STRUCTURE OF THE VERTEBRATES 



153 



posterior. The caudal vertebrae of the human are fused to form 

 a terminal process, the coccyx. 



Embryology of the Vertebrae. The developmental history 

 of the vertebrae varies in the different classes of vertebrates. 



[nlage of intercentrum 



of neurocentrum 



A. Early Stage 



' Notochord 



\ 



B. Later Stage 

 nlage of neurocentrum^ 



A'. Cross section of "A' 



B'. Cross section of "B" 



Intercentrum 



Jeurocentruml 



Fig. 88. Embryology of the Vertebrae. (A) and (B) show the isolated 

 cartilages forming rings around the notochord, equivalent to the inter- 

 centrum and neurocentrum. Drawings (C) and (D) show the intercentra 

 being crowded ventrally during development, and lost in the definitive 



structure (E). 



The following is a generalized description. The vertebrae first 

 appear as eight minute cartilages in each body segment, forming 

 two incomplete rings around the notochord. As these cartilages 

 grow they fuse into two rings, thus parallelling the fossil history. 

 The more anterior ring of each segment enlarges, developing a 

 neural arch, and pushes anteriorly across the myoseptum. In 



