56 



COMPARATIVE ANATOMY 



other hand, as in Elasmobranchs, Teleosts, Bony Ganoids, and the 

 higher Vertebrates, the vertebrae are preformed in cartilage, and 

 true articulations are always formed between them : as a rule, but 

 by no means always, the convexity is posterior and the concavity 

 anterior (proccelous form). A further difference is seen in the 

 relations of the notochord, which persists 

 intravertebrally longer than interverte- 

 brally, in this respect resembling Lepi- 

 dosteus and Reptiles. 



The configuration of the caudal region 

 of the vertebral column must also be 

 remarked upon, as it differs in tailed and 

 tailless Amphibians. The long caudal 

 portion of the vertebral column in Anuran 

 larva?, which is very similar to that of 

 Urodeles, undergoes during metamorphosis 

 a gradual retrogressive modification, and 

 the vertebra? of its proximal end become 

 fused and co-ossified to form a long, un- 

 segmented, dagger-like bone, the urostvle 

 (Fig. 45). 



Ha?mal arches are present in the 

 caudal region of Urodeles only. The 

 neural spines, as well as the transverse 

 processes, which in Urodeles are as a rule 

 bifurcated at the base and are present 

 from the second 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 attach- 

 ment to the pelvis, are particularly 

 Ob, upper arch of first ver- strongly developed, especially in the An- 

 tebra; Pa, articular ura (Fig. 45), in which the number of 

 presacral vertebra? is usually only eight. 



Articular processes (zygapophyses) are 



transverse processes of we ll developed in all Vertebrates from 

 Urodeles onwards, and consist of two 

 pairs of projections arising from the an- 

 terior and posterior edges respectively of 

 the neural arch. Their surfaces are 

 covered with cartilage and overlap one 

 another from vertebra to vertebra, and 

 in some Urodeles the neural spines also articulate with one 

 another : thus a well-articulated and mobile, chain-like vertebral 

 column results. 



The first or cervical vertebra becomes differentiated from the 

 others, and consists of a comparatively simple ring which articulates 



FIG. 45. VERTEBRAL 

 COLUMN OF Discoglossus 

 pictus. 



Ps, 



neural spine ; Pt, 



caudal vertebrae (uro- 

 style, Oc) ; 7?, ribs ; Sg, 

 condylar facets of first 

 vertebra ; 8 W, sacral 

 vertebra. 



