54 COMPARATIVE ANATOMY 



As a rule Elasmobranchs and Ganoids possess a greater number 

 of vertebras (up to nearly 400) than Teleosts, in which we seldom 

 meet with more than 70 : the Eel, however, possesses more than 

 200, while amongst the Plectognathi there may be as few as 15. 



The tendency towards a fusion of the various components such 

 as occurs in the ossified vertebras of Bony Ganoids and Teleosts is 

 also seen in the Amphibia and Amniota, the notochord being of 

 less importance and the vertebrae becoming more consolidated and 

 secondarily modified in various directions. Thus the homology of 

 the different elements of which they are composed can only be 

 traced by a study of their development ; but even in the adult, 

 parts are frequently present which recall the primarily composite 

 nature of the vertebras, as will be seen in the following pages. 



Amphibians. Amongst Amphibians, the vertebral column is 

 more or less distinctly differentiated into cervical, thoraco-himbar, 

 sacral, and caudal regions, and these regions can be recognised, 

 except in certain modified types, in all the higher Vertebrates. On 

 account of the absence of extremities in Cascilians, there is no sacral 

 region, and in Anura, the caudal portion is modified to form a 

 urostyle (Fig. 45). 



The notochord of Urodele larvae, like that of most Fishes, 

 undergoes intravertebral constrictions, while intervertebrally it 

 remains thicker, and accordingly appears expanded. Thus the 

 centra here also are ampliicoelous. In the course of their develop- 

 ment, a gradual reduction of the cartilage may be observed, and 

 the bone, originally perichondral in origin, becomes correspond- 

 ingly independent (Fig. 44). The cartilage is more and more 

 limited on the one hand to the arches (Fig. 52), and on the other 

 to the intervertebral regions round the notochord, extending to a 

 greater or less degree into the anterior and posterior ends of the 

 individual bony vertebral bodies, thus constricting or even entirely 

 obliterating the notochord in these regions. The bony centra are 

 formed from the bases of the arches, which, before ossification, 

 only rest on the notochord and do not enclose it. Finally a 

 differentiation, as well as a resorption, extending inwards from the 

 periphery, occurs in these cartilaginous parts : in the interior of 

 each an articular cavity is formed, so that in the vertebras of many 

 higher Urodeles an anterior convexity and a posterior concavity 

 may be distinguished, both covered with cartilage ; they are, 

 therefore, opisthocodous (Fig. 44). 



In the development of the vertebral column of Urodeles we 

 can thus distinguish three stages : (1) A connection of the indi- 

 vidual vertebras by means of the intervertebrally expanded 

 notochord; (2) a connection by means of intervertebral masses 

 of cartilage ; and finally (3) an articular connection. These 

 three different stages of development find a complete parallel 

 in the phylogeny of tailed Amphibians, inasmuch as many of 



