44 



COMPARATIVE ANATOMY 



Oe 



vidual vertebrae by means of the intervertebrally expanded 

 notoctiord ; (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 



the Stegocephala of the Carboniferous 

 period, as well as the Perennibranchiata r 

 Derotremata, and many Salamanders, 

 possess simple biconcave vertebras without 

 differentiation of definite articulations. 1 



The bony parts of the vertebrae of 

 Urodeles are not formed from the carti- 

 laginous sheath of the notochord, but in 

 the surrounding connective-tissue, there 

 being only an intervertebral cartilaginous 

 zone, extending into the ends of the centra. 

 In the Anura, on the other hand, as in 

 Elasmobranchs, Teleosts, bony Ganoids r 

 and the higher Vertebrata, the vertebrae 

 are preformed in cartilage, and true arti- 

 culations always arise between the 

 vertebrae : as a rule the convexity is 

 posterior and the concavity anterior (pro- 

 ccelous form). A further difference is 

 seen in the relations of the notochord, 

 which persists intravertebraliy longer 

 than intervertebrally, a condition which 

 leads towards the Reptiles. 



The configuration of the caudal region, 

 of the vertebral column must also be re- 

 marked upon, as it differs in tailed and 

 tailless Amphibians. The long caudal 

 portion of the vertebral column in Frog 

 larvae, which is very similar to that of 

 Urodeles, undergoes during metamor- 

 phosis a gradual retrogressive change, 

 and the vertebrae of its proximal end 

 become fused together and ossified to form 

 a long unsegmented dagger -like bone, the 

 urostyle (Fig. 34). 



Both neural and haemal arches arise 

 in direct continuity with the centra. 

 Haemal arches are, however, present in the caudal region of 

 Urodeles only. 



The neural spines, as well as the transverse processes, which are 

 as a rule bifurcated at ttie base and are present from the second 



1 In certain of the Stegocephala incomplete hoops of bone, the inter- and 

 pleuro-centra, twice as numerous as the arches, surrounded the persistent notochord.. 



FIG. 34. VERTEBRAL 

 COLUMN OF Disco<jlo*u 

 pic t us. 



Pa, articular processes ; 

 Pa, neural spine ; Pt, 

 transverse processes of 

 trunk vertebrae ; Ptc, 

 transverse processes of 

 caudal vertebras (uro- 

 style, Oc) ; 8 W, sacral 

 vertebra ; Ob, upper 

 arch of first vertebra ; 

 Sy, its condylar facets ; 

 Po, its anterior pro- 

 cess ; R, ribs. 



