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 (procodous 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 

 larvae, which is very similar to that of 

 Urodeles, undergoes during metamorphosis 

 a gradual retrogressive modification, and 

 the vertebras of its proximal end become 

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

 segmented, dagger-like bone, the urostvle 

 (Fig. 45). ' 



Haemal 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 



FIG. 45. - VERTEBRAL body. The transverse processes of the 

 COLUMN OF Discoylossu* single sacral vertebra, which give attach- 

 pi ctus - 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 



s 



neural spine ; Pt, Articular processes (zygapophyses) are 

 transverse processes of well developed in all Vertebrates from 



trunk vertebra; Ptc, Uroc [eles onwards, and consist of two 



t ran verse processes of ... 



caudal vertebra (uro- pairs of projections arising from the an- 



style, Oc) ; R, ribs ; Sy, terior and posterior edges respectively of 



condylar facets of first th { h Thir surfaces are 



vertebra ; S W , sacral 



vertebra. 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 ci'rcic.td vertebra becomes differentiated from the 

 others, and consists of a comparatively simple ring which articulat es 



