land-vertebrates which have
subsequently returned to an aquatic mode of life and become
adapted to it. The adaptation takes the form of a modification
of the limbs into flippers or paddles, which superficially may
come to resemble the fins of fish, but which betray their
descent from the pentadactyl structure of the land-vertebrate's
limb in their internal structure. This adaptation has taken
place at least nine separate times, in independent groups.
Three of these are mammals : the whales, the Sirenia, and the
seals. Among the birds, the penguins have modified the
wing into a paddle. In the reptiles, the turtles (Chelonia),
Ichthyosaurs, Plesiosaurs, Mosasaurs, Thalattosuchia, and
Thalattosaurs all show the same modification of the limbs
into paddles, and in several fossils it is possible to trace the
evolution from normal pentadactyl limbs.

Y

322 COMPARATIVE ZOOLOGY OF CHORDATES

In the more highly modified of these paddle-like limbs (as in
the whales, for example), it is common to find that the number
of phalanges is increased (a condition known as hyperphalangy).
In the broad paddles of Ichthyosaurs, the number of rows
of phalanges exceeds five, producing the condition called
hyperdactyly.

The 5th metatarsal bone is an object of interest. Normally
this bone is straight, as in the amphibia, the most primitive
reptiles (Cotylosauria), the Theromorph and allied reptiles
and the mammals. In other groups of reptiles, however, it
is peculiar in being hook-shaped, and the possessors of this
modified type of 5th metatarsal are : Sphenodon, lizards,
tortoises, crocodiles, Dinosaurs, and Pterosaurs. It is worthy
of note that these groups all have characters in common in the
structure of the heart or of the skull, and are regarded as
belonging to the great Sauropsidan branch of the reptiles which
culminates in the birds. It is probable that the hook-shaped
metatarsal is characteristic of this group, and distinguishes it
from the other main stem of reptiles (Theropsida) which evolved
in the direction of mammals. The evidence from the 5th
metatarsal fits in with that obtained from other sources.

Mention must be made of those animals which have lost
their limbs. They have totally disappeared in some of the
eels. Among the amphibia, the pelvic limbs and girdle have
been lost in the Sirenidae, and the worm-like Gymnophiona
have lost all the limbs and girdles. Coming to the reptiles, the
snakes have lost the girdles and the pectoral limbs altogether,
while only very small vestiges of the pelvic limbs remain.
Several families of lizards have independently assumed the
snake-like form by loss of the limbs, such as the slow- worm
(Anguis), some of the Scincidae and the Amphisbaenidae.
These forms furnish an interesting example of convergent
evolution. Among mammals the pelvic girdle and limbs
vanish almost completely in the whales (Cetacea) and Sirenia.

Literature

Goodrich, E. S. Vertebrata Craniata, Cyclostomes and Fishes. Black,
London, 1909.

FINS AND LIMBS 323

Goodrich, E. S. On the Classification of the Reptilia. Proceedings of

the Royal Society, Ser. B, vol. 89, 1916.
Gregory, W. K. Present Status of the Problem of the Origin of the

Tetrapoda. Annals of the New York Academy of Sciences, vol. 26

1915.

Watson, DM. S. The Evolution of the Tetrapod Shoulder Girdle and
rorehmb. Journal of Anatomy, vol. 52, 1917.

. The Evolution and Origin of the Amphibia. Philosophical Trans-
actions of The Royal Society, Ser. B, vol. 214, 1926.

CHAPTER XXIII

THE TAIL

An extension of the body behind the anus, containing all the
chief tissues of the body, is a structure characteristic of chordate
animals. Its original function was to assist the animal in swim-
ming, for it contains myotomes and a portion of the notochord,
and so is able to take part in the undulatory movements from
side to side which propel the animal forwards through the
water. The area of the tail is commonly increased by the
formation of a fin in the middle line, in the lower chor dates.
In Amphioxus, the fin is not very large, but it extends sym-
metrically from the middorsal and midventral lines of the tail,
and tapers to a point behind. This primitive type of tail
is called diphycercal. It is present also in the Cyclostomes,
where it is supported by cartilaginous radials, and in early
stages of development of other forms.

In Selachians the tail is asymmetrical, for the vertebral
column is bent slightly dorsally, and the dorsal (epichordal)
lobe of the caudal fin is reduced while the ventral (hypochordal)
lobe is increased. The ventral lobe is supported by the
elongated haemal arches of the vertebral column, known as
the hypurals, and not by separate radials. This type of fin
is called heterocercal. In addition to the Selachii, it is present
in the sturgeon, the Osteolepidoti and the fossil Dipnoi.
Since the axis of the tail in such forms is bent up, in swimming,
the head of the fish tends to be turned down, as when the
fish noses along the bottom in search of its prey.

In the higher bony fish (Teleosts) the dorsal lobe of the
caudal fin is further reduced and the ventral lobe enlarged,
with the result that the tail presents an externally symmetrical

324

THE TAIL 325

(usually forked) appearance. Internally, however, the skeleton
reveals the fact that this homocercal type of tail is derived from
the heterocercal, and the axis can be seen to bend up at the
tip. It is found also that during development, the homocercal
tail passes through a heterocercal stage.

In other forms the tail tapers symmetrically to a point, and
so comes to resemble the diphy cereal type. This secondarily
simplified type of tail (shown by the eel, for example) is called
gephyrocercal, and is the result of reduction from the hetero-
cercal or homocercal condition. The tail-fin of Gadus is

Fig. 165. — Skeleton of the tail of the salmon, showing the homocercal
pattern of tail-fin characteristic of most Teleost fish.

Note the up -turned vertebral column, h, hypurals ; /, lepidotrichia ;
vt vertebra.

peculiar, for it is formed from the hind portions of the median
dorsal and ventral fins. Such a tail is called pseudocaudal.

In Ceratodus, the tail seems to be diphycercal (and there-
fore primitive), because its ventral lobe is supported by separate
radials, and not by hypurals. There is, however, doubt about
this, because many of the fossil Dipnoi had heterocercal tails,
and if it can be proved that Ceratodus is descended from them,
the structure of its tail must be gephyrocercal.

In amphibia, the tail-fins are present in the larval stages,
which live in water ; but they disappear when the animals
come out on land, to grow again in some during the water
sojourn of the breeding season. In the Anura (frogs and toads)

326 COMPARATIVE ZOOLOGY OF CHORDATES

the tail disappears altogether in the adult terrestrial form ; in
the Urodeles (newts) it persists as a more or less tubular
structure. In the Gymnophiona there is scarcely any tail at
all, for the anus is almost at the hind extremity of the animal.
In land-animals, the tail ceases to have the function which
it exercised in the water, and it is often consequently much
reduced. Instead of being a posterior prolongation of the
body, it has the appearance of being merely an appendage,
and it is of use to the animal in the maintenance of its balance,
as a covering for the anus and genitalia, and in some cases as
a fly- whisk.

Lizards have an interesting modification in that the
vertebrae of the tail are cleft transversely, and it is at these
points that the tail can be detached from the rest of the body.
This faculty (autotomy) is of service to the animal in enabling
it to escape from its enemies.

The primitive birds had long tails, with separate vertebrae,
as is shown by Archaeopteryx. In living birds the caudal
vertebrae are fused together to form the pygostyle, and the tail
itself is much reduced. The so-called tail of birds consists
of the tail-feathers.

In some arboreal animals, such as the chamaeleon and the
American monkeys, the tail is prehensile and capable of grasping
objects.

It is common to find that in those vertebrates which
have returned to the water, the tail is well developed and
expanded into fins. While superficially not unlike the tails
and caudal fins of fish, they show in their structure fundamental
differences. So in Ichthyosaurus, the vertebral column passes
back into the ventral lobe of the fin ; in the whales the two
lobes of the caudal fin are not dorsal and ventral but right and
left, for the tail is expanded horizontally.

In the apes and man the external tail has disappeared
altogether.

CHAPTER XXIV

THE VASCULAR SYSTEM

The vascular system is remarkably uniform in its main
features in all chordates. It consists essentially of four
longitudinal vessels running along the whole length of the
animal. Of these, one runs under the gut in the gut-wall
(subintestinal vessel) ; the other three run in the body-wall,
and are the dorsal aorta and the paired cardinal veins
respectively. The subintestinal vessel connects with the dorsal
aorta at the anterior end of the animal by a number of paired
vessels which run up round the gut on each side passing in
between the gill-slits. The anterior prolongations of the dorsal
aorta (which is paired in the anterior region) are the internal
carotids. Farther back the dorsal aorta gives off small vessels
in each septum (between the segments) to the tissues of the
body-wall, and other vessels which pass down the mesentery
supporting the gut to supply the gut- wall. The blood in the
gut-wall is collected up into the subintestinal vessel and is
led forwards again. On the way, it breaks up into capillaries
again in a glandular diverticulum of the gut — the liver, and
deposits much of the digested and absorbed material which it
has picked up in the posterior region of the gut (intestine).
In this way a hepatic portal system is formed. The blood in
the body- wall makes its way to the cardinal veins, and from
them it crosses the coelomic cavity between the body-wall and
the gut- wall by the ductus Cuvieri (or superior vena cava),
running in the transverse septum, to the subintestinal vein.
This is the fundamental type on which the peripheral vessels
are arranged in all chordates, and the details in the various
groups can be considered under the headings, veins, heart,

327

328 COMPARATIVE ZOOLOGY OF CHORDATES

Q

THE VASCULAR SYSTEM

329

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330 COMPARATIVE ZOOLOGY OF CHORDATES

and arteries. It may be remembered that arteries are vessels
leading blood away from the heart, and veins lead blood towards
the heart, whatever be the kind of blood which they contain.
Further, arterial blood is rich in oxygen, and venous blood poor
in oxygen, whatever may be the nature of the vessel which
contains it. Actually, the purest arterial blood in the body is
in a vein (pulmonary), and the foulest venous blood is in an
artery (also pulmonary).

The Veins. — The description given above applies to the
venous system of Amphioxus. In the Craniates, the presence
of mesodermal kidneys (pronephros and mesonephros), lying
in the track of the posterior cardinal veins, brings about the
formation of a renal portal system. The anterior cardinal
veins give rise to the jugulars, and in the Gnathostomes there
are veins returning blood from the fins or limbs. Those from
the anterior limbs are the subclavian veins which run into the
ductus Cuvieri ; those from the hind limbs are the pelvic
veins which run into the renal portals and into the lateral
abdominal veins. The two latter veins often join in the
middle line on the ventral side and give rise to the anterior
abdominal vein of Ceratodus and higher forms. In the
amniotes the lateral abdominal veins receive the blood from the
allantois in the embryonic stages of development. Beginning
in the Dipnoi, there is another connexion between the circula-
tion of the body- wall and that of the gut-wall, apart from the
superior venae cavae. This is the inferior vena cava. Pul-
monary veins are present in Polyp terus, Dipnoi, and Tetrapods,
returning blood from the lungs to the heart. In the amniotes
the renal portal veins tend to diminish owing to the fact that
the functional kidney of the adult is no longer a mesonephros
but a metanephros, and in the birds and mammals there is no
renal portal system.

The Heart. — In Amphioxus there is no specialised heart
in which the blood is pumped forwards, but, apart from the
specialised bulbils, the whole vascular system is contractile
and propels the blood along. Beginning in the Cyclostomes,
there is a definite portion of the subintestinal vein in front of
the liver and behind the gill-slits which is set apart as a muscular

THE VASCULAR SYSTEM 331

pump, and forms the heart. The veins from the liver and the
ductus Cuvieri are received by a sinus venosus, which in turn
leads into a thin-walled auricle. The latter passes the blood
on to the thick-walled muscular ventricle, by which it is
propelled into the anterior portion of the subintestinal vessel
which is called the ventral aorta. The arteries are surrounded
by smooth muscle, but the musculature of the heart is peculiar
and unique in that it shows a number of cross-striations and
its fibres branch. The openings between the various sub-
divisions of the heart are guarded by valves which prevent a
return flow.

In Scyllium the ventricle is produced forwards into a
muscular and contractile conus, which contains several rows
of valves. In front of this, the base of the ventral aorta is
swollen into a non-contractile bulbus. (The walls of the conus
contain heart-muscle, those of the bulbus smooth muscle.)
In the higher bony fish the conus tends to disappear while
the bulbus enlarges. Amia is primitive in showing a fairly
large conus with three rows of valves. In the Dipnoi, the
valves of the conus are well developed, and they give rise to
a spiral septum which almost or quite divides the conus into
two. These same forms are further very interesting in that
the ventral aorta is very much shortened up into a truncus
(instead of extending forwards all the way beneath the gills
as in Scyllium), and also because in Ceratodus there is a
beginning of the subdivision of the auricle into two, with the
pulmonary veins running into the left subdivision.

In the frog, the heart is not unlike that of Ceratodus, except
that the auricles are completely divided into two, and that the
spiral septum in the conus and truncus is better developed,
dividing a pulmonary channel (leading to the pulmonary
arches) from an aortic channel (leading to the aortic and
carotid arches).

In the water-breathing forms, the heart is always full of
venous deoxygenated blood, while in air-breathing forms there
is always a double stream of blood in the heart. One of these
streams is arterial and oxygenated, and the other venous and
deoxygenated. Since in the frog there is only one ventricle,

332 COMPARATIVE ZOOLOGY OF CHORDATES

and both the arterial blood from the left auricle and the
venous blood from the right auricle open into it, there is a
mixture in the ventricle which is sorted out into the two
channels in the truncus by the spiral septum and valves.
In newts, the septum between the auricles tends to break down,
as does the septum in the truncus. In the embryonic stages
of amniotes the septum between the auricles remains incom-
plete also, until the time of hatching or birth, in connexion
with functional details of the embryonic circulation.

The hearts in the amniotes fall into two classes, neither of
which can be derived from the other, and which must have
been separately evolved from the amphibian condition. The
conus is reduced and incorporated in the wall of the ventricle,
but while in one group which may be called Sauropsidan the
truncus is split right down to the ventricle into three channels,
in the other or Theropsidan group it is split into only two
channels.

The three channels in the Sauropsida are the pulmonary,
the right systemic, and the left systemic. The two latter cross
over one another so that the right systemic springs from the
left side of the ventricle, while the left systemic arises with the
pulmonary from the right side of the ventricle. In the lizards,
snakes, tortoises, and Sphenodon, the ventricle is still single,
although there is a septum which divides it incompletely.
The left auricle, as always, contains the arterial blood, most of
which goes into the right systemic arch. In the crocodile,
the interventricular septum is complete, but it is formed in
such a way that while the right systemic arch gets all the
arterial blood from the left auricle, the left systemic arch
together with the pulmonary, gets only venous blood from the
right auricle. There is a small foramen (of Panizza) between
the right and left systemic arches which allows a little inter-
change of blood. The condition in the bird is like that of the
crocodile except that the left systemic arch has been abolished
altogether, which is not surprising, seeing that it could only
distribute blood which is almost purely venous. In the bird,
therefore, with its four-chambered heart, there is no mixture
of arterial with venous blood ; all the venous blood in the

THE VASCULAR SYSTEM 333

heart goes to the lungs and only to the lungs. In reptiles and
birds, the carotids arise from the right systemic arch.

The two channels of the truncus in the Theropsida are the
pulmonary and the single systemic aorta. These forms
include the mammals, and the Theromorph reptiles, although
the latter (fossils) are obviously only known from their
skeleton. The heart is four-chambered, and the ventricle is
completely divided into two, so that all the venous blood from
the right auricle goes into the pulmonary arch, and all the
arterial blood from the left auricle into the systemic aorta, and
there is no mixture. It so happens that the aortic arch of the
right side does not persist, and only the left one remains, but
it is of the utmost importance to realise that the reason why
there is a single systemic arch in the bird is totally different
from that which is responsible for the single arch in the
mammal. The structure of the heart in the amniotes shows
that the reptiles contain two main lines of evolution (besides
other less important lines), the one culminating in the birds
and the other in the mammals. The sinus venosus disappears
in the highest forms, birds and mammals, and is represented
by the so-called sino-auricular node. This structure is of
great functional importance, for it acts as the pace-maker to
the heart. It is here that the contraction originates, which
contraction then becomes taken up by the other parts of the
heart, and constitutes its " beat." In birds and mammals the
superior and inferior caval veins open direct into the right
auricle. The sino-auricular valves give rise in the mammals
to the Eustachian and Thebesian valves.

The Arteries. — In the fish typically, each of the visceral
arches has an afferent branchial artery leading from the ventral
aorta to the gills, and an efferent branchial artery connecting
the gills to the lateral dorsal aorta. The vessels in the mandi-
bular arch become reduced. The general arrangement of
these vessels is necessitated by the presence of the visceral
clefts, which make it impossible for the vessels to reach the
dorsal side of the gut from the ventral side except by passing
in the visceral arches between the clefts. Since gill-slits or
pouches are present in the embryos of all chordates, the same

334 COMPARATIVE ZOOLOGY OF CHORDATES

reason accounts for the arrangement of the arterial arches in
the higher forms. In the air-breathing vertebrates, the gills
are reduced and there is a continuous vessel in each visceral
arch running from the truncus arteriosus (ventral aorta) to the
lateral dorsal aorta. In Salamandra all the vessels in the 3rd
to 6th visceral arches persist. The 3rd becomes the carotid,
the 4th and 5th become systemics, and the 6th is the pul-
monary. All these arterial arches place the truncus in com-
munication with the lateral dorsal aorta. The lateral dorsal
aortas are, however, interrupted between the dorsal ends of
the 3rd and the 4th arterial arches ; i.e. there is no ductus
caroticus. The conditions in Triton are similar except that
the 5th arterial arch has completely disappeared. In Lacerta
(as in all higher forms) the 3rd arch persists as the carotid, the
4th as the systemic, and the 6th as the pulmonary. In Lacerta,
the connexion between the dorsal ends of the arteries of the
3rd and 4th arches persists, forming the ductus caroticus.
The lateral dorsal aorta is here accordingly uninterrupted.
The ductus caroticus is absent in the adult of higher forms.
The connexion between the pulmonary arch and the lateral
dorsal aorta is the ductus arteriosus. This connexion is
important in the embryonic stages of Amniotes. It enables
the blood from the right side of the ventricle (or the right
ventricle, if it is separated off) to reach the lateral dorsal aorta
through the pulmonary arteries, instead of going to the lungs.
At these early stages of development the lungs are not yet
open. In the adult amniote, the ductus arteriosus usually
degenerates into a ligament, as, for example, in the mammal
(on the left side), or disappears. It persists, however, in some
turtles, and their case is interesting, for they are in the habit of
diving, and during the submerged period the lungs are not
working. The blood in the pulmonary artery can then escape
into the general circulation without going through the lungs.
The ductus arteriosus is also called the ductus Botalli.

In the frog, there is neither ductus caroticus nor ductus
arteriosus in the adult.

In the Sauropsidan reptiles, the right and left systemic
arteries of the 4th arch are separate right down to the base of

THE VASCULAR SYSTEM 335

the truncus. The left one of these arches is absent in the
bird. The subclavian arteries come off from the right
systemic arch in lizards (dorsal type of subclavian) ; in
Chelonia, crocodiles, and birds, the subclavian arteries are
given off from the carotids (ventral type).

In the mammal, the right and left arteries of the 4th
(systemic) arch differ from those of the Sauropsidan reptiles
in that the aorta is undivided, instead of being split to its base.
The artery on the right side does not reach round to the dorsal
aorta ; it is given the name of innominate artery, and it leads
to the right carotid and subclavian arteries. That on the left
side forms the so-called aorta, gives off the left carotid and
subclavian arteries, and continues back as the dorsal aorta.
It is connected with the pulmonary arch by the ductus arteriosus
as already mentioned.

The internal carotid arteries are the anterior prolongations
of the lateral dorsal aortae, and they enter the skull by passing
up between the trabecular, close to the pituitary body. The
external carotids are the anterior prolongations of the ventral
aorta, on each side of the thyroid.

The proximal ends of the arteries and veins are joined at
the heart. The distal ends of the arteries are connected with
those of the veins by the capillaries, so that the whole vascular
system is a closed one. When a vein starts from capillaries
and breaks down into other capillaries again before reaching
the heart, it is known as a portal vein. The hepatic portal
vein occurs in all chordates, the renal portal appears in the
Cyclostomes and disappears in the amniotes.

The blood of Amphioxus is colourless, but in all higher
forms, haemoglobin, a respiratory pigment, is present in
corpuscles, which become known as " red blood-corpuscles. "
In the adult mammal, these corpuscles are peculiar in being
non-nucleated. The white corpuscles of the blood play an
important part in the defence of the organism against invasion
by foreign bodies. In the embryo, the blood arises from blood-
islands, between the mesoderm and the endoderm in the
region of the yolk. In the adult, blood-corpuscles are formed
in the marrow of the bones, and in the lymphatic organs. The

336 COMPARATIVE ZOOLOGY OF CHORDATES

blood is under pressure in the arteries and capillaries, owing to
the contraction of the smooth muscle surrounding the former
and of the Rouget-cells which compose the walls of the
latter.

Lymphatics. — Attention may now be turned to the lym-
phatic system. In addition to the blood-vessels, the body
contains a system of vessels, channels, and spaces in which
lymph circulates, forming the lymphatic system. It is in
communication with the ccelomic cavity. Lymph is blood-
plasma and white corpuscles which exude from the capillaries
and bathe all the tissues of the body, supplying them with
nutritive products. From the tissues, the lymph (which may
thus be regarded as blood minus the red blood-corpuscles) is
gathered up into thin-walled channels, called the lymphatics.
These start from blind ends and eventually join the veins, in
particular the subclavians, the left of which receives the main
lymphatic trunk which is known as the thoracic duct. In the
amphibia the space between the skin and the muscles of the
body-wall is occupied by lymph, and in certain regions " lymph-
hearts " are present, with muscular walls, which propel the
lymph along. These lymph-hearts are lacking in mammals.
Lymphatic vessels are present in the wall of the intestine, and
are known as lacteals, for they absorb the fatty products of
digestion, and the milk-like emulsion which they contain gives
them a white appearance. Here and there along the lym-
phatics, lymph glands are formed. To these belong the
spleen (which first appears in the Selachii), the tonsils (derived
from the 2nd pair of visceral pouches), and Peyer's patches
along the intestine in the mammals.

Literature

Goodrich, E. S. Vertebrate Craniate, Cyclostomes and Fishes. Black,
London, 1909.

Goodrich, E. S. On the Classification of the Reptilia. Proceedings of
the Royal Society, Ser. B, vol. 89. 1916.

CHAPTER XXV

THE RESPIRATORY SYSTEM

All chordates have a closed vascular system and haemoglobin
as a convenient transporter of oxygen. Their respiratory
systems involve structures in which blood-vessels are brought
into close contact with the surrounding medium (water or air)
with as little intervening tissue and as great an exposed surface
as possible. The former requirement is met by the very thin
nature of the epithelium covering the blood-vessels, and the
latter by reducing the size of the blood-vessels to capillaries,
which therefore have a large surface compared with their
volume.

The respiration of embryos within their membranes is
effected by various means, such as the circulation of the yolk-sac
or of the allantois, as has been described in connexion with the
development of the frog, chick, and rabbit.

After the embryonic stage has been passed, chordates
breathe either by gills, or by gills and lungs (sometimes
assisted by the skin), or by lungs alone.

Gills are groups of capillaries in the walls of the gill-slits,
through which water passes out from the pharynx. In
Amphioxus the current of water is caused by the action of the
cilia on the under side of the oral^ hood and in the gill-slits
themselves. Fish breathe in the following manner : the gill-
slits are shut and the floor of the mouth is lowered, which
causes water to enter the mouth. The mouth is then closed,
its floor is raised, and the water escapes through the gill-slits.
When Cyclostomes are feeding, they are firmly attached to
their prey by their mouth and the sucker surrounding it.
They cannot therefore take in water through the mouth, and

337 Z

338 COMPARATIVE ZOOLOGY OF CHORDATES

the gill-pouches are modified into sacs which pump water in
and out again. In the larvae of some fish (Polyp terus,
Lepidosiren), and in those of amphibia, external gills may be
developed in the form of tuft-like structures projecting out
from the body into the water, and which enable the blood to
be oxygenated before the gill-slits are pierced. The larval
amphibia afterwards develop ordinary gills on the outer faces
of the gill-arches, and their respiration is like that of the fish.
In all these cases the respiratory movements are brought about
by means of the contraction of visceral muscles, innervated by
dorsal cranial nerve-roots, and controlled by a centre in the
medulla oblongata.

The first visceral cleft or spiracle is open in the Selachii,
but it is closed in all higher forms with the exception of
Polypterus and Acipenser (the sturgeon). There may be a
spiracular gill, which is called a pseudobranch because its
capillaries receive blood which has already been oxygenated
in the next posterior (true) gill. In the Tetrapods the cavity
of the spiracular cleft gives rise to the tympanic cavity and
Eustachian tube.

The rays are Selachii adapted for living on the sea-bottom,
and they are of a flattened shape, with the gill-slits on the
under side. The spiracle is on the upper side, and serves to
admit water into the pharynx. In the Selachii, the gill-slits
are uncovered, but in the bony fish (Dipnoi and Teleostomes)
they are covered over and protected by an operculum. An
analogous operculum develops in the larva of the frog, and it
may be remembered that in Amphioxus the gill-slits are
protected by being enclosed in the atrial cavity.

The gill-sacs of Petromyzon all open independently to the
exterior, whereas those of Myxine have a single joint opening
on each side.

The number of gill-slits in Amphioxus is large (up to 180).
In Selachii, not counting the spiracle, it is five, except in
Heptanchus which has seven, and Hexanchus and Pliotrema
which have six. Five is also the number in bony fish. It is
important to remember that gill-slits or pouches are present
in early stages of development of all chordates up to and

THE RESPIRATORY SYSTEM 339

including mammals, and that they play a part in the disposition
of the arterial arches although they cease to function as
respiratory organs.

The adult Amphibia (or most of them, i.e. those which
have not lost the lungs) and all higher vertebrates breathe by
lungs. (The use of the skin as a breathing organ in Amphibia
is made possible by the fact that their skin is moist and
uncovered.)

Lungs are also present in some fish. In Polyp terus, there
is a trachea leading out from the ventral side of the oesophagus,
and forking into two lungs. The cavity of these lungs is
divided into small spaces or " cells," which has the result of
increasing the internal surface. Such lungs are called cellular,
and they are supplied with blood by pulmonary arteries, i.e.
branches from the last (6th) pair of branchial arterial arches.
From them, blood returns (to near the sinus venosus) by paired
pulmonary veins. In the Dipnoi, there are paired lungs in
Protopterus and Lepidosiren, but a single one only in
Ceratodus. Their relations are similar to those of Polyp terus,
except that the lungs, together with the pulmonary arteries and
veins, have been displaced to a dorsal position by passing round
the right side of the oesophagus. In Ceratodus the pulmonary
veins open into the left side of the auricle. Lungs were
almost certainly present in the Osteolepidoti. These animals
lived or live in fresh water in which the oxygen- content is
low (owing to desiccation and accumulation of decomposing
organic debris), and branchial respiration is supplemented by
the intake of bubbles of air through the mouth. Indeed,
Protopterus is able to withstand periods of drought when the
swamps in which it lives dry up, by burying itself in the mud
and breathing by its lungs. The lungs of higher vertebrates
are easily derived from those of the fish just described. It is
possible that the lungs respresent a pair of gill-pouches behind
the remainder, and which ceased to open to the exterior. They
are formed from the endoderm and communicate with the
alimentary canal, and they preserve their blood-supply from
the vessel of the last branchial arch.

In the higher bony fish, the lung is single and modified.

340 COMPARATIVE ZOOLOGY OF CHORDATES

In the primitive form Amia, it is still supplied with blood from
the last branchial artery and its walls are cellular, but in all
the rest it derives blood from the coeliac artery and dorsal
aorta, and its walls are not adapted for the diffusion of gases
through them, except in a restricted vascular area. In some
forms it remains connected with the alimentary canal by an
open tube, but in others it is completely shut off (in the adult
condition). In these higher bony fish, the lung no longer
functions as a respiratory organ, but it has become a hydro-
static organ. The quantity of gas which it contains is regulated
by the vascular area just referred to (where oxygen may be
passed from the blood into it or vice versa), and the fish is able
to adapt its specific gravity to that of the depth of the water
at which it is swimming. It is therefore able to maintain
its depth without muscular effort. In these forms it is no
longer a lung, but an air-bladder or swim-bladder. In some
Teleosts, such as the catfish (Amiurus), the swim-bladder
enters into relations with the auditory vesicle, and is connected
with it by a chain of small bones called the Weberian ossicles,
which are derived from the first three vertebrae. In some other
Teleosts, the swim-bladder disappears in the adult, and these
are often found to be bottom-living forms, which live at a more
or less constant depth.

Strange as it may seem, therefore, it is probable that the
lungs were evolved while the vertebrates were still in the
water, and that they gave rise to the swim-bladder by
specialisation.

It is now necessary to turn to the relations which the
olfactory organs bear to the respiratory system. In the
Selachii and the higher bony fish, the nasal sacs have no
connexion with the mouth, but this is not the case in the most
primitive bony fish. In Osteolepis and in the Dipnoi there
are external nostrils on the snout, and they lead to internal
nostrils which open into the mouth-cavity. This condition
is also present in all the Tetrapods. In these forms, therefore,
the olfactory organs are subservient to the respiratory system
in that they enable the respiratory medium (water or air) to
enter the mouth- cavity without having to pass through the

THE RESPIRATORY SYSTEM 341

mouth itself. It may be remembered that in Petromyzon
the nostril is single and confluent with the opening of the
hypophysial sac. The same is true of Myxine, but here the
hypophysial sac opens into the alimentary canal. This con-
nexion between nose and gut is, however, quite different from
that of the other forms just mentioned, and was independently
acquired.

The amphibia when adult breathe air into their lungs, but
the mechanism for doing so is similar to that which the fish
use for breathing with their gills. The floor of the mouth is
lowered and air is taken into the mouth cavity. The mouth
and nostrils are then closed, and the floor of the mouth raised,
which forces the air down the throat and larynx into the lungs.

The method of respiration in the amniotes is more efficient.
The volume of the lungs is increased by the expansion of the
thoracic box, and this is accomplished by movements of the
ribs (assisted in the mammals by movements of the diaphragm).
The muscles concerned in these movements are somatic and
innervated by ventral nerve-roots of the neck and thorax.
The tortoises, whose ribs are, of course, fixed to the carapace
which surrounds them, replenish the air in their lungs by
movements of the neck, arms, and legs.

The lungs of Polypterus, Dipnoi, and amphibia are more
or less hollow sacs. In reptiles the internal surface of the
lungs are increased by foldings of the walls, with the result
that the lungs can no longer be described as simple hollow sacs.
In birds and mammals, this process has been carried still
further, and the lungs are spongy masses of tissue penetrated
by innumerable small air-spaces. In mammals, the internal
surface-area of the lungs may be thirty times that of the
external surface of the body.

The lungs of the chamaeleon are of interest in that they are
produced into a number of blind diverticula or air-sacs.
These air-sacs reach their highest degree of development in
the birds, in which they may occupy a large volume. Air is
led into the air-sacs from the bronchi passing straight through
the lungs, and it then passes back into the lungs where it
oxygenates the blood, and out again through the trachea. The

342 COMPARATIVE ZOOLOGY OF CHORDATES

efficiency of this mechanism lies in the fact that there is a
through- draught right through the lungs. All the air can be
renewed, whereas in other forms, the lungs are blind sacs and
there is always a certain amount of stale residual air at the
bottom of them which cannot be renewed. The efficiency
of the respiratory system has played a large part in the evolution
of the birds, which require a high rate of metabolism in order
to perform the very arduous muscular exertion of maintaining
the body in the air during flight.

Attention may now be turned to two modifications which
may occur in connexion with the respiratory system. The
first concerns the formation of the false palate. This structure
is a secondary roof to the mouth, closing over the original
internal nostrils, and enclosing the nasal passage as far back
as the secondary choana. The secondary choana is opposite
the glottis (the opening through which the pharynx com-
municates with the larynx and trachea, and so with the
lungs), and the whole structure is an adaptation enabling
the animal to breathe and yet have its mouth full of food or
water at the same time. It is especially developed in aquatic
forms such as the crocodile and the whale, but it is character-
istic of the higher Theromorph reptiles and mammals in
general. In the whales the glottis can be pushed right up into
the secondary choana, thus making a closed communication
between the external nostrils (above the surface of the water)
and the lungs, without running the risk of water entering
the latter from the mouth. In the higher vertebrates, and
especially those which frequent deep waters, the windpipe or
trachea is prevented from collapsing by rings of cartilage or
bone.

The fact that respiration in terrestrial vertebrates involves
the pumping of air in and out of the body, has been made use
of in connexion with the production of sound. Bands of
connective tissue stretch across the cavity of the larynx, and
can be thrown into vibration by the passage of the air. These
bands are the vocal cords. In the male frog there are vocal
sacs at the corners of the mouth, and these become distended
with air when the animal " croaks " and act as resonators.

THE RESPIRATORY SYSTEM 343

The larynx and its vocal cords are the organ of voice-
production in the mammal, and the pitch of the sounds can be
controlled by the tension of the cords and the laryngeal muscles.
The false palate acts as a resonator. In the birds there is a
special organ called the syrinx situated at the fork where the
trachea divides into the two bronchi, and it is to the vibrations
of this that the song of birds is due.

It is interesting to note that the power of producing vocal
sounds has evolved parallel with the capacity for appreciating
them, or in other words, the differentiation of the cochlear part
of the ear.

Literature

Goodrich, E. S. Vertebrata Craniata. Cyclostomes and Fishes. Black,
London, 1909.

Oppel, A. Atmungsapparat : Lehrbuch der Vergleichenden Mikrosko-
pischen Anatomie der Wirbeltiere. Part 6. Fischer, Jena, 1905.

/$ viO

CHAPTER XXVI

THE ALIMENTARY SYSTEM

The alimentary system comprises the tube which leads from
mouth to anus, together with the glands attached to it which
aid in the processes of digestion. There is a slight invagination
of the ectoderm at the mouth and anus, forming the stomodaeum
and the proctodeum ; but the remainder, which forms by far
the larger part of the alimentary system, is formed exclusively
from the endoderm. In addition to the digestive glands, the
alimentary canal has a number of derivatives which have been
considered in connexion with other organ-systems. So the
gill-pouches and the larynx and lungs belong to the respiratory
system ; the allantoic bladder forms part of the excretory
system ; while the thyroid gland, which in Gnathostomes and
adult Cyclostomes is one of the endocrine organs, belongs to
the alimentary system in Amphioxus and the larval Cyclostome
(Ammocoete).

The primitive method of obtaining food is by the creation
of a current of water towards the mouth by means of cilia.
This is the case in Amphioxus (and the Ascidians). Here the
endostyle is accessory to the alimentary system in that it
ensures that the particles of food reach the intestine instead of
being lost with the current of water flowing out through the
gill-slits. The method of feeding by means of a sucking
mouth and a rasping tongue which is characteristic of the
Cyclostomes, is secondary and specialised. In all the Gnatho-
stomes, the most anterior visceral arches, between the mouth
and the first visceral cleft, become modified and adapted for
seizing food, and give rise to the jaws. This method enables
food of larger size to be obtained than is possible by the ciliary

344

THE ALIMENTARY SYSTEM 345

method, and the Gnathostomes were thereby able to evolve
to greater size. In these forms also, the jaws are garnished
with teeth, and the nature and shape of the teeth varies with
the kind of diet. Not only do teeth assist in seizing prey,
but in the higher forms they serve to grind it up small, which
is an aid to the processes of digestion. In the higher verte-
brates, the tongue may also be used for obtaining food as in
the case of the chamaeleon, and it assists in the process of
swallowing.

In the primitive forms the alimentary canal or gut runs
straight from mouth to anus, as in Amphioxus and the Cyclo-
stomes. In these two forms the lining of the gut is ciliated,
but in higher forms the ciliation is restricted to certain anterior
regions. Food is propelled along by peristaltic action of the
smooth muscle in the gut- wall.

Beginning in the Selachians, a special part of the gut is
modified as a receptacle in which food is treated with digestive
juices secreted by its walls, and in which absorption does not
take place. This is the stomach, and in all Gnathostomes it is
an enlarged region of the gut, kinked to the left side of the
body, and situated between the non-digestive supply-tube or
oesophagus and the absorbent intestine. The intestine of the
Gnathostomes is greater in length than the space which con-
tains it, with the result that it is more or less coiled. In the
higher forms the intestine is very considerably longer than the
body itself. The effect of this is to increase the surface of
absorption. A modification which serves the same function
is the spiral valve in the intestine, which is feebly developed
in Petromyzon and well developed in the Selachians. The
spiral valve is also present in Ceratodus and in a few primitive
bony fish (Teleostomes), but it is lost in all higher forms. A
peculiarity of the stomach of the higher bony fish (Teleostei)
is the development of a number of blind outgrowths (pyloric
coeca) from the hinder end of the stomach. The wall of the
intestine is well supplied with blood-vessels belonging to the
hepatic portal system, and with lymphatic vessels or " lacteals."

The oesophagus in birds is modified and enlarged into a
crop or temporary storage place. The stomach is divided

346 COMPARATIVE ZOOLOGY OF CHORDATES

into two regions. The first of these, the proventri cuius, has
soft walls provided with glands. Next comes a hard- walled
gizzard, in which the food is crushed with the help of stones,
for the bird has no teeth and so cannot perform this function
in the mouth.

In mammals, the stomach is simple except in a group of
the Ungulates called the Ruminants, where it is divided into
several parts. These animals " chew the cud," and their
stomach is modified in consequence. The food (grass) is
swallowed down (without being masticated) into the anterior
divisions of the stomach composed of the paunch and the
" honey-comb." When the animal ceases feeding, the food
is brought up to the mouth again and thoroughly chewed and
salivated. It then redescends to the other divisions of the
stomach, termed the maniplies and the abomasum. The latter
has glandular walls, and secretes digestive juice.

The region between the intestine and the anus is short and
straight in the lower forms, and is called the rectum. It is
usually marked off from the intestine by the development of a
constriction, the ileo-colic sphincter, and by one or two blind
diverticula or coeca. In the Tetrapods the region between the
intestine and the anus becomes longer and coiled, and it becomes
possible to distinguish a so-called large intestine (on account
of its diameter) or colon which is coiled, from the terminal
straight rectum. The intestine proper is then called the small
intestine. The large intestine is concerned with the absorp-
tion of water from the non- digested remains of the food, a
function of importance for animals which inhabit dry land.

The ccecum in mammals may be very large, as in the rabbit,
and this condition is common in herbivorous animals. The
ccecum contains a colony of bacteria whose function it is to
attack the cellulose of the food and to digest it. In other
forms the ccecum is reduced, and may be represented only by
its tip, the vermiform appendix, as in man.

The anus primitively opens to the outside in conjunction
with the urino-genital ducts, forming a cloaca. This con-
dition is departed from in the higher bony fish (Teleostomes)
and in the higher mammals or Ditremata (Marsupials and

THE ALIMENTARY SYSTEM 347

Placentals), in which the alimentary and urino-genital systems
open separately to the exterior.

The first special digestive gland to appear is the liver,
which is present in Amphioxus. In the higher forms it
connects with the anterior part of the intestine by the bile-
duct, and usually possesses a gall-bladder. A rudimentary
pancreas appears in the Cyclostomes, and in higher forms it is
well developed, connecting with the intestine by one or more
pancreatic ducts. It may be mentioned that in mammals at
least, the secretion of pancreatic juice is started by a substance
" secretin," which is released from the lining of the intestine
into the blood-stream and carried therein to the pancreas
which it stimulates. This is of interest, for secretin was the
first of the hormones (chemical stimulants with specific effects
and which are carried about in the blood) to be properly
recognised.

Salivary glands are lacking from the lower water-living
chordata, as is readily understood when it is remembered
that a current of water is constantly sweeping through the
mouth to the gill-slits. Salivary glands make their appearance
in the Amphibia. In the snakes, some of the salivary glands
may be modified into poison-glands.

Literature

Oppel, A. Der Magen, Schlund und Darm, Mundhohle, Bauchspeichel-
driise und Leber. Lehrbuch der Vergleichenden mikroskopischen
Anatomie der Wirbeltiere, Parts I, 2, and 3. Fischer, Jena, 1896.

CHAPTER XXVII

THE EXCRETORY AND REPRODUCTIVE SYSTEMS

Amphioxus is unique among chordate animals in possess-
ing true nephridia. These organs are situated above the
gill-slits, their solenocytes project into the lateral dorsal
coelomic cavities, and their external openings lead into the
atrial cavity. The gonads of Amphioxus are segmental, and
situated at the ventral ends of the original myoccelic cavities.
The germ-cells of each segment make their way independently
to the exterior (actually into the atrial cavity) by pores in the
body- wall.

It is possible that the region of the ccelom, which in all
higher Chordates is concerned with the formation of the ex-
cretory organs, corresponds to that region which in Amphioxus
forms the gonads.

Originally there must have been a continuous row of little
tubes on each side of the body, leading out of the splanchnocoel
into a duct which collected from them all, and opened to the
outside at or near the anus. These little tubes represent the
original connexion between the myoccel and the splanchnocoel
(the nephrocoel, in the intermediate cell-mass), and consequently
they are segmental in arrangement : one tubule on each side
to each segment. Such an arrangement has been called an
archinephros, and the duct the archinephric duct, and this
condition is almost fulfilled in the Cyclostome Bdellostoma.
Here a continuous row of tubules is formed, but an inter-
mediate section of them disappears, thus separating an anterior
batch — the pronephros — from a more posterior set — the
mesonephros. In other forms the pronephros appears first,
and the duct which is formed by the backward growth of the
ends of the tubules is the pronephric duct. The pronephros

348

EXCRETORY AND REPRODUCTIVE SYSTEMS 349

is the functional larval kidney in the lower vertebrates, and the
pronephric duct grows back to the cloaca without waiting for
the mesonephric tubules to develop. When these form, they
find the pronephric duct ready-made to receive them. After
receiving the mesonephric tubules the pronephric duct becomes
known as the mesonephric duct, and the pronephros
degenerates (except in the bony fish Fierasfer and Gobiesox).
The functional kidney in the adult fish or amphibian is the
mesonephros.

The cavity of each tubule (pronephric or mesonephric)
becomes shut off from the splanchnocoel, although the opening
of the tubule into the splanchnocoel (the ciliated funnel or
ccelomostome) may persist (as in Selachians and amphibia)
on the median side of the occlusion. The cavity of the tubule
now becomes known as a Bowman's capsule, and its wall is
indented by capillaries from the dorsal aorta and leading to the
posterior cardinal vein, forming the glomerulus. Bowman's
capsule and the glomerulus together form a Malpighian
corpuscle. Primitively, these corpuscles are segmen tally
arranged, and this condition is retained in Myxine. In other
forms the number of Malpighian corpuscles is greatly increased
by the formation of others by budding.

In the Cyclostomes, the germ-cells in the two sexes are
shed into the coelomic cavity, and make their way to the exterior
by a pair of pores at the base of the mesonephric ducts. In
all higher forms the sperms are never shed into the ccelom,
but led by vasa efferentia to the vas deferens, primitively
passing through the tubules of the mesonephros. The vasa
efferentia are the ccelomic funnels leading into the tubules,
and the mesonephric duct forms the vas deferens or Wolffian
duct. In addition, on each side there is another duct, in the
embryo. This is the Miillerian duct or oviduct, which develops
in the females but becomes reduced in the males. The
Miillerian duct in the Selachians arises by splitting off from the
Wolffian duct, but in other forms it grows back independently
from its opening into the ccelom (the oviducal funnel or
Fallopian tube) in front, to the cloaca behind. The eggs then
are shed into the ccelom whence they enter the oviducts,

350 COMPARATIVE ZOOLOGY OF CHORD ATES

whereas the sperms pass down a duct which serves for them as
well as for the evacuation of urine from the kidney. Thus,
while the Cyclostome has a single kidney-duct on each side in
both sexes, and the germ-cells do not pass through it, in the
fish and amphibia typically the females have two ducts on each
side. One of these is the Wolffian duct evacuating the urine,
the other is the Mullerian duct leading out the eggs. In the
males of fish and amphibia the Wolffian duct evacuates both
urine and sperms ; the Mullerian duct is reduced, and in the
Selachian is represented only by the funnel and the sperm-
sacs. This condition is typically represented in the frog and
newt.

In several different groups of fish and amphibia, this
arrangement is slightly altered by the separation of a part of
the Wolffian duct conveying the sperms (vas deferens) from
another part which drains the kidney (mesonephric ureter,
not a true ureter). By this means, the sperms avoid going
through the excretory part of the kidney, and this condition
is found in the Dipnoan Protopterus, Polypterus, the Teleosts,
and in such toads as Alytes, in all of which it has been inde-
pendently developed. In Scyllium, it will be remembered
that only the hinder part of the mesonephros is excretory in
function, and the sperms pass through the anterior part.

In Lepidosteus and many Teleosts, the ccelomic wall
surrounds the ovary forming a sac which joins on to the oviduct.
In this manner the ovary is completely shut off from the
ccelomic cavity, and consequently the eggs are not shed into
it, but led directly to the exterior.

In the amniotes, the functional kidney in the adult is the
metanephros, and the metanephric duct or ureter is an out-
growth from the Wolffian duct. The Wolffian duct is therefore
spared the function of evacuting urine, and it persists only
in the male, where it functions solely as a vas deferens for
the sperm. The mesonephric tubules form the epididymis.
The Mullerian duct disappears in the male, and the Wolffian
duct disappears in the female. The Mullerian duct persists
in the female as the oviduct. In the adult bird, only the left
ovary and oviduct persist.

EXCRETORY AND REPRODUCTIVE SYSTEMS 351

Except in the Monotremes, the base of the oviduct in the
mammals becomes specialised to form the uterus, in which the
embryos undergo development. According as to whether
the bases of the two oviducts remain separate or become
fused together the uterus may be double or single.

Another peculiarity of the mammalian reproductive
system, is the fact that in the male, the testes usually leave their
position in the roof of the abdominal cavity, and descend into
scrotal sacs (see p. 150).

While claspers or copulatory organs are present in the
males of several fish, the amphibia lack them (except the
Gymnophiona), and fertilisation has to take place in water
since the sperm require a fluid medium. In Anura the eggs
and sperm are shed together into the water. In the newts, as
a rule, the male lays a packet of sperm, and then gives a display
of " courtship " in front of the female to stimulate her to
pick up the packet with her pelvic limbs and place it in her
cloaca. During the breeding season the male has specially
developed secondary sexual (epigamic) characters, such as the
crest and the colour of the belly, which assist in the courtship
activities.

In the amniotes, fertilisation is internal, and the sperms
are introduced into the cloaca of the female by the copulatory
organ or penis of the male. In this way, the amniotes are
independent of water for fertilisation.

A feature of considerable interest is the increase in care
of the young after they are hatched, by the parents. This
increases in the higher groups of vertebrates, and all stages
can be found in the evolution of the family, from the condition
of Amphioxus where fertilisation takes place in the sea water
outside the parents which are in no way concerned with the
development of the young, to that of man. This evolution
has involved the development and perfection of characters of
behaviour as well as those of structure. The first step in this
direction is usually the habit of protecting the eggs until the
young hatch. In several species of fish, the eggs are laid in
holes or in nests specially prepared by the parents, and the
male remains on guard. This habit is resorted to by Proto-

352 COMPARATIVE ZOOLOGY OF CHORD ATES

pterus and Lepidosiren among the Dipnoi, by Amia, several
catfish, and the stickleback, only to mention a few. In some
of these cases there are interesting adaptations for ensuring a
sufficient supply of oxygen to the eggs. So in Lepidosiren,
the pelvic fin of the male becomes modified into a tuft-like
organ well supplied with blood, from which oxygen diffuses
out into the water. In some catfish, the eggs are carried about
by the parent (usually the male), and so are continually exposed
to fresh sea water. Ichthyophys (Gymnophiona) coils itself
round its eggs in a burrow, as do some snakes such as the
python. Several Anura lay their eggs in nests specially
prepared ; others make living nests of themselves. In Pipa
the eggs are placed on the female's back, where they sink into
pits and undergo development ; the male Rhinoderma carries
the eggs in large vocal sacs ; Hylambates carries the eggs in
its mouth. Alytes is peculiar in that pairing takes place on
land, and the eggs, which are tied together by strings of slime,
are carried about by the male, wound round his legs. When
the young are about to hatch, the male takes them to the water
and abandons them. In some viviparous snakes such as the
viper, the young remain with the parent for a time. It
happens in some forms (e.g. viper) that the egg is hatched
while still in the oviduct, without being laid. This condition
is called ovo-viviparous.

The eggs of birds require a constant high temperature for
their development, and this necessitates the uninterrupted
attention of the parents. (In the Megapodes, the eggs are
laid in heaps of decaying vegetable matter, the heat of which
enables them to incubate.) Nearly all birds lay their eggs in
nests, which serve to protect the eggs from enemies, cold and
damp. Nests are constructions in which as a rule both sexes
take part, and courtship and display play an important part in
keeping together the members of a pair to perform the various
duties which devolve on them. The male is usually the
active partner in courtship, and often possesses brilliant
secondary sexual characters used for the purpose. After the
nest has been built, these duties include the collecting of food
for the sitting partner, and for the young when they hatch in

EXCRETORY AND REPRODUCTIVE SYSTEMS 353

a helpless condition. In some species, this food is freshly-
caught, in others it is regurgitated, while in the pigeon the
lining of the crop comes away forming a mucous secretion
known as " pigeon's milk."

In all mammals, the young are fed on milk produced by the
mammary glands of the mother, and they are born alive in all
except the Monotr ernes, in which they are hatched from eggs.
The mammary glands of the Monotremes are primitive in that
they do not have proper teats, but merely exude the milk
which is lapped up by the young. In the Monotremes and
Marsupials, the ventral surface of the belly of the female is
modified to form a pouch, or marsupium. In the Monotreme,
the egg is hatched in the pouch. The young Marsupial is born
very early after a short period of gestation in the uterus, and
in a very undeveloped condition. It becomes attached to a
teat of the mammary glands which are situated in the pouch.
There it completes its development. In the higher mammals,
or Placentals, the allantoic placenta is well developed, and the
period of gestation is long. During this time the embryo is
able to develop to a high degree of perfection, such as would
not be possible without a lasting physiological connexion with
the mother in the form of a placenta. After birth the young
is supplied with milk by the mother until it is weaned and
able to feed for itself. Another feature of the mammals is
that they go through a period of " childhood," during which
they play, fed by the mother and protected by her and the
father. Parental instincts reach their highest form in man,
whose superiority in mind and body is conditioned by the
relatively very great length of time spent in development,
both before and after birth.

There may be one or several Embryos developing in the
uterus at the same time. Each embryo develops from a
separate egg except in cases of true twinning ; here, as in the
armadillo and possibly in man, the twins arise by a process
of fission of one blastocyst which has developed from one
egg-

2 A

CHAPTER XXVIII

THE HEAD AND NECK

The structure of the head in an adult vertebrate animal is
somewhat complicated, and bears little resemblance to the
simpler segmented nature of the trunk-region. The segmenta-
tion is obscured, added to which there is the complication
introduced by the presence of the special paired sense-organs
(nose, eye, and ear) and of the gill-slits. The somites do not
all form straightforward myotomes as in the trunk, but give rise
to the eye-muscles ; and lastly, it is difficult to recognise the
segmental nerves because the dorsal and ventral nerve-roots
remain separate. Nevertheless, the head is built strictly on a
segmental plan, and it is easy to unravel its structure by
considering early stages of development.

The embryo of the dogfish, for example, passes through a
stage in which the mesoderm on each side of the body is
segmented into a complete row of somites, from the front to
the hind end of the body. There is no difference between
the somites of the future head-region and those of the trunk,
and they grade insensibly into one another. The ist somite
is, however, peculiar in that it is connected with its fellow of the
opposite side by a strand of mesoderm-cells which passes in
front of the tip of the notochord. Such a connexion would
be impossible between somites situated farther posteriorly,
because the notochord separates those of one side from those
of the other. The ist somite is called the premandibular
somite, and it is innervated by a ventral nerve-root : the
oculomotor. The 2nd somite is rather larger than the others,
it is called the mandibular somite and is innervated by the
trochlear nerve. The 3rd somite is the hyoid somite, and it is

3 "54

THE HEAD AND NECK

355

innervated by the abducens. These first three somites will
become differentiated into the extrinsic eye-muscles, and they
all lie in front of the auditory vesicle, for which reason they are
called the prootic somites.

The 4th somite is the 1st of the metotic somites, and it is
similar to the ones following it. It and the 5th somite in the
dogfish eventually disappear, being squashed underneath the
large developing auditory sac, and they either do not have,
or do not retain, any ventral nerve-roots. In Petromyzon,
however, no somites are lost, and the 4th becomes the most

Fig. 167. — Reconstruction of the head of a dogfish embryo, showing the

segmentation.
77/, oculomotor ; IV, trochlear ; V, trigeminal ; VI, abducens ; VII,
facial ; IX, glossopharyngeal ; X, vagus ; nerves ; as, auditory sac ;
gi to #3, first to third gill-slits ; hm, hypoglossal muscles ; hn, hypoglossal
nerve ; op, ophthalmicus profundus nerve ; s, spiracle ; si to s8, first to
eighth somite ; sn, spinal nerve ; the arrows show the position of the posterior
limit of the neurocranium : P, in Petromyzon ; Sc, in Scyllium ; Sq, in
Squalus.

anterior of the myotomes of the body. In the dogfish, it is
the 6th somite which gives rise to the most anterior myotome
of the body.

It is now necessary to turn to the relations which the dorsal
nerve-roots bear to the somites. Above the premandibular
somite, the cells of the neural crest group together to form the
ganglion of the ophthalmicus profundus nerve. This nerve
is lost in adult Scyllium, but it is present in Squalus, and it
is the dorsal root of the 1st segment, corresponding to the
oculomotor.

356 COMPARATIVE ZOOLOGY OF CHORDATES

Above the mandibular somite is the ganglion of the tri-
geminal nerve which is the dorsal root of the 2nd segment,
corresponding to the trochlear. The hyoid somite lies under
the ganglion of the facial nerve, which is accordingly the dorsal
root of the 3rd segment, corresponding to the abducens. It
is possible, therefore, to recognise three prootic segments.

The glossopharyngeal nerve is the dorsal root of the 4th
segment, overlying the 4th somite. The vagus represents
parts of four dorsal roots joined together, and it corresponds
to the 5th to 8th segments. The ventral roots of the 4th and
5th segments which disappear in the dogfish, are present in
Petromyzon. The ventral roots of the 6th and following
segments are present in the dogfish, innervating the anterior
myotomes, and contributing to the hypoglossal nerve which
accompanies the growth downwards and forwards of portions
of the myotomes to form the hypoglossal muscles.

So far, then, the only difference between the head and trunk-
regions is that in the former, the dorsal and ventral nerve-roots
remain distinct from one another, and that in the three prootic
somites the ganglia of the dorsal roots lie outside the little
somites, instead of median to them as in the trunk.

It is now time to turn to the gill-slits, which arise as out-
growths from the pharynx on each side, and connect with the
ectoderm. The gill-slits are formed at a level below that of
the somites, in the region of the lateral plate, or unsegmented
mesoderm. The connexion of the endoderm of the pharynx
with the ectoderm in the formation of the gill-slits necessarily
obliterates to the mesoderm in places and confines it to the bars
between the gill-slits. These are the gill-bars (gill-arches, or
visceral arches). The remnants of the splanchnocoelic cavity
in this region are restricted to the cavities in the gill-bars
(as in the primary gill-bars of Amphioxus).

Now, down each of these gill-bars or visceral arches there
passes a large branch of a dorsal nerve-root. The most
anterior visceral slit is the spiracle, and separating it from the
mouth is the mandibular arch (or 1st visceral arch) down
which the trigeminal nerve passes. Between the spiracle and
the 2nd visceral slit (1st gill-slit) is the hyoid arch (or 2nd

THE HEAD AND NECK 357

visceral arch), and down this there passes the facial nerve. In a
similar way, the glossopharyngeal nerve passes down the 3rd
visceral arch, behind the 1st gill-slit ; and a branch of the vagus
runs down each of the 4th, 5th, 6th, and 7th visceral arches.

Since the dorsal nerve-roots are segmental in arrange-
ment, the visceral arches are segmental also, for they correspond.
This means that the spiracle and gill-slits are intersegmental
in arrangement. It must be remembered, however, that this
segmental arrangement of the visceral arches is not the same
thing as the primary and fundamental segmentation of the
somites, because the visceral arches lie in the lateral-plate
mesoderm (not in the segmented vertebral plate). The cavities
enclosed in the mesoderm of the visceral arches are really
part of the originally continuous splanchnocoel, and not
myoccelic cavities. This is important, for it explains why the
muscles to which the mesoderm of the visceral arches gives
rise are innervated by dorsal and not ventral nerve-roots,
although they are striped and voluntary. Ventral nerve-roots
only innervate somatic striped muscles derived from the
segmented myotomes of the vertebral plate. Muscles formed
from the visceral mesoderm (inner wall of the splanchocoelic
cavity) in the region behind the gill-slits are of course the smooth
muscles of the gut, innervated by the autonomic system. That
the muscles of the visceral arches should differ from these
latter in being striped and voluntary is due to the fact that,
unlike them, they are attached to skeletal structures. These
skeletal structures support the jaws and the branchial arches,
and their movements are involved in the processes of biting
and breathing, which are related to the outside world. Smooth
muscles are only related to the events which go on inside the
animal.

Since the visceral arches correspond to the segmentation
of the body, the structures in them correspond also. These
consist of the skeletal elements just mentioned, and of the
blood-vessels which run up round the gut from the ventral
to the dorsal aorta. So Meckel's cartilage and the quadrate
correspond to the trigeminal nerve and the 2nd segment, and
the hyomandibula and ceratohyal correspond to the facial

358 COMPARATIVE ZOOLOGY OF CHORDATES

nerve and the 3rd segment. The blood-vessels in these two
arches disappear in the higher vertebrates, but that in the 3rd
visceral arch corresponding to the glossopharyngeal nerve
and the 4th segment of the body becomes the carotid.
Similarly, the systemic blood-vessel corresponds to the 4th
visceral arch (5th segment of the body) down which the first
branch of the vagus nerve runs. The pulmonary artery
corresponds to the 6th visceral arch (7th segment of the body)
down which the 3rd branch of the vagus runs.

The segmentation of the head is now clear, and it may be
asked how many segments of the body does the head occupy ?
Before this can be answered it is necessary to be clear as to
whether " the head " is to be regarded as everything in front
of the hindmost part of the skull (occipital arch), or whether it
extends as far back as the gill-slits. In point of fact, it is
necessary to distinguish between the dorsal or " neural head,"
and the ventral or " visceral head," for they differ in extent.
The hindmost region of the neural head is indicated by the
position of the occipital arch of the skull, that of the visceral
head by the position of the last visceral arch. It is interesting
to find that the number of segments in either kind of " head "
varies in different animals. The neural head of Petromyzon
occupies 4 segments, that of Scyllium 7, that of Squalus 9,
that of Amphibia 6, that of Amniotes probably 8. Similarly,
the number of segments in the visceral head varies from 10
in Petromyzon to 8 in Scyllium, 9 in Hexanchus, and 10 in
Heptanchus, while the number is reduced in land- vertebrates
which no longer breathe by gills. In the formation of the
neural head, more and more segments of the trunk are incor-
porated during evolution. The occipital arch is therefore not
formed by the same segment in different groups of vertebrates,
but this fact does not affect the homology of the occipital
arches. This structure has a representative in the common
ancestor of Craniates, whatever segment of the body it may be
in. In the more primitive forms the neural head is short,
and the occipital arch becomes displaced backwards. The
primitive extent of the visceral head is probably about 10
segments, for not only is this the number in Petromyzon and

THE HEAD AND NECK

359

•33

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3rd.

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Rectus superior,
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myzon.

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myzon and

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360 COMPARATIVE ZOOLOGY OF CHORDATES

in the primitive selachian Heptanchus (both of which have 8
visceral slits, although Petromyzon loses one), but Amphioxus
in its development passes through a stage (the so-called
" critical stage ") when it has 8 pairs of symmetrically arranged
gill-slits.

The relations and destinies of the three prootic somites are
constant in all vertebrates above the Cyclostomes (in which the
eyes are degenerate), and they may now be considered.

In the first place, it is interesting to note that the ist or
premandibular somites correspond to the anterior head-
cavities or anterior gut-diverticula of Amphioxus, and that
the front ends of all chordates correspond. Just as in
Amphioxus the anterior head- cavity (of the left side) opens into
an ectodermal pit (the preoral pit), so in Selachians (Torpedo)
the premandibular somites open into an ectodermal inpushing
(the hypophysis), and this connexion between premandibular
somites and hypophysis also occurs in some reptiles and birds.
The hypophysis is therefore probably homologous with the
preoral pit of Amphioxus. This connexion between a meso-
dermal pouch and the ectoderm is similar to that which occurs
in Balanoglossus and the larvas of Echinoderms, forming the
co-called " water-pores " and " proboscis-pores." (It may
be mentioned that the so-called " anterior head-cavities " of
some Selachians are merely parts of the premandibular
somites, and have no segmental value.)

The morphological anterior end of the body in Craniates is
a point near the middle of the mesodermal strand connecting
the premandibular somites with one another. Just behind
this point is the front end of the notochord, and the preoral
gut ; just in front of it the hypophysis grows in from the
superficial ectoderm, and just above it is the floor of the fore-
brain near the optic chiasma and the point of closure of the
neuropore. This morphologically anterior point of the animal
is represented in many skulls near the dorsum sellae, which
lies immediately behind the pituitary body. That part of the
head which lies in front of this is the result of secondary
forward growth.

In the conversion of the prootic somites into the eye-muscles

THE HEAD AND NECK

361

in the dogfish, for example, the walls of the somites become
thickened by the formation of muscle-fibres, and the contained
ccelomic cavity is obliterated. The premandibular somite
wraps round the optic nerve from behind, and becomes divided

Fig. 168. — Reconstructions showing stages in the conversions of the first
three somites into the extrinsic eye-muscles in a dogfish.

A to E, successive stages, ab, abducens ; exr, external rectus muscle ;
/, facial nerve ; hy, hyoid or 3rd somite ; infr, inferior rectus muscle ;
inob, inferior oblique muscle ; inr, internal rectus muscle ; m, mandibular
or 2nd somite ; oc, oculomotor nerve ;" opn, optic nerve ; p, profundus
ophthalmicus nerve ; pa, trochlear nerve ; pm, premandibular or 1st
somite ; ros V and VII, superficial ophthalmic branches of trigeminal and
facial nerve ; suob, superior oblique muscle ; sur, superior rectus muscle ;
tr, trigeminal nerve.

into four pieces. The two dorsal portions are the internal and
superior recti muscles, the two ventral portions are the inferior
oblique and the inferior rectus muscles. The mandibular
somite grows forwards above the premandibular and gives

362 COMPARATIVE ZOOLOGY OF CHORDATES

rise to the superior oblique ; and the hyoid somite, also growing
forwards, becomes attached to the posterior part of the eyeball,
forming the external rectus muscle.

In higher vertebrates, the prootic somites are not always
separately recognisable as such, and in these cases the eye-
muscles appear to arise from masses of mesenchymatous
mesoderm-cells. In others, the manner of development is
the same as that described for the dogfish.

In the lower vertebrates, the head has no greater mobility
than any other part of the body ; indeed, in several bony fish
it has none, for the back of the skull is connected with the
pectoral girdle by a chain of bones. The neck has not yet
evolved in these animals. The neck is a region of flexibility
which enables the head to be moved without moving the body.
This is made possible by the specialisation of the most anterior
vertebrae. In reptiles, the differentiation of the first two
vertebrae into the atlas and axis appears, and the head is then
able to hinge on the transverse axis (as in signing " yes"), and
on the longitudinal axis (as in signing " no "). Movement of
the head to the side is effected by the flexibility of the next
posterior vertebrae, the ribs of which do not get attached to
the sternum. In this way, the cervical vertebrae differ from
the thoracic. When the neck is very long and capable
of extensive twisting, it is common to find vertebrarterial canals,
formed between the centra and the ribs fused on to them.
These canals protect the artery from being kinked when the
neck is twisted. In mammals, the neck contains seven
vertebrae, except in three species only.

Literature

Goodrich, E. S. " Proboscis Pores " in Craniate Vertebrates. Quarterly-
Journal of Microscopical Science, vol. 62, 1917.

On the development of the Segments of the Head in Scyllium.

Quarterly Journal of Microscopical Science, vol. 63, 1918.

van Wijhe^J. W. Ueber die Mesodermsegmente und die Entwicklung der
Nerven des Selachierkopfes. de Waal, Groningen, 1915.

CHAPTER XXIX

THE FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM

It is usual to describe and to refer to a nerve with regard to
the segment of the body in which it finds itself. So one may
speak of the facial (7th) nerve, or of the 2nd spinal nerve, and
designate by these terms well-marked structures, visible by
dissection. Nerves are composed of fibres formed of long
filaments (or axons) which are produced by cells (neurons),
the " bodies " and nuclei of which are situated in the brain
and spinal cord, or in the swellings on certain nerves, called
ganglia. But all the fibres of any given segmental nerve do
not serve the same function. The function of a nerve is to
conduct impulses. If the conduction is towards the brain
and spinal cord (which together are called the central nervous
system) from sense-organs, the fibres are called afferent or
sensory. If the conduction is from the central nervous
system outwards towards muscles or glands, the fibres are
called efferent or motor. Sense-organs may be of many
different kinds and appreciate various sorts of stimuli, such as
light, sound, pressure, vibration, pain, etc., but from the fact
that they do receive these stimuli they are called receptors.
On the other hand, muscles and glands are structures which
" do something," and are consequently called effectors.

A large part of the life of an animal is taken up with adjust-
ing itself to different conditions, and these conditions may be
of two kinds. There is the outside world with which the
animal keeps in touch by means of its receptors at or near the
skin : eyes, ears, lateral-line organs, and the skin itself. These
are the exteroceptors. The movements which the animal
makes in response to the outside world are largely locomotory,
and brought about by the muscles of the body- wall and limbs.

363

364 COMPARATIVE ZOOLOGY OF CHORDATES

These muscles are striated and voluntary. In order that such
movements may be properly coordinated, the animal must
have some information (unconscious, of course) of the existing
state of its muscles, tendons, and joints. This is supplied by
sense-organs which are situated in these structures, and are
called proprioceptors.

At the same time, there is a " world " within the animal,
and sensations arise from stimuli which start from organs

X

da

amg-l-p

-ama.

Fig. 169. — Diagrammatic transverse section through the trunk of a verte-
brate showing the relations of the nerve-roots, sympathetic ganglia,
and the functional components.

ama, anterior mesenteric artery ; amg, anterior mesenteric ganglion ;
da, dorsal aorta ; dr, dorsal nerve-root ; g, gut ; n, notochord ; re, ramus
communicans ; sg, spinal ganglion ; sm, somatic motor region of grey
matter ; ss, somatic sensory region ; sy, sympathetic ganglion ; vm, visceral
motor region ; vr, ventral nerve-root ; vs, visceral sensory region.

such as the stomach, intestine, or bladder, and the functions
connected with them. The sense-organs of taste are largely
of use in connexion with what is about to enter the alimentary
canal, and they also belong here. Such sense organs are called
interoceptors. The reactions to these stimuli take the form of
secretions on the part of glands, and contractions of the
muscles of the alimentary canal, bladder, arteries, or oviduct.
Such muscles are always smooth and involuntary.

FUNCTIONAL DIVISIONS OF NERVOUS SYSTEM 365

It is possible, therefore, to make out four main divisions
of the nerves according to their function :

those which convey sensory impulses from the outside
world, somatic sensory, or afferent ;

those which convey sensory impulses from the inner
world, visceral sensory, or afferent ;

those which convey motor impulses to the smooth muscles
of the viscera, visceral motor, or efferent ;

those which convey motor impulses to the striped muscles
of the body-wall and limbs, somatic motor, or efferent.
Each of these functional systems are called components, and
as the same components can be found in several different
nerves, it is interesting to study the nerves according to the
components which they contain. In this way a classification
of nerves is obtained which, as it were, runs at right angles to
the classification according to the segment of the body in which
they lie. Further, the different components occupy special
parts of the central nervous system, and the evolution of the
latter, and especially of the brain, has been largely controlled
by the positions and relations of these " centres."

In an ordinary spinal nerve of any vertebrate above the
Cyclostomes, there are two roots : one dorsal and one ventral,
and they join to form a mixed nerve. The mixed nerve also
sends a branch (ramus communicans) to a sympathetic ganglion.
Now, the dorsal root is made of fibres of afferent (sensory)
neurons, and the ventral root is composed of efferent (motor)
ones. Accompanying the anatomical division into dorsal and
ventral roots, there is therefore an important physiological
distinction.

The cell-bodies of the afferent neurons are situated in the
ganglion which is always present on the dorsal root in all
chordates above Amphioxus. This means that the receptor
cell itself does not convey the impulse to the central nervous
system, this function being served by the afferent neuron of
the ganglion of the dorsal root. (In Amphioxus, and in the
nose of all vertebrates, on the other hand, the primitive con-
dition characteristic of many invertebrates persists : that is,
the receptor sensory cell itself produces an axon which runs

366 COMPARATIVE ZOOLOGY OF CHORDATES

into the central nervous system and conveys the impulse
thither. There is, therefore, no ganglion on the dorsal root
of the nerves of Amphioxus, nor on the olfactory nerve in any
vertebrate.)

After running into the central nervous system through the
dorsal root, the afferent fibres terminate and make synaptic
connexions with other neurons. Now the neurons in the
spinal cord have their cell-bodies in the grey matter which is
central, while the surrounding white matter is made up of the
axons (fibres) which pass up and down the cord to higher or
lower levels. The grey matter of the cord can be separated
into four longitudinal regions on each side. The most dorsal
strip is where the fibres of somatic afferent neurons terminate.
Beneath this is the place where the visceral afferent neurons
end. Under this again is the region which contains the cell-
bodies of the efferent visceral neurons ; and lastly the most
ventral part of the grey matter contains the cell-bodies of the
efferent somatic neurons. Thus the dorsal half of the spinal
cord is related to afferent and the ventral half to efferent fibres.
As will be seen later, this arrangement is also the fundamental
plan on which the brain is built.

The axons of the efferent neurons run out of the spinal
cord through the ventral root. The somatic efferent neurons
go straight to the striped voluntary muscles of the body- wall,
and to the muscles of the limbs (or fins) and end in them. All
muscles which are innervated direct in this way by ventral
roots are somatic, striped, voluntary muscles derived from the
segmented myotomes. On the other hand, the visceral
efferent fibres leave the mixed nerve by the ramus communi-
cans, and end in the sympathetic ganglia. There they make
synaptic connexions with other neurons which run to the
smooth muscles of the viscera and form the sympathetic
(autonomic) nervous system. The sympathetic system will
be dealt with in greater detail below, but it may be noticed
now that the visceral efferent fibres belonging to this system
never run all the way to the smooth muscle or gland. There
is always another neuron intercalated in the circuit, and
carrying the impulses on from the sympathetic ganglion. The

FUNCTIONAL DIVISIONS OF NERVOUS SYSTEM 367

muscles so innervated are never striped, voluntary nor derived
from the segmented myotomes.

The ramus communicans serves not only for the passage of
the visceral efferent fibres, but also for the visceral afferent
fibres, which then continue to the spinal cord through the
dorsal root.

In the region of the head, a slight complication is introduced
owing to the development of special sense-organs, and to the
fact that the anterior region of the alimentary canal is modified
in connexion with the jaws and gill-arches. There is further
the fact that the dorsal and ventral nerve-roots of the cranial
segments remain separated and do not join to form a mixed
nerve.

The various nerve-components in the head can conveniently
be studied in the dogfish. Leaving aside for the moment the
very specialised visual and olfactory organs, the somatic
afferent system is divided into two owing to the development
of the lateral-line system.

There is, therefore, a general somatic afferent system which
receives impulses from simple sense-organs in the skin corre-
sponding to those in the region of the trunk and spinal nerves.
This component is present in the trigeminal, glossopharyngeal,
and vagus, and their fibres end in the dorsal portion of the
medulla oblongata in a region which may be called the " skin-
brain."

The special somatic afferent system is concerned with the
lateral-line organs and the special member of these which is
the ear. This component is present in the facial (superficial
ophthalmic, buccal and hyomandibular branches), auditory,
glossopharyngeal and vagus, and its centre is also in the dorsal
part of the medulla oblongata. ^3o great is the number of
fibres which end in this way, that the neurons in the medulla
with which the afferent fibres make connexion are also multi-
plied. The result is that this region, which may be called the
" ear-brain," bulges out, forming the tuberculum acusticum.
The special somatic afferent system is also called the lateralis
system, and arises in relation to the dorso-lateral placodes of
the 7th, 9th, and 10th cranial nerves (see p. 194).

368 COMPARATIVE ZOOLOGY OF CHORDATES

The proprioceptive organs are innervated by nerves which
(in the head) run in to the brain through most of the cranial
nerves, including the oculomotor, trochlear, and abducens.
The ear, as an organ of balance, can also be considered as
belonging to the proprioceptive organs.

The visceral afferent fibres collect impulses from the mucous
surface of the pharynx, mouth, and other viscera, and from
the taste sense-organs. In fish, the taste sense-organs are not
confined to the mouth, but may be found all over the surface
of the body. The afferent visceral fibres run in the branches
of the facial, glossopharyngeal, and vagus from the pharynx
and from the anterior and posterior faces of the gill-slits. In
the brain they converge in the medulla oblongata in the
visceral lobe or " taste-brain," beneath the centres for the
somatic afferent system. The visceral afferent system is also
called the communis system. The fibres innervating the
sense-organs of taste are sometimes regarded as forming the
special visceral afferent system, and they arise in relation to
the epibranchial placodes (see p. 194).

The visceral efferent system is complicated by the fact that
the anterior end of the alimentary canal enters into relations
with the outside world. Its opening, the mouth,. is bounded
by the jaws which are under voluntary control, and so enable
the animal to aim at its prey and bite it. In connexion with
this, it is found that the muscles which actuate the jaws are
striated and voluntary, although they are visceral in origin.
The muscles attached to the gill-arches and which perform
respiratory movements are likewise striated. But although
voluntary and striated, these jaw and gill-arch muscles are not
innervated by ventral roots, for they are not derived from
segmented myotomes. Instead, they are innervated direct
by fibres of the special efferent visceral system which run in
the branches of the trigeminal, facial, glossopharyngeal, and
vagus, that pass down behind the mouth, spiracle, and the
several gill-slits respectively. In higher vertebrates, a portion
of the fibres of the vagus become grouped together more
posteriorly, and form the spinal accessory or nth nerve.

The general efferent visceral system innervates smooth

FUNCTIONAL DIVISIONS OF NERVOUS SYSTEM 369

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370 COMPARATIVE ZOOLOGY OF CHORDATES

muscles and glands, and forms part of the autonomic (para-
sympathetic) system. The fibres run through the oculomotor,
facial, glossopharyngeal, and vagus nerves. The centre of
origin of the visceral efferent neurons is for the most part in
the medulla oblongata, beneath the visceral lobe.

The somatic efferent system is concerned with the innerva-
tion of striated voluntary muscles derived from the segmented
myotomes. In the head these are represented by the muscles
which move the eyeballs, and the hypoglossal muscles. This
component is, therefore, to be found in the oculomotor,
trochlear, abducens, and hypoglossal nerves. The centres of
the oculomotor and trochlear are in the mid-brain, those of
the abducens and hypoglossal are in the medulla oblongata.

Fig. 170, G. — The proprioceptive fibres of the general somatic sensory
component.

It may be noticed that the arrangement in the medulla
oblongata of the centres concerned with the various components,
is similar in a general way to that which holds in the spinal
cord. The medulla is the least specialised portion of the brain.

The eyes themselves are part of the brain, and therefore the
optic nerve is not an ordinary nerve. Its fibres are strictly
intra-cerebral throughout their course. They run through
the optic chiasma and end in the roof of the midbrain, which
is enlarged to form the optic lobes, or " eye-brain."

The nasal sacs are lined by sensory epithelium, the cells
of which produce axons growing back into the end-brain.
The latter becomes enlarged to form the olfactory lobes or
" nose-brain."

Expressed in tabular form, the component nerve-systems
are as follows : —

FUNCTIONAL DIVISIONS OF NERVOUS SYSTEM 371

Somatic. Afferent. General.

Visceral.

Eye.

Nose.

Efferent.
Afferent.
Efferent.

Special.

Interoceptors.
General.

Special.

Exteroceptors

Proprioceptors

Lateral-line organs

To myotomic striped muscles.

Taste organs and mucous surfaces.

Autonomic (sympathetic and para-
sympathetic) to smooth muscles
and glands.

To striped visceral muscles of jaw
and gill-arches.

Literature

Herrick, C. Judson. An Introduction to Neurology. Saunders Co.,
Philadelphia and London, 1922.

Johnston, J. B. The Nervous System of Vertebrates. John Murray,
London, 1907.

CHAPTER XXX

THE BRAIN, AND COMPARATIVE BEHAVIOUR

The brain is the anterior region of the spinal cord, modified,
specialised, and enlarged in connexion with the development
of special sense-organs in the anterior region of the body.
That these sense-organs should be accumulated here rather
than elsewhere is due to the fact that chordate animals are
bilaterally symmetrical and move along a definite axis with
one end constantly leading. This end is the first to come into
contact with new surroundings, information concerning which
is of the highest value to the animal.

In order to understand the evolution of the spinal cord and
brain, it is necessary to consider what is known as a reflex arc.
An afferent fibre brings an impulse from a receptor, and if this
afferent fibre were to connect with only one efferent fibre going
to a particular muscle, whenever the receptor was stimulated
the response would be the contraction of this muscle. Nothing
else in the way of response would be possible. But actually
the afferent fibre when it has run into the brain or spinal cord
makes a large number of connexions with other neurons.
Some of these may be efferent neurons and connected with
various effectors ; others may be neurons which carry the
impulse to other parts of the spinal cord or brain : the so-
called association-neurons. By this means a receptor can be
connected up with several effectors, or one effector may be
stimulated by impulses coming from several different receptors.
This possibility of one efferent neuron being used by impulses
coming from several afferent neurons, as a " final common
path " for their reflex circuits, is of the greatest importance.
The efficiency and economy of using what may be called

372

THE BRAIN, AND COMPARATIVE BEHAVIOUR 373

interchangeable standard units (the neurons), capable of an
infinite variety of combinations is one of the main factors of
the success of the higher vertebrates. An animal possessing
this type of nervous system can make many kinds of response,
and indeed by suitable connexions and adjustments there is
no limit to the number of combinations which may be formed
between receptors and effectors. These adjustments are made
in the central nervous system, and they are its function, just
as that of a telephone exchange is to make adjustments between
calling and answering subscribers. The key to the whole
system is the neuron, which is not rigidly fused on to any
other cell, but which can make synaptic connexions with a
great number of other cells and pass impulses on to them.
New connexions can be made, and new kinds of response can
be evolved, which become " conditioned " reflexes, or habits.

The places in the central nervous system where these
adjustments are made are called centres, and they lie in the
grey matter. When the skin of a dog is stimulated by a small
irritation, the receptor in the skin sends an impulse through
an afferent neuron which runs into the spinal cord by the
dorsal root. This neuron makes a synaptic connexion with
an association-neuron in the grey matter of the spinal cord.
The fibre of this association-neuron runs down the spinal
cord in the white matter to the segment of the body where
the hind leg is situated. There it makes a synaptic connexion
with an efferent neuron (in the grey matter) which passes out
through the ventral root to the muscle of the leg. The result
of the stimulus is a jerk or " scratch " on the part of the leg.
This reflex arc illustrates the fact that the function of the
spinal cord is twofold. It contains a number of reflex adjust-
ment-centres (in the grey matter), and it conducts impulses
up or down the cord to different levels (in the white matter).

In the brain there are the primary centres, connected with
the different functional systems of components. These are
the " skin brain," " ear-brain," " taste-brain " (in the medulla
oblongata), the " eye-brain " (in the midbrain) and the " nose-
brain " (in the forebrain). Each of these is a centre where
impulses are received of a particular type (from a particular

374 COMPARATIVE ZOOLOGY OF CHORDATES

component-system), and where adjustments are made with
association and efferent neurons so as to complete the reflex
circuit.

Now if these primary centres are marked off in the brain
of a dogfish, it is found that except for the cerebellum, they
occupy nearly the whole of the brain. Those regions of the
brain which conform to the organisation of the spinal cord are
called the " segmental apparatus " or " brain-stem," and are

Fig. 171. — Transverse sections through the end-brains of, A, dogfish ; B,
frog ; C, Chelonian (reptile) ; and D, shrew (mammal).

Showing the development of the cerebral hemispheres and lateral
ventricles, and the migration of nerve-cells to the surface forming a cortex.

to be distinguished from the additions in the shape of the
cerebellum, and in higher forms the cerebral cortex, which are
" suprasegmental " structures.

The various centres of the brain of the fish are mainly
concerned with their own functional component-system ; there
is not much " team work " between the different centres.
The result is that the behaviour of fish largely takes the form
of reflex responses to stimuli of certain kinds without much

THE BRAIN, AND COMPARATIVE BEHAVIOUR 375

ability for variation or modification by experience. When
any particular sensory system is very highly developed, the
corresponding centre in the brain is enlarged. So in the carps,
which are well supplied with taste-organs, the medulla oblon-
gata is enlarged owing to the expansion of the visceral lobe.
This expansion is due to the increase in number of neurons
in the centre, parallel with the increased number of afferent
fibres coming from the numerous receptors. In the catfish,
the lateral-line system and the " ear-brain " are well developed.

A certain amount of correlation exists between the primary
centres. For example, in the catfish, the " skin-brain " is
connected with the " taste-brain," so that food may be recog-
nised by touch and by taste, and these two types of sensation
co-operate in producing those movements which lead to
feeding. In other words, the reflex arc can pass from one
functional component system to the other.

But this interrelation and team work between primary
centres is best brought about by special correlation- centres,
which are not related to any single primary centre but to
several. The history of the development and evolution of
these correlation-centres really makes up the evolution of the
brain in vertebrates.

In the fish, the correlation-centres are not well developed,
with the exception of the cerebellum. The cerebellum lies
on the dorsal side of the medulla, and from its position its
connexions are mostly with the neighbouring centres : " ear-
brain " and " eye-brain." The ear-brain is concerned with
the balance of the animal as reported from the semicircular
canals, and the eyes report its position relatively to external
objects. At the same time, fibres of the general somatic
system run to the cerebellum and convey impulses of tactile
sensations, and of the state of the muscles and joints of the
body (proprioceptive). As a result of the commingling of
these impulses, the cerebellum comes to be an organ for the
regulation of the posture of the body and of bodily movements.
It keeps the muscles in " tone," and as a whole regulates the
execution of reflexes. It may, in a sense, be compared with
the steam steering gear of a ship, which smoothly carries out

376 COMPARATIVE ZOOLOGY OF CHORDATES

the directions of the man at the wheel ; and it has been called
the head of the proprioceptive system.

In bony fish, the cerebellum is enlarged to form the so-
called valvula which projects forwards beneath the roof of the
midbrain. In most amphibia and all higher vertebrates the
lateral-line system is lost except for the ear, and the cochlea
or organ of hearing is better developed. This affects the
cerebellum to some extent. In mammals two new features
arise, the superficial cerebellar cortex and the pons Varolii.
These develop in connexion with the cerebral cortex.

Apart from the cerebellum, the correlation-centres are
mostly concerned with responses to the outside world. In the
fish there are correlation-centres of this kind in the forebrain
and the midbrain, but the most important are those which
become evolved above the evolutionary stage of the fish, and
which are situated in the sides of the between-brain (thalamus),
the floor of the end-brain (corpus striatum), and the roof of
the end-brain (cerebral cortex).

It is characteristic of these higher centres of correlation
that they are more or less isolated from the primary sensory
centres ; in other words, the correlation-centres are not mono-
polised by any single sensory system. In much the same way,
if the government of a nation sat in the ordinary town-hall of
one of its cities, much of its business would be taken up or
influenced by local municipal matters, and it would be less
able to deal with business affecting not the city but the nation
as a whole.

The thalamus is related by fibres to most of the sensory
centres, and it is among other things the centre where impulses
are analysed into pleasurable and painful. As such, it is of
great importance, for a negative reaction to danger and a
positive reaction to food and to a mate go far to ensure the
perpetuation of the species. Consequently the thalamus has
great survival value in evolution.

The corpus striatum reaches a great development in birds,
in which it is responsible for the correlation of the many and
varied reactions and movements which form part of the
instinctive behaviour. Instinct in birds is highly developed,

THE BRAIN, AND COMPARATIVE BEHAVIOUR 377

and its hereditary nature is due to the fact that the reflex arcs
and association-neurons in the thalamus and corpus striatum

Fig. 172. — Dorsal views of the brains of A, Petromyzon ; B, Scyllium ; C,
Gadus ; D, Ceratodus ; E, Triton ; F, Lacerta ; G, Columba ; and
H, sheep. (Not all drawn to the same scale.)

c, cerebellum ; ch, cerebral hemisphere ; /, flocculus ; hb, hindbrain ;
mb, midbrain ; ol, olfactory lobe ; on, olfactory nerve ; op, optic lobe ; ot,
olfactory tract ; p, pineal ; v, vermis-

conform to a certain pattern which is the result of development.
This also accounts for the fact that instincts are specific, that

378 COMPARATIVE ZOOLOGY OF CHORDATES

is, they occur in all members of a species, just as they all have
kidneys or livers. But because instinct is determined by the
hereditary pattern of the neurons, such behaviour is not easily
modified to meet unusual circumstances. A good example of
such shortcomings is to be found in the meadow pipit, a bird
which is parasitised by the cuckoo. In the pipit's nest the
cuckoo lays an egg, which hatches into a young cuckoo. This
young parasite proceeds to eject the young pipits from the
nest. It was observed on one occasion that the young pipit
so ejected remained just outside the nest, under the mother-
bird's nose, where it lay helpless and squeaking. It never
occurred to the mother-bird to put it back in the nest under
her, and so the young one died. The situation was novel and
had not presented itself to the bird before, and it could not
rise to the occasion. The necessary correlation of neurons
could not be made ; and if it could, the bird would probably
not have been able to act on the experience of a similar previous
occasion. The corpus striatum is not well adapted for such
powers of individual adaptability, though it is very suitable
for ready-made correlations which make the species as a whole
well adapted to a particular routine of life. It is interesting
to note that the behaviour of birds resembles that of insects
in this respect, and that both the brain of the insect and the
corpus striatum of birds are solid compact masses of neurons.
For really effective and unusual correlations such an arrange-
ment appears to be ill suited. The cerebral cortex which
fulfils this very function is shaped not as a solid mass, but as
a layer of neurons, the number of which is augmented by
increasing the area of the layer. The hollow tubular nerve-
cord of vertebrates is very suitable for such an arrangement,
and it is probable that its possession enabled vertebrates to
evolve as they have done, while its absence from insects
prevented them from progressing any further.

The cerebral cortex is a layer of grey matter near the surface
of the end-brain. It is scarcely represented in the fish, and
in the amphibia most of the neurons remain in the primitive
position for grey matter ; that is, near the central cavity.
Some neurons, however, migrate towards the surface. At

THE BRAIN, AND COMPARATIVE BEHAVIOUR 379

the same time, the end-brain has been evolving in another
direction, in that the cerebral hemispheres are formed as
outgrowths containing each a cavity (the lateral ventricles)
communicating with that of the between-brain through the
foramina of Monro. Cerebral hemispheres first appear in the
Dipnoi, and it is possible that they are an adaptation to deficient
oxygen-supply : a matter of great importance, for the brain
requires the purest arterial blood in the body. The formation
of cerebral hemispheres increases the surface of the brain-
tissue relatively to its volume, not only on the outside in contact
with the vascular pia mater, but also on the inside which is
bathed by the cerebro-spinal fluid, itself oxygenated by the
choroid plexus. The migration of the neurons to the surface
to form a cortex may also be an adaptation to oxygen require-
ments, for solid masses of neurons would require large arteries
to enter the brain, and there are indications that the pulse
of large arteries is injurious to the delicate workings of the
neurons.

Another advantage of the cortex type of structure is that
it allows of the arrangement of centres on its surface after
the fashion of a chequer board. The cortex deals with
impulses from the outside world, in animals with sense-organs
sufficiently well developed to give them good representations
of the relations of different objects and events in space. It
is apparently necessary that these representations of objects
in space should remain separate in the brain until finally co-
ordinated. In the same way it would be impossible to judge
which of a number of threads was which, if they were all
tangled up together in a ball. This analogy also introduces
the fact that the function of the cerebral cortex is to receive the
impulses which have already been sorted out in the correlation-
centres, and to judge which of many possible is the best
response to make. The cortex introduces hesitancy and
arbitration into behaviour, which, on the level of the reflex arc,
is immediate and determined.

Another factor to be borne in mind is that the cerebral
cortex is principally concerned with impulses coming from
the exteroceptors, and especially those which, like the eye,

380 COMPARATIVE ZOOLOGY OF CHORD ATES

ear, and nose, can perceive objects at a distance : the distance-
receptors. Responses to stimuli which touch the animal
usually (when successful) abolish the stimulus which evoked
them. So the flea tickling the dog on its skin evokes the
scratch which incapacitates the flea from tickling any more.
Such a response is consummatory. If, however, an animal
sees some of its food at a distance, the response which it makes
to start with does not abolish the stimulus. It sets its limbs
in motion towards the food ; this is an anticipatory response,
and the consummation is not complete until the food has been
reached and eaten. Until this time, the food occupies the
attention of the animal.

In the reptiles, there are three sheets of superficial grey
matter in each cerebral hemisphere. The median sheet is
the hippocampal and the lateral sheet the pyriform cortex.
Both these regions are predominantly concerned with impulses
coming from the nose ; they are not really " impartial "
arbitrators of behaviour. That the cerebral hemispheres
should in early stages of evolution be largely under the influence
of olfactory sensations follows from the proximity of the
olfactory lobes, and from the fact that at these stages the
vertebrates had recently emerged from life in water to dry
land, for the nose is a more highly developed and efficient
organ in air than in water. Being at the most anterior end of
the brain, it naturally took time in evolution before fibres
from all the correlation-centres farther back in the central
nervous system reached them. Part of the middle sheet in
the cerebral hemispheres of the reptile appears to be the fore-
runner of the true cerebral cortex, which reaches such a high
development in the mammals. The hippocampal and pyri-
form cortex are called archipallium, to distinguish them from
this neopallium in which olfactory impulses do not predominate.

In the birds the cerebral cortex is less well developed than
in the reptiles, and the corpus striatum with the attendant
highly instinctive type of behaviour is specialised instead.

In the mammals, the cerebral cortex is developed out of
proportion to the rest of the brain. In the higher mammals
(but not in Monotremes or Marsupials) a special commissure

THE BRAIN, AND COMPARATIVE BEHAVIOUR 381

is developed to link together the neopallium of the two hemi-
spheres ; this is the corpus callosum. The dorsal commissure
of the reptiles, which links together the hippocampal archi-
pallia, persists in the mammals as the hippocampal commissure.

The volume of the neopallium is increased in higher
mammals without much increasing its thickness by throwing
it into folds.

The various regions of the neopallium are connected with
the other centres by projection-fibres, and in addition, these
regions are interconnected by association-fibres. The number
of possible combinations between the neurons is so large that
it baffles the power of the mind to grasp it. As an example,
one million neurons connected together in all possible ways
in groups of two neurons each, gives a number of combinations
with nearly three million figures in it. There are not far off
ten million neurons in the human cerebral cortex.

The neopallium is therefore well fitted to correlate all the
stimuli which the animal receives and to make delicately
adjusted responses to them. It also serves as a storehouse
for impressions which are collected during experience, and an
animal which, in determining the response to be made to a
set of stimuli, considers the results of experience, is said to
show intelligent behaviour. Such an animal has the power of
learning, which is not the same thing as the establishment of
a habit. Habits can be formed in the lower simple correlation-
centres, by means of neurons between certain afferent and
certain efferent neurons. The oftener an impulse passes
along a reflex arc the easier does its passage become, with the
result that the " habitual " response is given to a stimulus.
Some habits so formed may be quite complicated, as when
a piece of music is " learned by heart." This learning is,
however, not necessarily intelligent, because it often happens
that when the musician breaks down he is unable to adapt
himself to the immediate circumstances and continue, but
has to start again at the beginning.

In a similar way animals can be trained to do tricks, or to
thread the " Hampton Court " maze without going down any
of the blind alleys. If a rat be so trained as to " know " a

382 COMPARATIVE ZOOLOGY OF CHORDATES

maze perfectly, and then be placed in a similar maze but with
different lengths of alleys and distances between the turnings,
it will try to run the distances which it ran in the original
maze, and turn where the turnings were in it, and in so doing
it bumps into the walls of the new maze. Its learning was
therefore not intelligent.

It is interesting to compare this case with that of a chim-
panzee confronted with a novel situation. In order to reach
food which was placed out of its reach, it hit suddenly on the
idea of piling packing-cases on one another and climbing up
on them. There is a good deal of evidence to show that in
order to " see " what to do in a set of circumstances, the ape
must really see the goal and the object which it may use as an
instrument, in the same field of view at the same time. There
is little doubt that the eyes have played an important part in
the evolution of the brain : in man the number of afferent
fibres running in from the retina is greater than that running
in from all the spinal nerves of one side put together.

The possession of a cerebral cortex and neopallium does
not adapt the species to any particular set of environmental
circumstances, but instead, it makes all the members of the
species individually adaptable to a large variety of circumstances.
This is one of the chief differences between the higher and
lower vertebrates. All are well supplied with sense-organs,
but the lower vertebrates can only make a small number of kinds
of responses to the stimuli which they receive. The higher
vertebrates have much the same amount of information given
them by their sense-organs, but they use it to much better
advantage owing to the integrative and retentive properties of
the neopallium. The intelligent being does not waste time
on trial and error like Paramecium ; the probable results of
possible actions are weighed up in what must be called the
mind, with the help of experience stored up as memory, and
by means of thought, and the action when taken is intentional.
Lastly, it must be noticed that the possession of such a mind
and its physical basis the neopallium, confers an enormous
advantage on its possessor, and has survival value in evolution.

THE BRAIN, AND COMPARATIVE BEHAVIOUR 383

Literature

Elliot Smith, G. Some Problems Relating to the Evolution of the Brain.

The Lancet, 1910 (1), pp. 1, 147, and 221.
Herrick, C. Judson. Brains of Rats and Men. University of Chicago

Press, 1926.
. Neurological Foundations of Animal Behaviour. Henry Holt & Co.,

New York, 1924.
Kohler, W. The Mentality of Apes. Kegan Paul, London, 1925.
Kuhlenbeck, H. Vorlesungen liber das Zentralnervensystem der Wirbel-

tiere. Fischer, Jena. 1927.
Sherrington, C. S. The Integrative Action of the Nervous System. Yale

University Press, 1920.
Smith, E. M. The Investigation of Mind in Animals. Cambridge

University Press, 1923.

CHAPTER XXXI

THE AUTONOMIC NERVOUS SYSTEM

It has been mentioned that the smooth muscles and glands
of the body are innervated by fibres of the general visceral
efferent component system. It is characteristic of such fibres
that they do not reach all the way from the central nervous
system to the effector in question, but they make synaptic
connexions with other neurons which carry the impulses on
to the muscle or gland as the case may be. There are, there-
fore, two members in each efferent circuit of this kind : a
connector neuron and an exciter neuron. The cell-body of
the exciter neuron may be in a sympathetic ganglion, or it
may be by itself near the muscle which it innervates. In the
former case, the connector neuron is often called the pre-
ganglionic fibre, and the exciter the postganglionic fibre.
Impulses conveyed in this way through the visceral efferent
system to smooth muscles and glands are involuntary, and the
neurons and ganglia concerned in the conduction of these
impulses form the autonomic or involuntary nervous system.
It may be noticed that the autonomic system is essentially
efferent. Although the afferent visceral neurons run up from
the viscera through the ramus communicans, and accompany
the efferent neurons, they conform to the type of the somatic
afferent fibres in that their cell-bodies are in the ganglia on
the dorsal roots, and that they stretch all the way from the
sense-organ to the central nervous system. After separating
off the autonomic nervous system, what is left is called the
cerebro-spinal nervous system, including the brain, spinal
cord, and the somatic fibre-systems.

The autonomic nervous system can be separated into two
divisions, each of which works against the other. The visceral

384

THE AUTONOMIC NERVOUS SYSTEM 385

efferent fibres which come out from the spinal cord in the neck,
thorax, and lumbar regions together constitute the sympathetic
system ; those which leave the central nervous system in the
head from the brain, and from the spinal cord in the sacral
region, constitute the parasympathetic system. The word
" sympathetic " is sometimes loosely used as synonymous with
" autonomic," which introduces confusion. The sympathetic
system may be called the " thoracico-lumbar " outflow, and
the parasympathetic system the " cranio-sacral outflow."

The autonomic system may now be described in greater
detail, in a typical mammal, and commencing with its sympa-
thetic constituent.

The visceral efferent fibres in the cervical, thoracic and
lumbar regions of the spinal cord run out through the ventral
roots and down the rami communicantes to the sympathetic
ganglia situated on each side of the aorta. These fibres are
preganglionic or connectors, and their cell-bodies are in the
grey matter of the spinal cord ; they are surrounded by
medullary sheaths and these rami communicantes are conse-
quently white.

Some of the preganglionic fibres stop in the sympathetic
ganglion corresponding to the segment in which they emerge
from the spinal cord, others continue to the next sympathetic
ganglia in front or behind and end there. In this way, the
sympathetic ganglia of each side become connected together
forming the lateral sympathetic chains, and the ganglia on
them are called the lateral ganglia. In the region of the neck,
several of these lateral ganglia join up close together, forming
the large anterior and posterior cervical ganglia and the stellate
ganglion.

Yet other preganglionic fibres run out through the lateral
ganglia, but do not stop there. Instead, they run on and end
in groups of ganglia situated near the base of the coeliac,
anterior and posterior mesenteric arteries. The most im-
portant of these ganglia, which are called collateral, are the
anterior mesenteric and the posterior mesenteric ganglia.
The long rami communicantes which connect these ganglia
with the spinal nerves are the splanchnic nerves.

2 c

3 86 COMPARATIVE ZOOLOGY OF CHORD ATES

In the lateral and collateral sympathetic ganglia are the
cell-bodies of the postganglionic or exciter neurons. These
run out of the ganglia as non-medullated and therefore grey
fibres, to the muscles of the blood-vessels, heart, stomach,
intestine, oviduct, bladder, and skin ; and some of them run
to the ciliary and iris muscles inside the eye.

The effect of stimulation through the sympathetic system
is to slacken the ordinary muscles surrounding the gut, but to
tighten the sphincters, to tighten the heart and artery muscles,
to tighten the muscles under the skin (which make hair stand
" on end "), to tighten and slacken the muscles of the oviduct,
to slacken the sphincter and tighten the radial muscles of the
iris so that the pupil enlarges.

The structures enumerated above are also innervated by the
parasympathetic system (except the muscles of the oviduct).
The visceral branch of the vagus contains connector fibres
which run to exciter neurons situated on the lungs, heart, and
the muscles of the gut as far as the end of the small intestine.
In the region of the intestine, the exciter neurons lie between
the muscle coats of the gut, forming the plexus of Auerbach.
The remainder of the gut is innervated by connector fibres
which leave the spinal cord in the sacral region through the
ventral nerve-roots, and form the pelvic nerve. These con-
nector fibres run to excitor neurons on the muscles of the large
intestine, on the bladder, on the skin round the anus, and on
the blood-vessels near the urethra.

The ciliary and iris eye-muscles receive innervation by
means of connector fibres which run in the oculomotor nerve
to the ciliary ganglion. This ganglion contains the cell-
bodies of the exciter neurons which run to the muscles in
question in the eye. Two sets of autonomic connector fibres
run through the facial nerve. One goes down the palatine
branch (" greater superficial petrosal ") to the spheno-palatine
ganglion from which exciter neurons run to the lachrymal
glands and the glands of the nose. The other set runs
in the chorda tympani (ramus mandibularis internus facialis
of the dogfish) to the submaxillary ganglion, whence exciter
neurons run to the submaxillary salivary glands. Another set

THE AUTONOMIC NERVOUS SYSTEM 387

of connector fibres runs out in the glossopharyngeal nerve
through the lesser superficial petrosal nerve to the otic
ganglion, from which exciter neurons innervate the parotid
salivary glands.

A very interesting feature of the connector fibres of the
parasympathetic autonomic nervous system is, that while
those of the oculomotor (midbrain outflow) and of the sacral
outflow connect with the central nervous system through
ventral nerve- roots, those of the facial, glossopharyngeal, and
vagus (hindbrain outflow) run in dorsal nerve-roots.

The anatomy of the autonomic system in the head is
slightly complicated. The anterior prolongation of the lateral
sympathetic chain of the trunk continues forwards, accompany-
ing the internal carotid artery as the internal carotid nerve. A
branch of it (the deep petrosal) joins the palatine nerve (forming
the Vidian nerve) and runs to the spheno-palatine ganglion.
This ganglion is also connected to the maxillary branch of the
trigeminal. Another sympathetic branch runs to the ciliary
ganglion, which is also connected to the ophthalmic branch of
the trigeminal. The sympathetic exciter neurons from the
anterior cervical ganglion are thus able to make their way into
the eye to the iris-muscles. The mandibular branch of the
trigeminal connects with the chorda tympani and the sub-
maxillary ganglion.

Further mention must be made of Auerbach's plexus,
which lies between the circular and longitudinal coats of
muscles on the intestine. The neurons which compose it
are the exciters of the parasympathetic outflow through the
vagus, and these neurons branch, the two axon fibres having
different destinations. A mass of food in the intestine stimu-
lates the muscles above it to contract, and those below it to
slacken, thus causing peristaltic action. This is particularly
interesting because peristalsis can occur when all the nerves
to the intestine are cut, which means that local reflex arcs are
formed in Auerbach's plexus. Another plexus (Meissner's),
which lies within the muscle-coats of the intestine, has an
unknown function.

The effect of impulses travelling out through the para-

388 COMPARATIVE ZOOLOGY OF CHORDATES

sympathetic outflows is to contract the ordinary muscles
round the gut, but to slacken the sphincters, to slacken the
muscles of the heart and of the blood-vessels near the urethra
(causing erection of the penis), to tighten the ciliary muscle
and the sphincter of the iris, to slacken the radial muscles of
the iris (which allows the pupil to be contracted, and to secrete
saliva and tears).

The antagonism between the effects of the sympathetic and
parasympathetic systems is remarkable. It may be expressed
in the form of a table.

Ordinary muscles Sphincters Radial muscles Sphinctex

of the gut. of the gut. Heart. of the iris. of the iris.

Sympathetic. Slackens. Tightens. Tightens. Tightens. Slackens.

Parasympathetic. Tightens. Slackens. Slackens. Slackens. Tightens.

It is also interesting to note that the action of the sym-
pathetic system can partly be simulated by the injection of
adrenalin, and that of the parasympathetic by injection of
acetyl-cholin. The similar effects of adrenalin and the sym-
pathetic are less surprising when it is remembered that the
supra-renals and the medulla of the adrenal bodies are derived
from cells similar to sympathetic neurons, and which like
them have migrated out from the spinal cord.

The case of the gut is particularly interesting, because the
ordinary muscles of its coat are antagonistic in their effects
to those of the sphincters. It stands to reason, that if the
ordinary gut-musculature contracts and propels the contents
of the gut along, contraction of the sphincters would prevent
this movement of the contents. Now the parasympathetic
system tightens the ordinary musculature and slackens the
sphincters, and the sympathetic system contracts the sphincters
and slackens the ordinary musculature. Further, the cell-
bodies of the neurons which tighten the sphincters and slacken
the ordinary muscles are in the same ganglion (anterior or
posterior mesenteric ganglion, according to the region of the
gut). It is possible that it is one and the same neuron which
produces two axon fibres, one tightening the sphincters and
the other slackening the ordinary muscles. This provides an
explanation of how the co-ordination between antagonistic sets
of muscles may be brought about.

THE AUTONOMIC NERVOUS SYSTEM 389

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390 COMPARATIVE ZOOLOGY OF CHORDATES

No autonomic system is known in Amphioxus. In Petro-
myzon, neurons are found along the gut, connected with the
vagus and probably with " pelvic " nerves. The parasympa-
thetic system is therefore present. The sympathetic system
is, on the other hand, not well developed, and imperfectly
differentiated from the supra-renal elements. Groups of these
cells are found near the spinal nerves and the blood-vessels, but
they are not joined together by sympathetic chains. Parallel
with this poor development of the sympathetic component of
the autonomic system in Petromyzon, it may be mentioned
that that animal has no oviduct or bladder, and no smooth
muscle under the skin. In the head the eyes are degenerate,
and there are no salivary glands, and this is parallel with the
absence of differentiated cranial autonomic ganglia. In
Selachians, the sympathetic ganglia are joined together by
the longitudinal lateral chains, and the ciliary ganglion is
present in the head. With the land-vertebrates the full
development of the autonomic system appears.

It is not easy to see why the exciter neurons for smooth
muscles and glands should migrate out of the central nervous
system as they do, and take up positions outside it. It
is also very remarkable that some of them should connect
with the central nervous system through dorsal nerve-roots
(hindbrain outflow of parasympathetic), while others should
connect through ventral nerve-roots (midbrain and sacral
outflow of parasympathetic and the entire sympathetic). In
this connexion it may be noted that in Amphioxus the smooth
muscles of the body are innervated through the dorsal nerve-
roots, while the ventral roots contain only fibres belonging to
the somatic system. The primitive course for fibres innervat-
ing smooth muscle, therefore, appears to be through the dorsal
nerve-roots, and this primitive feature is retained in the case
of the hindbrain (facial, glossopharyngeal and vagus) outflow
of the parasympathetic system, but lost in all the rest.

Literature

luntary Nervoi
Lanoley, J. N. Autonomic Nervous System. Heffer, London, 1921.

Gaskell, W. H. The Involuntary Nervous System. Longmans, Green,
London, 1920.

CHAPTER XXXII

THE SENSE-ORGANS

The Eye. — With regard to the eyes, two points of interest
present themselves. The first concerns the method of
accommodation of the eye for seeing objects at different
distances, and the second relates to the capacity of some
animals to see a single object with both eyes at the same time.

Accommodation is a simple optical problem concerning
the focal length of the lens, the distance of the viewed object,
and the distance between the lens and the retina. These
three terms must be in relation according to the laws of optics
if there is to be a clear image of the object on the retina. The
first and the third term are within the animal, and are there-
fore variable, while the second, the distance of the object, is
obviously external to the animal and not under its direct
control. It is found that some animals accommodate by alter-
ing the distance between the lens and the retina, and others by
altering the focal length of the lens itself.

Cyclostomes and Selachians may be left out of account, for
their eyes can accommodate but little if at all. In the bony
fish, the eye when at rest is accommodated for near vision.
This fact is in relation to the optical nature of the medium in
which they live, water, through which it is not possible to see
very far. The lens is attached to the eye-cup by a retractor
lentis muscle, and when this contracts, the lens is brought
nearer to the retina, and the eye can then focus objects which
are farther away. Land-vertebrates always have their eyes
focussed at rest for distant vision, which enables them the
earlier to see their prey or their enemies. So, in amphibia,
the lens is attached to the eye-cup by a protractor lentis muscle.

391

392 COMPARATIVE ZOOLOGY OF CHORDATES

By its contraction, the distance between the lens and the retina
is increased, and the eye can then focus near objects.

In all the cases so far mentioned, the lens is a rigid body
with a fixed and definite focal length, and which has to be
moved bodily in order to accommodate the eye. In the
remaining vertebrates, the lens is elastic and capable of varying
its convexity and focal length. In reptiles, accommodation for
near vision is brought about by contraction of the circular
muscle of the iris, which has as its effect the increase in con-
vexity of the lens, which thus tends to become spherical.
In the birds, there is in addition a striated muscle called
Crampton's muscle, contraction of which decreases the diameter
of the eyeball in the neighbourhood of the junction between
the cornea and the sclerotic. This causes the surface of the
cornea to become more convex, and assists the lens to bring
rays of light from near objects to a focus on the retina.

The method of accommodation in the mammals differs
from that in other vertebrates. The lens is suspended by the
suspensory ligament, which is kept tense by the elasticity of
the lens trying to revert to the spherical shape. The suspensory
ligament is attached to the ciliary process. The ciliary muscle
is attached to the cornea in front and to the choroid behind, so
that when it contracts, the choroid and ciliary process are
brought forwards. This forwards movement of the ciliary
process reduces the tension on the suspensory ligament, and
the lens is allowed to become more spherical, which increases
its refractive power and enables it to accommodate the eye to
near objects. The change in focal length of the lens is there-
fore only indirectly due to the action of the ciliary muscle.

In some vertebrates, and especially those of nocturnal
habits, the eyes do not accommodate for distance at all, which
fact does not prevent them from enjoying good sight, as does
the owl. In daylight, the pupil may be so contracted as to
simulate a " pinhole " camera, in which accommodation is
unnecessary.

In mammals the ciliary muscle is contracted by impulses
passing in fibres of the parasympathetic system through the
oculomotor nerve and the ciliary ganglion. Other fibres

THE SENSE-ORGANS 393

following the same path constrict the pupil (contract the
sphincter and relax the radial muscles of the iris). The pupil
is dilated by impulses in fibres coming from the sympathetic
system of the neck.

In the lower vertebrates, the eyes are on each side of the
head, and there is little, if any, overlap in the two fields of
vision. In these forms, the decussation or crossing- over of
the fibres at the optic chiasma is complete : the fibres from
an eye run to the opposite side of the brain. In the higher
vertebrates, on the other hand, it is common for the fields of
vision of the two eyes to overlap considerably, and even to
coincide. In these cases both eyes can be brought to bear
on a single object, which enables the animal to estimate
distance. This is of importance in arboreal animals which
have to gauge the strength of their efforts in leaping from branch
to branch. This binocular vision is present in the monkeys
and man, in the owls, and to a varying extent in other animals.

The possession of binocular vision is a great advantage,
but it robs the animal of vision over a large radius around it,
which it would have if its eyes diverged widely on each side
of the head. It is found as a rule that the more timid mammals
have widely divergent axes of vision, amounting to nearly two
right angles in the case of the rabbit. The rabbit therefore
can see objects almost everywhere all round it ; it uses its
eyes qualitatively to warn it of the approach of enemies. The
axes of vision of the lion, on the other hand, are almost parallel ;
it sacrifices a large field of vision for the advantage of using its
eyes quantitatively in estimating distance and spatial relations.

In mammals with binocular vision, it is important that the
movements of the two eyes should be co-ordinated so that their
axes of vision remain more or less parallel with one another.
In other animals each eye can be moved separately, and this
faculty is extremely developed in Chamaeleo.

The fibres from the eyes of mammals such as the rabbit
decussate almost completely at the optic chiasma. In the
monkeys and man, on the other hand, the decussation of the
fibres is incomplete. Fibres from the lateral portion of the
retina of each eye do not cross-over, but go to the same side

394 COMPARATIVE ZOOLOGY OF CHORDATES

of the brain. It is the fibres from the median portions of the
retinae which cross-over and go to the opposite side of the
brain. The images of one object fall on corresponding points
in the two retinae, and the fibres from these corresponding
points run to one and the same side of the brain.

Many of the lower vertebrates have been shown to be
sensitive to different colours. It is supposed that the cones
of the retina are sensitive to colour, and that the rods only
perceive light and dark. Nocturnal animals such as owls and
bats have scarcely any cones, and are presumably colour-blind.
The proportion of cones in the retina increases as a rule from
the lower to the higher vertebrates. In the higher Primates
and in man, the eyes have " corresponding points " of optimum
sensitiveness (the macula lutea or " yellow spot "), in which
the retina consists of cones only, without any rods.

In some vertebrates the eyes have been lost. They are
very degenerate in some of the Cyclostomes, which lead a
semi-parasitic life, and in the Urodele Proteus, which inhabits
the dark caves of Carniola. Fish which live in the dark of
the abyss of the ocean or in caves may be blind and eyeless,
as, for example, Ipnops, Amblyopsis, and Lucifuga. Among
mammals, the eyes are often reduced in forms which live in
the dark in burrows underground. The common mole is an
example, and a comparable but even more far-reaching reduc-
tion of the eyes has taken place independently in the " marsupial
mole " Notoryctes.

The Pineal. — There is no doubt that the early vertebrates
were capable of seeing by means of their pineal organs, through
the pineal foramen in the roof of the skull, though possibly
not of forming an image. Among living forms, Petromyzon
has two pineal organs ; other forms have only one, which may
represent the original right or left organ. The pineal is least
degenerate in Sphenodon. It is in the form of a vesicle of
which the upper wall forms the lens and the lower the retina,
which is connected by nerve-fibres with the brain. This
retina is not " inverted," as is that of the paired eyes. Sur-
rounding the retina is pigment, and the organ is sensitive
to light.

THE SENSE-ORGANS 395

In birds and mammals there is no pineal foramen in the
skull, and the pineal organ remains beneath the bone. It is
reduced to a solid vestige and its function is changed from that
of a visual organ to an organ of internal secretion, or ductless
gland.

The Ear. — The most primitive part of the ear is the
utricular portion with its semicircular canals and ampullae.
Myxine has one, and Petromyzon has two semicircular canals
on each side. All other Craniates have three, in planes at
right angles to each other. In the ampullae are the statolithic
particles which are supported on sensory cilia. Gravity makes
these particles weigh on the cilia immediately beneath them,
whatever the position of the animal, and so the animal is
informed of its position with regard to the vertical according
as to which of the cilia are so stimulated. The semicircular
canals contain fluid, the endolymph, as do all parts of the
auditory sac. When the animal starts or ceases moving, a
flow of endolymph takes place in the semicircular canals,
which resolve the direction of the movement into resultants
in the three planes of space in which they lie. While the
statoliths are static, the semicircular canals are dynamic organs
of balance.

Hearing is the perception of mechanical vibrations of low
frequency. Fish are capable of hearing with their auditory
organs, but this sense only becomes important in the verte-
brates which have left the water, and are therefore subject to
vibrations in air. This is significant because these animals
are also the first to emit vocal sounds. Since these animals
are autostylic and no longer breathe by gills, the spiracular
cleft and the hyomandibula are no longer needed to subserve
their primitive functions ; they give rise to the tympanic
cavity (and Eustachian tube) and columella auris (stapes)
respectively. The vibrations of air impinge on the tympanic
membrane or ear-drum, and are conveyed by the columella
auris across the tympanic cavity to the auditory capsule. The
wall of the auditory capsule is imperforate in the fish and
in the most primitive Stegocephalia (Eogyrinus). In the
remaining vertebrates the auditory capsule has two openings

396 COMPARATIVE ZOOLOGY OF CHORDATES

in its wall. One of these is the fenestra ovalis which enables
the vibrations to be imparted to the fluid (perilymph) which
bathes the auditory sac. The other is the fenestra rotunda ;
it is covered by a membrane which absorbs the vibrations in
the perilymph and so brings them to an end.

That part of the auditory sac which is actually concerned
with hearing is the cochlea, rudimentary in amphibia but well
developed in the higher vertebrates. The vibrations of the
perilymph are imparted to the endolymph within the cochlea,
which in its turn stimulates the sensory cells. In mammals
where the development of the ear is at its highest, the auditory
ossicles are three in number, the cochlea is long and coiled,
and an external ear assists in collecting the air vibrations.

Cyclostomes, fish, and larval amphibia possess a system of
sense-organs known as the lateral-line organs, and which serve
to appreciate vibrations in water of low frequency. The ear
itself is to be regarded as a specialised organ of the lateral-line
(or " neuromast ") system.

The Nose. — The olfactory organ or nose contains an
epithelium which is sensitive to very minute quantities of
chemical substances, dissolved or suspended in water, or
suspended in air. In Dipnoi and Tetrapods the nose has an
open connexion with the mouth cavity, and so enters into the
service of the respiratory system, enabling air to reach the
lungs without opening the mouth. This connexion does not
exist in forms below the Dipnoi (except in Myxine, where the
hypophysial sac opens into the gut).

Taste-organs. — The nose is a distance-receptor, appreciat-
ing chemical substances from afar. Taste-organs, on the other
hand, serve for appreciating substances in contact with the
animal, and especially in connexion with the opening of the
alimentary canal. Taste is a visceral sense, while smell is a
somatic sense. While in most vertebrates the taste-organs
are restricted to the mouth, in some fish, such as the catfish,
they are distributed over the surface of the body.

Jacobson's Organ.— Associated with the nose in land-
vertebrates is a pair of pouches which constitute Jacobson's,
or the vomero-nasal organs. Their function is doubtful, but

THE SENSE-ORGANS 397

it is probably concerned with smelling the food in the mouth,
with which they are in communication. In some forms, in-
cluding man, Jacobson's organs disappear. In the snakes
they are very highly developed, and the tips of the forked
tongue enter their openings in the roof of the mouth. Sub-
stances gathered on the tongue when protruded are thus
placed in contact with the sense-organ.

Literature

von Buddenbrock, W. Grundriss der vergleichenden Physiologic. Vol. i.

Borntraeger, Berlin, 1924.
Herrick, C. Judson. An Introduction to Neurology. Saunders Co.,

Philadelphia and London, 1922.

CHAPTER XXXIII

THE DUCTLESS GLANDS

The ductless glands, or endocrine organs, are a group of
structures remarkable no less for their function than for their
mode of development, and their evolutionary history. The
method of pouring out a secretion into the blood-stream
instead of leading it away by a duct, is secondary, and some
glands which are now ductless doubtless possessed ducts at
earlier stages in evolution. Others, comprising the majority
of the endocrine organs, were originally not glands at all, but
structures which have become useless in the sense that their
original function is not or cannot any longer be performed.
They have become modified and their functions have changed
in a remarkable manner. It is perhaps not without significance
that so many of the ductless glands should have such a his-
tory of structural and functional transformation. Another
peculiarity which applies to several at least of these organs is
that in development they arise from two separate rudiments,
distinct in manner and place of origin, and even in the germ-
layer from which they are formed.

The method of secreting into the blood-stream carries
with it a property which cannot be possessed by glands secreting
by means of definite ducts, for the latter can only communicate
with definite and restricted spaces in the body, and the effects
of such secretions must be only local. On the other hand,
the blood circulates all over the body, carrying the endocrine
secretions with it. These can therefore affect the body as a
whole, and they are of immense importance both during
development and during adult life in effecting correlations of
the various parts with one another. The ductless glands act

398

THE DUCTLESS GLANDS 399

as a chemical mechanism of integration relying on the trans-
portation of the stimulus (the secretions) through the vascular
system; and this mechanism is complementary to that of nervous
correlation and integration which involves not transportation
of stimuli but conduction of impulses arising from stimuli
along special paths, the nerves. " Secretin,' ' which is produced
by the lining of the intestine and stimulates the pancreas to
secrete, has been mentioned in Chapter XXVI.

The Thyroid. — The thyroid was originally a longitudinal
tract of ciliated and mucous-producing cells on the floor of
the pharynx, called the endostyle. The endostyle is typically
represented in Amphioxus (and in the Ascidians), where it is
correlated with the ciliary method of feeding, and serves to
make a moving " fly paper," on to which particles of food
adhere and get carried safely back into the intestine (along the
hyperpharyngeal groove), instead of getting carried out through
the gill-slits by the outgoing current of water and lost. Such
an endostyle is also present in the Ammoccete larva of
Petromyzon. In the adult, however, it becomes closed off from
the pharynx and sunk beneath it, and it gives rise to the vesicles
of the thyroid. In all Gnathostomes the thyroid arises in
development from the floor of the pharynx, and in some
Selachii its cells still show traces of flagella. In the bony
fish, the thyroid is not enclosed in a capsule of connective
tissue, with the result that when it undergoes abnormal
growth (goitre) it may become carcinomatous and give rise
to a malignant cancer which invades the neighbouring tissues,
including the bones. In the higher forms the thyroid is
enclosed in a capsule.

The secretion of the thyroid increases the speed of the
processes of metabolism in the body, and it has been said that
it stands in the same relation to the body as the draught does
to the fire. It plays an important part in the metamorphosis
of amphibia, by promoting the growth of the (previously
invisibly determined) regions into the organs which distinguish
the tadpole from the adult frog or newt.

The Pituitary. — In all Craniates, the pituitary body is a
composite organ formed from the hypophysis which grows in

4oo COMPARATIVE ZOOLOGY OF CHORDATES

from the superficial ectoderm of the front of the head, and the
infundibulum which is a down-growth from the floor of the
forebrain. In Myxine these two constituents remain separated
by connective tissue, but in all the remaining animals they are
intimately connected and fused. In the Tetrapods it is possible
to distinguish four parts in the pituitary, of which three (the
anterior, intermedia, and tuberalis) arise from the hypophysis,
and one (the nervosa) arises from the infundibulum. The
intermedia is always (except in Myxine) plastered on to the
nervosa, and the two together form the neuro-intermediate
lobe. This is separated from an anterior lobe (formed of the
anterior part) by the hypophysial cleft which represents the
original cavity of the hypophysial ingrowth, Rathke's pocket.

Fig. 174. — The pituitary body of a cat, seen, A, from the left side ; B, in
longitudinal section.
al, anterior lobe ; he, hypophysial cleft ; ic, infundibular cavity ; nil,
neuro-intermediate lobe ; pa, pars anterior ; pi, pars intermedia ; pn, pars
nervosa ; pt, pars tuberalis ; tc, floor of the brain.

In some animals, the hypophysial cleft becomes obliterated
in the adult.

In evolution, the hypophysis appeared before the infundi-
bulum, for in Amphioxus the latter is not represented, whereas
the hypophysis is present in the form of the preoral pit. The
preoral pit communicates with the (left) anterior head-cavity
just as the hypophysis communicates with the premandibular
somite in a number of Craniates. In the adult Amphioxus
the preoral pit becomes absorbed in the oral hood and gives
rise to the ciliated organ which produces a current of water
towards the mouth. Thereafter it probably sank into the
tissues and became a gland secreting by a duct into the mouth.
This duct (which represents the open mouth of the cavity of

THE DUCTLESS GLANDS 401

Rathke's pocket) is preserved in Polyp terus, and Cyclostomes.
In the latter, however, the duct has given rise to the large
hypophysial sac which extends beneath the brain and has lost
contact with the pituitary body. At the next stage in its
evolution it must be imagined that the gland entered into
relations with the infundibulum of the brain, and that it adopted
the method of secreting into the blood-stream.

The functions of the pituitary are many, and they are only
very imperfectly known. It must suffice to say that among
these functions are those of : promotion of growth, control of
blood-pressure, causing contractions of the uterus, expanding
the black pigment-cells in the skin of amphibia, and stimulating
the mammary glands to secrete milk.

The Adrenal. — Like the pituitary, the adrenal bodies of
the Tetrapods are composite structures. They are made up
of an external cortex derived from the (mesodermal) coelomic
epithelium, and a central medulla (chromafrlne tissue, so-called
from its staining reactions) derived from the (ectodermal) cells
which have migrated out from the nerve-tube in connexion
with the sympathetic nerve-cells. In the fish, these two
components are quite separate. The cortex of the adrenal is
in them represented by the inter-renal, which, as its name
implies, is situated between the kidneys. The medulla is
represented by a number of supra-renal bodies which lie on
or near the sympathetic nerve-chains, on each side of the aorta ;
they are roughly segmental in arrangement. In the Cyclo-
stomes, the supra-renals are closely associated with the
ganglia of the dorsal roots, but the inter- renals are not well
known.

Coming to the Tetrapods, the inter-renals and supra-renals
are fused together to form the adrenal bodies, but in the more
primitive forms such as the newts, these still resemble the fish
in that they are not compact but form separate strips extending
along the sympathetic nerve-chains, from the kidney to the
anterior region of the thorax. The carotid gland, which is
situated at the joint of the internal and external carotid arteries,
is one of these.

The secretion of the medullary portion of the adrenal

2 D

402 COMPARATIVE ZOOLOGY OF CHORDATES

(adrenalin) has been synthetically prepared, but in spite of
this fact, little is known of the functions of the gland, except
that it produces effects similar to those due to stimulation through
the sympathetic autonomic nervous system.

The Thymus. — The thymus first appears in the fish as a
series of paired upgrowths from the roof of the gill-slits. In
the Selachians it is more or less segmental in its arrangement,
but in higher forms the correspondence is lost, and the number
of slits which contribute to it is reduced. It controls the
formation of the shell, shell-membranes, and albumen in
birds' eggs.

The Parathyroid. — The name parathyroid is given to
bodies which are usually situated close to or even in the thyroid,
but which differ from the latter in their structure and method of
development. They arise from the ventral regions of the 3rd
and 4th visceral pouches in the Tetrapods, and are apparently
absent in the fish.

The Pineal. — The pineal eye has already been described in
connexion with the sense organs. In the higher vertebrates
this structure degenerates and is transformed into a gland.

The Pancreas. — In addition to its function of producing
enzymes for the purpose of digesting the food in the intestine,
whither the enzymes are conducted by the pancreatic duct,
the pancreas also functions as an organ of internal secretion.
The tissue responsible for producing this internal secretion
is that known as the islets of Langerhans, and its production
is called insulin . The function of insulin is to store up glycogen
in the liver, in which respect it is antagonised by the adrenalin.
Diabetes is the result of faulty or non-functioning of the islets
of Langerhans. In some Teleost fish, the endocrine islet-
tissue may form little masses separate and apart from the
ordinary pancreatic tissue, which secretes the digestive pan-
creatic juice.

The " Puberty " Gland. — The reproductive glands, ovary
and testis, in the birds and mammals produce internal secretions
which are concerned with the development and maintenance
of the characters which distinguish one sex from the other.
Since these secretions are essential for the proper sexual

THE DUCTLESS GLANDS 403

differentiation of the developing animals, the glands producing
them have been called " puberty " glands.

The Corpus Luteum. — The corpus luteum is the name
given to what is really a temporary endocrine organ in the
mammals. After an egg has vacated its Graafian follicle, the
follicle undergoes changes resulting in the increase in size
of the follicular cells, and the invasion of the follicle by
connective tissue and blood-vessels. Should the egg liberated
not get fertilised, the corpus luteum soon disappears. Should
fertilisation result, however, and the blastocyst become
attached to the wall of the uterus, the corpus luteum persists
and increases in size, until the end of pregnancy. During this
time it produces a secretion the functions of which are to
prevent other eggs from being released from the ovary, and
to control the growth of the uterus and the secretion of milk.

Literature

Riddle, O. Internal Secretions in Evolution and Reproduction. The

Scientific Monthly, vol. 26, 1928.
Swale Vincent. Internal Secretion and the Ductless Glands. Arnold,

London, 1924.

CHAPTER XXXIV

REGULATORY MECHANISMS

All animals below the birds and mammals are what is usually
called " cold-blooded," or poikilothermous. Actually, these
animals are not so much cold as dependent on the environ-
mental temperature, which may be hot. It is a mistake to
regard " cold-blooded " animals as necessarily cold, lethargic
and sluggish, for in a tropical climate their temperature is high
and they may be very active. Nevertheless, since the processes
of life can only go on within a certain limited range of tempera-
ture, the fact that an animal is dependent on its environment
for its temperature necessarily restricts the kinds of environ-
ments in which it is capable of living. Further, within the
suitable habitat, the degree of activity of the animal will depend
on the temperature. This inconstancy of thermal conditions
is a serious bar to the further evolutionary progress of the
poikilothermous animals.

The advantage which the birds and mammals have in being
" warm-blooded " (homothermous) is not only the fact that
the temperature at which their biological processes go on is
high, but still more the fact that this temperature is constantly
maintained, regardless of the temperature of the environment.

The processes of metabolism, and especially muscular
activity, entail the production of heat. Some warm-blooded
animals shiver when they are cold, and their muscles are then
thrown into series of contractions. There is therefore a source
of heat within the organism which tends to make the tempera-
ture rise. At the same time, heat is continually being lost by
radiation from the surface of the animal. The maintenance
of a constant temperature within the animal therefore depends

404

REGULATORY MECHANISMS 405

on a regulation and balance of the amounts of heat produced
and lost. Poikilothermous animals have a temperature only
slightly higher than that of the environment. Some seem
to be able to raise their temperature slightly for a period by
muscular contractions, such as the python when it is coiled
round its eggs. But these animals have no means of combating
really cold external temperatures, during which they must
either hibernate or die. Within limits, the hotter the tempera-
ture, the better are the conditions for poikilothermous forms.
Some lizards, however (Varanus, Uromastix), when exposed
to great heat, increase their rate of breathing very considerably,
and so resort to panting. Panting results in the lungs getting
rid of large quantities of water vapour, and as heat is absorbed
in the conversion of water into vapour, panting means loss of
heat also. Uromastix, which inhabits deserts, is dark in colour
up to a temperature of 41 ° C, but as the temperature rises
above this point, it tends to become white. Since dark colours
absorb heat and light colours reflect it, Uromastix has a peculiar
mechanism which tends roughly to regulate its intake of heat
from the environment. This method, however, is quite
different from that of homothermous animals, birds, and
mammals. In the first place, the homothermous animals have
an external covering which is a bad conductor of heat ; this
takes the form of feathers in birds, hairs in terrestrial mammals,
and oil or blubber in birds and mammals which lead an aquatic
existence. The effect of such a layer is to minimise the loss
of heat by radiation. Next, they have more efficient respiratory
and vascular systems, notably a four-chambered heart with
complete separation of the arterial and venous circulations.
In the Monotreme Echidna, the temperature is regulated by
varying the amount of heat produced, but it has no method of
varying the amount of heat which it loses. It has no sweat-
glands, no increase in the amount of blood in the skin (vaso-
dilatation), and it does not resort to panting. The heat-
production of Echidna varies according to the difference
between its temperature and that of the environment. How-
ever, this regulation is not very efficient, for if the environ-
mental temperature varies from 350 to 50 C, the temperature

406 COMPARATIVE ZOOLOGY OF CHORDATES

of the animal will vary by about io° C. Not only is the
constancy of the temperature less than that of higher mammals,
but the actual normal internal temperature is lower, being
about 300 C. In cold weather, Echidna hibernates. Its
protective covering of hair is poor, and, like a few other
mammals (such as the marmot), it becomes almost poikilo-
thermous. On the other hand, in hot weather when the
temperature rises above 350 C, Echidna dies of apoplexy
(unless it is activating, deep beneath the ground), for its only
method of countering a rise in the environmental temperature
is to reduce its own internal heat-production, and a point
is reached below which it cannot reduce its metabolism and
still live.

The other Monotreme, Ornithorhynchus, has a slightly
higher normal temperature, 3 2° C, and it keeps it a little more
constant. Not only can it vary its heat-production, but it
can also vary its loss of heat by means of evaporation of water
from its sweat-glands.

The higher mammals regulate their temperature almost
entirely by controlling the heat-loss. This they do by three
methods : by the evaporation of water from the sweat-glands,
by the dilatation of the blood-vessels in the skin, and by the
acceleration of respiration or ' ' panting. ' ' The heat-production
in these animals is not increased unless the external temperature
drops considerably. The Marsupials are intermediate between
the Monotremes and the higher mammals in the efficiency
of their temperature-regulations.

In birds, heat is lost by evaporation of water through the
lungs and air-sacs.

The advantages accruing from the possession of a high
and constant internal temperature are very great. Not only
does it allow of a higher rate of living, since chemical reactions
are accelerated at high temperatures, but it enables differentia-
tions and specialisations to arise which would be wrecked if
the speed of the metabolic processes (or in other words, the
internal temperature) were not constant. Further, it enables
the animals to inhabit climates in which poikilothermous forms
either cannot live, or have to spend considerable time hiber-

REGULATORY MECHANISMS 407

nating against the cold or aestivating against the heat. So it
is found that the supreme and dominant animals in arctic
regions are the birds and mammals, while in the tropics,
reptiles can compete successfully with birds and mammals.

It is interesting to notice that during most of the period
of incubation, the embryo chick is poikilothermous. It is
only shortly before hatching that it acquires the capacity
of maintaining a uniform temperature. The same is true of
new-born mice, which become homothermous by the tenth
day after birth.

Another matter for which a regulatory mechanism has been
evolved in the vertebrates is the osmotic pressure of the blood.
Of aquatic invertebrates it may in general be said that their
body-fluids have roughly the same osmotic pressure and the
same percentage of salts as the water in which they live, and
that these vary as the water varies. It is interesting to find
that in the Selachii, the osmotic pressure of the blood is not
constant either, but varies with the water. The salts in the
blood are only about half as concentrated as in sea water, but
the blood of Selachians contains urea, which makes up the
difference. In the Teleosts, the osmotic pressure of the blood
is about one-third that of sea water, but it is kept more or less
constant. This regulation is more efficient in some forms than
in others ; the osmotic pressure varies with that of the sur-
rounding water slightly in the cod, varies more in the plaice,
and varies still more in the eel, which alternates between fresh
and sea-water. As a rule the osmotic pressure of the blood
of fresh- water Teleosts is lower than that of the marine forms.

In the land- vertebrates, the osmotic pressure of the blood
is kept constant, and regulated by the kidneys, in spite of
variations in food and drink.

In an animal like a Selachian, living in the sea, it is not of
much importance if water and salts be lost from the body, as
they can be replenished from the medium in which it lives.
In a land-vertebrate the case is different, and the loss of water
and salts is regulated. The importance of maintaining a
constant osmotic pressure of the blood lies in the fact that it
entails constancy in the concentration of salts, or in other words,

408 COMPARATIVE ZOOLOGY OF CHORDATES

a stable " internal environment " ; and stability of conditions
is essential for highly specialised and co-ordinated processes
of life.

The relation between the quantities of oxygen and C02
in the blood is regulated by the respiratory system, controlled
by a centre in the brain. If the blood is rich in C02 the
respiratory movements are accelerated, and conversely they
are retarded if the quantity of C02 is low. In this connexion
it must be remembered that the respiratory movements of the
fish and amphibia are effected by the muscles of the visceral
arches. These are visceral muscles, innervated by visceral
efferent fibres in the dorsal cranial nerve-roots, and the centre
which controls them is in the visceral sensory lobe of the
medulla oblongata. In the Selachian (Raia) it is perhaps
better to speak of several centres, one corresponding to each
of the 7th, 9th, and 10th cranial nerves. Each of these seg-
mental centres in Raia has a degree of autonomy of its own, for
if separated from the others by cutting across the medulla,
it continues to regulate the muscular movements in the visceral
arch or arches to which it is connected.

In the amniotes, however, the respiratory movements are
effected by the intercostal muscles (moving the ribs) and the
muscles of the diaphragm. These are somatic (myotomic)
muscles innervated by somatic efferent fibres through ventral
nerve-roots in the region of the neck and trunk. Neverthe-
less, the " respiratory centre " is still in the medulla oblongata,
in the primitive position which it occupied in the fish and
amphibia, but it no longer shows the simple segmental
arrangement.

Lastly, attention may be paid to two features which the
higher vertebrates possess, and which though not strictly
regulatory (compensating) mechanisms, nevertheless serve
to ensure maximum constancy of conditions. The first of
these is concerned with the fact that the ovary and testis in
birds and mammals serve not only for the production of
reproductive cells, but they also furnish a chemical secretion
which evokes and maintains the development of the secondary
sexual characters.

REGULATORY MECHANISMS 409

The other feature refers to the method of ossification of
certain cartilage-bones by means of a diaphysis and two
epiphyses, which is characteristic of the mammals. This
method enables the bones in question to function as supports
and hinges, and at the same time to grow and enlarge so long as
the diaphysis and the epiphyses remain separated by cartilage.
But once the diaphysis becomes firmly united by bone with the
epiphysis at each end of it, the growth of the bone as a wrfole
ceases. The maximum size of such bones is therefore limited,
as is that of the animal. In several respects, therefore, the higher
vertebrates differ from the lower. With the temperature, the
osmotic pressure and the acid-base relations of the blood
regulated and constant, the higher vertebrates are largely
independent of the environment. Indeed, they have a constant
internal climate and " environment " of their own, in which
they live sheltered from external agencies, with, in mammals,
a constant final adult size.

The possession of this " internal environment " is not only
one of the chief means of survival of the higher vertebrates,
but it has also enabled them to become as specialised and
perfected as they are.

Literature

Dakin, W. J. The Osmotic Concentration of the Blood of Fishes taken
from Sea Water of naturally varying Concentration, and Variations in
the Osmotic Concentration of the Blood and Coelomic Fluids of Aquatic
Animals, caused by Changes in the External Medium. Biochemical
Journal, vol. 3, 1908.

Haldane, J. S. Respiration. Yale University Press, 1922.

Hide, I. H. Localisation of the Respiratory Centre in the Skate. American
Journal of Physiology, vol. 10, 1904.

Krehl, L., und Soetbeer, F. Untersuchungen iiber die Warmeokonomie
der Poikilothermen Wirbeltiere. ^Pfliiger's Archiv. f. d. Gesammte
Physiologie, vol. 77, 1899.

Martin, C. J. Thermal Adjustments and Regulatory Exchange in Mono-
tremes and Marsupials. Philosophical Transactions of the Royal
Society, Ser. B, vol. 195, 1903.

Scott, G. G. A Physiological Study of the Changes in Mustelus Canis
produced by Modifications in the Molecular Concentration of the
External Medium. Annals of the New York Academy of Science,
vol. 23, 1913.

CHAPTER XXXV

BLOOD RELATIONSHIPS AMONG THE CHORDATES

The various species of animals differ not only in their structure,
their method of development and their habits, but also in the
chemical composition of their tissues. The most useful tissue
to take in this connexion is the blood. Now, chemical methods
are not sufficiently refined to detect the difference between the
bloods of two animals and to estimate the degree of similarity
which they show. It is possible, however, to have recourse to
biological methods by making use of the property which animals
possess of developing immunity. If horse 's blood , for example ,
is injected into the vascular system of a rabbit, the rabbit will
after a time produce a substance in its blood which reacts to
horse's blood, and precipitates it. This is the same principle
as that used for preparing antitoxins for certain diseases. As
to how the antitoxin or antiserum is produced, little is known,
but it suffices for present purposes to realise that in the hypo-
thetical case just described, rabbit's blood immunised against
horse's blood will always precipitate horse's blood, to the
extent of ioo per cent. This means that anti-horse serum,
as it may be called, is specific against horse, and it is a matter of
no importance what kind of animal has been used to produce
the antiserum. But the specificity against horse is not quite
complete. Anti-horse serum, as it may be called, will produce
no effect whatever if mixed with, say, blood of a bird ; but it
will produce a slight precipitation with blood of pig, and still
more with blood of ass. This means that the blood of horse is
more similar to that of ass than to that of pig, as regards its
chemical composition, and this is just what would be expected

410

BLOOD RELATIONSHIPS 411

from a knowledge of the comparative anatomy and embryology,
and from the palaeontology regarding these three species.

The precipitin blood-tests therefore furnish a means for
estimating the relative similarities between the bloods of
different animals, and they are not only a biochemical proof
of the theory of evolution, but also an index for classification.

The following are a few tables showing the relative affinities
between the bloods of a number of vertebrates : *

Anti-human serum, mixed with blood of : —

Man

gives

100 per

cent.

precipitation

Gorilla

>y

64

>

»>

Orang Outang

n

42

>

*»

Baboon

t>

29

>

»

Ox

>>

10

1

tt

Sheep

it

10

>

>i

Deer

>>

7

7

jj

Horse

„

2

,

>>

Marsupial

»»

0

,

»

These results show several interesting points. In the first
place, the great similarity between the blood of man and that
of the gorilla should dispel any doubt (should any be left)
concerning the evolution of man from lower mammals. The
precipitation percentages show that human blood is more like
that of the apes than that of the baboon, and more like the
latter than the blood of animals like horse and deer. This
fits in perfectly with evidence derived from other sources.
It is also interesting to note that two animals which are believed
to be closely related to one another like ox and sheep, should
show the same degree of dissimilarity to man. The relation-
ship between sheep and ox can also be tested by immunising
a rabbit to sheep blood .

Anti-sheep serum mixed with blood of : —

Sheep gives ioo per cent, precipitation.

Ox „ 75

Antelope ,, 67 „ ,,

Reindeer ,, 35 „ „

The relationship between sheep and ox is here definitely
shown to be close.

* FromNuttall.

4i2 COMPARATIVE ZOOLOGY OF CHORDATES

Some of the most interesting results are those which refer
to the relative affinities between the various groups of reptiles,
and between them and the birds.

Anti-fowl egg serum mixed with blood of : —

Crocodile, gave a positive result (precipitation) in 50 per cent, of cases.
Chelonian ,, ,, ,, ,, 40 ,, ,,

Lacertilian ., ,, ,, ,, 7 ,, ,,

Ophidian ,, „ ,, ,, 6 ,, ,,

These results show that the reptiles (alive now) nearest akin
to the birds are the crocodiles, which again corroborates all
the evidence from other sources. It further indicates that the
crocodiles and turtles are more closely akin to one another
than they are to the lizards and snakes, which, again, are fairly
closely allied to one another. This is further shown by the
following : —

Anti-chelonian serum mixed with blood of : —

Chelonian gave a positive result in 87 per cent, of cases.
Crocodile „ „ „ 25

Lacertilian ,, ,, ,, o „ ,,

Ophidian „ ,, ,, 6 „ ,,

Besides the precipitin tests, there are other methods of
estimating the blood-relationships of vertebrates. For one
thing, it is found that blood of any particular species has the
power of destroying the blood-corpuscles of other species,
to an extent varying with the remoteness of the relationship
between them. Again, it is found that the degree of virulence
with which an animal will suffer from a human disease varies
with its degree of kinship to man. So syphilis attacks the
chimpanzee more seriously than the orang, and the latter more
than the baboon. Lastly, attention may be called to the
so-called blood-groups, into which the human race is divided.
There are four of these blood-groups, and they are due to two
agglutinating substances, which may be absent, or one, or the
other, or both may be present, in the blood of a man, and
cause clotting when the blood is mixed with that of another
incompatible group. Incidentally these groups are further
interesting in that there is reason to believe that they are
inherited by means of Mendelian factors, but their main

BLOOD RELATIONSHIPS

4i3

interest from the present point of view lies in the fact that the
blood-groups and agglutinating substances are also found in
monkeys. Here, therefore, are definite biochemical characters
which are shared by monkeys and man, and which were
derived from a common ancestor.

Literature

Landsteiner, K., and Miller, C. P. Serological Studies on the Blood of
the Primates. Journal of Experimental Medicine, vol. 42, 1925.

Nuttall, G. H. F. Blood Immunity and Blood Relationship. Cam-
bridge University Press, 1904.

PART IV
EVOLUTIONARY MORPHOLOGY

CHAPTER XXXVI

THE BEARING OF PHYSICAL AND CLIMATIC FACTORS ON CHORD ATES

To understand their evolution and life it is essential to consider
animals in relation to their environment. During the time
since chordate animals first appeared, the environment has
changed very considerably at one time or another. Of the
most primitive forms there is no record preserved, for the
simple reason that these animals did not possess structures
capable of preservation by fossilisation. The earliest known
vertebrates are from the Silurian period, and they were fish.
The earth was at this time covered with shallow seas containing
coral-reefs which are indicative of a mild climate. In the
ensuing Devonian period, shallow lagoons and enclosed basins
were in abundance, and the land which had emerged enjoyed
desert conditions with little rainfall. It is towards the end of
this period that the first land-vertebrates (Stegocephalian
amphibia) appeared. The next or Carboniferous period was
one of tropical climates, during which luxuriant forests covered
the land. The trees had no rings of growth, which fact proves
that there were no seasons. True reptiles first appeared here.
In the late Carboniferous and Permian period the climate
became colder as the continents rose and mountain chains
were formed, resulting in an ice-age or glacial period. In the
following Trias, warm conditions returned, without seasonal
variation. The earliest known mammals belong to this period.
Warm conditions persisted throughout the Jurassic period, in
which the first birds are found, but this period is pre-eminently

415

4i 6 EVOLUTIONARY MORPHOLOGY

the " age of reptiles," not only on account of the number of
different types which flourished, but also because of the
gigantic size to which many of them grew.

In the Cretaceous, cold conditions returned with seasonal
variations. Mountain-building and glaciation occurred in
some parts of the earth, the temperature of which was now
considerably reduced. At this time and perhaps for this
reason the majority of the reptiles which had hitherto been so
successful, went extinct and were superseded by mammals as
the dominant animals. After this time, hot conditions set in
again for the main part of the Tertiary era, gradually diminish-
ing towards its close when a fresh bout of mountain-building
erected the Alps. Then followed the great Ice- Age. Mam-
mals continued evolving during this period, towards the end
of which man appeared.

The most important changes in the environment as far as
the vertebrates were concerned were the drying-up of the
lagoons and estuaries in the Devonian, and the variations of
temperature.

It is a characteristic feature of desiccated areas that the
water expanses which they possess shrink to ponds, and the
oxygen-content of the water decreases owing to the quantities
of decomposing organic matter with which the ponds become
filled. Under such circumstances it is obvious that fish which
are provided with means of supplementing their branchial
respiration would have a much greater chance of surviving,
and the first step in this direction was the habit of taking air
into the pharynx when at the surface. At the present day,
inhabitants of such waters show diverse adaptations, but by
far the most important of these from the present point of view
are the Dipnoi, with their lungs. There is little doubt that
the ancestors of the Tetrapods encountered and mastered
conditions of desiccation in fresh water, in the same way as
the modern Dipnoi. There is the further danger that under
these circumstances the water may dry up altogether, as it
does in the case of the swamps in which Protopterus lives, and
then the possession of a means for pulmonary respiration is
the only condition for survival.

PHYSICAL AND CLIMATIC FACTORS 417

Temperature may vary in several different ways, either in
space or in time, or in both. So the tropics and the temperate
and polar regions differ in temperature, as do day and night
or summer and winter.

Homothermous animals are largely independent of tem-
perature variation in the outer environment since they live in
a constant internal environment of their own. However, the
outer environmental temperature has a bearing on their size.
This follows readily from a consideration of the ratios of surface
to volume at different sizes. The surface increases as the
square, but the volume increases as the cube of the linear
dimensions, so that there is relatively more surface in small
animals than in large ones. The importance of this for
homothermous animals is that the amount of internal heat
produced (by metabolism) and lost (by radiation) varies
relatively with the surface. So, of two dogs weighing 20 and
3 J kg. respectively, the former will have a surface of 7,500 sq.
cm., the latter 2,423 sq. cm. For every kg. of dog, there is
in the large dog 375 sq. cm., and in the small one 757 sq. cm.
of surface, and the amount of heat given off from the dogs
per kg. is twice as high in the case of the small dog as in the
case of the large one.

Small homothermous animals therefore radiate relatively
more heat from their surface than large animals, and this
heat-loss has to be compensated by relatively more active
metabolism and intake of food. In spite of the fact that
mammals and birds grind their food up small (in the mouth
in mammals : in birds, in the gizzard) so that the processes of
digestion are accelerated, a stage of smallness is reached when
the animals have to spend all their time feeding. Shrews and
humming-birds are of about this size. If they were smaller
than this they would need to consume quantities of food which
they would not have time to eat. Especially true is this of
regions in which because of seasonal variation the days are
short for a period in each year. The ratio of surface to volume
therefore establishes a minimum limit of size for homothermous
animals in a given outer environmental temperature.

In cold climates, such as prevail in polar regions, homo-

2 E

418 EVOLUTIONARY MORPHOLOGY

thermous animals tend to be large. They profit by their
relatively small surface from which they lose heat, and also by
the fact that they do not require to spend all their daylight
eating as they would if their surface/volume ratios were large
and they were small in size. On the other hand, tropical
homothermous animals can afford to be small and to have
large surface/volume ratios. The intensity of heat radiation
is less than in polar regions because of the higher temperature
of the air, and small size enables them to get rid of their heat.
Also, there is ample food, and daylight to eat it in, to make up
for the heat lost. Homothermous animals as small as humming-
birds could not live in really cold climates.

It is worth noticing that fat, which is a poor conductor of
heat, forms a layer underlying the skin in the animals inhabiting
polar regions (seal, penguin), and so assists in minimising the
amount of heat lost by radiation. When, on the other hand,
fat is stored by homothermous animals living in hot climates,
it is not distributed under the skin all over the body, where it
would interfere with heat- radiation, but it is localised and
forms humps as in the camel or the zebu.

Whereas homothermous animals tend to be large in polar
regions and small in the tropics, poikilothermous animals show
precisely the opposite tendency, and for the same reasons.
The reptile depends on the outer environment for its heat. In
cold climates, when it is not hibernating, it is to its advantage
to absorb as much as possible of what heat there is. This is
assisted by a large surface/ volume ratio, and consequently a
small size. Effectively, it is found that the fish, amphibia, and
reptiles inhabiting cold climates are smaller than their relatives
living under warmer conditions. For in tropical climates,
these animals can afford to be large. The giant frogs, turtles,
lizards, snakes, and crocodiles of the tropics illustrate this
point well. The huge size of the reptiles in the Jurassic
period must have been made possible by the hot conditions
which prevailed then.

It is further to be noticed that in tropical regions, the
poikilothermous animals can compete successfully with the
homothermous ; whereas in polar regions, the homothermous

PHYSICAL AND CLIMATIC FACTORS 419

animals dominate over the poikilothermous by reason of their
constant internal temperature. It follows that if a region of
high temperature, populated by poikilothermous and homo-
thermous animals, were to undergo a reduction of temperature
(as by greater elevation of the land above sea-level or the
approach of an ice-age), the homothermous animals would
survive, whereas the poikilothermous forms would be very
likely to go extinct, especially if they were of large size. This
may be what happened at the cold end of the warm secondary
era (Trias to Cretaceous inclusive, the " age of reptiles "),
when the reptiles all but went extinct, and were only survived
by the present-day forms, which furnish a miserable sample
of former richness of the reptilian fauna. At the same time,
the homothermous birds and mammals survived, the latter to
become the dominant animals.

It is seen, therefore, that certain of the greatest episodes
in the history of the vertebrates, such as the evolution of the
amphibia, may have been largely conditioned by climatic
changes in the earth's crust. Other episodes were probably
related to adaptations to more fixed climatic conditions. Of
these, two only will be mentioned here. The first concerns
the evolution of the early fish. The original ancestors of the
chordates must have been marine forms, but there are certain
considerations which suggest that the evolution of the early
chordates took place in fresh or estuarine water. The typical
chordate method of locomotion by undulations of the body
from side to side may be regarded as an adaptation to life in
rivers in which there is a more or less constant flow of water
in a certain direction.

The other episode concerns the evolution of man, part of
whose ancestral history is related to the habit of living in
trees. It is common for arboreal animals to retain unspecialised
limbs, and to acquire the capacity of opposing one or more
digits to the others, and so be able to grasp branches firmly.
At the same time the sense of smell becomes less important,
while that of sight becomes dominant, leading to binocular and
stereoscopic vision, and the capacity to estimate distance. This
is of importance to an arboreal animal in estimating the strength

420 EVOLUTIONARY MORPHOLOGY

of its leaps from branch to branch. The neurological changes
which accompany these anatomical ones are the subordination
of the olfactory cortex of the cerebral hemisphere (hippocampus)
and the elevation of the non-olfactory cortex or neopallium to
a dominant position. In other words, arboreal life favoured
the development and evolution of the brain, which is the organ
which most distinguishes the Primates, and especially man,
from the remainder.

In previous paragraphs it was shown how the minimum
limit of size of homothermous animals was determined, and
it was found to be affected by the climatic temperature. The
minimum size of poikilothermous Tetrapods has no relation
to temperature, but is determined by the capacity of the
muscles to actuate the skeleton and move the animal about.

The maximum size of land-vertebrates is limited by the
ratio between the weight of the body and the supporting
strength of the legs. The weight varies with the volume
which is proportional to the cube of the linear dimensions of
the animal. But the strength of the legs is measured by the
cross-sectional area, which is proportional to the square only
of the linear dimensions. The larger the animal is, therefore,
the relatively heavier will the load be which the legs have to
carry. If the length of a rabbit is 10 times more than that of
a mouse, the weight which the rabbit's legs carry is iooo times
greater than that which the legs of the mouse support. Against
this, the cross-sectional area of the rabbit's leg is ioo times that
of the leg of the mouse. The result is that the weight per
square millimetre on the legs of the rabbit is 10 times more
than that on the legs of the mouse. As the strength of the
skeletal material (bone) cannot be increased, a stage is reached
at which the legs can no longer safely carry the weight of the
body, or they must be so large as to be almost immovable.
Already in the elephants they are like pillars, and these animals
are near the maximum size for land- vertebrates. For aquatic
forms, the conditions are of course different, since by Archi-
medes' principle the buoyancy of the water reduces the relative
weight of the animal, which is usually not borne on the limbs
at all. So the whales and sharks can reach sizes which are

PHYSICAL AND CLIMATIC FACTORS 421

impossible for land forms. For this reason, it is likely that
the largest of the Dinosaurs were more or less aquatic.

Literature

Brooks, C. E. P. Climate through the Ages. Benn, London, 1926.

d'Arcy Thompson. Growth and Form. Cambridge University Press,
1917.

Haldane, J. B. S. Possible Worlds. Chatto and Windus, London, 1927.

Hesse, R. Tiergeographie auf okologischer Grundlage. Fischer, Jena,
1924.

Przibram, H. Form und Formel im Tierreiche. Deuticke, Leipzig and
Wien, 1922.

X§M$t

CHAPTER XXXVII

THE ORIGIN OF CHORD ATES, AND THEIR RADIATION AS AQUATIC

ANIMALS

From studies on all the groups of chordates and comparisons
between them, it is possible to arrive at an idea as to what the
original chordates must have been like. They were small,
bilaterally symmetrical, and no part of them was sufficiently
hard or resistant to be capable of preservation by fossilisation.
They were marine animals, as are their lowest representatives
at the present day. Some of these may now be considered,
for, although they are specialised often to the point of de-
generacy, and are of no use in the interpretation of the higher
chordates, they show some characters which assist in estima-
tions of the relations which the chordates bear to other animals.
Balanoglossus is a " worm-like " form, with the following
chordate features : gill-slits, ciliated grooves assisting in the
process of feeding, a dorsal nerve-cord which for a short
portion of its length is tubular, and a skeletal structure in
the anterior region of the body which is held to represent
a rudimentary notochord. From the latter possession it is
classed as a Hemichordate. The body is divided into three
regions, " proboscis," " collar," and " trunk," and is adapted to
its mode of life, which is burrowing in the sand. Its chordate
affinities are obvious from the characters just mentioned, but
other features ally it to a number of invertebrates, and espe-
cially the Echinodermata. The free-swimming larval form of
Balanoglossus, the Tornaria, is very similar indeed to the
larval forms of the Echinoderms, in general form, in the
arrangements of the bands of cilia which it carries, and in the
method of origin of the mesoderm. The latter arises as three

422

ORIGIN OF CHORDATES 423

pairs of pouches from the archenteron (strictly, two pairs and
an anterior median pouch which represents a fused pair).
Such coelomic sacs are enterocoelic, like the anterior gut-
diverticula of Amphioxus. The three sets of coelomic pouches
persist in the adult Balanoglossus, and it is interesting to notice
that the first two sets, forming the cavities of the " proboscis "
and of the " collar," have openings to the exterior. These
openings are coelomostomes, comparable to the water-pores of
the Echinoderms, and the connexions which are occasionally
found in Craniates between the premandibular somites and
the hypophysis (" proboscis-pores ").

Allied to Balanoglossus are the Pterobranchia, which show
a slight trace of a notochord, but no dorsal tubular nerve-
cord. One of them, Cephalodiscus, has the three sets of
coelomic pouches, each with a coelomostome, and a pair of
gill-slits. It is not free-swimming but sessile, reproducing
actively by budding. The other, Rhabdopleura is not only
sessile but colonial, for the buds formed remain in con-
nexion with the parent stock. Rhabdopleura has the three
sets of coelomic pouches, and coelomostomes, but no gill-
slits.

The next form to consider is Phoronis, which is worm- like
with the anterior end modified into a row of tentacles. The
anterior region of the body corresponding to the proboscis is
reduced to a flap overhanging the mouth, so that the body
contains only two sets of coelomic pouches. The larval form
of Phoronis which is called the Actinotrocha, has ciliated bands
reminiscent of those of the Tornaria. Phoronis has nephridia,
and as these structures are also present in Amphioxus, it is
possible that they were present in the original common ancestor
from which all these forms were descended.

Phoronis is related to the Ectoproctous Polyzoa, and to
the Brachiopoda. All these forms, so far as is known, tend to
have coelomic pouches developed as enterocoels, and usually
showing a tripartite arrangement. Many of them have open
coelomostomes. The larval forms usually have a ciliated band
passing behind the mouth, and cleavage of the egg is in-
determinate. These features distinguish the chordates and

424

EVOLUTIONARY MORPHOLOGY

ORIGIN OF CHORDATES 425

their allies from the other great group of invertebrates com-
prising the Annelida, Arthropoda, and Mollusca.

There is reason to believe that the concentration of
nerve-cells to form a central nervous system out of the more
primitive diffuse nerve-net took place in the region of greatest
stimulation. This is the ventral side in Annelida, Arthro-
poda, and Mollusca, all of which typically crawl on the
ventral surface. The fact that the central nervous system of
chordates is dorsal seems to show that the ancestral chordates
were not ventral crawlers, but pursued a free-living pelagic
existence, receiving the greatest stimulation on the dorsal
side from the surface of the sea.

Returning now from these more or less distant allies to
true chordates, the next group to consider is one which, like
the Hemichordates, has left the main line of chordate evolu-
tion and become specialised in different directions : the
Urochordata. These preserve the notochord in the tail in
the larval stage only, and the dorsal tubular nerve-cord of the
larva degenerates in the adult. They possess gill-slits, and a
typical well-developed endostyle, used in connexion with the
ciliary method of feeding. Their development is also typical
of chordates. One group of these animals, the Larvacea,
retain the larval structure throughout life, with the tail and
notochord. The others pass through a free-swimming larval
stage, and then undergo a retrograde metamorphosis into
sessile animals, losing the tail, notochord, and larval eyes and
organs of balance. These are the Ascidiacea or sea-squirts.
Some of these are solitary, but most are colonial, reproducing
extensively by asexual reproduction or budding, as is commonly
the case with sessile forms. Others, forming the group of
Thaliacea or salps, have returned to a free-swimming existence,
but retaining many traces of former sessile habits ; in particular
the habit of budding, which is very prevalent. Some of them
have a true alternation of sexually produced (from fertilised
eggs) and asexually produced (from buds) generations, and one
form is further interesting in that the sexually produced genera-
tion is nourished during its development by the mother by
means of a placenta (Salpa).

426 EVOLUTIONARY MORPHOLOGY

None of these forms, however, exhibit the typical chordate
segmentation of the body, which enables them to swim in
definite directions instead of being carried aimlessly about at
the mercy of currents . The immediate ancestors of Amphioxus
and of the higher chordates were elongated, compressed from
side to side and deep from dorsal to ventral edge. As a
consequence, they were able to bend the body from side to
side, and perform undulatory movements. The body was
made up of several segments. Each segment was separated
from the ones in front and behind by septa or partitions, and
stretching from septum to septum were the myotomic muscle-
fibres. When the myotome on one side of a segment contracts,
the septa bounding that segment come closer together, and
the body becomes concave on that side. The advantage of
having several segments made it possible for the body to bend
in several places. By bending alternately right and left in
successive regions of the body, and making the bends pass
down the length of the body by throwing the myotomes into
contraction in succession, the undulatory movements are
produced which enable the organism to swim. These move-
ments were made still more efficacious by lengthening the body,
which was accomplished by the development of an extension
behind the anus forming the tail. After bending in any
place, the body became straight (before bending in the opposite
direction), and this was effected not only by the relaxation of
the myotome on that side, and the contraction of the myotome
on the opposite side, but by the possession of a stiff yet elastic
rod running along the whole length of the animal : the
notochord. This is the typical primitive method of chordate
locomotion which persists not only in the fish, but also in the
lowest land-vertebrates.

The fact that the animal moved in a definite direction had
the consequence that the front end was further specialised by
a concentration of sense-organs, which ultimately was to bring
about the formation of a head.

It has been held, with some degree of probability, that the
habit of swimming in a definite direction was evolved in re-
sponse to the constant direction of flow of water in rivers or

ORIGIN OF CHORDATES 427

large estuaries, and that the evolution of the early true chordates
took place in such surroundings.

The ciliary method of feeding which these animals possessed
limited the size of the particles of food which they could ingest,
and the size to which they could grow.

The earliest chordates of which fossil remains are known
are the Ostracoderms (from the upper Silurian and Devonian),
which recent work has shown to be related to the Cyclostomes,
especially as regards the brain, auditory organs, and blood-
vessels. This raises some interesting problems, because the
Ostracoderms possessed denticles, bone, and paired fins, all
of which structures are lacking in Cyclostomes. The mode of
life of Petromyzon, Bdellostoma, and Myxine is undoubtedly
degenerate with their sucking mouth, but they would be more
degenerate than otherwise expected if they had lost the struc-
tures possessed by the Ostracoderms. The curious fossil
Palasospondylus (from the Devonian) may be related to these
forms, in all of which evolution had proceeded far enough for
the formation of a definite head.

The first true fish appear to have been cartilaginous
(together, the cartilaginous fish are called Chondrichthyes) and
related to the Selachii. Among them may be mentioned
Acanthodes (upper Silurian), Cladoselache (Devonian) in-
teresting for the structure of its paired fins, and Pleuracanthus
(Permian). All these forms had true biting jaws, two pairs of
paired fins, and heterocercal tails. True Selachii related to
Heterodontus (the Port Jackson Shark) appeared in the
Carboniferous. At the present day, the Selachii are repre-
sented by the true sharks (and dogfish), and by the rays (Raia,
Torpedo), which have become adapted to living on the sea
bottom and have become flattened in consequence. Their
pectoral fins have expanded and fused with the sides of the
body. The gill-slits are on the under surface, the spiracle
is above. One member of the rays, Pristis the saw-fish,
has returned to an active mode of life. The angel-fish
Rhina is intermediate in form between the sharks and the
rays.

Another group of cartilaginous fishes diverged in the

428 EVOLUTIONARY MORPHOLOGY

Devonian and gave rise to the Holocephali, represented at the
present day by Chimaera.

The bony fish or Osteichthyes appear in the Devonian,
and the lungs which they possess were probably in connexion
with the poor oxygen- content of the fresh water in which they
lived. The Dipnoi were represented by Dipterus (Devonian),
and the non-Dipnoan bony fish, or Teleostomi were represented
by Osteolepis (also Devonian). These two forms were closely
related, and they had the following characters in common :
blunt lobate fins, a pair of external and a pair of internal
nostrils, the general arrangement of the bones of the roof of the
skull, heterocercal tails, and, most important of all, cosmoid
scales. There is no doubt that they had a common ancestor,
perhaps in the Silurian, and from a close relative of this
ancestor the Tetrapods arose. One of the Osteolepidoti,
vSauripterus, had fins from which the structure of the penta-
dactyl limb of the Tetrapod might be derived. In the Ccela-
canths, which are Teleostomes related to the Osteolepidoti,
there is definite evidence of the presence of a lung, for it was
calcified and fossilised. The Dipnoi evolved into the forms
living at the present day, and became more and more adapted
to life in rivers which are liable to dry up. The wide and dis-
continuous distribution of Ceratodus (Australia), Lepidosiren
(South America) and Protopterus (Africa) to-day is evidence
of the antiquity of the group. The evolution of these forms
went on parallel to that of the early Tetrapods, and inde-
pendently from them.

On the Teleostome side, another group arose in the
Devonian from some relatives of the Osteolepidoti : the
Palaeoniscoidea. These fish are characterised by the possession
of scales of the type called palseoniscoid . Cheirolepis resembled
Osteolepis in the structure of its skull, but its eyes were larger
and the heterocercal tail was more accentuated. This pro-
vision for more active swimming was probably connected with
the improvement of the eyes as sense-organs. Polyp terus,
alive to-day, may be regarded as a descendant of the Palaeo-
niscoids. It has palaeoniscoid scales, and preserves the open
spiracle. It inhabits certain rivers in Africa. On the other

ORIGIN OF CHORDATES 429

hand, the Palaeoniscoids also gave rise to the sturgeons.
Chondrosteus (Jurassic) is already like the sturgeon Acipenser.
These animals preserve the open spiracle, but the palaeoniscoid
structure of the scales is lost. Sturgeons are both fluviatile
and marine.

Another line of evolution from the Palaeoniscoids leads to
the higher bony fish or Holostei. These fish lose the open
spiracle and their tails assume the homocercal pattern. At
the same time the radials of the paired fins become reduced,
and the web of the fin is supported mostly by the dermal
fin-rays or lepidotrichia. In the median dorsal and ventral
fins the lepidotrichia correspond in pairs to the radials, so that
the fins can be lowered and raised. Of the Holostei, two
groups are primitive. One of these contains Amia, an in-
habitant of the rivers of North America. Its lung is still
highly vascular and supplied by pulmonary arteries, and in
the region of the tail its vertebral column consists of separate
hypo- and pleurocentra. The other group contains Lepi-
dosteus, likewise an inhabitant of North American rivers. Its
scales are of the peculiar pattern known as lepidosteoid, with
a covering of ganoin. The lung is vascular, but supplied by
arteries from the dorsal aorta. It is worth noticing that the
primitive Osteichthyes are almost exclusively inhabitants of
fresh water.

In the remaining highest bony fish or Teleostei (a term
not to be confused with Teleostomi), the scales lose the layer
of ganoin and become thin and transparent. The lung
becomes modified into a swim-bladder, and loses the vascular
spongy walls characteristic of a lung. It functions as a hydro-
static organ of adaptation to different depths, and this illustrates
the fact that the Teleostei are a group which has reinvaded the
sea from fresh water. Rivers do not possess sufficient depth
to necessitate a swim-bladder. A number of Teleosts, how-
ever, are inhabitants of fresh water, to which they have pre-
sumably returned from the sea. The Teleosts have radiated
into a great many different lines, and are the most successful
of the fish. They have become specialised to various modes
of life, but they must be regarded as a sterile side branch on

430 EVOLUTIONARY MORPHOLOGY

the tree of vertebrate evolution, for their specialisations have
prevented them from evolving into anything further. Although
some of them, such as Periophthalmus, are capable of coming
out on dry land and hobbling about, they cannot compete with
the true land- vertebrates, which are less specialised but more
progressive descendants of their ancestors, the pre-Osteolepids.
Of the adaptations which Teleostei have undergone, one of
the most interesting is the modification in connexion with the
habit of living on the sea-bottom, and which has resulted in
the " flat fish." When hatched, these fish, of which Solea
(the sole) is an example, are normal and symmetrical in form,
but they undergo a metamorphosis as a result of which they
lie on one side on the bottom. The head becomes twisted so
that the eye of the " underside " (right or left, according to the
species) moves on to the " upper side." It is interesting to
compare this flattened condition of the body with that of the
rays. The modifications in the two groups are totally different,
but both are adaptations to one and the same mode of life,
and this accounts for what similarity there is between them.

The so-called flying fishes, of which Exocoetus is an
example, have enlarged pectoral fins, and are capable of
prolonged leaps through the air rather than of true flight.
Lastly, attention may be called to certain deep-sea fish
(Edriolychnus) which are not only of a peculiar shape, but
are remarkable in that the males are dwarfed and degenerate,
and live attached to the females on which they are parasitic.

Literature

Delage, Y., et Herouard, E. Zoologie Concrete, 8, les Procordes.
Schleicher Freres, Paris, 1898.

Regan, C. T. Dwarfed Males parasitic on the Females in Oceanic Angler-
Fishes. Proceedings of the Royal Society, B, vol. 97, 1925.

CHAPTER XXXVIII

THE EVOLUTION OF THE AMPHIBIA .* THE FIRST LAND CHORDATES

That the amphibia arose from fish there is no doubt, and their
ancestor must have been one of the primitive Osteichthyes,
related to the stock which also gave rise to Osteolepis and
Dipterus. For purposes of comparison Osteolepis may be
taken as approaching the structure of this ancestor.

The resemblances between Osteolepis, on the one hand,
and one of the earliest Stegocephalian amphibia such as
Loxomma on the other extend to the following features. In
both, the skull is a complete bony box, the dermal bones of
which can in most cases be identified with certainty because
the amphibia also had lateral-line canals which occupied
grooves in the bones. The bones of the palate are similar,
and both had nostrils which lead through into the cavity of the
mouth. The amphibian Eogyrinus had a shoulder girdle the
dermal bones of which were attached to the post-temporal
bone of the skull by the supra- cleithrum, as in the fish. Also,
these early amphibia had no sacrum, for the ilium was not
attached to the ribs. The walls of the teeth were folded, in
the Labyrinthodont pattern. The amphibia are autostylic,
as are the Dipnoi including Dipterus. The otic process in
Osteolepis did not reach the auditory capsule, however, and
it is a question as to whether the common ancestor of Osteo-
lepids, Dipnoi, and Tetrapods was autostylic or not. A lung
was almost certainly present in Osteolepis.

The ancestor of the Tetrapods must, however, have had
pectoral and pelvic fins equally developed and similar in
structure, and this condition has not yet been found in any
Osteolepid (or other) fish. The really distinctive feature of

431

432

EVOLUTIONARY MORPHOLOGY

all the Tetrapods is the possession of limbs ending in five
digits, and it has already (see p. 315) been shown that the skele-
ton of the pectoral fin of the Osteolepid fish Sauripterus is
such as to render it easy to suppose that the pentadactyl limb
arose from a fin like that of the Osteolepids. Osteolepis is
Devonian, and the earliest known amphibia are from the Lower
Carboniferous. It is fairly certain, therefore, that at some time
in the Devonian, fish living in the estuaries and fresh-water
basins became subjected to the desiccation which characterised
this period. They were able to breathe atmospheric oxygen

Fig. 176. — A few examples of different types of Amphibia. (Not drawn

to scale.)

a, restoration of Stegocephalian ; b, male newt in breeding season
(Urodele) ; c, Amblystoma (larval form or Axolotl showing the external
gills) ; d, frog (Anuran) ; e, Ichthyophis (Gymnophiona) . (e after
Sarasin).

by means of their nostrils and lungs, and as they floundered
about in the mud, the number of rows of radials in their fins
became reduced to five, separate from one another instead of
being united by the web of a fin. The persistence of the
lateral-line canals shows that these animals still spent much
of their time in the water, and their excursions on land
probably took the form of wandering from pond to pond.
In fact, the amphibia never succeeded in making themselves
completely independent of water, and for three reasons. In
the first place the eggs had to be laid in water, and the larval

EVOLUTION OF THE AMPHIBIA 433

stages which breathed by gills required then, as they do now, a
watery medium. Next, fertilisation was external, and for the
sperms to be able to find the eggs, there must be a liquid
medium for them to swim in. Lastly, amphibia breathe largely
through their skins, and these must be moist to enable the
gaseous exchange to take place.

The transition from water to air necessitated a development
of the olfactory organs to greater sensitiveness, for the concen-
tration of substances in water is very much greater than that
which can be obtained in air. The result was an increase in
development of the olfactory organs and of the corresponding
centres in the forebrain. The latter development accompanied
and perhaps assisted the formation of the cerebral hemispheres,
which are regarded as connected with an adaptation to the
poor oxygen- content of the water in which the amphibia and
their ancestors evolved.

It appears, therefore, that the transition from aquatic to
terrestrial life was accomplished without any very striking
changes or modification of organs, but it must be remembered
that the function of these organs is controlled by the pattern
of nerve-fibres in the central nervous system, and it becomes
necessary to inquire whether the transition necessitated any
great neurological rearrangement. Two aspects of the transi-
tion will be considered, regarding breathing and locomotion.
In connexion with respiration, it will be remembered that the
amphibia breathe by means of respiratory movements per-
formed by the visceral muscles in the floor of the mouth, in a
manner very similar to that of the fish . The only real difference
is that whereas the fish take in water and pass it back and out
through the gill-slits, the amphibia take in air and pass it
back and into the lungs. The mechanism is the same, and it
is obvious that the transition from water to air involved no
functional rearrangement of importance as regards respiration.

The same holds true with regard to locomotion. The
amphibia were clumsy sluggish beasts with bodies dispropor-
tionately large in comparison with their limbs. As a conse-
quence, the body was not supported by the limbs but its ventral
surface dragged along the ground. The limbs stuck out at

2 F

434 EVOLUTIONARY MORPHOLOGY

right angles to the body, and as the body performed the same
undulatory movements by means of the myotomes as does a
fish when swimming, the limbs were moved forwards and
backwards. In other words, the limbs were used as oars to
row the animal along on land, and the same muscles and
nervous connexions came into play as in the aquatic ancestor.
In the very earliest Amphibia, the sacrum was absent {e.g.
Eogyrinus). In the others it was present, and by anchoring
the pelvic girdle on to the vertebral column, it strengthened
the hind limbs.

One consequence must be mentioned of the possession of
an autostylic method of suspension of the jaws, and of the
abolition of branchial respiration and the closure of the visceral
clefts, in particular the spiracle. The hyomandibula being
no longer required to suspend the quadrate from the auditory
capsule, its function became converted into that of conveying
vibrations from the skin covering the spiracular cleft to the
auditory capsule. In this way the hyomandibula became the
columella auris ; the covering of the spiracular cleft became
the tympanic membrane, and the cavity of the spiracular cleft
became the middle-ear and Eustachian tube ; all quite simply
and without involving any great rearrangement. So the ear
became an organ for the delicate appreciation of sound as well
as balance.

The early amphibia had a covering of dermal bones more or
less all over the body. The vertebral column of the earliest
forms or Embolomeri is remarkable in that each vertebra
possessed two centra. There was an anterior hypocentrum
and a posterior pleurocentrum. The later amphibia preserved
the hypocentrum at the expense of the pleurocentrum, which
disappeared. It will be seen that in the reptiles the opposite
occurred.

Collectively, the early amphibia are known as the Stego-
cephalia or Labyrinthodonts, the former term referring to the
complete bony covering of the skull. They flourished in the
Carboniferous, and persisted until the Triassic period, when
they were extinguished by the competition of their more
successful descendants the reptiles, leaving only the frogs

EVOLUTION OF THE AMPHIBIA 435

(Anura), newts (Urodela) and Gymnophiona alive to-day.
An important feature in the amphibia is the outgrowth from
the gut to form a bladder. It is homologous with the allantois
of the Amniotes.

With regard to the living amphibia, it is most important
to realise that they have departed far from the primitive type
of their Stegocephalian ancestors. This is shown by the great
reduction in the bones of the skull and other parts of the
skeleton. The Gymnophiona have evolved a worm-like
burrowing habit ; the frogs have become modified in connexion
with the habit of leaping with the hind legs, and the newts
have become secondarily readapted to living in water, even
going so far as to develop median fins which differ entirely from
those of fish in not possessing radials or dermal fin-rays. Now,
in several points, the newts of to-day resemble living Dipnoi
(such as Ceratodus) very closely. It is of the utmost import-
ance, however, to realise that these resemblances are due to
parallel evolution and convergence, and not to genetic affinity.
It is only necessary to look at the list of specialised characters
of Ceratodus to see that forms like it could not have given rise
to the Tetrapods : they are cousins and not ancestors.
Similarly, an examination of the specialised characters of
Triton and a comparison between it and the Stegocephalia
show that all the points in which Triton resembles Ceratodus
have been evolved within the amphibia, and a long time after
the amphibia came on land.

Literature

Watson, D. M. S. The Structure, Evolution and Origin of the Amphibia.
Philosophical Transactions of the Royal Society, Ser. B, vol. 209, 1919.

The Evolution and Origin of the^Amphibia. Philosophical Trans-
actions of the Royal Society, Ser. B, vol. 214, 1926.

CHAPTER XXXIX

THE EVOLUTION OF THE REPTILES

That the Reptiles were evolved from the amphibia there is no
doubt whatever, and indeed, in some cases it is difficult to
decide whether a fossil is an amphibian or a reptile. The
most important distinctive features are the fact that the centra
of the vertebral column are formed from the pleurocentral
elements while the hypocentral elements are very much reduced,
and the absence of grooves for lateral-line canals on the skull.
This latter point shows that the reptiles had become definitely
terrestrial. They emancipated themselves from the water by
overcoming the three obstacles which checked the amphibia ;
viz. the necessity of water for breathing, for copulating, and
for the embryo to develop in.

The first of these was countered by a better development of
the lungs and the adoption of the method of expanding the
thoracic cavity by means of the ribs, for replenishing their
content of air. The skin was thus enabled to become dry and
horny, and to be of greater efficiency in protection.

The next difficulty was surmounted by the development of
copulatory organs with which the sperm could be inserted
straight into the oviducts of the female, and fertilisation was
internal. The sperm swam to the egg in the mucous fluid of
the oviduct instead of in the pond water.

The last obstacle was overcome by a group of adaptations.
The egg was laid on land, and the albumen surrounding it was
itself surrounded by a shell composed of lime salts secreted by
the oviducts. In this way the egg was protected from drying
up, and from becoming flattened and collapsed like a " poached
egg," which it would otherwise be. The embryo became

436

THE EVOLUTION OF THE REPTILES 437

surrounded by upgrowths of the blastoderm forming the
amnion and enclosing the amniotic cavity. The embryo
developed in the fluid contents of this cavity, which may thus
be regarded as an artificial enclosed pond. The food require-
ments of the developing embryo were met as in lower forms by
a store of yolk in the yolk-sac. This entailed no new modifi-
cations by itself, but as embryonic development took a longer
time, the quantity of yolk in the egg was relatively greater and
this necessitated the modification of the process of gastrula-
tion, and the formation of a primitive streak. There remained
the difficulty of breathing, for although the gill-slits were
developed they opened into the amniotic cavity, the oxygen-
content of which could not be renewed. The problem was
solved by the development of the allantois, representing the
bladder of the amphibia. The allantois became applied to
the inner surface of the porous shell, and as it was highly
vascularised, the respiratory exchange took place in it. At the
same time, the allantois served as a receptacle for the non-
volatile excretory products of the embryo during development.
Because of these structural adaptations, the reptile does not
pass through a metamorphosis, but hatches from the egg as a
more or less perfect miniature replica of the. adult.

In the reptiles, the head is capable of extensive independent
movement, and a definite neck is formed. In this connexion,
the two first vertebrae become modified into the atlas and the
axis.

The most primitive known reptile is Seymouria (Permian),
and it is remarkable for the fact that its characteristics are not
intermediate between those of amphibia and of reptiles, but
some of its characters are frankly .amphibian and others rep-
tilian. Seymouria is therefore a mosaic transitional form. It
is probable that the transition from amphibia to reptiles took
place in the Carboniferous.

Seymouria belongs to the group of Reptiles known as
the Cotylosaurs, and they preserve the complete covering of
dermal bones over the skull which they inherited from their
Stegocephalian amphibian ancestors. The nature of the skull
is of importance in tracing out the lines of evolution of the

438

EVOLUTIONARY MORPHOLOGY

THE EVOLUTION OF THE REPTILES 439

reptiles, and those forms in which the roof is complete and
imperforate are often grouped together as the Anapsida.

The Chelonia are often classified among the Anapsida
because their skull-roof is not fenestrated, although it may be
reduced by emargination ; i.e. bones may be lost round the
edge but there is no separation of bones by a perforation
forming a fossa. The Permian fossil Eunotosaurus which had
osteoscutes and expanded ribs appears to be intermediate
between Cotylosauria and Chelonia. The Chelonia preserve
the osteoscutes (which covered the body of the Stegocephalia)
and they contribute to the formation of the carapace which is
so distinctive a feature of the Chelonia. The Triassic Triasso-
chelys still had teeth and the cleithrum ; these structures are
absent from existing forms. The clavicular pectoral girdle of
existing Chelonia, consisting of clavicles and interclavicle, is
associated with the ventral covering of osteoscutes that form
the plastron. The scapular pectoral girdle consists of a cora-
coid and a scapula bearing a large process as big as itself,
directed forwards and inwards. This girdle and the pelvic
girdle are remarkable in that they are situated within the ribs
instead of outside them as in normal forms. It is interesting
to note that some Chelonia have become secondarily adapted to
life in water, and their limbs have been modified into paddles
or flippers, as in the turtles.

The osteoscutes of the carapace are covered by corneo-
scutes (" tortoise-shell ") except in Sphargis, the " leathery
turtle," which form is further interesting in that the carapace
attached to the expanded ribs as in other Chelonia is not present.
Instead there is a bony shell formed of a great many little poly-
gonal osteoscutes bearing no relation to the ribs.

The 5th metatarsal is hook-shaped in the Chelonia, but
normal in the other Anapsida (Cotylosauria).

The next group of reptiles to consider is the Synapsida.
They are characterised by the fact that the skull-roof is per-
forated by one inferior temporal vacuity or fossa, on each side,
and the 5th metatarsal is normal. Here belong the Plesiosaurs
and Theromorphs.

The Theromorphs are a very important group. They

44© EVOLUTIONARY MORPHOLOGY

appear in the Permian, and preserve many primitive characters.
They may have a precoracoid as well as a coracoid in the
shoulder girdle, and some even retain the cleithrum. The
most highly developed forms are the Theriodonts, which are
the ancestors of the mammals, and they foreshadow the
characters of the latter in many respects. The skull had two
occipital condyles, a false palate was present, and the teeth
were modified into incisors, canines, premolars and molars.
The dentary was large and beginning to take on the articulation
with the squamosal, while the quadrate became small and loose.
In the pelvic girdle the ilium showed the mammalian character
of pointing forwards, and the limbs were long and supported
the body clean off the ground. A typical Theriodont is Cyno-
gnathus, but it is probable that several of its characters were
evolved parallel with the mammals, having been derived from
a more primitive ancestor common to it and to the mammals.

Among Synaptosauria, the Sauropterygia or Plesiosaurs
have become secondarily adapted to an aquatic mode of life.
They preserve primitive features such as the gastralia, which
are remnants of the ventral dermal bones or osteoscutes of the
Stegocephalian amphibia, but their limbs become modified
into paddles. This modification has not proceeded as far in
the Triassic Nothosaurus as in the Jurassic Plesiosaurus. The
Plesiosaurs reached lengths of 50 feet.

In the next group or Parapsida, the roof of the skull was
perforated by a single superior temporal vacuity, above the
postorbital and squamosal bones. The hind border of the
vacuity is formed by a bone concerning the homology of which
doubt remains (see p. 104), but which may be the supra-
temporal. Here belong the Ichthyosaurs and the Squamata
(Lacertilia and Ophidia).

The Ichthyosaurs are primitive in retaining the gastralia,
and a foramen for the pineal eye between the parietals, but
otherwise they are specialised in adaptation to an aquatic mode
of life. Median dorsal and tail-fins are developed, and the
limbs become modified into paddles, so much so that it is
impossible to determine the nature of the 5th metatarsal. A
series of progressive modification can be traced from the

THE EVOLUTION OF THE REPTILES 441

Triassic Mixosaurus, through the Jurassic Ichthyosaurus to
the Cretaceous Ophthalmosaurus. They reached lengths of
30 feet.

All the Squamata which possess limbs have a hook-shaped
5th metatarsal. The first group of these are the Lacertilia,
first appearing in the Jurassic, and represented now by the
lizards, geckos, and chamaeleons. In the Cretaceous, a group
of Lacertilia became adapted to an aquatic life, — the Mosa-
sauria. They reached a length of as much as 40 feet, and their
limbs became modified into paddles. The second group of
the Squamata are the Ophidia or snakes. It is characteristic
of the Squamata that the quadrate is loose, and in the Ophidia
the two halves of the lower jaw are separate, which enables
relatively enormous mouthfuls to be swallowed.

The remaining reptiles form the group Diapsida, for their
skull roof is perforated by two temporal fossae or vacuities on
each side. So far as is known, all of them have a hook-shaped
5th metatarsal. Here belong the Rhynchocephalia, the
Crocodilia, the Dinosaurs and the Pterosaurs, and the Diapsida
also contained the ancestors of the birds.

The Rhynchocephalia appear in the Triassic with Rhyncho-
saurus, and are represented to-day by Sphenodon. They are
primitive in retaining the gastralia. The Triassic Thalatto-
saurs, which had paddle-like limbs, were probably related to
the Rhynchocephalia.

The Crocodilia form a large group of generalised reptiles,
possibly dating back to the Permian. The Triassic Pseudo-
suchia, of which Euparkeria is an example, are regarded as
related to the ancestors of the Rhynchocephalia, the Dinosaurs
and Pterosaurs, the existing crocodiles, and the birds. The
living crocodiles and alligators retain the gastralia, and other
dermal ossifications. Mention may be made of the Jurassic
Thalattosuchia, yet another group of reptiles which became
secondarily adapted to aquatic life with paddle-like limbs.

The Dinosaurs were the dominant animals in the Jurassic
and Cretaceous. The skull had two temporal vacuities on
each side, and in addition a prelachrymal vacuity. Some
were quadrupedal and herbivorous, such as Diplodocus

442 EVOLUTIONARY MORPHOLOGY

(Jurassic), reaching the immense length of 90 feet. Others
were carnivorous with formidable teeth, and the hind limbs
larger than the fore limbs, so that they were probably bipedal.
An example of such a form is Tyrannosaurus, Cretaceous,
reaching almost 50 feet in length. The foregoing types of
Dinosaurs had a pelvis of normal shape, and are grouped
together under the term " Saurischia." The remainder, Pre-
dentata (or Ornithischia), have an additional postpubis which
stretches back beneath the ischium, and a predentary bone
in the lower jaw. The Predentata include the herbivorous
bipedal Iguanodon from the Cretaceous, over 30 feet long ;
the absurd-looking Stegosaurus with its armour of large bony
plates (Jurassic, 20 feet long) ; and the horned Triceratops
(Cretaceous, 25 feet long).

The Pterosaurs were closely allied to the Dinosaurs, and
like them had a pair of temporal vacuities and a prelachrymal
vacuity on each side. They also preserved gastralia, and while
some had teeth others were toothless. The fore limbs were
modified for flying, by means of a web of skin stretched from
the greatly elongated fourth finger. The Cretaceous Ptera-
nodon had an expanse of wings measuring 25 feet. Of all this
enormous wealth of reptilian life which dominated the land,
water, and air, in the Jurassic and Cretaceous periods, only
the Squamata, the Chelonia, the Crocodilia, and the Rhyncho-
cephalia have survived, and in very reduced numbers. The
rest went extinct before the Eocene. It may be that a reduc-
tion of temperature put an end to them, or that the food
supply became deficient. Certain it is that their brains were
ridiculously small, and they can have been no match for the
small and agile mammals in intelligence.

Two main points for consideration arise out of a study of
the reptiles. The first concerns the structure of the 5th
metatarsal bone. In the Cotylosaurs it was of a normal shape,
as also in the Synapsida. Now, the Synapsida are to be
regarded as having been derived from the Cotylosaurs, and one
group of them, the Theromorphs, gave rise to the mammals.
The line of mammalian descent is therefore characterised by
the possession of a normal-shaped straight 5th metatarsal.

THE EVOLUTION OF THE REPTILES 443

On the other hand, the other reptiles such as the Chelonia,
together with the Squamata, and all the Diapsida have a hook-
shaped 5th metatarsal. The birds were derived from a Diapsid
stock, and so it may be said, therefore, that the line of avine
descent is characterised by the possession of a hook-shaped
5th metatarsal. Actually what this modification means or
what function it serves is unknown, but it is to be noticed that
among the animals possessing it are forms which live on land,
in the water, and in the air, so that it would seem not to have
an adaptive significance, nor to be capable of modification by
different modes of life. It looks, therefore, as if it could be
used as a diagnostic feature inherited from a common ancestor
by all the forms possessing it. This common ancestor was
probably a late Anapsidan, and the importance of this matter
is that from this point onwards the reptiles were divided into
two main and divergent branches. One branch which may be
called the Sauropsidan includes the Chelonia, the Parapsida,
the Diapsida, and the birds. The other or Theropsidan branch
includes the Synapsida perhaps (the Synaptosauria), and the
mammals. As a result of these considerations, it appears that
the term " Reptilia " is applied not so much to a unified group
of related animals as to two divergent stocks. It therefore
refers to a grade of structure and degree of evolution ; and when
the knowledge of fossil forms is more complete, it will be
possible to abolish the class " Reptilia," or to restrict it to the
primitive Anapsida, and to substitute the classes " Sauropsida "
and " Theropsida," containing the birds and mammals
respectively.

That these conclusions are sound is shown by a considera-
tion of the aortic arches. It is to be noticed that all the living
reptiles belong to the Sauropsidan branch, and in all of them
the systemic aorta is split into two right down to the ventricle
of the heart. The result is that there are right and left systemic
arches springing respectively from the left and right sides of
the ventricle. The condition of the bird fits into this scheme,
for it differs from the arrangement in the crocodile only by the
loss of the left arch. Now, in the mammal, the systemic aorta
is single and undivided. The point is that it is impossible to

444 EVOLUTIONARY MORPHOLOGY

derive the Sauropsidan type of aorta from the mammalian, or
vice versa, and it is necessary to go back to a primitive type
like that of the amphibia where the aorta is not only undivided
but the pulmonary arch has not yet become separated off.
The primitive Cotylosaurs may have been of this type. It is
certain that the Synapsida must have resembled the mammal
(for the latter was derived from the former), and therefore
differed from the Sauropsida as regards the structure of the
aortic arches. This is the same divergence which appeared
from a consideration of the hook-shaped 5th metatarsal.

Literature

Goodrich, E. S. On the Classification of the Reptilia. Proceedings of the
Royal Society, Ser. B, vol. 89, 191 6.

Nopcsa, F. Die Familien der Reptilien. Fortschritte der Geologie und
Palaeontologie, 2, 1923.

Watson, D. M. S. On Seymouria, the Most Primitive known Reptile.
Proceedings of the Zoological Society, London, 1918.

CHAPTER XL

THE EVOLUTION OF THE BIRDS

The birds present so many similarities to the reptiles that they
have been classified together with them in the group Sauro-
psida. The resemblances extend to the following features.
The heart and arteries of the bird are the same as those of the
crocodile with the exception of the left systemic arch, which
in birds is abolished. The perivisceral coelomic cavity of
birds is divided up into pulmo-hepatic recesses and pleural
cavities, by means of the pulmo-hepatic ligaments and oblique
septa ; this arrangement is also present in the crocodile. The
lung of birds gives rise to a number of diverticula or air-sacs
which ramify about inside the body ; small air-sacs are formed
by the lung of the chamasleon. With regard to the nervous
system, the brain of birds is an elaboration of the grade of
structure shown by the brain of crocodiles, and its distinctive
feature is that the corpus striatum has been especially developed
while the cerebral cortex remains small and thin. The
cerebellum of birds presents many resemblances to that of the
Pterosaurs, which can be explained as due to the action of
similar modes of life working on related materials. The early
stages of development, amnion arid allantois, are very similar.
Coming to the skeleton, the single occipital condyle, the inter-
orbital septum, the limb girdles, the hollow nature of several
of the bones and the mesotarsal articulation of the feet, are all
characters which appear in some or most of the reptiles of the
Sauropsidan branch. In addition, the Jurassic fossil Archaeo-
pteryx had gastralia, a prelachrymal and remnants of two
temporal vacuities, a long tail with several separate caudal

445

446 EVOLUTIONARY MORPHOLOGY

vertebrae, and a lower jaw perforated by a foramen as in croco-
diles. It is possible that Archaeopteryx had cartilaginous
uncinate processes on its ribs.

Nothing can be regarded as more certain than that the birds
were evolved from reptiles of the Sauropsidan branch, and the
only point left to consider in this connexion, is which.
Birds share with : —

Crocodiles : the structure of the heart and arteries, the
arrangement of the ccelom, the large corpus striatum,
the foramen in the mandible (of Archaeopteryx) ; and
according to the precipitation blood-tests a high degree
of blood-relationship ;
Dinosaurs : the prelachrymal fossa, the mesotarsal
articulation, the hollow bones, and the bipedal mode of
progression ;
Pterosaurs : the structure of the cerebellum ;
Rhynchocephalia : the uncinate processes of the ribs ;
with all the above-mentioned reptiles, the two temporal
vacuities.
Now, the ancestors of all these reptiles were probably
closely allied to the Pseudosuchia, of which Euparkeria is an
example from the Triassic ; and it is very probable that these
forms were the ancestors of the birds also.

The characteristic of birds is their peculiar method of
flight, and most if not all of their modifications are adaptations
to life in the air, foremost among which are the feathers.

Feathers are one of the most marvellous cases of adaptation
known in the animal kingdom. Firmly anchored by the rachis
or quill to the skeleton of the forelimb, the vane of the feather
gives perfect air-resistance at the downstroke coupled with
lack of resistance in the upstroke. The feather is proverbially
light, and the structure of the hooks or hamuli and barbules
enables the feather to be repaired if damaged, by the bird itself,
by simply drawing the vane through the beak. The result of
this process is to rehook the hamuli on to the barbules should
the barbs have become torn apart.

Other feathers are smaller and serve not for flight but as
a non-conducting layer for heat, and enable the bird to maintain

THE EVOLUTION OF THE BIRDS 447

a constant and high internal temperature. Birds are homo-
thermous. This condition with its consequent increased
efficiency of muscular and nervous processes is necessitated
by the great exertion required to maintain the weight of the
body in the air during flight.

This was accomplished with the aid of the feathers by the
perfection of the vascular system (abolition of the left systemic
arch) and the development of the air-sacs. The latter ensure
a complete " blow- through " of the lungs and avoid the
inefficiency due to the presence of stagnant residual air in
blind-ended sac-shaped lungs. Further, the air-sacs assist in
regulation of the internal temperature by varying the amount
of heat lost.

The characteristic development of the keel (carina) on
the sternum in flying birds is a direct adaptation to the need
for a firm origin for the large muscles concerned with
flight.

As to the evolution of the method of flight itself, there is
not yet any certainty, and several theories have been pro-
pounded.

The avine method of flight differs from that of all other
forms in that no use is made of stretched membranes. From
a study of the feathers and scales on the leg of the ostrich,
there is reason to believe that feathers were evolved in con-
nexion with scales, but not from them. Be that as it may,
the development of feathers must have resulted in an increase
in the surface of the animal without any material increase
in its weight. It is easy to see how this would have benefited
the ancestral birds if they inhabited trees and were in the
habit of leaping or parachuting from one branch to another.
At the same time it must be remembered that the structure of
the pelvic girdle and limbs of the bird are adapted to a cursorial
mode of life with a bipedal method of walking or running.
The biological success of birds is probably due in no small
measure to the fact that the adaptations for flight in no way
interfere with or involve the hind pair of limbs. The ancestors
of the birds must therefore have been terrestrial and cursorial
in habits before becoming arboreal.

448

EVOLUTIONARY MORPHOLOGY

The early birds like Archaeopteryx (Jurassic) had a long
tail, jaws garnished with teeth, wings with three well-formed
ringers ending in claws, and a more or less flat sternum. At
the present day, birds have no teeth, and their tails are shortened
up, the fused vertebrae forming the pygostyle.

Living birds are divided into two groups : Palaeognathae,
and Neognathae, according to the structure of the palate. In

Fig. 178. — A few examples of different types of birds (not drawn to scale).

a, Archaeopteryx ; b, bird of prey ; c, swift ; (both b and c are examples
of the Neognathas snowing highly perfected aerial adaptation) ; d, ostrich
(Palaeognathae, flightless) ; e, dodo (one of the Neognathae which lost the
power of flight) ; /, penguin (aquatic adaptation), (a after Pycraft.)

the former (which include what used to be called the " flight-
less birds " or Ratites, plus the Tinamus), the prevomer is
large and touches the pterygoids ; whereas in the latter (the
so-called Carinates minus the Tinamus) the prevomer is small
and does not touch the pterygoids. In Tinamus and Neo-
gnathae the sternum bears a large keel or carina, on to which the
pectoral muscles (which are used in flight) are attached, and
the well-developed clavicles fuse together at their base to form

THE EVOLUTION OF THE BIRDS 449

the " merrythought " or furcula. The carpals, metacarpals,
and phalanges are much reduced and fused together, but in
the young Hoatzin (South American) among others, the hand
is well developed and represented by three digits ending in
claws. With the help of these claws the young Hoatzin
clambers about on trees in a manner suggestive of the presumed
habits of its ancestors. The barbs of the flight-feathers are
connected to one another by the hooks on the barbules, and
most of them are active flyers. All make nests of one kind
or another and incubate the eggs, except the cuckoo which is
parasitic in that it lays its eggs in the nests of other birds for
them to incubate, and the Megapodes which lay their eggs in
decaying vegetable matter, relying on the heat engendered by
the latter to incubate them.

A point of no small importance in these birds is the high
development of courtship-activities and structures, which play
a large part in knitting parents together into a family. Normally
it is the male which is the active partner in courtship and
possesses well-developed " secondary sexual " or courtship
characters. In some, such as the Great Crested Grebe, males
and females are similar in appearance and in the ardour of
their behaviour. In others, of which the Phalarope is an
example, the female is the more brilliant and active, and the
male incubates the eggs, and in fact does almost everything
except lay them.

As regards their anatomy, these birds are very much alike ;
so much so that it is a matter of extreme difficulty to arrive
at a satisfactory scheme of classification for them. This is
partly because the birds are a comparatively recent group, and
because they have been so successful in the walk of life to
which they have become adapted that little extinction has
taken place. Most of them are strictly aerial animals. Others
such as the common fowl and the extinct Dodo of Mauritius
have secondarily become terrestrial as a result of reduction of
the powers of flight. Many have become adapted to swim-
ming on water, and have developed webs of skin between the
toes of the feet, which are then described as webbed feet.
This has taken place independently in a great many groups,

2 G

45© EVOLUTIONARY MORPHOLOGY

such as the Petrels, Penguins, Divers, Cormorants, Flamingos,
Ducks, and Gulls.

In the Penguins, in addition, the wings have been modified
into paddles or flippers and the birds can no longer fly. It is
worth noticing that these aquatic birds, with the exception of
the penguins, have their powers of flight in no way impaired
by the adaptation of webbed feet ; this is a consequence of
the avine structure of the wing which leaves the hind limbs
unencumbered. A peculiar modification which may occur is
the absence of the 5th flight-feather carried on the ulna (5th
secondary remex). This condition is known as aquinto-
cubitalism or diastataxy, and it occurs sporadically in some
groups and not in others, or even in some members and not in
others of the same group. The significance of this modifica-
tion which is so peculiar is a mystery.

The Palaeognathae include the Struthiones (Ratites) or
" flightless birds " and the Tinamus. In the former the flight-
feathers have lost the hamuli on the barbules, so that the vanes
are no longer resistant to the air. This character is associated
with the loss of the power of flight on the part of these birds,
and accounts for the well-known structure of the ostrich plume.
Also connected with their flightless condition, the Palaeo-
gnathae (again with the exception of the Tinamus) have lost the
keel on the sternum. These characters are specialisations and
losses from a more primitive type of flying bird. At the same
time the palate of the Palaeognathae is more primitive than that
of Neognathae, and some preserve primitive characters such as
the claws at the ends of the fingers in the wing in the ostrich.
In addition to the ostrich (Struthio, South African and Arabian),
the Struthiones include the Rhea (South American), the Emu
and Cassowary (Australasian), and the Kiwi (Apteryx, New
Zealand). The extinct Moa of New Zealand had reduced its
wings altogether.

It is interesting to note that some Neognathae also lost the
power of flight, such as the extinct Dodo of Mauritius, and
the Solitaire of Rodriguez. Flightless birds, whether Palaeo-
gnathae or Neognathae, are more or less restricted to the southern
hemisphere or to islands, where there is little competition to

THE EVOLUTION OF THE BIRDS 451

fear from other animals. Their degeneracy was therefore, so
to speak, allowed because of the greater leniency of natural
selection in these regions.

Literature

Heilmann, G. The Origin of Birds. Witherby, London, 1926.
Pycraft, W. P. A History of Birds. Methuen, London, 1910.

CHAPTER XLI

THE EVOLUTION OF THE MAMMALIA

In considering the evolution of the mammals it is necessary
to revert to the Theromorph reptiles, in the more highly
developed members of which, such as Cynognathus, it was
found that the following characters were present. The skull
had two occipital condyles, a false palate, heterodont teeth in
sockets with the mammalian method of replacement. The
dentary was large, the remaining membrane-bones of the lower
jaw were small, and the jaw articulation was beginning to be
taken on by means of the squamosal ; the quadrate was loose
and small. The limb girdles were of the mammalian type,
and the limbs were long and supported the body clean off the
ground. These characters point unmistakably to the fact
that the mammals were derived from ancestors which were
Theromorph reptiles.

The dominant factor in mammalian evolution appears to
have been the development of the brain along the lines of
increase in size of the roof of the cerebral hemispheres, and the
formation of a special area of cerebral cortex called neopallium,
which was no longer under the dominance of the fibres coming
from the olfactory centres. The neopallium became an organ
for the retention of past sensations and for the delicate co-
ordination of the activities of the body of the animal, which
thus became capable of more efficient response to external sets
of circumstances, and capable of profiting by experience. It
enabled the animal to improve the speed and precision of its
method of locomotion with the help of the long and delicately
formed limbs ; and the fact that the skin lost its hard horny
scales and became supple and covered with hairs, enabled it to

452

THE EVOLUTION OF THE MAMMALIA 453

increase its sensitiveness. The hair covering, furthermore,
was a non-conductor of heat, and this fact together with the
greater activity of the animal and more intense metabolism
enabled the mammals to become warm-blooded. Later on,
with the development of the sweat-glands in the skin, the
mammals were able to regulate their loss of heat, and so become
constant- temperatured or homothermous. The modification
of some of the skin-glands into mammary glands made it
possible for the young mammals to pass through a protected
period of infancy during which the finishing touches to their
development were put on, and they became apprenticed
under the care of the family to the conditions of their adult
life.

The transition from Theromorph reptiles to mammals
probably took place in the Permian period, for in the Triassic,
fossils are found which show an advance in grade of structure.
Of these, the Multituberculata are a group which persisted
until the Eocene. They advanced in general evolution and
grade of structure as far as the Marsupials. The pelvis was
narrow as in the reptiles, and the lower jaw which contained
a single bone, had inflected angles. The single bone (dentary)
in the lower jaw is a characteristic mammalian feature. The
Multituberculata were, however, specialised, and possessed
molar teeth with a large number of cusps. They are probably
a divergent line which evolved parallel with but independently
from the remaining mammals.

At this stage it must be imagined that the primitive mam-
mals had seven cervical vertebrae as a constant number, and
that they had evolved the characters enumerated above,
together with the diaphragm and the non-nucleated red blood-
corpuscles. The epipterygoid had been converted into the
alisphenoid, and the quadrate (incus) and articular (malleus)
into auditory ossicles. They retained, however, the reptilian
characters of the presence of the coracoid, precoracoid, and
interclavicle, the cloaca and the habit of laying eggs. They
had not yet evolved the viviparous habit or the formation of a
placenta, epiphyses were not yet well developed in the bones,
the mammary glands were unprovided with teats, and the two

454 EVOLUTIONARY MORPHOLOGY

halves of the neopallium were not connected by that special
transverse commissure : the corpus callosum.

The Monotremes must have diverged from the main stem
at this point, and they are represented to-day by Ornitho-
rhynchus and Echidna, to which the description just given fits
well. They are inhabitants of the Australasian region.

The remaining mammals were the ancestors of the Mar-
supials and of the Placentals. These two groups are fairly
closely allied, and have the following characters in common :
the mammary glands have teats, they are viviparous, a placenta
of some kind is present, the ear has an external pinna, the bones
mostly have epiphyses ; the coracoids,interclavicle, and cloaca
have been lost.

It is clear from the reduction of the milk- dentition and of
the allantoic placenta (which is only preserved in Perameles)
in Marsupials, that they are derived from a stock with two
sets of teeth and with a well-formed allantoic placenta. On
the other hand, some primitive Placentals show evidence of
descent from forms with marsupioid characters, such as
alleged traces of a marsupial pouch, of coracoids, and other
features. The conclusion to be drawn is that Marsupials and
Placentals had a common ancestor perhaps in the Jurassic.
Now, in the Jurassic the fossil Trituberculata are found, and
they are regarded as related to the Marsupials by some, and
to the Insectivores (Placentals) by others. The number of
teeth was large. The Trituberculata which derive their name
from the pattern of the cusps on the molars were probably
related to the common ancestors of Marsupials and Placentals.
It is a significant fact that the arrangement of the cusps on the
molar teeth in several primitive groups of mammals is of this
type, regardless of the diet for which the teeth of the higher
members of these groups are modified. Molars with separate
cusps like this are called bunodont.

In the Eocene period, the Marsupials had a wide distribu-
tion over the earth, but at the present day they are restricted
to the Australian and southern and central American regions.
These regions are characterised by their isolation and the
comparative absence of mammals of the Placental type. If it

THE EVOLUTION OF THE MAMMALIA 455

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456 EVOLUTIONARY MORPHOLOGY

had not been for the latter fact, there is little doubt that the
Marsupials would have become extinct, for they cannot com-
pete with the Placentals. Instead, in the security of their
isolation, they radiated out into a number of types which are
especially interesting in that they have evolved parallel with
several groups of Placentals, and by becoming adapted to
equivalent biological environments have developed a con-
vergent resemblance to these Placentals. Nearly all Marsupials
have a marsupial pouch in the female and epipubic bones
supporting it.

The opossum (Didelphys) and Cagnolestes are American ;
all the remainder are restricted to Australasia, though fossils
related to these are also found in South America. Dasyurus
is the Marsupial equivalent of the cats, while the dogs are
represented by Thylacinus, Perameles (the bandicoot) is an
attempt at a rabbit, Petaurus (the phalanger) resembles the
flying squirrels, while Notoryctes is a remarkable imitation of
the mole. Phascolarctos (the koala) is the " marsupial bear,"
Phascolomys (the wombat) is the " marsupial rodent," the
extinct Thylacoleo was the " marsupial lion," while Macropus
(the kangaroo) represents the swift-moving Ungulates.

The Cretaceous strata of Mongolia have revealed fossils of
apparently Placental mammals, of which Deltatheridium is an
example, and which can be regarded as intermediate between the
Jurassic Trituberculata and the true Placentals of the Eocene.

The Placentals are characterised by the possession of an
allantoic placenta, a corpus callosum joining the two halves of
the neopallium, and a typical dental formula of if, c], p|, m|.
This number of teeth is, however, often modified and reduced.

At the beginning of the Eocene period there appeared a
group of true Placentals which were primitive in that they were
of small size, with tritubercular short-crowned molars, five
fingers and toes, and walking on the flat of the hand and foot.
Among them can be recognised some with a tendency to
modification of the teeth for a carnivorous diet — the Creodonta,
— others for a herbivorous diet — the Condylarthra. Others
again were generalised Insectivora. Very early, a branch
diverged from the Condylarthran stock and gave rise to the

THE EVOLUTION OF THE MAMMALIA 457

Amblypoda, large, clumsy, premature rhinoceros-like forms
such as Uintatherium, and which soon went extinct.

In the later Eocene divergent evolution has progressed, and
it is very interesting to notice that a number of the Orders of
Mammals have become differentiated, and that these are not
yet split up into the various families. The Creodonta had given
rise to the Carnivora, which branched out into the Pinnipedia
or seals, and Fissipedia or dogs, cats, bears, civets, and badgers.
The Rodentia came off from near the primitive Insectivora, as
did also the Primates (Lemuroids and Tarsioids) and the
Edentates. The Perissodactyla or odd- toed Ungulates emerged
from a stock intermediate between Condylarthra and Insecti-
vora, and blossomed out into the huge Titanotheres which soon
went extinct, the horses, tapirs, and rhinoceroses. The even-
toed Ungulates or Artiodactyla emerged from some form
between the Creodonta and the Insectivora, and, apart from a
number of short-lived groups, radiated out into the pigs and
hippopotamuses on the one hand, and the camels, antelopes,
deer, cattle, and giraffes on the other. Related to the Ungulates
are the conies (Hyracoidea) and the elephants (Proboscidea).
The whales (Cetacea) are regarded as having arisen from a
stock related to the Creodonta, and the Sirenia may have a
common descent with the Proboscidea. South America
became inhabited by a peculiar collection of archaic forms
which were all doomed to extinction, but of which some such
as the Thoatheria had evolved into a very remarkable imitation
of the horses. The Edentata include armadillos, sloths, and
ant-eaters.

The Cheiroptera or bats are related to the Insectivora,
while the Dermoptera have affinities with the Insectivora and
Primates. In some cases sufficient fossil forms are known
from successive strata to enable lines of descent to be traced
with considerable precision. This applies especially to the
horses, the camels, and the elephants. The evolution of the
horses from Eohippus (Eocene) through Mesohippus (Oligo-
cene), Miohippus (Miocene), Pliohippus (Pliocene) to Equus,
was accompanied by a progressive increase in size, lengthening
of the teeth which become " rootless,'' development of ridges

458 EVOLUTIONARY MORPHOLOGY

on the molars, fusion of ulna with radius and tibia with fibula,
specialisation of the wrist and ankle joints into articulations
allowing movement in only one plane, enlargement of the 3rd
digit in hands and feet, and reduction of all the other digits
until their disappearance.

The evolution of the camels from the Eocene Protylopus
through Poebrotherium (Oligocene), Procamelus (Miocene) to
the present day is likewise a history of gradual increase in size,
increase in length of the teeth and development of selenodont
ridges on the molars, reduction of the upper incisors, enlarge-
ment of the 3rd and 4th digits in hands and feet with suppres-
sion of the remainder, and fusion of the 3rd and 4th meta-
carpals and metatarsals.

So far as is known the history of the elephants starts with
the Eocene Moeritherium, of about the size of a pig, and
with the primitive dental formula of i|, c^, pf, nig. Its
ridged (lophodont) molars had only two ridges. In the upper
Eocene, Palseomastodon was larger, and had a not inconsider-
able trunk. The canines and all the incisors except one pair
in each jaw had disappeared, and the molars had three ridges.
Tetrabelodon from the Pliocene was still larger and its incisors
were elongated into tusks, with persistent pulps. The molars
had as many as six ridges and were so large that there was
not room in the jaws for more than two teeth in each jaw on
each side. Furthermore, instead of being replaced from
beneath as in ordinary mammals, they were replaced from
behind, the new tooth pushing the old one out forwards in
front of it. It is worth noticing that although these animals
grew large and tall, their necks were very short, and it is only
by means of the long trunk that they were able to reach down
to the ground for eating and drinking. The next step, shown
by the Pliocene Mastodon, was accomplished by a shortening
of the lower jaw and the loss of the lower incisor- tusks. Lastly
in Elephas the grooves between the ridges on the molars become
filled with cement. The ridges may be a dozen in number,
and the maximum number of molars on each side in each jaw
in use at one time is one and a half.

The history of the Primates is reserved for the next chapter.

THE EVOLUTION OF THE MAMMALIA 459

Literature

Broom, R. On the Origin of Mammals. Philosophical Transactions of

the Royal Society, Ser. B, vol. 206, 1914.
Gregory, W. K., and Simpson, G. C. Cretaceous Mammal Skulls from

Mongolia. Nature, vol. 118, 1926.

Matthew, W. D. The Evolution of the Mammals in the Eocene. Pro-
ceedings of the Zoological Society of London, 1927.

Osborn, H. F. The Age of Mammals. Macmillan, New York, 19 10.

Weber, M. Die Saiigetiere. Fischer, Jena, 1927.

CHAPTER XLII

THE EVOLUTION OF THE PRIMATES AND MAN

The Primates originated from a stock related to the Insectivora
probably in Cretaceous times. Plesiadapis, from the Early
Eocene, had characters in common with the Insectivora and
the Lemurs, which are the lowest Primates. The characteristic
of the higher Primates is that the bony bar separating the orbit
from the temporal fossa is complete ; or in other words, the
eye-socket is round and protected all round by bone. At the
same time, both eyes look more or less forwards so that their
fields of vision overlap and may coincide (stereoscopic vision).
There are five fingers and toes, and the first digits are opposable
to the others, except in the case of the first toe of man. This
opposability makes the limbs efficient grasping organs, and is
evidence for the fact that the early Primates lived on the
branches of trees. It will be seen in the sequel that this arboreal
habit had consequences of the highest importance in the
evolution of the Primates . Lastly , the most important character
of all in the Primates is the great development of the neopallium
in the cerebral hemispheres.

In the Eocene the fossil Notharctus is found, representing
the earliest member of the group of the Lemuroidea. It was
very generalised, for whereas the primitive dental formula in
mammals is i| , c \ , p| , m| , that of Notharctus was i| , c \ , pf , mf .
From forms of this type the Lemurs must have descended.
The Lemurs alive to-day are nearly restricted to Madagascar,
though a few occur in Africa, Ceylon, and Malay. They are
fairly primitive animals, but show certain specialisations which
rule them out from the main line of Primate evolution. Among
these may be mentioned the peculiar procumbency of the
incisors of the lower jaw, with which they comb their fur.

460

EVOLUTION OF THE PRIMATES AND MAN 461

The tail of a Lemur is long but not prehensile, and its skull
may be recognised by the fact that the cavity of the orbit can
still communicate with that of the temporal fossa beneath the
postorbital bar.

Another Eocene fossil allied to Notharctus is Tetonius,
the earliest representative of the group Tarsioidea. Tetonius
had an enlarged rounded brain-case and a small face. Its
brain must have been relatively larger than in any other
known Eocene animal. It also was not on the direct line
of descent owing to specialisations such as the loss of the
lower incisors, but a close relative of it must have been the
ancestor of Tarsius which lives at the present day. In Tarsius
the postorbital bar is splayed out and almost but not quite
prevents communication between the orbit and the temporal
fossa. It shows important advances in the structure of the
brain, and of the external ear. In the fact that it has a discoidal
placenta with a thickened trophoblast hollowed out into
lacunae filled with maternal blood, and in the fact that the
mesoderm appears very early in the development of the
embryo, Tarsius resembles the higher Primates and Man, and
differs from the Lemuroidea.

In the true monkeys, apes and man, or Anthropoidea, the
orbit is completely shut off from the temporal fossa. From
some Tarsioid ancestor with affinities to Notharctus there
diverged a branch which gave rise to the New World monkeys.
These forms which are included in the group Platyrrhinas have
a broad internasal septum with the nostrils wide apart, and a
tail which is usually prehensile. They show a considerable
advance in the structure of the brain, and in the fact that their
dental formula is reduced to i§, c\, p|, m-?. At the same time
they are definitely off the main line of Primate evolution
because of the structure of the tympanic bone which forms a
ring.

The Old World monkeys, apes and man, form the group
Catarrhinae, in which the internasal septum is narrow, the
tympanic bone forms a tubular external ear, the tail when
present is never prehensile, and the dental formula is reduced
to if, cf, p|, inf.

462 EVOLUTIONARY MORPHOLOGY

The Catarrhinae must have emerged in the Eocene period
from Tarsioid ancestors related to those which gave rise to the
Platyrrhinae. In the Oligocene, Parapithecus is found, and
from forms related to it the ordinary monkeys or Cercopithe-
cidae must have been derived. These forms are again ruled
out from the main line of Primate evolution by specialisations
such as the development of two transverse ridges on the
molars. At the same time, the Cercopithecidae, which include
the baboons and mandrills, show a great development of the
brain, which must have undergone an evolution parallel to
that which went on in the stock leading to the apes and man.

The main stem of the Primates leading to the anthropoid
apes and man was represented in the Oligocene by the little
Propliopithecus. The fact that it was small is important, for
so many divergent branches became specialised in the direction
of large size, and in the search for the ancestors of the apes
and man, choice is limited to forms considerably smaller than
those to which they might have given rise. The anthropoid
apes have lost the tail, they show a tendency to walk erect,
the mechanism of pronation and supination of the hand is
perfected, and the brain is greatly enlarged and developed.

A descendant of Propliopithecus was the Pliocene form
Pliopithecus, which itself was an ancestor of the gibbon
(Hylobates), the smallest of the apes. Other lines of descent
from forms like Propliopithecus led to the Orang, and to the
chimpanzee and Gorilla, while the main stem culminated in
the Hominidae and man. The apes while having highly
developed brains and retaining the power to oppose the first
toe to the others, have not got brains large enough to enable
them to do otherwise than remain brutes, relying on their
strength and their long canines instead of on memory, skill, and
the neopallium. There are some characters possessed by adult
modern man which are present in the young but lost in the
adult apes. An example of these is the absence of large
brow-ridges in young apes and man.

The fossil record of the Hominidae is not by any means as
complete as would be desired, but there is already sufficient
evidence to enable an outline to be given of the more important

EVOLUTION OF THE PRIMATES AND MAN 463

changes and modifications which accompanied the evolution of
modern man : Homo sapiens.

The Hominidae include all the members of the human
family and it must be noted that they differ from other Primates
not so much in matters of kind as in matters of degree. Essen-

Fig. 180. — Professor John I. Hunter's reconstruction of the Piltdown skull,
drawn by T. L. Poulton. (From Elliot Smith.)

tially, the evolution of the Hominidae is a story of enlargement
of the brain, reduction of the nose, face, and jaws, perfection
of the erect position, and loss of opposability of the large toe,
which last feature converted the Quadrumana or apes into the
Bimana or men.

464

EVOLUTIONARY MORPHOLOGY

The nearest approach to the human condition without
achieving it on the part of an ape is Australopithecus, the
Taungs skull, from South Africa. This fossil betrays kinship
to the Gorilla and chimpanzee, but its brain is slightly larger
and its face smaller.

The earliest known member of the human family is Pithe-
canthropus from Java. This form had a much enlarged brain
with a cubic capacity of about 950 c.c, while the maximum

Fig. 181. — Skull of Homo rhodesiensis, drawn by T. L. Poulton.

Elliott Smith.)

(From

volume of an ape's brain is 650 c.c. From the structure of its
femur, it walked almost erect. In some respects it preserves
primitive features such as the continuity between the occipital
and temporal crests on the skull, and many features in the
conformation of the brain, but in others it is specialised, as in
the development of the large brow-ridges.

The most important of all the human fossils is Eoanthropus,
the Piltdown skull from Sussex, The bones of the skull are

EVOLUTION OF THE PRIMATES AND MAN 465

very thick, and the lower jaw is strikingly similar to that of a
chimpanzee, with no chin whatever, and large canine teeth.
But the brain-case is dome-shaped and large, with a cubic
capacity of about 1170 c.c, and there were no brow-ridges.
The latter fact, together with the primitive nature of certain
features in the brain, makes it possible to regard Eoanthropus
as very close to the line of man's descent, if not his ancestor.

Pithecanthropus and Eoanthropus date from the latest
Pliocene or earliest Pleistocene periods. The remaining
fossil men come definitely within the period of the great
Ice- Age.

The most primitive known member of the genus Homo is
Homo rhodesiensis, from Broken Hill in Rhodesia. It had a
brain- volume of about 1250 c.c, but still lacked any semblance
of a forehead. There was no boundary between the nose and
the face, and the palate was very big, but its chief specialisation
is the development of enormous brow- ridges. The lower jaw
is unknown, but it was probably not very different from that
of Homo heidelbergensis from Heidelberg, which unfortunately
is known only from the lower jaw. It is large and massive,
without any chin. The teeth were definitely human, and
primitive in that the last molars preserve five cusps, while
those of modern man are often reduced to four.

Incomparably the best known of the fossil men is Homo
neanderthalensis, specimens of which have been found from a
number of localities in Europe, from Gibraltar to Palestine.
In addition to the bones themselves, there is considerable
evidence concerning these men from the weapons and tools
which they fashioned from flint, and which consequently have
been preserved. An indication of the degree of mental develop-
ment of these people is obtained from the fact that some of the
individuals which have been discovered appear to have been
intentionally buried. The brain is very big (about 1350 c.c.
on an average), but it preserves numerous primitive features.
At the same time, the brow- ridges were large as were also
the face, palate, and jaws. There was no chin, and the lower
jaw preserves the large attachments for the digastric muscles.
The vertebrae and the legs show that the neanderthal man

2 H

466 EVOLUTIONARY MORPHOLOGY

did not stand straight up, but stooped considerably. The
hip-girdle was still long, and the foot rested mostly on its
outer border, as in young children and certain savage races
to-day. While having lost the capacity to be opposed to the
other toes, the large toe was considerably separate from the
remainder. The neanderthal race has gone extinct, doubtless
because of its specialisations, and the insufficient development
of the brain which handicapped it in the competition with
Homo sapiens. Here, the brain has achieved maximum
development, so much so that its front wall has been pushed
forwards to form a more or less vertical forehead. This
vertical wall of bone provides the necessary resistance for the
reduced lower jaw to bite against. In the apes, Pithecan-
thropus, rhodesian and neanderthal man, where the lower
jaw is still large and there is no forehead, the strain of the bite
is taken up in the large brow-ridges which are developed in the
adult. These brow-ridges are therefore not a primitive
feature, but independently acquired as an adaptation in certain
groups. Their presence, however, rules their possessors out
from modern man's ancestry.

The face in Homo sapiens is relatively smaller than in any
mammal, the lower jaw is slender and provided with a promi-
nent chin, and the canine teeth are small. The hip-girdle is
short and wide, and the vertebral column and legs enable man
to stand bolt upright.

In connexion with the expansion of the brain and the
assumption of an erect attitude, it is consistently found, on
ascending the scale of Primate evolution to man, that the
foramen magnum through which the spinal cord joins the brain
is movedr elatively farther and farther forward. This fact is
obvious when it is considered that the head of an ordinary
lower mammal projects forwards horizontally from its neck,
whereas man's head is carried vertically above his neck. At the
same time, the eyes of lower mammals and of man look hori-
zontally from about the middle of the front of the face. There
has therefore been a progressive expansion of the hinder and
upper part of the skull accompanying the development of the
brain, and which moves the face-region farther and farther

EVOLUTION OF THE PRIMATES AND MAN 467

forwards. The ordinary superposition of median vertical
sections through skulls suffers from the fact that it is then
difficult to distinguish between differences of actual size and
differences of development. This difficulty vanishes when the
sections are superposed on a common centre, and then rotated
so that certain standard radius-lines coincide. Other lines

Fig. 182. — The skeletons of Neanderthal man and of modern man com-
pared. (From JBoule.)

can then be read-off by angular measurement regardless of the
actual size of the skull. The centre of gravity is chosen as the
common centre since it is the morphological centre of form.
It may be called Sollas' centre. The sections are then rotated
so that the radius-lines from the centre to the middle of the
foramen magnum coincide. The sections are then " set,"
and reference-lines are made by continuing the radius of

468 EVOLUTIONARY MORPHOLOGY

coincidence up to the top of the skull, and drawing a line at
right angles to it through Sollas' centre.

One of the most instructive readings is the measurement
of the angle made between the line of the foramen magnum,
and the line from the centre to the point of junction between
the nasal and frontal bones (the nasion). It is essential for
this comparison that the sections be taken from specimens of
equivalent age, for during development the angle changes.
Nevertheless, taking adult material it is possible to make out
the following : —

Angle between foramen magnum and nasion in adult :

Gibbon

. . 2380

Chimpanzee

.. 239°

Pithecanthropus

25 1 ° (conjectural)

Homo neanderthalensis

• • 253°

Homo rhodesiensis

. . 2620

Eoanthropus

2640 (conjectural)

Homo sapiens

circ. 2700

These measurements show that the periphery of the brain-
case in modern man amounts to three right angles, and it is
interesting to note in comparison with lower forms that the
accommodation for the increased size of man's brain is obtained
by the angular increase in the periphery of the brain-case as
well as absolute increase in size.

It is also noticeable that Eoanthropus approaches nearer to
sapiens than do neanderthalensis and rhodesiensis, which is an
additional reason for including the former but excluding the
latter from sapiens' ancestry.

As regards Australopithecus, the following table shows the
comparison between it and juvenile specimens of other forms.

Angle between foramen magnum and nasion in young : —

Orang 2430

Chimpanzee . . . . . . . . . . . . . . 2520

Australopithecus . . . . . . . . . . . . 2580

Homo sapiens child . . . . . . . . . . . . 2820

It is clear, therefore, that the Taungs skull approaches the human
condition in this respect, though still remaining similar to the
Chimpanzee.

Having now reviewed the material on which all study of

EVOLUTION OF THE PRIMATES AND MAN 469

Fig. 183.— Diagrams of longitudinal sections of skulls, superimposed on
their centres. (After Sollas.)

Showing the difference in angular measurement between the nasion-line
(from the centre to the top of the nasal bone) and the line through the fora-
men magnum (from the centre) : in Gibbon 2380 ; in Chimpanzee 2390 ;

TJ fnP ^°PUS 25IVon NeaPderthal man 253°; in Rhodesian man
62 2 , in Piltdown man 2640 ; and in modern man 2700.

470 EVOLUTIONARY MORPHOLOGY

the evolution of man must be based, it remains to consider what
causes were probably operative during the history of human
descent. It may be said at once that just as the rise of the
mammals was due to the development of the brain and forma-
tion of a neopallium, so a continuation and perfection of that
process led to the rise of the Primates and man, and that this
development was largely associated with the sense of sight.

It has been seen that the history of the Primates can be
traced from Insectivore-like ancestors, through Tarsioid,
monkey, and ape stages, and that their evolution was accom-
plished under arboreal conditions of life. Now, the Insecti-
vora, Tarsioidea, monkeys and apes have living representatives
at the present day, some of which have changed but little from
their Eocene ancestors. Without in the least suggesting that
these living forms are on the main line of descent (which
indeed it has been shown carefully that they are not), they
may be taken and studied for their brains and organs of sight,
as showing grades of structure approximately representative
of the stages through which it is known that the Primates
passed.

Of the Insectivora, Macroscelides (the jumping shrew) may
be taken as a primitive mammal, in which the neopallium is
developed, but the archipallium related to the sense of smell
is still very large. In particular it is important to notice that
the region of the neopallium (parietal region) related to the
sense of sight is small.

Tupaia (the tree shrew) is related to Macroscelides, and the
difference which it shows in its brain is related to the habit of
living in trees. Life in trees is conducive to the better develop-
ment of the sense of sight, for jumping from one branch to
another, and inefficient perception of spatial relations would
lead to disaster. Accordingly, it is not surprising to find that
the visual area of the neopallium of Tupaia is better developed
than in Macroscelides, and that in the nature of its retina and
other features connected with the eyes, Tupaia approaches the
Lemurs.

The stage represented by Tarsius, which is also arboreal, is
of great importance, for here for the first time the sense of

EVOLUTION OF THE PRIMATES AND MAN 471

BRAIN OF THE JUMPING SHREW
(ttdcroocdidea)

BRAIN Of THE TREE SHREW

OLFACTORY

BRAIN OP THE TAR31ER x
Ctoius)

TACTILE

MOTOR

PREFRONTAL

ACOUSTIC

BRAIN OF THE* MAJWX^^^

Fig. 184.— Diagrams showing the left side of the brains of Macroscelides,
Tupaia, Tarsius and the Marmoset. (From Elliot Smith.)

Showing the increase in the area of the cortex associated with vision and
co-ordinate movements (prefrontal), and the decrease in the olfactory
region, in the evolution of Primates.

472 EVOLUTIONARY MORPHOLOGY

smell is reduced below the level of the sense of sight, which
becomes the dominant sense in the body. The eyes of Tarsius
look forwards, and the fact that they have rotated onto the front
of the face necessitates the reduction of the nose and snout.
At the same time, the senses of hearing and touch are better
developed, together with their respective temporal and tactile
areas in the neopallium. The development of the tactile area
is important because it is associated with that area of the cerebral
cortex which is concerned with the performance of delicately
adjusted and skilful muscular movements. Such movements
are essential for an active arboreal animal, but there is another
reason for referring to this part of the neopallium, and that is
that a portion of it (the prefrontal region) is concerned with the
co-ordination of the movements of the two eyes.

In the prefrontal area of Marsupials there are centres
which control the eye-muscles and therefore the movements of
the eyeball of the opposite side. The movements of the two
eyes are linked together in higher forms, and this is especially
significant in the Primates, where the visual axes of the eyes
become parallel. Further, whereas in lower vertebrates the
fibres from each eye all go to the other side of the brain (the
crossing at the optic chiasma is complete), in the mammals a
certain number of fibres remain uncrossed, and go to the same
side of the brain. Now, in the Platyrrhine stage of evolution,
represented by the Marmoset, the co-ordination between the
movements of the two eyes is perfected, and both eyes are able
to follow one and the same object. A consequence of this is
that " corresponding points " are developed in the retinae of
each eye, on which the images of one object are formed, and
the most important of these points is the macula lutea or spot
of optimum sensitiveness.

Consequent on the power of making conjugate eye-move-
ments, the Anthropoidea have evolved a macula lutea, and this
still further increases the importance of the parietal (visual)
and prefrontal (skilled movement) areas, which features already
distinguish the brain of the Platyrrhine from that of Tarsius.
A continuation of the process of enlargement and perfection
of the parietal, prefrontal, and temporal areas can be gradually

EVOLUTION OF THE PRIMATES AND MAN 473

and serially traced through the Catarrhine, the ape (Gorilla),
Australopithecus, Pithecanthropus, Eoanthropus, Homo rhode-
siensis, Homo neanderthalensis to Homo sapiens. There is
further the very interesting fact that in human develop-
ment, the regions of the neopallium which are the last to be
formed are precisely these parietal, prefrontal and temporal
areas.

There is therefore good reason to believe that the per-
fection of these areas of the cerebral cortex and of the functions
with which they are associated played the major part in the
evolution of man. The brain developed first, and other
features such as the reduction of the face and assumption of
the erect attitude followed. It is to be noted that the perfec-
tion of the parietal and prefrontal areas is directly or indirectly
concerned with the function of vision, so that it may be said
that sight was of capital importance in the evolution of man.
In this connexion, mention may be made of some other aspects
of the bearing of sight on evolution.

In the first place, it will be remembered that the eyes are
" distance-receptors, " and that the responses which they evoke
on the part of the animal are anticipatory rather than con-
summatory movements. Next, there is the fact that in man,
the number of nerve-fibres entering the brain from one eye
vastly exceeds the number of all the other afferent nerve-
fibres of one side put together. From the physiological side,
it is found that in the higher Primates including the monkeys,
apes, and man, the eyes assume great importance in regulating
the posture of the organism, a regulation which in lower forms
is principally dependent on the semicircular canals of the ear.
Lastly, from the psychological point of view, experiments on
the behaviour of chimpanzees when confronted with problems
shows that the eyes play a very important part in solving the
problem. Cases of great interest are those in which there
lies close at hand some instrument, such as a stick, and by
using which the ape would be able to solve its problem
easily. Unless the instrument to be used is seen by the ape
in the same field of vision as the object or goal for which it is
to be used, it pays no attention to it. Without this optic

474 EVOLUTIONARY MORPHOLOGY

co-presence, the ape does not " see " the solution to the
problem.

Perhaps the most important of all the consequences of the
perfection of the sense of sight in the Primates is the fact that
it is the neopallium which undergoes commensurate develop-
ment in the brain, and the neopallium is the physical companion
of memory, of the ability to profit by experience, and of the
arbitrator of possible responses, known as the will. There
is also to be noticed here the importance of remaining un-
specialised. For if the great development of the sense of sight
had taken place earlier in evolution, in an ancestor of the
mammals, it would have been not the neopallium, but the
optic lobes which would have undergone specialisation, and
for a number of reasons these are unsuited for the development
of the higher mental faculties. The success of man is therefore
also due to the fact that his ancestors did not shoot their bolt
of specialisation prematurely.

A consequence of binocular vision and conjugate move-
ment is the power to converge the eyes on an object. In the
first place, this enables an estimate of distance to be made, which
is important in leaping from branch to branch. Feeling of
the degree of convergence is conveyed by stimuli from pro-
prioceptive sense-organs in the eye-muscles by afferent fibres
in the eye-muscle nerves. When, however, the eyes are
converged on an object, that object occupies the attention of
the animal, and the stereoscopic vision which it now enjoys
enables it to become aware of the true geometrical and spatial
relations of the objects in the world around it.

To return to the face, it is obvious that when the nose and
snout are reduced as a result of the eyes coming on to the
front of the face, the mouth itself can no longer so easily be
used as a food-obtaining organ, as it is in lower forms. Here,
the hands come to the rescue, and being five-fingered and
with opposable thumbs, capable of pronation and supination,
they undertake the function of carrying food to the mouth.
At the same time, the development of the prefrontal area of
the neopallium enables delicate movements to be made, in the
course of which the animal acquires skill. It is an interesting

EVOLUTION OF THE PRIMATES AND MAN 475

fact that in the higher Primates, the focal length of the eyes
for most acute vision should be just within the reach of the
hands. The assumption of the erect posture which is made
possible by the increased power of co-ordination of the brain,
relieves the hands from the service of locomotion, which is
performed solely by the feet. The latter therefore lose the
opposability of the big toe.

Lastly, in connexion with the greater development of the
temporal region of the neopallium, the power of hearing
became more acute, and with it came the development of speech.
There is clinical evidence that in man, one of the lobes of the
temporal region is concerned with the faculty of stringing
words together into sentences with a logical meaning, and it
has been shown above that this is one of the regions of the
neopallium which has undergone progressive development in
the evolution of man. It is not claimed that man is nothing
more than a mammal which sees, hears, and co-ordinates his
movements better than other mammals. All that is intended
is to show that the development and perfection of these
functions of sight, auditory discrimination with which must
be coupled speech and language, and muscular skill, bring
about changes which are prerequisite for the development of
that peculiarly human attribute — the higher mental faculties.

Literature

Boule, M. Les Hommes fossiles. Masson, Paris, 1921. (English trans-
lation, Oliver & Boyd, Edinburgh, 1923.)

Elliot Smith, G. Essays on the Evolution of Man. Oxford University

Press, 1927.
Gregory, W. K. The Origin and Evolution of the Human Dentition.

Williams and Wilkins, Baltimore, 1922.

Sollas, W. J. Ancient Hunters. Macmillan, London, 1924.

Sonntag, C. F. The Morphology and Evolution of the Apes and Man.
John Bale, Sons and Danielsson, London, 1924.

Thomson, A. A Consideration of the more Important Factors concerned
in the Production of Man's Cranial Form. Journal of the Anthro-
pological Institute. Vol. 33. 1903.

PART V
CHAPTER XLI1I

CONCLUSIONS

Not the least of the interests aroused by the study of Verte-
brates is due to the fact that they form a group which lends itself
perhaps better than any to a consideration of general principles
and matters of wide importance. This is largely because,
although imperfect, present knowledge covers a considerable
amount of the results of vertebrate evolution, and still more
because between the most widely separated members of the
group, between Amphioxus and man, there is sufficient simi-
larity in plan of structure to enable comparisons to be made
with advantage. Comparative Anatomy as an intellectual
weapon is the more satisfactory when the number of corre-
spondences of kind which can be established is great, regardless
of course of matters of detail. So it is not astonishing that the
Anatomy of, for example, Nematodes and Echinoderms when
compared should be less fertile in conclusions of general
interest than a comparison between Vertebrates as distant
from one another as are fish and mammals. From the fact
of the general homogeneity of the group as a whole, the varia-
tions to be observed in different vertebrates become all the more
interesting.

It is very striking to find organs such as notochord, nerve-
tube, dorsal and ventral nerve-roots, essentially the same in
Amphioxus and man, but the most striking case of homologous
organs is that of the thyroid. From the endostyle of
Amphioxus, through Petromyzon with its tell-tale Ammocoete
larva, to all the Craniates, the chain is complete, and not the

477

4?3 CONCLUSIONS

least remarkable feature of it is the great change in function
which has taken place from an organ connected with the
ciliary method of feeding to a ductless gland regulating the
metabolism of the body. This case is a good illustration of
the fact that function is no criterion whatever in questions of
homology, and that the sole condition which organs must
fulfil to be homologous is to be descended from one and the
same representative in a common ancestor.

A fact which the vertebrates illustrate well, is that the
numerical correspondence of segments which give rise to
particular structures is not a necessary criterion for homology.
This is well shown by a consideration of the pectoral and
pelvic limbs. The fore limb is formed from trunk-segments
2, 3, 4, and 5 in the newt (Salamandra), whereas in the lizard it
arises from segments 6, 7, 8, and 9. Similarly the hind limb
arises from segments 16, 17, and 18 in the newt, but segments
26 to 31 in the lizard. Countless similar examples are afforded
by other vertebrates, and it is to be noticed that the limbs
not only vary in their position, but also in the number of
segments which have contributed to their formation. Yet
wherever they may be and however many or few segments
they may contain, fore limbs are homologous throughout the
vertebrates, and so are hind limbs. During evolution trans-
position has occurred ; new adjacent segments have taken to
contributing to the formation of the limb, and at the opposite
end segments which hitherto contributed may cease to do so.
In this way the limbs may become transposed over the trunk
of the animal much as a tune can be transposed over the keys.
But it is the same tune and the same limb.

Another case is that of the position of the occipital arch at
the back of the skull. The neurocranium of Scyllium occupies
7 segments while that of a form as closely related to it as
Squalus occupies 9. Although they are situated in different
segments, there can be no doubt that the occipital arches of
these two animals are descended from the occipital arch of a
common ancestor, and are therefore homologous.

A very interesting example of the same kind is furnished by
the number of gill-slits in various Selachii. Heptanchus has

CONCLUSIONS 479

7, Hexanchus and Pliotrema have 6, and the remaining Selachi
have 5 gill-slits and branchial arches on each side. Now the
remarkable thing is that the last branchial arch has a typical
structure whether it be the 7th, 6th, or 5th. Its peculiarity
consists in the fact that its pharyngobranchial element is
attached to that of the preceding arch, and it receives a portion
of the trapezius muscle. The function of the last branchial
arch is to anchor the branchial basket on to the shoulder-
girdle. This being so, in the course of the evolution of forms
with 5 branchial arches from forms in which there were 6,*
it is impossible to imagine that the transformation took place
gradually by reduction from behind ; for if this had occurred,
there would have been stages in which the " old last branchial "
arch had partly disappeared and the " new last branchial "
arch had partially been modified to replace the former, and it is
difficult to see how such an arrangement could have fulfilled
the function of providing attachment between the branchial
basket and the shoulder-girdle. This case is therefore different
from that of the limbs, for in the latter there is nothing to
prevent gradual transposition of the limb by means of partial
modification of adjacent segments. Still less can it be imagined
that the number of branchial arches has been altered by
reduction from in front because the first two visceral arches,
the mandibular and hyoid, are constant throughout the Gnatho-
stomes. The only explanation left is that there has been a
sudden change in evolution, and that a formwith,say,6 branchial
arches gave rise to offspring with 5, without any intermediate
stage of functional inefficiency. This conclusion is, of course,
interesting from the point of view of evolution, but it is also
not without importance as regards the relation of metameric
segmentation to differentiation during development. The
only difference between this hypothetical offspring with 5,
and its parent with 6 branchial arches, is that the raw material
for the production of the branchial arches has in the one case
been divided up between 5 segments and in the other between
6 segments, during development. It is this raw material which
is homologous in the two forms, regardless of the numerical

* Or vice versa.

48o CONCLUSIONS

position of the segment in which it is situated. A matter like
this is worth some attention, for it is an example of how
principles of general and wide interest can be derived from
comparative anatomical studies.

In sharp contrast to homologous structures are the re-
semblances between different and unrelated groups of animals
as regards characters which can be proved to have been
separately and independently evolved. These resemblances are
analogies, and they give rise to the phenomenon of convergence
in evolution which is well illustrated by vertebrates. The
instances of convergence which might be given are so numerous
that only very few need be mentioned here. A good example
is the modification of the pentadactyl limb into a paddle,
thereby losing its typical appearance and presenting a super-
ficial resemblance to the fins of fish. But the interesting thing
is that this process has occurred not once but several times,
independently, in different groups of Tetrapods : Chelonia,
Ichthyosaurs, Plesiosaurs, Mosasaurs, Thalattosaurs, Thalatto-
suchia, Penguins, Cetacea (whales), Carnivora pinnipedia (seals),
and Sirenia. The Ichthyosaurs and some of the Cetacea are
further interesting in that they have developed median dorsal
fins which are superficially very similar to those of fish. The
Urodela also have median fins ; but in all these cases, a little
study suffices to show that these structures not only differ
very much from the fins of fish, but also that they differ
between themselves.

Convergence is also to be found in the case of the elongated
and limbless condition of Gymnophiona, certain lizards such
as Anguis, Amphisbaena, Scincus, and the snakes. Or again,
the fore limb has been modified into a wing independently
in Pterosaurs, birds, and bats. The marsupial " mole "
Notoryctes is very similar to the true placental moles (Talpa).

Now it is noteworthy that these cases of convergence are
each of them related to a particular mode of life. So the paddle-
like modification of the limbs is an adaptation to life in the
water, just as wings are adapted to life in the air ; the limbless
condition is a form of adaptation to a burrowing habit, while
another form of this habit characterises the " moles." It is

CONCLUSIONS 481

because of their adaptations to their environment that these
animals come to resemble one another, and these adaptations
have of course no value in determining affinities or descent.

Another phenomenon may now be considered, which is
in some ways intermediate between homology and convergence.
It is often the case that in two groups of related animals which
have recently diverged the same evolutionary changes take
place. This may be called parallelism, and it is illustrated
in certain groups of Ungulates such as the Titanotheres and
the rhinoceroses. In several distinct stocks of Titanotheres
peculiar bony knobs appear on the skull. These structures
were not visibly present in the common ancestor of the
forms which have evolved them ; the structures cannot there-
fore strictly be called homologous, yet they are so similar
that it is impossible to avoid the impression that they
have some common cause. The independent development of
such similar structures in related groups of animals is often
ascribed to a so-called process of " Orthogenesis,' ' or variation
along " straight " and constant lines. The working of this
process in two or more related groups is supposed to result
in parallel evolution.

Now it is worthy of note that when tracing lines of descent
through fossil forms, it is rarely possible to identify one form
as the direct ancestor of another. Instead, it is more usual
to find that one fossil form is related to the ancestor of another,
because it possesses characters which that ancestor must have
possessed, while at the same time showing other characters
which proclaim that it had diverged from that ancestor. The
characters of the ancestor in question are, of course, to a certain
extent deducible from those of the form descended from it.

The incompleteness of knowledge of the fossil record
makes it difficult to find " fathers," but it supplies a number of
" uncles." The question now is this : why do the " fathers "
and " uncles " resemble one another ? Cynognathus itself
is not the ancestor of the mammals, for in several respects it is
too specialised, but it must have evolved parallel with the
ancestor of the mammals or it would not possess so many
similarities. In the same way it can be shown that the later

2 1

482 CONCLUSIONS

Stegocephalians, which were not on the line of descent of the
reptiles, nevertheless show a number of changes in evolution
which took place parallel to those which were going on in their
" cousins " the reptiles.

The answer must be that the " fathers " and the " uncles "
inherited something from the " grandfather " which deter-
mines the course of their evolution. This something need
not, however, have been visible in the " grandfather," so that
the " fathers " and the " uncles " in which the something does
become visible appear to have evolved it independently. In
these cases there appears what may be called a latent homology
between the structures in question, and which accounts for the
so-called " Orthogenesis." In any case, it is most important
to avoid the impression that " Orthogenesis " implies a purpose-
ful or directive force, or that evolution takes place in straight
lines. Such impression is quickly dispelled by a consideration
of the record of success and failure of the different groups of
animals during evolution. If a directive force were responsible
for evolution, it would seem to be peculiarly malicious, for
most groups of animals have been " directed " to their doom
by extinction.

An insight into what " Orthogenesis " really means is
given by a study of the relative sizes of parts of animals to the
whole animals, at different absolute sizes. It is found, for
example, that the size of the antlers in Red deer is relatively
larger in large animals than it is in small ones. That is to say,
that the larger a Red deer grows, the relatively larger do its
antlers become, on the average. These cases are susceptible
of mathematical treatment, and it is found that the antlers not
only grow faster than the body, but they grow faster at a
constant rate, for the ratio of the growth-rates of antlers and
body remains constant. Organs to which this principle applies
are called heterogonic, and Heterogony is of wide occurrence
in the horns and bony nobs of various groups of Ungulate
mammals. Now just as the heterogonic organ is relatively
smaller in small animals, it is found as a rule that in two
species of one genus both of which possess this organ, the larger
species will have the relatively larger heterogonic organ. So

CONCLUSIONS 483

the antlers of the little Muntjack are relatively smaller than
those of the larger Red deer. There are of course exceptions
and complications, but from the present point of view, the main
thing to notice is that for an organ which shows heterogonic
growth to appear at all, the animal's body must have reached
a certain absolute size. Now as the different races of Titano-
theres evolved, their size increased, in common with nearly
all the groups of mammals. Independently, each of these races
of Titanotheres developed bony knobs on the skull, and as the
size-increase of the animals continued, the bony knobs became
relatively larger still. The bony knobs are heterogonic organs,
and their independent appearance in different races is not due
to any directive force, but automatically to the increase in size
of the body of the animal. This increase of body-size was
probably due to random variation selected by natural selection
in the direction of greater size because it is (up to a point)
advantageous, and has survival value. From the common
ancestor of the different races it is only necessary to assume
that the capacity was inherited to produce bony knobs if and
when a certain body-size is reached. On this view, therefore,
" Orthogenesis " does not mean directed evolution, but merely
directional. It also enables an explanation to be given for the
cases of extinction of animals in which the size of the hetero-
gonic organ (consequent on the large size of the body) had
become so great as to reduce the animal's chances of survival.
This applies to the Irish elk, which was a very large deer with
relatively immense antlers.

Attention must now be paid to the terms " primitive "
and " specialised," which were defined early in this book, and
which have been consistently used throughout. In the first
place, it is necessary to notice that their meaning is relative, so
that it is possible to find an animal which is primitive when
compared with one and specialised when compared with another
animal. A specialised animal is one which is committed to a
particular line and so has a restricted potency of evolution. As
a rule, specialised animals are adapted to a particular mode of
life, and this adaptation has entailed either the development or
loss of certain structures which render the animals unfit to live

484 CONCLUSIONS

in any other environment but their own. Once committed,
they are committed for always, for in its broad lines evolution
is irreversible.

Primitive animals, on the other hand, are not committed to
any particularly restricted mode of life ; they do not have any
delicate adaptations with the structural modifications which
they involve, and they are, in a word, generalised.

It is from generalised ancestors that the main groups of
animals have evolved, and as these groups radiated out they
became specialised in their various ways. Specialisation and
evolutionary capacity are roughly inversely proportional.

The significance of primitiveness and specialisation is thus
related to evolution. Amphioxus is primitive because it
possesses many characters which the early ancestral Chordates
must have had. But its specialised characters show that it was
not itself that ancestor. Amphioxus is with regard to the higher
Chordates not a " father " but an " uncle."

It is worth noticing that the primitive arrangement of several
structures was segmental, and that as evolution proceeded this
simple scheme was departed from. So the gonads of Amphi-
oxus, myotomes of Amphioxus, kidney tubules of Myxine,
ribs of Cotylosaurs and respiratory centres of Raia show that
" a pair of each in each segment " was the primitive outfit,
on which evolution has worked.

When man is considered in relation to his ancestors, a
significant fact emerges. Man is not adapted to any restricted
mode of life at all ; instead he is fitted for almost all sorts of
habits and circumstances ; he is generalised not specialised,
and that is one of the secrets of his evolutionary success. His
ancestors must have been among the most primitive and
generalised of the mammals ; they did not live on the capital of
their evolutionary capacities and spend it in exchange for
delicate adaptations, which, while perhaps allowing of " easier
living," would have resulted in side-tracking the race into a rut
or backwater of life.

Lastly, mention may be made of the material which the
vertebrates supply for a consideration of what is often called
the Law of Recapitulation. It is not astonishing that a group

CONCLUSIONS

48S

as broad and as well known as the vertebrates should provide
several examples of embryos which seem to reflect something
in the ancestral stages of the forms to which the embryos in
question belong. As an example, the gill-slits (or rather gill-

■^\Ca

Fig. 185. — Views of embryos of A, dogfish ; B, lizard ; C, chick ; D, rabbit ;
and E, man ; showing the similarity at early stages between embryonic
forms of related animals.

pouches) of the mammals may be taken. It is rightly held
that these structures in the embryo mammal represent the gill-
pouches and slits of the fish-stage ancestor of the mammals.
But the most important thing to notice is that it is the gill-
pouches of embryo fish and not those of adult fish which the gill-

486 CONCLUSIONS

pouches of mammalian embryos resemble ; indeed, not much
observation is needed to see that between the gill-pouches of the
mammalian embryo and the gill-slits of an adult fish there is but
little resemblance, whereas the gill-pouches of embryonic stages
are very similar in all groups of vertebrates. This explanation
covers all cases of so-called recapitulation. It follows that it
is inaccurate and misleading to say that Ontogeny (the develop-
ment of the individual) recapitulates Phylogeny (the evolution
of the race). What may be true is that Ontogeny recapitulates
the Ontogeny of the ancestor, and even then, it is not necessarily
true of all embryonic forms. While the gill-pouches do
recapitulate in this sense, other organs such as the primitive
streak or the extra- embryonic ccelom do not. It is also to be
noted that the order of appearance of structures in Ontogeny
is not necessarily the same as in Phylogeny. Denticles
appeared early in evolution, bat they arise late in the develop-
ment of the dogfish. The embryo is phylogenetically older
than the amnion, but in the development of the mouse, the
amnion arises first and the embryo afterwards.

The real value of embryology from the point of view of
evolution lies in the fact that embryonic forms are like the
embryonic forms of related animals. As a rule, the younger
the embryos are, and the closer akin the species to which
they belong, the more closely do the embryos resemble one
another. The more closely allied the species are, the longer
does the resemblance between the embryos persist. Embry-
ology furnishes valuable evidence therefore as to affinities,
but it cannot profess to give definite information concerning
the adult forms of ancestors.

Literature

Bateson, W. Problems of Genetics. Yale University Press, 1913.
Garstang, W. The Theory of Recapitulation : a Critical Restatement of

the Biogenetic Law. Journal Linnean Society, London, Zoology, vol.

35> 1922.
Goodrich, E. S. Metameric Segmentation and Homology. Quarterly

Journal of Microscopical Science, vol. 59, 191 3.
Huxley, J. S. Constant Differential Growth-ratios and their Significance.

Nature, vol. 114, December 20th, 1924.

Versluys, J. Uber die Riickbildung der Kiemenbogen bei den Selachii.
Bijdragen tot de Dierkunde, vol. 22, 1922.

CLASSIFICATION OF THE ANIMALS AND GROUPS OF
ANIMALS MENTIONED IN THIS BOOK

(An asterisk denotes a totally extinct group)

Chordata.

Hemichordata.
Pterobranchia, e.g. Cephalodiscus.
Enteropneusta, e.g. Balanoglossus.
Protochordata (Acrania).
Urochordata.

Ascidiacea, e.g. Ascidia.
Thaliacea, e.g. Salpa.
Larvacea, e.g. Fritillaria.
Cephalochordata, e.g. Amphioxus.
Craniata.
Anamnia.
Cyclostomata.
Cyclostomata.

Petromyzontia, e.g. Petromyzon

(? Palaeospondylus).
Myxinoidea, e.g. Myxine, Bdello-
stoma.
Qstracoderma,* e.g. Cephalaspis.
Gnathostomata.
Pisces.
Chondrichthyes.

Selaehii, e.g. Scyllium, Squalus,
Heptanchus, Hexanchus, He-
terodontus, Pristis, Rhina,
Pliotrema, Raia, Torpedo.
Holocephali, e.g. Chimaera.
Acanthodii,* e.g. Acanthodes.
Pleuracanthodii* e.g. Pleura-
- canthus

Cladoselachii,* e.g. Cladoselache.
Osteiehthyes.
Teleostomi.

Osteolepidoti * e.g. Osteolepis,

Sauripterus.
Coelacanthini,* e.g. Undina.
Polypterini, e.g. Polypterus.
Palseoniscoidei,* e.g. Cheiro-
lepis.
487

Phylum.

Subphylum.

Class.

Class.

Subphylum.

Class.

Order.

Order.

Order.

Class.

Subphylum.

Grade.

Branch.

Class.

Subclass.

Subclass.

Class.

Branch.

Class.

Grade.

Order.

Order.
Order.
Order.

Order.
Grade.
Subclass.
Order.

Order.
Order.
Order.

488

CLASSIFICATION OF ANIMALS

Order. Acipenseroidei, e.g. Acipenser.

Order. Amioidea, e.g. Amia.

Order. Lepidosteoidei, e.g. Lepido-

steus.
Order. Teleostei, e.g. Gadus, Amiurus,

Ipnops, Periophthalmus,
Fierasfer, Gobiesox, Ambly-
opsis, Lucifuga, Solea, Exo-
ccetus, Edriolychnus.
Dipnoi, e.g. Ceratodus, Lepidosiren,
Protopterus, Dipterus.
Amphibia.
Labyrinthodontia * (Stegocephalia),
Embolomeri, e.g. Eogyrinus,
Loxomma.
Urodela, e.g. Triton, Salamandra,

Proteus, Siren.
Anura, e.g. Rana, Pipa, Rhino-
derma, Alytes, Hylambates.
Gymnophiona, e.g. Ichthyophys,
Hypogeophys.
Amniota.
Reptilia.
tCotylosauria * e.g. Seymouria.
Sauropsida.
tChelonia, e.g. Testudo, Chelone,
Sphargis, Eunotosaurus, Tri-
assochelys.
Parapsida.
Squamata.

Lacertilia, e.g. Lacerta, Vara-
nus, Uromastix, Gecko,
Anguis, Chalcides, Scincus,
Amphisbaena, Chamaeleo,
Mosasaurus.
Ophidia, e.g. Vipera.
Ichthyosauria, e.g. Mixosaurus,
Ichthyosaurus, Ophthalmo-
saurus.
Diapsida.

Rhynchocephalia, e.g. Spheno-

don (? Thalattosaurus).
Crocodilia.
Pseudosuchia,* e.g. Eupark-

eria.
Thallatosuchia,* e.g. Geosau-

rus.
Eusuchia, e.g. Crocodilus.

t The Cotylosauria and Chelonia are often grouped together as
Anapsida.

Subclass.

Class.

Order.

Suborder.

Order.

Order.

Order.

Grade.

Class.

Subdivision

Subdivision

Order.

Group.

Superorder

Order.

Order.

Superorder and Order.

Group.
Order.

Order.
Suborder.

Suborder.

Suborder.

CLASSIFICATION OF ANIMALS

489

Order.
Suborder.

Suborder.

Order.
Subdivision.
Group.
Order.

Group.
Order.

Class.
Grade.
Grade.
Subclass.

Subclass.

Class.

Grade.

Grade and Subclass.

Grade.

Subclass.

Subclass.

* Subclass.
Order.

Order.
Order.

Suborder.

Suborder.

Order.

Order.

Dinosauria.*
Saurischia, e.g. Diplodocus,

Tyrannosaurus.
Predentata (or Ornithischia),
e.g. Iguanodon, Stegosaurus,
Triceratops.
Pterosauria,* e.g. Pteranodon.
Theropsida.
Synapsida.

Theromorpha,* e.g. Cynogna-
thus.
Synaptosauria.
Sauropterygia,* e.g. Nothosau-
rus, Plesiosaurus.
Aves.

Archaeornithes,* e.g. Archseopteryx.
Neornithes.
Palseognathse, e.g. Struthio,
Emu, Rhea, Cassowary, Ap-
teryx (Kiwi), Moa, Tinamu.
Neognathse, e.g. Columba, Gal-
lus, Megapode, Grebe, Petrel,
Diver, Gull, Flamingo, Duck,
Phalarope, Dodo, Solitaire,
Humming-bird, Penguin.
Mammalia.
Multituberculata.*
Monotremata, e.g. Ornithorhyn-

chus, Echidna.
Ditremata.
Trituberculata.*

Marsupialia, e.g. Didelphys,
Ccenolestes, Dasyurus, Thy-
lacinus, Perameles, Phasco-
larctos, Phascolomys, Pha-
langer, Notoryctes, Thyla-
coleo, Macropus.
Placentalia.
DeltatheridiidaB,* e.g. Deltathe-
ridium.
- Creodonta.*
Carnivora.
Fissipedia, e.g.Canis, Felis,

Ursus, Civet, Badger.
Pinnipedia, e.g. Seal.
Condylarthra.*
Amblypoda,* e.g. Uintathe-

Order.

Suborder.

Family.

Ungulata.
Perissodactyla.
Titanotheridse,* e.g. Tita-
notherium.

490

CLASSIFICATION OF ANIMALS

Family.
Family.

Family.

Suborder
Tribe.
Family.
Family.

Tribe.

Family.

Tribe.

Family.

Family.

Family.

Order.
Order.

Order.
Order.

Order.
Order.
Order.

Order.

Order.

Order.
Order.
Order.
Suborder.

Suborder.

Suborder.
Series.
Series.
Family

Tapiridae, e.g. Tapir.
Rhinocerotidae, e.g. Rhino-

cerus.
Equidae, e.g. Eohippus, Me-
sohippus, Miohippus,
Pliohippus, Equus.
Artiodactyla.
Suina.
Suidae, e.g. Sus.
Hippopotamidae, e.g. Hippo-
potamus.
Tylopoda.
Camelidee, e.g. Protylopus,
Poebrotherium, Proca-
melus, Camelus.
Pecora.
Giraffid33, e.g. Giraffe.
Cervidas, e.g. Cervus, Rein-
deer, Muntjack, Irish Elk.
Bovidae, e.g. Ox, Zebu,
Sheep, Goat, Antelope,
Antilocapridae, e.g. Anti-
locapra.
Hyracoidea,e.g. Hyrax (coney).
Pro boscidea, e.g. Mceritherium ,
Palaeomastodon, Tetrabelo-
don, Elephas.
Sirenia, e.g. Manatus.
Cetacea, e.g. Whale, Dolphin,

Porpoise.
Litopterna,* e.g. Thoatherium.
Edentata, e.g. Bradypus, Cho-
loepus, Armadilloe, Pango-
lin.
Rodentia, e.g. Lepus, Mus,

Squirrel, Porcupine.
Inseetivora, e.g. Mole, Hedge-
hog, Shrew, Plesiadapis,Ma-
croscelides, Tupaia.
Cheiroptera, e.g. Bat.
Dermoptera.
Primates.
Lemuroidea, e.g. Notharc-

tus, Lemur.
Tarsioidea, e.g. Tetonius,

Tarsius.
Anthropoidea.
Platyrrhini,e.g. Marmoset.
Catarrhini.
Parapithecidae,* e.g.
Parapithecus.

CLASSIFICATION OF ANIMALS

491

Family.
Family.

Family.

Cercopithecidae, e.g.
Cercopithecus, Man-
drill, Baboon.
Simiidse, e.g. Proplio-
pithecus, Pliopithe-
cus, Hylobates, Simia
(Orang), Chimpan-
zee, Gorilla, Austra-
lopithecus.
Hominidse, e.g. Pithe-
canthropus, Eoan-
thropus.
Homo rhodesiensis,
Homo heidelbergen-

sis,
Homo neandertha-

lensis,
Homo sapiens.

INDEX

References to generic names are in italics.

Abdominal pore, 37, 53, 276
Abducens nerve, 26, 46, 355, 368,

37o
Abomasum, 346
Acanthodes, 427
Accessory mesentery, 115, 272
Accommodation of eye, 391
Acetabulum, 93
Acetyl-cholin, action of, 388
Acipenser, 429
— , clavicle of, 313
— , spiracle of, 338
Acrodont dentition, 261
Acromion, 141
Actinotrichia, 66
Actinotrocha larva, 423
Adrenal, 62, 101, 152, 401
— , development of, 194
Adrenalin, 388
Afferent branchial artery, 13, 31, 58,

187
After-birth, 234, 253
Aftershaft, 119, 224
Air-bladder, 77, 340
Air-chamber, 199
Air-sac, 127, 341
Ala temporalis, 283
Albumen egg-coat, 198, 402
Albumen sac, 213
Alisphenoid, 135, 295, 300
— canal, 137
Allantoic placenta, 232
Allantois, 213, 248
Alytes, 350, 352
Amblyopsis, 394
Amblypoda, 457
Amia, 429
— , heart of, 331
— , vertebrae of, 303
Ammocoete larva, 28, 399
Amnion, 208, 231, 253
Amniota, head of, 358
Amphibia, head of, 358

Amphibia, skull of, 285
Amphioxus, sensory cells of, 365
Amphistylic skull, 283
Ampulla of ear, 24, 40, 395

— of Lorenzini, 39
Analogy, 480
Anapsida, 288, 439
Angular, 71, 73, 298, 300
Animal pole, 161, 172, 198
Ant-eater y 457

Anterior abdominal vein, 86, 98,
130

— cardinal vein, 31, 58, 97, 330

— cervical ganglion, 156, 385

— chamber of eye, 23

— commissure, 42, 155

— gut-diverticula, 165

— head-cavities, 165, 360

— intestinal portal, 211, 230

— laryngeal nerve, 155

— mesenteric ganglion, 156, 385
Anthropoidea, 461
Anticipatory response, 380
Antilocapra, 260

Antlers, 260, 482

Anura, 435

Aortic arches, development of, 187

Aqueduct of Sylvius, 41

Aqueous humour, 24

Aquintocubitalism, 450

Arachnoid membrane, 155

Archceopteryx, 445

— -, gastralia of, 258

— , tail of, 326

— , teeth of, 263

— , wing of, 321

Archenteron, 163, 177, 206

Archinephros, 34, 348

Archipallium, 380

Archipterygium, 81, 312

Area opaca, 200

— pellucida, 200

— vasculosa, 206, 231

493

494

INDEX

Armadillo, 258, 353, 457

Arrector muscles of hair, 235, 278

Articular, 71, 73, 138, 158, 298, 300

Artiodactyla, 320, 457

Arytenoid, 146

Ascending process, 82, 90, 105, 283

Ascidians, 425

— , endostyle of, 399

Association neuron, 372

Astragalus, 93, 141, 301

Atlas, 106, 307

Atriopore, 8, 11, 170

Atrium, 11, 16

Auditory capsule, 47, 68, 120, 135,

195, 279
— nerve, 46
Auerbach's plexus, 386
Auricle, 30, 58, 85, 98, 109, 127, 147,

219,233
Autostylic jaws, 82, 283, 431
Autotomy, 106, 326
Australopithecus , 464
Axis, 106, 307
Axon, 363
Azygos vein, in, 148

Baboon, 411, 462
Balanoglossus, 422
Barbs, 119, 224
Barbules, 119
Basal process, 82, 283
Basibranchial, 48, 69, 73, 300
Basidorsal, 28, 49, 72, 301, 302
Basihyal, 48, 71, 73, 300
Basioccipital, 68, 72, 300
Basipterygium, 51
Basipterygoid process, 105, 296
Basitemporal, 120, 300
Basiventral, 49, 72, 301, 302
Bdellostoma, kidney of, 33, 348
Beak, 117, 257
Between-brain, 41
Bile-duct, 30, 51, 115, 274
Bimana, 463
Binocular vision, 393
Bladder (allantoic), 94, 11 1

, devpt. of, 185, 189, 233

Blastoccel, 162, 173, 200
Blastocyst, 228
Blastoderm, 199, 240
Blastopore, 162, 176, 202, 240
Blastula, 162, 173, 200
Blood-groups, 412
Blood-islands, 188, 206, 247
Body-stalk, 249

Bowman's capsule, 31, 55, 221, 349
Brachial plexus, 100

Brachiopoda, 423
Brachyodont teeth, 268
Bradypus, neck vertebras of, 307
Brain-stem, 374
Branchial basket, 28

— rays, 49, 77

— tube, 29

Branchiostegal rays, 71, 73, 300
Brown funnels, 12, 15
Buccal cirrhi, 8

— nerve, 46, 367
Bulbils, 13

Bulbus arteriosus, 58, 331
Bunodont teeth, 268
Bursa Fabricii, 125

Ccenolestes, 456

Calcaneum, 93, 301

Calcigerous glands, 192

Campanula Halleri, 76

Camptotrichia, 81, 310

Canine tooth, 138, 263

Capitulum of rib, 121, 308

Carapace, 308

Cardinal veins, 13, 31, 58, 97, in,

148, 187, 327
Carina, 122, 309, 447
Carinates, 448
Carnassial tooth, 268
Carnivora, 457

Carotid arch, 95, 109, 196, 334
Carotid gland, 95, 401
Carpals, 93, 122, 301
Carpo-metacarpus, 122
Cartilage-bone, 66, 72
Cassowary, 450
Catarrhinas, 461
Cement, 261
Centrale, 93, 301
Central nervous system, 363
Centres, nerve, 365, 373
Centrum, 49, 72, 73, 302
Cephalodiscus, 423
Ceratobranchial, 48, 69, 73, 300
Ceratohyal, 48, 71, 73, 300
Ceratotrichia, 51
Ceratodus, 428
— , skull of, 81, 283
Cercopithecidae, 462
Cerebellar cortex, 376
Cerebellum, 25, 41, 375
Cerebral cortex, 113, 376
— hemispheres, 86, 99, 113, 130,

154, 378
Cetacea, 457
Chalazae, 199
Chalcides, placenta of, 248

INDEX

495

Cheiroptera, 457

Cheiropterygium, 314

Chelone, skull of, 288

Chelonia, 439

— , skull of, 288

Chevron-bones, 106, 306

Chimcera, 428

Chimpanzee, 462

Choana, 342

Choloepus, neck vertebras of, 307

Chamceleo, air-sacs of, 341

— , eyes of, 393

— , pigment-cells of, 260

— , tail of, 326

" Chicken-skin," 258

Chondrichthyes, 427

Chondrocranium, 279

Chondrosteus, 429

Chordal centra, 302

Chorda tympani, 386

Chorion, 211, 250

Choroid of eye, 22, 40, 190

— plexus, 41, 155
Chromafhne tissue, 401
Ciliary ganglion, 46, 387

— feeding, 11,30, 425

— muscle, 23, 40, 392
Circulus cephalicus, 78
Cladoselache, 427

— , fins of, 312

— , skull of, 283

Clasper, 37

Clavicle, 72, 83, 106, 122, 301, 313,

317
Clavicular girdle, 313
Cleithrum, 71, 73, 83, 301, 313, 440
Cloaca, 37, 64, 86, 346
Cloacal plate, 233
Club-shaped gland, 168
Coccygeo-mesenteric vein, 130
Ccelacanthini, 428
Coelom, 2, 31, 270
— , devpt. of, 165, 177, 206
Coelomoducts, 15, 190
Ccelomostomes, 31, 33, 55, 99, 188,

349
Cold-blooded animals, 404
Collateral ganglia, 385
Colon, 146, 346

Columella auris, 105, 113, 298, 395
Communis system, 368
Components, nerve, 365
Concha, 114

" Conditioned reflex," 373
Condylar foramen, 137
Condylarthra, 456
Condyle, 90, 105, 120, 135, 286

Confluence, 242

Conjunctiva, 23, 256

Connector neuron, 384

Consummatory response, 380

Contour feather, 119

Conus arteriosus, 58, 331

Convergence, 481

Coprodaeum, 125

Copulatory organs, 103, 351, 436

Coracoid, 51, 71, 73, 301, 313, 317

Cornea, 22, 190

Corneoscutes, 103, 257

Coronary vein, 148

Coronoid, 105, 297, 300

Corpora mamillaria, 154

— quadrigemina, 154
Corpus callosum, 154, 380

— luteum, 152, 227, 403

— striatum, 41, 99, 130, 376
Correlation centre, 375

" Corresponding points," 394
Cosmin, 258
Cosmoid scale, 259
Cotyledon, 251
Cotylosaurs, 288, 437
Courtship, 351
Crampton's muscle, 31, 392
Cranio-sacral outflow, 385
Creodonta, 456
Cribriform plate, 135
Cricoid, 146
Crocodile, 441
— , gastralia of, 258
— , heart of, 332
— , skull of, 290
Crop, 125
Crura cerebri, 154
Cuboid, 141, 301
Cuneiform, 93, 141, 301
Cutaneous artery, 97

— vein, 97

Cutis-layer, 2, 166, 270
Cynognathus , 440, 452
— , skull of, 297
Cyto-trophoblast, 231

Dasynrus, 456
Decussation, 393
Deep petrosal nerve, 387
Deltatheridium, 456
Demibranch, 58
Dental formula, 139

— lamina, 265
Dentary, 71, 73 , 297, 300
Denticle, 37, 261
Dentine, 258

Dermal bone, 66, 72

496

INDEX

Dermal fin-ray, 51, 310

— muscle, 133, 258, 278, 386
Dermatome, 270

Dermis, 7, 256

Dermoptera, 457

Diaphragm, 142, 274

— , devpt. of, 234

Diaphysis, 140, 409

Diapsida, 290, 441

Diastataxy, 450

Diastema, 138

Dichocephalous rib, 308

Didelphys, 456

Diencephalon, 41

Digital formula, 93

Digitigrade, 319

Dinosaurs, 290, 441

Diphy cereal tail, 80, 324

Diphyodont dentition, 138, 265

Diplodocus, 441

Dipnoi, 428

Dip terns, 428

Discontinuous distribution, 80, 428

Distance-receptors, 379

Dodo, 449

Dogfish, blastopore of, 240

Dorsal aorta, 13, 31, 58

, devpt. of, 186, 218

— mesentery, 2, 53, 271

— mesocardium, 185

— nerve, 365

— rib, 307

Dorso-lateral placode, 194
Down-feather, 119

Ductus arteriosus, 97, 147, 197,
218, 223, 233, 334

— Botalli, 97, 148, 197, 334

— caroticus, 95, 109, 197, 334

— cochlearis, 114, 158

— Cuvieri, 13, 31, 58, 271, 327
, devpt. of, 187

— endolymphaticus, 39, 191

— venosus, 218
Dumb-bell-shaped bone, 297
Duodenum, 145

Dura mater, 43, 155
Dwarf males, 430

Ear, 39, 395

— , devpt. of, 191

" Ear-brain," 367

Echidna, 454

— , temperature of, 405

Echinodermata, 422

Ectopterygoid, 71, 73, 296, 300

Edentates, 457

— , teeth of, 269

Edriolychnus, 430

Effectors, 363

Efferent branchial arteiy, 13, 31, 58,

187
Egg-membranes, 172, 198
Egg-shell, 198, 402, 436
Egg- tooth, 224
Elastica (of notochord), 302
Electric organs, 277
Elephas, 458
Embolomeri, 304, 434
Embryos, temperature of, 407
Emu, 450

Enamel, 37, 74, 257, 261, 265, 269
Endolymph, 113, 395
Endopterygoid, 71, 73, 300
Endostyle, 10, 29, 168, 399, 425
Enteroccel, 165, 276
Entypy, 254
Eoanthropus, 464
Eogyrinus, 431
— , ear of, 395
— , pectoral girdle of, 315
Eohippus, 457
Epaxonic muscles, 77
Epiboly, 163, 177
Epibranchial, 48, 69, 73, 300

— placode, 194, 368
Epidermis, 7, 20, 256
Epididymis, 111,151,350
Epigamic characters, 351
Epigastric vein, 130
Epiglottis, 145
Epihyal, 71, 73, 300
Epiotic, 68, 73, 300
Epiphyses (of bone), 140, 409, 453
Epiphysis (of brain), 41
Epipleur, 12, 170
Epipterygoid, 105, 295, 300
Epipubic bone, 301, 317, 456
Epoophoron, 149

Equus, 457

Eryops, 315

Ethmoid ligament, 49

Eunotosaurus, 439

Euparkeria, 441, 446

Eustachian tube, 107, 145, 183, 338

— valve, 333
Exciter neuron, 384
Exoccipital, 68, 72, 90, 104, 120,

135, 286, 300
Exocoetus, 430
External auditory meatus, 117

— gills, 183

— rectus muscle, 46, 362
Exteroceptors, 363
Extrabranchial, 49

INDEX

497

Extra-embryonic coelom, 206, 211,

486
Eye, 22, 40, 391
— , devpt. of, 190
" Eye-brain," 370
Eye-muscles, 46, 53
— , devpt. of, 360

Facial nerve, 26, 46, 75, 356, 367,

368, 370
Falciform ligament, 115, 146
Fallopian tube, 149, 349
False palate, 135, 143, 296, 342
Fat-distribution, 133, 276, 418
Feather, 117
— , devpt. of, 224
Feather-sheath, 224
Femur, 93, 301
Fenestra metotica, 279

— ovalis, 114, 396

— prootica, 280

— rotunda, 114, 396
Fertilisation, 351
Fibula, 93 , 301
Fibulare, 93, 301
Fierasfer, 349
Filoplume, 119

" Final common path," 372

Fin-ray box, 6, 167

Fissipedia, 457

Flat fish, 430

Flocculi, 131

Foramen lacerum anterius, 137

medius, 137

posterius, 137

— magnum, 27

— of Magendie, 155

— of Monro, 86, 99, 154

— of Panizza, 332

— of Winslow, 115, 143, 274

— ovale, 137

— rotundum, 137

— triosseum, 122
Forebrain, 25, 41
Frontal, 68, 72, 300
Funnel (ciliated), 31, 58, 99, i8£

220, 349
Furcuia, 122

Gall-bladder, 51, 77, 95, 146, 185

Ganglion, 17, 27, 181

Ganoin, 260

Gastralia, 258

Gastral mesoderm, 165, 179, 204

Gastrula, 163, 177

Genital pore, 34

Gephyrocercal tail, 80, 325

Gibbon, 462

Gill-pouches, 29, 234, 485

Gill-slits, 4, 12, 36, 48, 64, 90, 485

— , devpt. of, 168, 183, 217

Gizzard, 125

Glands of epidermis, 257

— of Schwammerdamm, 192
Glenoid cavity, 51, 92
Glomerulus, 31, 11 1, 189, 222, 349
Glomus, 189
Glossopharyngeal nerve, 26, 46,

356, 367, 368, 370
Glottis, 83

Glycogen (in placenta), 233, 253
Gobiesox, 349
Gonocoel, 167, 271
Gonotome, 167, 271
Gorilla, 462

Graafian follicle, 152, 227, 403
Greater superficial petrosal, 386
Great omentum, 143, 276
Grebe, 449
Grey crescent, 172

— matter, 42, 366

— rami, 386
Gubernaculum, 151
Gums, 143
Gymnophiona, 435
— , blastopore of, 242
— , scales of, 260
Gyri, 154

Habenular commissure, 41, 155

Hasmal arch, 50, 72, 302

Hair, 133

— , devpt. of, 234

Hamuli, 119

Harderian gland, 114

Hatschek's nephridium, 16, 171

— pit, 8, 167
Head-fold, 205

Heart, 30, 58, 78, 85, 98, 109, 127,

146, 330
— , devpt. of, 185, 217
Hemiazygos vein, 148
Hemichordata, 442
Henle's layer, 235
Hepatic portal vein, 4, 13, 30, 58, 115,

274,335

— sinus, 58
Heptanchus, 338, 478
— , head of, 358
Heterocercal tail, 32, 324
Heteroccelous centra, 106
Heterodont dentition, 138, 263
Heterodontus, 427
Heterogony, 482

2 K

498

INDEX

Hexanchus, 338, 479

— , head of, 358

— , skull of, 283

Hindbrain, 25, 41

Hippocampal commissure, 155, 381

— cortex, 380
Hoatzin, 321, 449
Holoblastic cleavage, 161, 173
Holocephali, 428
Holocephalous rib, 307
Holostei, 429

Hominidae, 462
Homocercal tail, 64, 325
Homodont dentition, 138, 263
Homo heidelbergensis, 465
Homology, 477
Homo neanderthalensis , 465

— rhodesiensis , 465

— sapiens, 466
Homothermous, 404
" Honey-comb," 346
Horizontal septum, 53, 277, 307
Horn, 257, 260

Horny teeth, 20, 195, 269
Humerus, 92, 301
Huxley's layer, 235
Hylambates, 352
Hylobates, 462
Hyoid arch, 48, 49,356

— segmeat, 46, 49

— sinus, 58

— somite, 354
Hyomandibula, 48, 71, 73, 82, 298,

300
Hyomandibular nerve, 46, 367
Hyostylic jaws, 49, 71, 283
Hypaphophyses, 121
Hypaxonic muscles, 277
Hyperdactyly, 322
Hyperphalangy, 322
Hyperpharyngeal groove, 10
Hypobranchial, 48, 69, 73, 300
Hypocentrum, 301, 304
Hypoglossal muscles, 53, 181, 356,

370

— nerve, 47, 113, 356, 370
Hypohyal, 71, 73, 300
Hypophysial cavity, 20, 25, 341, 401

— fenestra, 281
Hypophysis, 61, 193, 399
Hypsodont teeth, 268
Hypural, 304, 324
Hyracoidea, 457

Ichthyophis, 352
Ichthyopterygium, 314
Ichthyosaurus, 440

Ichthyosaurus, gastralia of, 258

— , skull of, 292

Iguanodon, 442

Ilio-sciatic foramen, 123

Ilium, 93, 106, 123, 141, 301, 316

Incisor, 138, 263

Incus, 138, 298, 300

Inferior commissure, 155

— jugular sinus, 58

— oblique muscle, 46, 361

— rectus muscle, 46, 361

— vena cava, 85, 97, 128, 147

, devpt. of, 187, 218

Inflected angle, 297
Infra-orbital gland, 145
Infundibulum, 25, 41, 61, 154, 193,

400
Inner mass, 228, 254
Innominate artery, 128, 147

— vein, 148
Insectivora, 456
Instinctive behaviour, 376
Intelligent behaviour, 382
Intercentra, 306

Interclavicle, 106, 301, 309, 317
Interdorsal, 28, 50, 301, 302
Interhyal, 71, 73, 300
Intermediate cell-mass, 181, 188,

220, 348
Intermedium, 93, 301
Internal carotid, 61, 78, 109, 327,

334

— rectus muscle, 46, 361
Interoceptors, 364
Interopercular, 71, 73
Inter-renal, 62, 101,401
Interventral, 301, 302
Invagination, 162, 177
Inverted retina, 24
Involuntary muscle, 55, 271, 364,

384
Ipnops, 394
Iris, 40

Ischio-pubic foramen, 106
Ischium, 93, 106, 123, 141, 301
Islets of Langerhans, 152, 402
Isopedin, 259

Jacobson's organ, 115, 396
Jaw-muscles, 55, 271, 357, 368
Jugal, 135, 300
Jugular vein, 58, 97, 128, 187

Kiwi, 450

Kolliker's pit, 8, 16, 163

Labyrinthodontia, 258, 434

INDEX

499

Labyrinthodont teeth, 261, 431

Lacertilia, 441

Lachrymal, 69, 73, 300

Lacteal, 336, 345

Lacunae, 232

Lamina orbito-nasalis, 281

— terminalis, 41, 208
Larvacea, 425
Larynx, 146, 183

Lateral abdominal vein, 58, 86, 219

— ganglia, 385
Lateralis system, 367
Lateral-line ossicles, 74

— system, 26, 38, 74, 90, 285, 286,
367, 396

, devpt. of, 194

lateral mesocardia, 187

— plate, 4, 26, 181, 270
Laterosphenoid, 68, 73, 120, 295,

300
Lemuroidea, 460
Lepidosiren, 339, 352, 428
Lepidosteoid scale, 259
Lepidosteus, 259, 429
Lepidotrichia, 66, 73, 258, 260, 301
Lesser omentum, 115, 272

— superficial petrosal, 387
Limb, devpt. of, 222

— , pentadactyl, 89, 314

Lip, 143

Liver, 8, 30, 51

— , devpt. of, 184

Lophodont teeth, 268

Lower layer, 200, 229, 243

Loxomma, 285, 431

Lucifuga, 394

Lunar, 93, 301

Lung, 83, 95, 109, 127, 146, 339,

43i

— , devpt. of, 184
Lymph, 336
Lymph-gland, 149
Lymph-heart, 336

Macropus, 456

Macroscelides, 470

Macula lutea, 394, 472

Magnum, 141, 301

Malleus, 138, 298, 300

Malpighian corpuscle, 31, 11 1, 189,

221, 349
Mammary glands, 134, 353
Manatus, neck vertebras of, 307
Mandibular arch, 47, 49, 356

— segment, 36, 49

— somite, 354
Maniplies, 346

Marmoset, brain of, 472
Marrow, of bone, 335
Marsupials, 454
— , placenta of, 248
— , teeth of, 265
— , yolk of, 247
Marsupium, 353, 454
Mastodon, 458
Maxilla, 71, 73, 293, 300
Meckel's cartilage, 48, 71, 281
Mediastinal septum, 143
Medulla oblongata, 42, 370
Megapodes, 353, 449
Meibomian glands, 134
Meissner's plexus, 387
Membrane-bone, 66, 72, 258
Meningeal membranes, 155
Meroblastic cleavage, 199, 240
Mesencephalon, 41
Mesethmoid, 68, 73, 135, 300
Mesocolon, 276
Mesodermal pouch, 165, 276
Mesohippus, 457
Mesometrium, 228, 276
Mesonephric duct, 33, 55, 189, 220
Mesonephros, 33, 55, 98, in, 189,

221, 349
Mesopterygium, 51
Mesorchium, 276
Mesotarsal joint, 107, 124, 4 1-6
Mesovarium, 276
Metacarpal, 93, 301
Metameric segmentation, 165, 181,

206, 478 .
Metamorphosis, 196
Metanephros, in, 221, 350
Metapleural fold, 8, 12, 170
Metapterygium, 51
Metapterygoid, 71, 73, 300
Metatarsal, 93, 107, 322, 442
Metencephalon, 41
Metotic somites, 355
Midbrain, 25, 41
Middle ear, 113
Miohippus, 457
Mitral valve, 147
Mixed nerve, 43, 47
Mixosaurus, 441
Moa, 450
Moeritherium, 458
Molar, 139, 263
Monimostylic skull, 294
Monotremes, 454
— , brain of, 380
— , mammary glands of, 353
— , pectoral girdle of, 317
— , yolk of, 247

500

INDEX

Morula, 162

Mosasauria, 321, 441

Miillerian duct, 5s, 99, IIJ, J3°,

149, 349

, devpt. of, 190, 222

Muller's organ, 8, 170
Multituberculata, 453
Muscle-buds, 277, 311
Mustelus, placenta of, 248
Myelencephalon, 41
Myoccel, 2, 13, 34, 166, 270
Myodome, 285
Myotome, 2, 7> *3> 35, 53, 166, 181,

270
Myxine, ear of, 395
— , hypophysial sac of, 341
— , kidney of, 33, 349
— , pituitary of, 400

Nasal, 68, 72, 300

— pit, 64, 193
Naso-lachrymal duct, 158
Navicular, 93, 301
Neognathae, 448
Neopallium, 380
Neossoptiles, 120
Nephridia, 15, 33, 170, 348
Nephroccel, 2, 31, 271
Nephrotome, 2, 31, 181, 188, 220,

271
Nestling down, 224
Nestling feathers, 119
Neural arch, 50, 72, 302

— crest, 181, 208

— fold, 163, 180, 207

— plate, 163, 180, 206
Neurenteric canal, 164, 180
Neurocranium, 28, 49, 279
Neuromast organs, 39, 396
Neuron, 363, 373
Neuropore, 17, 163, 208
Nictitating membrane, 103
Nose, devpt. of, 193

" Nose-brain," 370

Notharctus, 460

Nothosaurus, 440

Notochord, 1, 16, 28, 47,49,72,302

— , devpt. of, 164, 177, 203

Notoryctes, 456

Oblique septum, 125, 274
Obturator fissure, 124
Occipital arch, 279, 358, 478

— condyle, 90, 105, 120, 135, 286
Oculomotor nerve, 26, 46, 354, 368,

37o
Odontoid peg, 106, 140, 307

OEstrus, 227
Olfactory bulb, 41

— lobe, 370

— nerve, 45, 366

— pit, 8

— tract, 75
Omental bursa, 143

— cavity, 115, 274
Omphaloidean placenta, 231
Opercular, 71, 73, 300
Operculum, of fish, 64, 338
— , of tadpole, 196
Ophidia, 441

Ophthalmic profundus nerve, 45,

355

— superficialis nerve, 46, 367
Ophthalmosaurus , 441
Opisthotic, 68, 72, 300
Optic chiasma, 41, 393, 472

— cup, 22, 190

— foramen, 137

— lobe, 25,41, 154, 37o

— nerve, 45

— thalamus, 41, 154, 376

— vesicle, 22, 190
Oral hood, 8

— plate, 233
Orang, 462
Orbit, 68

Orbital cartilage, 279

— sinus, 58

Orbitosphenoid, 135, 300
Ornithischia, 442
Ornithorhynchiis , 454

— , horny teeth of, 257

— , skull of, 294

— , temperature of, 406

Orthogenesis, 4S1

Osmotic pressure regulation, 407

Osteichthyes, 428

Osteolepis, 428, 431

— , fins of, 312

— , nostrils of, 340

— , scales of, 258

— , skull of, 284

Osteoscutes, 103, 258

Ostracoderma, 427

Otic ganglion, 387

— process, 82, 283
Oviducal gland, 57
Oviduct : see Mullerian duct
Ovisac, 350

Ovo- viviparous, 249, 352
Ovulation, 227

Palaeognathee, 448
Palceo7iiastodon, 458

INDEX

5°*

Palaeoniscoidea, 428
Palaeoniscoid scale, 259
Palceospondylus , 427
Palatine, 71, 73, 300

— nerve, 46
Pancreas, 30, 53, 402
— , devpt. of, 185
Pangolin, scales of, 257
Panniculus carnosus, 133, 258, 278
Parachordal, 194, 279
Parapineal eye, 25
Parapithecus , 462

Parapsida, 292, 440
Parasitic males, 430
Parasphenoid, 68, 72, 297, 300
Parasympathetic, 47, 385
Parathyroid, 101, 402
Parietal, 68, 72, 292, 300
Paroccipital process, 104
Paroophoron, 149
Parotid gland, 145
Patella, 124, 142, 301
Paunch, 346
Pecten, 131
Pelvic bone, 72

— nerve, 386
Penguin, 450
Penna, 117

Pentadactyl limb, 89, 314
Perameles, 456

— , placenta of, 248, 251

Periblast, 200

Pericardio-peritoneal canal, 53, 271

Pericardium, 30, 31, 53, 271

— , devpt. of, 185

Perichordal centra, 303

Perilymph, 113

Periesophageal ciliated band, 10, 28

Periophthalmus, 430

Periotic, 120, 300

Perissodactyla, 457

Peristomial mesoderm, 165, 179, 204

Peritoneum, 2

Petaurus, 456

Petromyzon, head of, 355, 358

— , horny teeth of, 257

Peyer's patches, 136, 336

Phalanges, 93, 301

Phalarope, 449

Pharyngobranchial, 48, 69, 73, 400

Phascolarctos , 456

Phascolomys, 456

Phoronis, 423

Phrenic nerve, 157

Pia mater, 41, 43, 155

" Pigeon's milk," 353

Pigment-cells, 260

Pigment-layer of eye, 22, 40, 190
Pila antotica, 279
Pineal eye, 24, 25, 41

— foramen, 284, 285

— gland, 402
Pinna of ear, 134
Pinnipedia, 457
Pipa, breeding of, 352
Pisiform, 142
Pithecanthropus, 464

Pituitary body, 25, 41, 61, 193, 399

Placenta, 231, 248, 425

Placentalia, 456

Placodes, 193

Plantigrade, 319

Plasmodi-trophoblast, 231

Plastron, 258

Platybasic skull, 281

Platyrrhinag, 461

Platysma muscles, 134, 258, 278

Platytrabic skull, 281

Plesiadapis, 460

Plesiosaurus, 258, 292, 440

Pleura, 143

Pie ur acanthus, 312

Pleural cavity (of birds), 125, 274

(of mammals), 143

— ribs, 83, 307
Pleurocentrum, 301, 304
Pleurodont dentition, 261
Pliohippus, 457
Pliopithecus , 462
Pliotrema, 338, 479
Plumuke, 119
Poebrotherium, 458
Poikilothermous, 404
Poison-fangs, 263

Polar bodies, 161, 172, 198, 227
Polyphyodont dentition, 138, 265
Polypterus, 428
— , kidney of, 350
— , lungs of, 330, 339
— , pituitary of, 401
— , ribs of, 307
— , scales of, 259
-> — , spiracle of, 338
Polyzoa, 423
Pons Varolii, 153, 376
Portal vein, 4, 335
Postcleithrum, 71, 73, 301
Posterior cardinal vein, 31, 58, 97;
111,148,329

— cervical ganglion, 156, 385

— chamber of eye, 23

— commissure, 41, 155

— intestinal portal, 21 1, 230
Postganglionic fibre, 384

502

INDEX

Posthepatic septum, 125, 274
Postorbital, 69, 73, 291, 300

— bar, 290, 294, 460
Postpubis, 317
Postspiracular ligament, 49
Post-temporal, 69, 73, 301, 313

— fossa, 287
Precipitin tests, 411
Precoracoid, 92, 301, 317
Predentary, 297, 300
Predentata, 442
Prefrontal, 68, 72, 300
Preganglionic fibre, 384
Prelachrymal fossa, 290
Premandibular segment, 45, 49

— somite, 354
Premaxilla, 71, 73, 300
Premolar, 139, 263
Preopercular, 71, 73, 300
Preoral gut, 234

— pit, 167, 170, 400
Pretrematic nerve, 46
Prevomer, 68, 72, 296, 300
" Primary " feathers, 119
Primary gill-slits, 168
Primates, 457

Primitive characters, 17, 483

— groove, 201, 245

— knot, 201, 245

— pit, 201, 245

— streak, 201, 229, 242
Pristis, 427
Proamnion, 206
Proboscidea, 457

" Proboscis-pores," 276, 360, 423
Procamelus, 458
Processus Folii, 298, 300
Procoelous centra, 106
Proctodeum, 183, 344
Profundus nerve, 26, 45, 355
Pronation, 318
Pronephric duct, 33
Pronephros, 33, 188, 220, 348
Prootic, 68, 72, 300

— somites, 355
Propliopithecus , 462
Proprioceptors, 364
Propterygium, 51
Prosencephalon, 41
Prostate gland, 150
P oteus, 394

Protopterus, 339, 350, 428
Protylopus, 458

Proven triculus, 125
Pseudobranch, 61, 338
Pseudocaudal fin, 64, 325
Pseudosuchia, 441

Pseudovilli, 252

Pteranodon, 442

Pterobranchea, 423

Pterosaurs, 258, 290, 309, 441

Pterotic, 68, 72, 300

Pterygoid, 90, 296, 300

Pterygo-quadrate, 48, 71, 281

Pteryke, 117

" Puberty gland," 402

Pubis, 93, 124, 222, 301

Pulmo-hepatic ligament, 115, 125,

273

— recess, 115
Pyloric cceca, 77
Pyriform cortex, 380

— lobe, 154

Quadrate, 71, 73, 138, 158, 297, 300
Quadrato-jugal, 120, 300
Quadrumana, 463

Rachis, 117

Radial, 51, 71, 73, 301

Radiale, 93, 301

Radius, 93, 301

Rata, 277, 408, 427

Ramus communicans, 365

Rathke's pocket, 400

Ratites, 448

Rauber's cells, 229, 253

Recapitulation, 484

Receptors, 363

Rectal gland, 53

Rectrices, 119

Recurrent laryngeal nerve, 156

" Red gland," 77

Red marrow, 149

" Red meat," 122

Reflex arc, 372

Reissner's fibre, 43

Remiges, 119

Renal portal vein, 61, 149, 222, 335

Restiform body, 41

Retina, 22, 24, 394, 472

Rhabdopleura, 423

Rhea, 450

Rhina, 427

Rhinoceros, 457

— , horn of, 257

Rhinoderma, 352

Rhombencephalon, 41

Rhynchocephalia, 441

Rhynchosanrus , 441

Rib, 50, 82, 92, 141, 301, 307

Rodentia, 457

" Rootless," teeth, 139, 268

Rouget-cells, 336

INDEX

503

Saccule, 39, 114, 158
Sacculus rotundus, 146
Saccus vasculosus, 43, 75
Sacrum, 92, 106, 12 r 307
Salamandra, 95, 334
Salivary glands, 93
Salpa, placenta of, 425
Sauripterus, 315, 428
Saurischia, 442
Sauropsida, 322, 332, 443
Sauropterygia, 440
Scaphoid, 93, 301
Scapula, 51, 71, 73, 301
Scapular girdle, 313
Schizoccel, 276
Sciatic plexus, 100
Sclerocoel, 167
Sclerotic, 22, 40, 190
Sclerotome, 167, 182, 270, 303
Scrotal sac, 134, 143, 150, 358
Scy Ilium, head of, 358, 478
Sebaceous glands, 134, 257
Secondary choana, 145
" Secondary " characters, 18

— feathers, 119

— gill-slits, 168
Secretin, 347
Segmental apparatus, 374
Selenodont teeth, 268
Sella turcica, 152
Semicircular canal, 24, 40, 395

, devpt. of, 192

Seminal vesicle, 55
Septomaxillary, 104, 300
Sero-amniotic connexion, 211, 213,

253
Sesamoids, 142, 301
Seymouria, 437
— , skull of, 286
— , vertebras of, 306
Shell of egg, 198, 402, 436
Shell-membrane, 198, 402
Sino-auricular node, 333
Sinus terminalis, 206, 231

— venosus, 30, 58, 333
Sirenia, 457

Skin, 7, 256

" Skin-brain," 367

Sloth, 457

Smooth muscles, 55, 270, 368, 384

Soft commissure, 154

Solea, 430

Solitaire, 450

Sottas' centre, 467

Somatic motor component, 365

Somatic muscles, 26, 55

— sensory component, 365

Somatopleur, 2

Somite, 2, 165, 181, 206, 270

Specialised characters, 18, 483

Spermatic cord, 150

Sperm-sac, 55

Sphargis, 439

Sphenodon, 441

— , gastralia of, 258

— , heart of, 332

— , pineal of, 394

— , skull of, 290

— , vertebrae of, 306

Sphenopalatine ganglion, 386

Sphenotic, 68, 72, 300

Spinal accessory nerve, 1.13, 308

Spiracle, 37, 48, 338

Spiral valve, 30, 53, 83, 345

Splanchnic nerves, 385

Splanchnoccel, 2, 31, 53, 166, 181,

270
Splanchnocranium, 28, 49, 279
Splanchnopleur, 2
, Spleen, 61, 152, 335
Splenial, 90, 300
Splint-bones, 320
Squalus, head of, 358, 478
Squamata, 441
Squamosal, 90, 291, 300
Stapes, 138, 298, 300, 395
Stegocephalia, 434

— skull of, 285
Stegosaurus, 442
Stellate ganglion, 385
Sternebrae, 142, 309
Sternum, 92, 301, 309
Stomach, 51, 345
Stomodaeum, 183, 344
Stratum corneum, 133, 256

— Malpighi, 133, 256
Streptostylic skull, 105, 292
Striated muscle, 55, 270, 370, 392

— visceral muscle, 55, 271 , 368
Struthiones, 450

Styloid process, 138
Stylo-mastoid foramen, 137
Subintestinal vessel, 30, 58
Sublingual gland, 145
Submaxillary ganglion, 386

— gland, 145
Subopercular, 71, 73, 300
Suborbital, 69, 73, 300
Sucker of tadpole, 195
Sudoriparous glands, 134, 257
Sulci, 154

Superior oblique muscle, 46, 361

— rectus muscle, 46, 361

— vena cava, 85, 97, 128, 148

5°4

INDEX

Supination, 318
Supra-angular, 298, 300
Supracleithrum, 71, 73, 301, 313.

316
Supraoccipital, 68, 72, 300
Supra-renal, 62, 101,401
Suprasegmental structures, 374
Supratemporal, 104, 300
Surface Volume ratio, 417
Swim-blad.er, 77, 340
Sympathetic, 385

— nerve-chain, 45, 62, 156, 385

— ganglia, devpt. of, 194
Symplectic, 71, 73, 300
Synapsida, 291, 439
Synapticula, 12
Synaptosauria, 292, 440
Syrinx, 126, 343

Systemic (aortic) arch, 95, 109, 128,
147, 196, 332

Tadpole, horny teeth of, 257
Tapetum, 190
Tarsibidea, 461
Tarsius, 461, 470
Tarso-metatarsus, 124, 222
"Taste-brain," 368, 373
Tectum synoticum, 281
Telencephalon, 41, 86, 113
Teleoptiles, 119
Teleostei, 429
Teleostomi, 428
Temporal cavity, 287

— fossa, 103, 288
Tetoriius, 461
Tetrabelodon, 458
Thalamencephalon, 41
Thalamus, 41, 113, 154, 376
Thalattosaurs, 441
Thalattosuchia,
Thebesian valve, 333
Thecodont dentition, 261
Theriodonta, 440
Theromorpha, 292, 439
Theropsida, 322, 332, 443
Thoatheria, 320, 457
Thoracic duct, 148, 336
Thoracico-lumbar outflow, 385
Thylacinus, 456
Thylacoleo, 456

Thymus, 62, 151, 402
— , devpt. of, 183
Thyrohyoid, 138
Thyroid cartilage, 146

— gland, 11, 29, 62, 152, 399

, devpt. of, 183

Tibia, 93, 222, 301

Tibiale, 93, 301

Tibio-tarsus, 124, 222

Tinamu, 448, 450

Titanotheres, 457

Tongue-bars, 12, 170

Tongue, of Petromyzon, 20

Tonsils, 145, 336

Tornaria larva, 422

Torpedo, 277, 427

Trabecula, 194, 279

Transpalatine, 105, 296, 300

Transverse process, 92, 106, 121,306

— septum, 31, 34, 53, 58, 271

— — , devpt. of, 187, 218
Trapezium, 141, 301
Trapezoid, 141, 301
Triassochelys, 439

— , teeth of, 263

TriceratopSy 442

Tricuspid valve, 147

Trigeminal nerve, 26, 46, 356, 367,

368
Tritubercular teeth, 267, 454
Trituberculata, 454
Trochanter, 141
Trochlear nerve, 26, 46, 354, 368,

37o
Trophoblast, 228, 250
Tropibasic skull, 281
Tropitrabic skull, 281
Truncus arteriosus, 95, 109, 331
Tuberculo-sectorial tooth, 267
Tuberculum acusticum, 367
— , of rib, 121, 308
Tubule of kidney, 31, 55, 98, in,

188, 220, 348
Tunica vaginalis, 143
Tupaia, 470
Turbinals, 135, 300
Turtle, 334
Twinning, 353
Tympanic bulla, 135, 298, 300

— cavity, 108, 113, 395

— membrane, 108, 113
TyrannosauruSy 442

Uintatherium, 457

Ulna, 93, 301

Ulnare, 93, 301

Umbilical veins, 219, 233

Umbilicus, 212

Unciform, 141, 301

Uncinate process, 121, 308, 446

Ungulata, 319, 457

Unguligrade, 319

Ureter, in, 130, 149, 350

— , devpt. of, 221

INDEX

505

Urethra, 150
Urochordata, 425
Urodaeum, 125
Urodela, 435
Uromastix, 405
Uropygial gland, 117
Uterus, 149, 351

— masculinus, 149
Utricle, 39, 114, 395

Vagina, 149

Vagus nerve, 26, 47, 155, ^56, 367,

368, 370, 386
Valvula, 74, 376
Varanus, skull of, 104, 297
— , temperature of, 405
Vas deferens, 55, 86, 99, 113, 149,

190, 349
Vasa efferentia, 55, 86, 98, 111, 190,

221, 349
Vegetative pole, 161, 172
Velum, 8, 28, 170

— trans versum, 41
Ventral aorta, 13, 30, 58, 331

— nerves, 4, 365

— ribs, 83, 307

Ventricle of heart, 30, 58, 98, 109,

127, 147, 33i
Ventricles of brain, 41, 86
Vermiform appendix, 146, 346
Vermis, 131

Vertebral plate, 4, 26, 181, 270
Vertebrarterial canal, 121, 308
Vestibule, 149
Vidian nerve, 387

Villi, of intestine, 146
— . of trophoblast, 251
Visceral arch, 48, 49, 271, 356

— lobe, 368

— motor component, 365

— muscles, 55, 271, 357, 368

— sensory component, 365
Vitelline membrane, 161, 172, 198,

227

— veins, 216
Vitreous humour, 24
Vocal cords, 146, 342
Voluntary muscles, 55, 270, 364, 370
Vomer, 135, 297, 300

Warm-blooded animals, 404
Webbed feet, 449
Weberian ossicles, 340
Wheel-organ, 8, 170
White matter, 42, 366

— meat, 122

— rami, 385

Wolffian duct, 55, 86, 98, in, 130,
149, 189, 220, 349

Xiphisternum, 142

Yellow spot, 394, 472

Yolk, 161, 172, 198, 240, 247

Yolk-sac, 213, 229, 231, 247

Zona pellucida, 227

— radiata, 198
Zygapophysis, 72, 307
Zygomatic process, 135

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