148 STRUCTURE OF THE VERTEBRATES 



C. Vertebrae 



The vertebral column is part of the axial skeleton. The indi- 

 vidual vertebrae develop from mesodermal tissues which are laid 

 down around the endodermal notochord. The latter becomes 

 constricted in the elasmobranch fishes, and completely obliter- 

 ated in the adults of the higher vertebrates. 



Cyclostomes have the most primitive vertebrae of any class. 

 Each vertebra is composed of six minute cartilages, three on 

 either side, which have no connection with each other. The verte- 

 brae are separated from each other by the myoseptum. It is 

 difficult to ascribe any function to these discrete elements. 



The Elasmobranchs have well developed, cartilaginous verte- 

 brae which are biconcave and constrict the notochord in the 



J 



Vertebra^ ^.'oiochord. ^^<-^^r<T7x ^'^^^^^rc^ ^Notochord 



V^ 



A. Cyclostome B. Dogfish C. Dogfish (sagittal section) 



Fig. 85. Cartilaginous Vertebrae. The Cyclostome vertebrae are isolated 

 metameric cartilages; the Dogfish (B) has centra surrounding the noto- 

 chord and a neural arch enclosing the nerve cord; (C) shows the centra 

 constricting the notochord, leaving large discs of notochordal tissue between. 



middle of each vertebra. The notochord is, however, continu- 

 ous, being shaped in the adult like a string of beads — a narrow 

 neck through the center of each vertebra, and a lens-shaped 

 area fitting into the cavities of the two contiguous vertebrae. The 

 circular part of the vertebra which surrounds the notochord is 

 the centrum, and is evidently the most ancient part of the verte- 

 bra. Evidence of this is found in a small fossil (Paleospondylus). 

 The Elasmobranchs have also developed processes for pro- 

 tection of the nerve cord and for the attachment of muscles. On 

 the dorsal side of each vertebra is an arch made of a pair of 

 processes, through which the nerve cord passes. This is the 

 neural arch of the vertebrae, and is found in all classes above the 

 cyclostomes. In the Elasmobranch there are inter-neural car- 

 tilages which, with the neural arch, completely enclose the neural 

 canal except where it is pierced by spinal nerves. On the lateral- 



