VERTEBRAL COLUMN. 



39 



Amphibia. The vertebral column of Urodeles may be 

 divided into cervical, thoraco-lumbar, sacral, and caudal 

 regions, and these regions can be recognised, except in certain 

 modified forms such as Snakes, from Urodeles up to the Mam- 

 malia. On account of the absence of extremities in Csecilians, the 

 vertebral column can only be divided into three regions, cervical, 



FIG. 29. TAIL OF Protoptcrus. 



thoracic, and a very short caudal. In Anura, no special lumbar 

 region can be recognised, and the caudal portion is modified to 

 form a " urostyle " (see p. 42). 



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

 undergoes vertebral constrictions, while intervertebrally it grows 

 thicker, and accordingly appears expanded. Thus the vertebrae here 



FIG. 30. TAIL OF Lcpidosteus. 



also are amphicrelous. Later, intervertebral masses of cartilage 

 become developed, which, together with the bone which is formed 

 at the same time in the surrounding connective-tissue, extend in- 

 wards towards the centre, gradually constricting the notochord so 

 that eventually it may become entirely obliterated. Finally a dif- 

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



