VERTEBRAL COLUMN 45 



vertebra onwards, show the greatest variety as regards shape and 

 size, differing in the several regions of the body. The transverse 

 processes of the single sacral vertebra, which give attachment 

 to the pelvis, are particularly strongly developed, especially in the 

 Anura (Fig. 34). 



Articular processes (zygapophyses, comp. p. 40) are well de- 

 veloped in all Vertebrates from TJrodeles onwards, and consist of 

 two pairs of projections arising from the anterior and posterior 

 edges respectively of the neural arch. Their surfaces are covered 

 with cartilage, and overlap one another from vertebra to vertebra 

 like tiles on a roof: not unfrequently, in Urodeles, the "neural 

 spines also articulate with one another, and thus a well- articulated 

 and mobile chain-like vertebral column results. 



The first vertebra (and this is the only cervical vertebra of 

 Amphibia), becomes differentiated from the others, and consists of 

 a simple ring which articulates with the two condyles of the skull, 

 and also with the base of the latter by means of a more or less 

 marked process often spoken of as the " odontoid " process (Fig. 

 34) ; thus a freer movement between the skull and vertebral column 

 is rendered possible. This vertebra, however, is not homologous 

 with the first vertebra (i.e., the atlas) of the higher Vertebrates, 

 as is demonstrated by a study of its development, which shows 

 that the real atlas loses its individuality as a separate mass, and 

 becomes united with the occipital region of the skull. 1 The 

 first vertebra of Amphibians is therefore more nearly comparable 

 to one of the next following cervical vertebrae of higher forms. 

 It possesses posterior zygapophyses only, and its condylar facets 

 -correspond to modified transverse processes. 



The number of vertebrae present in Urodeles is inconstant, and varies 

 greatly : it may reach to nearly 100 (Siren), and in Csecilians may be very 

 much greater (up to 275). In Anura, on the other hand, there are only 

 oight precaudal vertebras and one sacral, in addition to the urostyle. It is 

 evident from this fact alone that the recent forms of Urodela and Anura are 

 widely separated from one another. 



Reptilia. In contrast to the numerous fossil forms, only a few 

 existing Reptiles, viz., Hatteria (Rhynchocephala) and the Geckos 

 (Ascalabota), retain throughout life the primitive biconcave char- 

 acter of their vertebrae, with the notochord expanded interverte- 

 brally. 



In the generalised Rhynchocephala the formation of the vertebrae out of 

 several pieces, such as occurs amongst the Stegocephala (p. 44), is still indicated 

 by sutures, each vertebra consisting of two processes, a centrum proper 

 (pleurocentrum) and an iiitercentrum. 



In all the others, the notochord remains expanded longer in the 

 intravertebral regions than intervertebrally, but in the adult it be- 



1 A similar fusion of the anterior part of the vertebral column with the skull 

 occurs in some Fishes and in Dipnoi. ' 



