30 4 COMPARATIVE ZOOLOGY OF CHORDATES 



next posterior segment. The vertebrae are therefore inter- 

 segmental in position, which enables the myotomes, which of 

 course are intrasegmental, to be attached to two vertebrae. 



The most posterior haemal arches are enlarged to form the 

 hypurals which support the ventral lobe of the tail-fin. 



In the most primitive amphibia, the Embolomeri of the 

 Labyrinthodonts, the vertebrae have neural arches and two 



id n <k 



Gfi K/" 



he. pc. D D 



Fig. 153. — Origin of the vertebral column, A, in Scyllium ; B, diagram 

 showing the relations of the vertebral elements to the nerves ; C, the 

 vertebral column in the tail-region of Amia ; D, the vertebral column 

 in the Embolomerous Stegocephalia. 



dr, dorsal nerve-root ; he, hypocentrum ; id, interdorsal ; iv, inter- 

 ventral ; m, myotome ; pc, pleurocentrum ; vr, ventral nerve-root. Other 

 letters as Fig. 152. 



centra. The anterior centrum of each vertebra is formed from 

 the basiventrals and is called the hypocentrum ; the posterior 

 centrum is formed from the interdorsals and interventrals, 

 and is called the pleurocentrum. While in the later amphibia 

 the hypocentrum has been enlarged and the pleurocentrum 

 reduced, in the reptiles, birds, and mammals the opposite 



