Book - Vertebrate Zoology (1928) 20

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XX The Skull

The skull consists of the protective case round the brain (neurocranium) and of the skeletal supports of the jaws (splanchnocranium). It is formed in all chordates from Petromyzon upwards (whence the name Craniate) and is always cartilaginous at first. In Cyclostomes and Selachians the skull remains cartilaginous throughout life, but in other forms this cartilaginous chondrocranium becomes more or less thoroughly replaced by cartilage-bone, and membrane-bones are added to it. The chondrocrania of the various vertebrates may be compared with one another on the one hand, and on the other, the bony skulls may similarly be compared.

Cartilaginous Skull

The typical structure of the chondro- cranium may now be considered. The floor of the neuro- cranium is formed of paired trabecular in front (enclosing the hypophysial fenestra between them) and of paired parachordals (on each side of the notochord) behind. The auditory capsules are firmly anchored on to the parachordals on each side. Behind the auditory capsules the paired occipital arches rise up from the parachordals, and become attached to the hind part of the auditory capsule. In so doing they enclose a fenestra (metotica) through which the glossopharyngeal and vagus nerves and the internal jugular vein pass. In front of the auditory capsule paired pillars rise up from the para- chordals and join on to the orbital cartilages. The latter form the sides of the brain-case in front of the auditory capsules, and the pillars just mentioned are the pilae antoticae. The pila antotica joins the front part of the auditory capsule of its own side, and in so doing encloses the trigeminal, facial, and 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 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.

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

This diagram does not represent any particular form, but shows the type on which nearly all skulls are built, abn, abducens nerve ; ac, auditory capsule ; at, ala temporalis ; btp, basal process ; hf, hyomandibular facial nerve ; ic, internal carotid artery ; jv, jugular vein ; Ion, lamina orbito- nasalis ; 0, occipital arch ; oa, orbital artery ; oc, olfactory capsule ; ocn, oculomotor nerve ; oln, olfactory nerve ; on, optic nerve ; op, otic process ; pa, ascending process ; pf, palatine facial nerve ; pp, pila antotica ; pq, ptery go-quadrate ; pv, pituitary vein ; rop, profundus ophthalmicus nerve ; tc, trabecula ; vn, vagus nerve.

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 processes.

The processes of attachment of the upper jaw in autostylic skulls are typically three in number. The otic process abuts against the auditory capsule, and lies in front of the main branch of the facial nerve ; the basal process abuts against the floor of the neurocranium, and lies above and in front of the palatine nerve (facial) ; the ascending process rises up on the outside of the pila antotica with which it may or may not join, and lies between the ophthalmic (Vi) and the maxillary (V2) branches of the trigeminal nerves.

The autostylic vertebrates above Ceratodus are terrestrial animals which no longer use the gill-slits for respiratory purposes in the adult. So the spiracular cleft gives rise to the tympanic cavity and Eustachian tube, and the hyomandibula becomes the columella auris.

This description of the typical chondrocranium can be applied to most groups of vertebrates. In the mammals an important modification occurs in that the ascending process comes to lie between the maxillary (V2) and mandibular (V3) branches of the trigeminal nerve, and it is usually known as the ala temporalis.

Bony Skull. — Attention may now be turned to the bony skull. The replacing, or cartilage-bones, are fairly constant throughout the vertebrate series. In the neurocranium they surround the brain, the olfactory and auditory capsules ; while in the splanchnocranium they form the main skeletal 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 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.

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.

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

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 q: actuate the lower jaw. Below, the temporal cavity opens on to the palatine surface of the skull, in front of the auditory capsule, and through this opening the above-mentioned muscles pass. The roof of the temporal region typically has three borders : an 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 in the roof of the temporal cavity, it is not an opening into the brain-case.

Some reptiles have a single temporal fossa on each side. Others have a pair on each side, for which reason they are called the Diapsida. The Diapsida have a superior and an inferior temporal fossa, and these fossae are separated from the orbit by the post-orbital bar (usually formed by the post- frontal and post-orbital bones) ; they are separated from the post-temporal fossa by the post-temporal bar (supratemporal and squamosal bones) ; and they are separated from one another by the superior temporal bar (post-orbital and squa- mosal bones. The superior temporal fossa is bordered above by the parietal bone ; the inferior temporal fossa is bordered os.

  • 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.

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 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

  • Some authorities prefer to regard the Lacertilia as derived from Diapsida which have lost the inferior temporal bar.

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 mouth.

Strep tostylic skulls are also found in the birds, and especially in the parrots. Here the upper beak is freely movable relatively to the brain-case. Some of the Dinosaurs also had strep to- stylic skulls.

When the quadrate is fixed, and the upper jaw is incapable of separate movement (as in Sphenodon, crocodiles, and mammals, for instance), the skull is described as monimostylic.

In many Theromorph reptiles, as in mammals, it is common for the post-orbital bar to disappear, and the temporal fossa

Fig. 146. Hind view of the skull of Chelone, showing the rela- tions of the post-temporal fossa (through which an arrow is passed into the orbit).

Fig. 147. Hind view of the skull of Ornithorhynchus, showing the small post-temporal fossa, indi- cated by an arrow.

then becomes confluent with the orbit. In Ornithorhynchus, for example, the temporal fossa has extended upwards in the manner described above in other Synapsida, and its upper border is formed by the parietal. It is bounded behind by the squamosal, below by the squamosal and jugal (forming the zygomatic arch), and in front it has no border since it is confluent with the orbit. Behind, the temporal fossa of Ornithorhynchus communicates with a small post-temporal fossa between the squamosal and the auditory capsule (periotic bone) and which opens on the hind face of the skull. This is the last appearance of the post-temporal fossa, for in higher mammals it is obliterated, as, for instance, in the dog.

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 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 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.

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.

Fig. 150. Side views of the lower jaws of A, Varanus ; B, a dog. The mammalian lower jaw contains only one bone ; the dentary. an, angular ; ar, articular ; c, coronoid ; d, dentary ; ml, first molar ; par, supra-angular.

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 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.

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).

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. 26.1915.

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

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

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 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) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 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 | Figures | Historic Embryology

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