THE DEFINITIVE VERTEBRA 



339 



The Composition of the Definitive Vertebra. — A fascinating 

 but difficult chapter in Vertebrate morphology is that which deals 

 with the composition of the definitive vertebra. We have already, 

 in describing the development of the cartilaginous vertebral column, 

 mentioned the elements which go to build it up — neural, haemal, 

 and central. The difficulties , of interpretation arise from the fact 

 that great variety shows itself in the ultimate fate of these elements 

 and in the manner in which they undergo fusion with their neighbours. 

 This can perhaps best be illustrated by the case of Amia as described 



■v. 



166. — Variation in vertebral column of Aima. 

 according to Schauinsland (1906). 



A, tail region of a 7-5 ram. larva ; B, posterior trunk region 

 of an 18 cm. specimen ; C, mid trunk region (18 cm.) ; D, anterior 

 trunk region (18 cm.). Fig. A is more highly magnified than 

 B, C, and D. The position of the boundaries between success- 

 ive myotomes or segments is indicated by the intersegmental 

 blood-vessels (v). A, B, neural arch-elements ; a, 6, haemal 

 arch do. ; a, jS, central do. 



by Schauinsland. Here in some cases two amphicoelous centra 

 (a and fi) are developed corresponding to a single segment, each one 

 carrying its pair of neural and pair of haemal elements, those attached 

 to the anterior centrum {A and a) being relatively small, those on 

 the posterior centrum (B and b) on the other hand well developed. 

 Variations from this diagrammatic arrangement are found in different 

 parts of the body. 



In the tail region (Fig. 166, A) the original condition frequently 

 persists although in aged individuals the arch-elements (A, a) of the 

 anterior vertebra of the segment are liable to become completely over- 

 grown and hidden by bone. On the other hand there frequently 



