IV. VERTEBRATA 455 



In Amphibia, reptiles and birds intervertebral cartilages develop between the 

 centra, also constricting the chorda. In order that the column shall have the 

 necessary flexibility, joints arise in the intervertebral cartilage in different man- 

 ners: (a) Oplsthocxle vertebras have a socket on the hinder surface which 

 receives the convex anterior end of the succeeding centrum, forming a ball-and- 

 socket joint, ib) Proaeloiis vertebrce have these relations reversed, the socket 

 being in front, (c) The vertebrae may articulate by a 'saddle joint' (birds). 

 If the intervertebral cartilage become rudimentary, the amphiccclous condition 

 reappears, (d) Between two successive vertebrae an elastic intervertebral 

 ligament may occur (mammals). The neurapophyses may bear, in addition 

 to the transverse processes, anterior and posterior articular processes {zygapo- 

 physes) connecting the separate vertebra:. 



The skull, the anterior continuation of the axial skeleton, occurs in all 

 vertebrates; it appears before the vertebra?, for it is found in the cyclos- 

 tomes, which lack these. It surrounds the brain as the vertebrae do the 

 spinal cord; and, like them, its first stages are formed in the skeletogenous 

 layer surrounding the anterior end of the notochord. It is so related to 

 the surrounding parts that it may in general be said to l)e equivalent or 

 homodynamous with the vertebra:, although we cannot agree with Oken 

 and Goethe, the founders of the 'vertebrate theory of the skull,' that it 

 lias arisen by the fusion of vertebrae. On the other hand skull and verte- 

 brae are parts arising in the common basis of the skeletogenous layer, but 

 which have developed in different directions. The vertebral column is 

 metameric since the segmental muscles attached to it would otherwise be 

 ineffective. The cranium is a continuous capsule, because the most 

 important sense organs are on the head and they prevent the development 

 of locomotor muscles. Many facts of anatomy and development, especi- 

 ally the relations of the nerves (p. 472), tend to show that one part of the 

 skull, the palceocranium, has no relation to vertebra:. This alone is 

 found in cyclostomes. In other vertebrates this is joined by the occipital 

 region {neocranium) of vertebra; secondarily fused with the palaeocranium. 



Three stages are recognized in the development of the skull: the 

 membranous, the cartilaginous cranium {cliondrocranlum), and the bony 

 skull. The first, which consists of connective tissue, occurs only in the 

 early embryonic stages, scarcely a trace of it persisting in the adults. 

 It is early replaced by the cartilaginous skull, which may persist unaltered 

 throughout life in the lower fishes (elasmobranchs, sturgeon). In most 

 vertebrates, however, ossification sets in, embracing a part (fishes, am- 

 phibians) or practically the whole of the cartilage (birds, mammals), con- 

 verting it in the latter case into a bony capsule. In the bony skull two 

 kinds of bone, primary and secondary, are recognized, these var)dng in 

 their origin. The primary or cartilage hones develop from the cartilage 

 itself. The secondary or membrane bones are, in their origin, foreign to the 



