150 STRUCTURE OF THE VERTEBRATES 



tail vertebrae of some recent fish. Considering these facts, and 

 the embryology of the vertebrae, it becomes evident that the 

 primitive animals had two vertebrae to each body segment. 

 Wherever the condition is found the anterior vertebra has a 

 neural arch and is called the neurocentrum; and the posterior 

 is the inter centrutn. This structure of the vertebrae was car- 

 ried over from the early Crossopterygii to the Stegocephalian 

 amphibian. 



The more primitive fossil amphibia have the two alternating 

 vertebrae, but the later fossils show a forward growth of the 

 neurocentrum, with the intercentrum being pushed ventrally 

 to form a triangular element. The. mammal-like reptiles carried 

 this destruction of the intercentrum a step farther, the inter- 

 centrum being very small. In the mammals the intercentrum is 

 entirely crowded out in the body region, but remains in the 

 caudal region of many species as the small chevron bones. 



With the anterior growth of the neurocentrum, and its en- 

 largement, the position of the vertebrae has changed. Originally 

 the vertebrae lay between the myosepta, but the forward growth 

 carried each into the more anterior body segment so that the 

 myoseptum crosses its middle portion. In this way the vertebral 

 number has been reduced to half, although metamerism has not 

 been lost. Observe, however, that each body segment contains 

 two halves of vertebrae, the posterior half of one, and the ante- 

 rior half of another. 



Above the fish the vertebrae displace the notochordal tissue 

 completely, and articulate against each other. This is usually a 

 ball and socket joint, a rounded projection of one vertebra fitting 

 into a concavity of the adjacent one. The concavity may be 

 either on the anterior or posterior face. The mammals have an- 

 other modification, both faces of the centrum being fiat. Move- 

 ment is assisted by a disc of cartilage lying between the two 

 faces. 



Accessory processes are found in most groups of the verte- 

 brates. In the reptiles these are rather simple. The mammal 

 vertebrae are more complex. From the neural arch develop ante- 

 rior and posterior zygapophyses, the anterior articulating with 

 the posterior ones of the vertebra in front of it. This permits 

 flexibility, but gives added strength. 



