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Marine Biological Laboratory Library t 

Woods Hole, Mass. [I 

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Presented by \[ 

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fi the estate of ff 

jj Dr. Herbert W. Rand ft 

| Jan. 9, 1964 J 

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Edited by Julian S. Huxley, M.A. 

Fullerian Professor of Physiology, Royal Institution 

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




Edited by Julian S. Huxley 



ANIMAL ECOLOGY. By Charles Elton. 

Other Volumes in Preparation. 
Kathleen E. Carpenter. f n the Press. 

S. Huxley. 


troduction to Animal Morphology and 
Bionomics. By W. Garstang. 






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














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 


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 


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. 

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. 


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

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

G 2 


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


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. 

February, 1928. 


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 


once she has reached out her hand to sister branches of 

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 


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 


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. 




Morphological Types illustrating the Different Stages 
of Organisation and the Trend of Vertebrate 


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 


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) 





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 




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 


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 .... 





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

Classification of the animals and groups of animals mentioned 

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














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 

















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 






in trans- 
in longi- 



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 








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 


of the 



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 








328, 329 










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. 



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 

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. 


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. 


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 

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 


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. 



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. 



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. 


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. 




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 


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 



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 


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 


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. #-- 


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. 


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 


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 



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 : 


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. 


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. 


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. 



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. 




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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 


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 



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 


vitreous humour, the anterior chamber contains the aqueous 

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 



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 ■ 

hi oc - P b 

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


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 

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


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 


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 


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 



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 


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

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 

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 


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 





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-' P d c ** V 


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 c fP s ^%\ so f ^ ed th ^ 
archineohric duct (ad). With each Bowman's capsule is associated a 
SoSlu" torfomed from an arteriole from the dorsal aorta (da) and a 


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). 




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 


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 ; 


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. 


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. 



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 




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 



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 

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. 

4 o 


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 


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 


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- . 




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 



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 


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

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 



— gsi, 


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 

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 

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 ; e y 
eye ; er y 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 ; sr y superior rectus eye-muscle ; t, terminalis nerve. 


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 

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- 

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


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 

4 8 


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 Sv qc. 

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, 



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 

1 st = premandibular 
2nd = mandibular 

Arches or Bars. 
1st visceral = mandibular 


2nd visceral =hyoid 


3rd visceral = 1 st branchial 


4th visceral = 2nd branchial 


5th visceral = 3rd branchial 


6th visceral = 4th branchial 


7th visceral = 5 th branchial 



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. 

b y 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 


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 

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 




ab5. ■ 





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 

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 ; cl t 
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 ; p y 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. 


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. 



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 



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. 


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. 




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. 


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. 



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, 

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. 



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 



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 


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 

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



T? O* 



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 



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 



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 


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 

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 

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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 


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

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. 



Mixed bones, 

















Cartilage-bones. j 

Membrane-bones. Mixed bones 


Epiotic (a tendon-bone) 






Basisphenoid : not 

always present 

Splanchno-cranium . 


Endopterygoid Palatine 




















Branchiostegal rays 





Vertebral Column. 


Ra dials 

Appendicular Skeleton. 






Post- clei thrum 




Dermal Skeleton. 




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 


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 

7 6 


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 


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 

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


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 


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 ; 


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 


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

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



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>' 


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 



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 



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 
Y 1 and V 2 ) ; 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 

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 


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 


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 


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 


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 


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 

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 

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. 


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. 



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, 

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



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 

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 



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 

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 



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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 

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. 



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 



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 

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 


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 

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 




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. 



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 

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 


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 

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 


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 

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, 


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 

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 ; 


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. 



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 



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 


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. 


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- 

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. 



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 

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 



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 



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 

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 




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 




#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. 


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 



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 


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. 


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 


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


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 

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. 




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. 


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- 

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 


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 


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 

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. 


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 



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 


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 


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 


trachea from the angle between the bronchi, forming the 

p IG 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, 

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


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 42 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 


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 


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. 



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 


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 


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. 


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



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 

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 



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 

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 





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

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 


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 : 


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


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

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 



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 


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 

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 



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 


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 

i 4 4 


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 

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 


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. 



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 

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. 



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 


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. 

al y anterior laryngeal nerve ; bv 1, 3, 4, and 6, blood-vessels (arterial 
arches) running in the first, third, fourth, and sixth visceral arch ; ca y 
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 



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 


J5 1 

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


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 


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 



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 


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 


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 



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 


SCL.— i 



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 


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. 


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, 

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




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 30 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: 


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 


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 

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 



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 



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 


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 

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 


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 



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 


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 



3 2-d 

C w G 
+j bjo <u s ij g fl 



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 

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 



formation of the tongue-bars, so that in the adult there is a 
nephridium to every two gill-slits). Hatschek's nephridium 



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. 


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

MacBride, E. \V. Text-Book of Embryology, Vol. I. Macmillan, 

London, 1914. 



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 




(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 



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 



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. 


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 ; ap t 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. 



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 


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 

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 



the roof of the archenteron, which still forms a complete 


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 


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 



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 


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. 



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. 


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. 



From the posterior end of the larynx, the lungs develop as 
sacs stretching back parallel to the oesophagus on each side. 



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 

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 


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 

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 



veins, which run from the yolk-cells and the rudiment of the 

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 


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 

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 


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 

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 

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 


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 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 


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 

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 



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 



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. / 


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 


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 


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 

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 


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 

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 


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 

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 


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. 


Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon 
Press, 191 3. 

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

Morgan, T. H. The Development of the Frog's Egg. Macmillan, New 

York, 1897. 



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 



pigment which gives the " egg " its characteristic colour and 

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 



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, 



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 



A -° 

■J? £, w 93 


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 

.~^V:'^^^ ap. 

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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- 



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 


Mist M'.'O'."- ' 'rfi-jsa 



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 



(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 



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 



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 



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 


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 



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 ; 
pc t 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 






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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 




on each side of the embryo, and soon covers over the latter 
completely, much in the same manner as the neural folds 



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 


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 



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. 


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 


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 


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 



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 



respiratory organs, for they communicate with the amniotic 
cavity in which the oxygen cannot be renewed. 


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 



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 ; sh y 
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, 


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 

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 


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 



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 


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 

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 



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 


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 



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 


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 ; h t 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 


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. 


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 



as their predecessors, and when the adult feathers are properly 
formed, the nestling-down is worn off. 


Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon 

Press, 1913. 
Kellicott, W. E. Chordate Development. Henry Holt, New York, 

Lillie, F. R. The Development of the Chick. Henry Holt, New York, 




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 egg t 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 




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 

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, 

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 



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 


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 



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 


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). 


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 



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 


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. 


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 



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 

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. 



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. 



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.) 






Branch and Class. 


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. 
Craniata. With a specialised head and skull, 
paired eyes, ears, and noses, 
heart and coelomostomic 
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. 
























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 

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. 
Amniota. Embryo develops on land 
inside an amnion. 
Reptilia. Body covered with horny 
scales, cold-blooded. 
Sauropsida. Reptiles related to the 

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 
Aves (Birds). With feathers, warm- 

Palaeognathae. With a large pre- 
vomer, e.g. Struthio (Ostrich). 




Neognathse. With a small pre- 

vomer, e.g. Columba (pigeon), 

Gallus (fowl). 


Mammalia. Mammary glands, hair, 

diaphragm, warm-blooded. 

Grade and Subclass. 

Monotremata. With a cloaca, e.g. 

Ornithorhynchus (duck-billed- 



Ditremata. Anus and urinogenital 

apertures separate. 


Marsupialia. With marsupial 

pouch, e.g. Perameles. 


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. 



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 




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 


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, 



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 


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 


the archenteron to the surface of the yolk. The walls of the 
definitive alimentary canal are formed from the lower layer, 


t, r Jk. 



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 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 


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. 






Very little. 



Very much. 




Not enclosed. 

Not enclosed. 






and invagi- 

streak and 

streak, no 








Closed from 

Gut formed 

Wall of 

Wall of 

Lower layer. 

Lower layer. 





Assheton, R. Growth in Length. Cambridge University Press, 191 6. 
Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon 
Press, 1 91 3. 



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 



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 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 

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 

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 


the outermost layer of extra-embryonic ectoderm, known as 
the chorion in reptiles and birds, and the trophoblast in 

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&&™ 


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 



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 



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 


eh njc +• f- mb. e f 

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 


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. 


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 

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 


and intense period of embryonic development, when the 
efficiency of the embryonic membranes and placenta is of the 
utmost importance. 


Jenkinson, J. W. Vertebrate Embryology. Oxford, at the Clarendon 
Press, 191 3. 



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 

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. 



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 

Hairs and feathers are commonly moulted at intervals and 

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 



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 




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 


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. 


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



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 



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 ; Mc y Meckel's cartilage ; mp, mesodermal papilla ; pc, pulp-cavity ; 
t, tongue. (C/. Method of development of the denticle, Fig. 120.) 



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. 


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 


- 4.-:""^ 


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 


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 


Fig. 124. — Types of teeth (the different teeth are not drawn to the same 


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 


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. 


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 

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 


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. 


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. 



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, 



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- 

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 


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 ; n t 
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 



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 



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. 



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 


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- 

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. 


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 

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 

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 


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. 


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. 



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 



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 



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 


Fig. 129. — Diagram of a schematic chondrocranium seen from the left side, 
and showing the relations of the cartilages to the principal nerves and 

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 


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 


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 

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 


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 


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 



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 




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 


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 



en: — 
1. — / 



. \\VV-nrf 

i — v5v \ " • 


^ ar A/rr" 

s " 17 ~~ VI 



1 TV " \ T ~ st 

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Fig. 133. — Seymouria : dorsal view of a skull. (Drawn from a cast.) 

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 



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 


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 


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. 



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in the roof of the temporal cavity, it is not an opening into the 

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 
by the parietal bone ; the inferior temporal fossa is bordered 

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Fig. 140. — Left side view of a skull of a bird (Columba). 

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 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 


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. 



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, 


the fangs extend back parallel to and beneath the roof of the 

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. 



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 


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 


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, 

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 


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 


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 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. 


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. 

Huxley, T. H. Contributions to Morphology. Ichthyopsida. On 

Ceratodus forsteri. Proceedings of the Zoological Society of London, 


Williston, S. R. The Osteology of the Reptiles. Harvard University 
Press, 1925. 



Cartilage from 

Part of Skeleton. 


Cartilage- (and 
tendon-) bones. 

Bones of 

which the car- 
tilage-bone, or 
part of the 
mixed bone os- 




Basal plate 



Basal plate 

Postfrontal=-part of 




Trabecular plate 



Occipital arch 



Tectum synoticum 


Intertemporal = part 

Prootic ) Peri- 
Opisthotic 3 otic 

Auditory capsule 

of pterotic ? 

Auditory capsule 




Neuro- / 



cranium \ 





Trabecular inter- 
nasal septum, 
and nasal capsule 



Pila antotica 

Vomer = part of 






Basitemporal = part of 



Lamina orbito- 




nasal is 
Auditory capsule 

Auditory capsule 


Quadrate = Incus 







Processus ascen- 



Ectopterygoid = trans- 


Ala temporalis 


Articular = Malleus 




Pterygoid == Endo- 

= Columella auris 

pterygoid ? 

= Stapes 



Splanchno- , 


Epihyal ; 

Hyoid arch 


Angular = Tympanic 
Supra-angular = Pro- 
cessus Folii 
j Splenial 




Basihyal / 

Pharyngobranchial j 



1 Branchiostegal rays 

! Preopercular 

Branchial arch 

; Opercular 


! Subopercular 

Basibranchial / 

i Interopercular 







Part of Skeleton. 


Cartilage- (and tendon-) 

Cartilage from which the 
Cartilage-bones ossify. 

Axial skeleton 

Neural arch 


(Vertebral Column, 

Haemal arch 




Basi ventral 


Interdorsal and inter- 





Appendicular skele- 

ton (Limbs, Girdles, 

and Sternum) 


Scapula | 


Coracoid > 












Radi ale = Scaphoid \ 

Intermedium = Lunar 

Ulnar e = Cuneiform 

Centrale \ 
Trapezium / 




Unciform / 











Epipubic =* Marsupial 








Tibiale )=Astra-\ 
Intermedium/ galus ? 

Fibulare = Calcaneum 

Centrale = Navicular s 


Endocuneiform 1 



Cuboid ) 







Sesamoids and patella 


Dermal skeleton 






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 



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 ; cv y 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 


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 


has occurred, and the vertebras of these animals have centra 
which correspond to pleurocentra. In consequence of this, 


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 ; 
na y neural arch ; pc, pleurocentrum ; pz, prezygapophysis ; r, rib ; tp y 
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 


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 


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 

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. 


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 


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 


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. 



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, 




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. 


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 

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 



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, 



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 


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 



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 


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 



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 


in the dog. In the more primitive forms, and those which are 
specialised for tree-climbing and digging, the clavicle is usually 

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 180 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 

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 



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 


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 

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 


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 

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. 



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 

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. 


Goodrich, E. S. Vertebrata Craniata, 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. 
Gregory, W. K. Present Status of the Problem of the Origin of the 

Tetrapoda. Annals of the New York Academy of Sciences, vol. 26 


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. 



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 



(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 ; 
v t 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) 


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 

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 



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, 






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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 


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, 


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 

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 


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 


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 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 


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 

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. 


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. 



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 

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 


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 


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 

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. 


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 

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 


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 

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 


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 

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 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. 


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 



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 

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 



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 

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 


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 


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 

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. 


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. 



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 



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 

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, 


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 

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. 


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 

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- 


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 


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 

2 A 



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 



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 

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 

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 


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 


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 

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 


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 






-a J J3 

L* +-> +J 

co «■ »n 

u . 


ist or 


•5 -S -5 

^■' m no 





Hyoid vessel. 
Carotid arch. 


















Meckel's carti- 


ist pharyngo 
epi, cerato, & 
hypo bran- 



3rd ditto. 
4th ditto. 
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Part of Hypo- 
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Vagus 3rd 
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internal, in- 

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Rectus external. 

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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 

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 



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 


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. 


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. 



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. 



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 





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. 


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 

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 


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 

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 


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 

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- 

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). 


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 


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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 

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 : — 


Somatic. Afferent. General. 










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. 


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. 



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 



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 


component-system), and where adjustments are made with 
association and efferent neurons so as to complete the reflex 

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 


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 


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, 


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 


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 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 

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, 


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 


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 


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. 



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. 



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 



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 

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 


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 


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- 


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. 


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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. 


luntary Nervoi 
Lanoley, J. N. Autonomic Nervous System. Heffer, London, 1921. 

Gaskell, W. H. The Involuntary Nervous System. Longmans, Green, 
London, 1920. 



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. 



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 

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 


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 


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. 


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 

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 


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 


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. 


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. 



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 



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 


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 


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 

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 


(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 


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. 


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. 



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 



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 35 to 5 C, the temperature 


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 30 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 35 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- 


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, 


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 C0 2 
in the blood is regulated by the respiratory system, controlled 
by a centre in the brain. If the blood is rich in C0 2 the 
respiratory movements are accelerated, and conversely they 
are retarded if the quantity of C0 2 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 

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. 


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. 


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. 



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 



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 : — 



100 per 








Orang Outang 


































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. 


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 



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. 


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. 




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 



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 

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. 


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 


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 


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 


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 


impossible for land forms. For this reason, it is likely that 
the largest of the Dinosaurs were more or less aquatic. 


Brooks, C. E. P. Climate through the Ages. Benn, London, 1926. 

d'Arcy Thompson. Growth and Form. Cambridge University Press, 

Haldane, J. B. S. Possible Worlds. Chatto and Windus, London, 1927. 

Hesse, R. Tiergeographie auf okologischer Grundlage. Fischer, Jena, 

Przibram, H. Form und Formel im Tierreiche. Deuticke, Leipzig and 
Wien, 1922. 





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 



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- 

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 




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). 


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 


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 

Another group of cartilaginous fishes diverged in the 


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 


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 


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. 


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. 



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 




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 

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 


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 


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 

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 


(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. 


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. 



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 



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 

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 




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 


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 


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 


(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. 


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 

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 


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. 


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. 



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 



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 
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 


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 

As to the evolution of the method of flight itself, there is 
not yet any certainty, and several theories have been pro- 

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. 

44 8 


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 " 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 


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 


fear from other animals. Their degeneracy was therefore, so 
to speak, allowed because of the greater leniency of natural 
selection in these regions. 


Heilmann, G. The Origin of Birds. Witherby, London, 1926. 
Pycraft, W. P. A History of Birds. Methuen, London, 1910. 



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 



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 

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 


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 


rj ^ <4h C O 

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1 "VI p 

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03 ,-Q 


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 


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 

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 


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. 



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. 



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. 



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 

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. 


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 


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. 

4 6 4 


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.) 


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 


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 


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 


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 


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 : 


. . 238 


.. 239° 


25 1 ° (conjectural) 

Homo neanderthalensis 

• • 253° 

Homo rhodesiensis 

. . 262 


264 (conjectural) 

Homo sapiens 

circ. 270 

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 243 

Chimpanzee . . . . . . . . . . . . . . 252 

Australopithecus . . . . . . . . . . . . 258 

Homo sapiens child . . . . . . . . . . . . 282 

It is clear, therefore, that the Taungs skull approaches the human 
condition in this respect, though still remaining similar to the 

Having now reviewed the material on which all study of 


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 238 ; in Chimpanzee 239 ; 

TJ fnP ^° PUS 25I Vo n Nea P derth al man 253°; in Rhodesian man 
62 2 , in Piltdown man 264 ; and in modern man 270 . 


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 

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 

The stage represented by Tarsius, which is also arboreal, is 
of great importance, for here for the first time the sense of 











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. 


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 


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 

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 


co-presence, the ape does not " see " the solution to the 

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 


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. 


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. 



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 

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 



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 


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. 


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 


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 


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 


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 


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 



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- 


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- 


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. 


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. 


(An asterisk denotes a totally extinct group) 


Pterobranchia, e.g. Cephalodiscus. 
Enteropneusta, e.g. Balanoglossus. 
Protochordata (Acrania). 

Ascidiacea, e.g. Ascidia. 
Thaliacea, e.g. Salpa. 
Larvacea, e.g. Fritillaria. 
Cephalochordata, e.g. Amphioxus. 

Petromyzontia, e.g. Petromyzon 

(? Palaeospondylus). 
Myxinoidea, e.g. Myxine, Bdello- 
Qstracoderma,* e.g. Cephalaspis. 

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. 

Osteolepidoti * e.g. Osteolepis, 

Coelacanthini,* e.g. Undina. 
Polypterini, e.g. Polypterus. 
Palseoniscoidei,* e.g. Cheiro- 

























4 88 


Order. Acipenseroidei, e.g. Acipenser. 

Order. Amioidea, e.g. Amia. 

Order. Lepidosteoidei, e.g. Lepido- 

Order. Teleostei, e.g. Gadus, Amiurus, 

Ipnops, Periophthalmus, 
Fierasfer, Gobiesox, Ambly- 
opsis, Lucifuga, Solea, Exo- 
ccetus, Edriolychnus. 
Dipnoi, e.g. Ceratodus, Lepidosiren, 
Protopterus, Dipterus. 
Labyrinthodontia * (Stegocephalia), 
Embolomeri, e.g. Eogyrinus, 
Urodela, e.g. Triton, Salamandra, 

Proteus, Siren. 
Anura, e.g. Rana, Pipa, Rhino- 
derma, Alytes, Hylambates. 
Gymnophiona, e.g. Ichthyophys, 
tCotylosauria * e.g. Seymouria. 
tChelonia, e.g. Testudo, Chelone, 
Sphargis, Eunotosaurus, Tri- 

Lacertilia, e.g. Lacerta, Vara- 
nus, Uromastix, Gecko, 
Anguis, Chalcides, Scincus, 
Amphisbaena, Chamaeleo, 
Ophidia, e.g. Vipera. 
Ichthyosauria, e.g. Mixosaurus, 
Ichthyosaurus, Ophthalmo- 

Rhynchocephalia, e.g. Spheno- 

don (? Thalattosaurus). 
Pseudosuchia,* e.g. Eupark- 

Thallatosuchia,* e.g. Geosau- 

Eusuchia, e.g. Crocodilus. 

t The Cotylosauria and Chelonia are often grouped together as 

















Superorder and Order. 















Grade and Subclass. 




* Subclass. 






Saurischia, e.g. Diplodocus, 

Predentata (or Ornithischia), 
e.g. Iguanodon, Stegosaurus, 
Pterosauria,* e.g. Pteranodon. 

Theromorpha,* e.g. Cynogna- 
Sauropterygia,* e.g. Nothosau- 
rus, Plesiosaurus. 

Archaeornithes,* e.g. Archseopteryx. 
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. 
Monotremata, e.g. Ornithorhyn- 

chus, Echidna. 

Marsupialia, e.g. Didelphys, 
Ccenolestes, Dasyurus, Thy- 
lacinus, Perameles, Phasco- 
larctos, Phascolomys, Pha- 
langer, Notoryctes, Thyla- 
coleo, Macropus. 
DeltatheridiidaB,* e.g. Deltathe- 
- Creodonta.* 
Fissipedia, e.g.Canis, Felis, 

Ursus, Civet, Badger. 
Pinnipedia, e.g. Seal. 
Amblypoda,* e.g. Uintathe- 




Titanotheridse,* e.g. Tita- 




















Tapiridae, e.g. Tapir. 
Rhinocerotidae, e.g. Rhino- 

Equidae, e.g. Eohippus, Me- 
sohippus, Miohippus, 
Pliohippus, Equus. 
Suidae, e.g. Sus. 
Hippopotamidae, e.g. Hippo- 
Camelidee, e.g. Protylopus, 
Poebrotherium, Proca- 
melus, Camelus. 
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- 
Hyracoidea,e.g. Hyrax (coney). 
Pro boscidea, e.g. Mceritherium , 
Palaeomastodon, Tetrabelo- 
don, Elephas. 
Sirenia, e.g. Manatus. 
Cetacea, e.g. Whale, Dolphin, 

Litopterna,* e.g. Thoatherium. 
Edentata, e.g. Bradypus, Cho- 
loepus, Armadilloe, Pango- 
Rodentia, e.g. Lepus, Mus, 

Squirrel, Porcupine. 
Inseetivora, e.g. Mole, Hedge- 
hog, Shrew, Plesiadapis,Ma- 
croscelides, Tupaia. 
Cheiroptera, e.g. Bat. 
Lemuroidea, e.g. Notharc- 

tus, Lemur. 
Tarsioidea, e.g. Tetonius, 

Platyrrhini,e.g. Marmoset. 
Parapithecidae,* e.g. 





Cercopithecidae, e.g. 
Cercopithecus, Man- 
drill, Baboon. 
Simiidse, e.g. Proplio- 
pithecus, Pliopithe- 
cus, Hylobates, Simia 
(Orang), Chimpan- 
zee, Gorilla, Austra- 
Hominidse, e.g. Pithe- 
canthropus, Eoan- 
Homo rhodesiensis, 
Homo heidelbergen- 

Homo neandertha- 

Homo sapiens. 


References to generic names are in italics. 

Abdominal pore, 37, 53, 276 
Abducens nerve, 26, 46, 355, 368, 

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, 

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, 

— 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 




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, 

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 



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, 

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, 

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 



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, 

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 



Extra-embryonic coelom, 206, 211, 

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, 


— 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 

49 8 


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, 

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, 


— 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, 

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, 


— 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, 

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 



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, 

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, 


— , 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 



Morula, 162 

Mosasauria, 321, 441 

Miillerian duct, 5s, 99, IIJ , J 3°, 

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, 

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, 

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, 

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, 


— 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 



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; 

— cervical ganglion, 156, 385 

— chamber of eye, 23 

— commissure, 41, 155 

— intestinal portal, 21 1, 230 
Postganglionic fibre, 384 



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, 


— 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 



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, 

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, 

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 



Supination, 318 
Supra-angular, 298, 300 
Supracleithrum, 71, 73, 301, 313. 

Supraoccipital, 68, 72, 300 
Supra-renal, 62, 101,401 
Suprasegmental structures, 374 
Supratemporal, 104, 300 
Surface Volume ratio, 417, 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 
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, 

Tritubercular teeth, 267, 454 
Trituberculata, 454 
Trochanter, 141 
Trochlear nerve, 26, 46, 354, 368, 

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 



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, 


— 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