VERTEBRAL COLUMX. 41 



different stages of development find a complete parallel in the 

 phylogeny of tailed Amphibians, inasmuch as all fossil forms, e.g. 

 the Stegocephala of the Carboniferous Period and the Labyrintho- 

 donts, as well as the Perennibranchiata, Derotremata, and many 

 Salamanders, possess simple biconcave vertebrae, without differen- 

 tiation into definite articulations. 



The bony parts of the vertebra? of Urodeles are not formed from 

 the cartilaginous sheath of the notochord, but in the surrounding 

 connective-tissue, there being only an intervertebral cartilaginous 



FIG. 32. VERTEBRAL COLUMS OF Discoglossus pidus. 



Pa, articular processes ; Ps, spinous processes ; Pt, transrerse processes of trunk 

 vertebra ; Ptc, transverse processes of caudal vertebrae (urostyle, Oe) ; S W, 

 sacral vertebra ; Ob, upper arch of first vertebra ; Sg, its lateral articular 

 surfaces ; Po, its anterior process ; R, ribs. 



zone, extending into the ends of the centra. In the Anura, on the 

 other hand, as well as in Elasmobranchs, bony Ganoids, and the 

 higher Vertebrata, the vertebra are preformed in cartilage. 

 In the Anura true articulations are always formed between the 

 vertebrae, and, as a rule, the convexity is posterior, and the concavity 

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

 tions of the notochord, which persists verteb rally longer than 

 iitervertebrally, a condition which leads towards the Reptiles. 

 The configuration of the caudal region of the vertebral column must 



