546 



NOTOCHORD. 



in the trunk forms the basis round which the vertebral column 

 is moulded. 



The early histological changes in the cells of the notochord 

 are approximately the same in all the Craniata. There is 

 formed by the superficial cells of the notochord a delicate sheath, 

 which soon thickens, and becomes a well- 

 defined structure. Vacuoles (one or more to 

 each cell) are formed in the cells of the 

 notochord, which enlarge till the whole noto- 

 chord becomes almost entirely formed of 

 large vacuoles separated by membranous 

 septa which form a complete sponge-like 

 reticulum 'fig. 313). In the Ichthyopsida 

 most of the protoplasm with the nuclei is 

 carried to the periphery, where it forms a 

 special nucleated layer sometimes divided 



into definite epithelial-like cells (fig. 314), 



.... . FIG. 314. SECTION 



while in the meshes of the reticulum a few THROUGH THE SPINAL 



nuclei surrounded by a little protoplasm still COLUMN OF A YOUNG 



SALMON. (From Ge- 



remain. In the Amniotic Vertebrata, pro- 

 bably owing to the early atrophy of the 

 notochord, the distribution of the nuclei in 

 the spaces of the mesh-work remains fairly 

 uniform. 



genbaur.) 



L-S. sheath of noto- 

 chord ; k. neural arch ; 

 /,'. hremal arch; ;;/. 

 spinal cord; a. dorsal 

 aorta ; z'. cardinal 

 veins. 



In the early stages of development the spaces in the notochordal sponge- 

 work, each containing a nucleus and protoplasm, probably represent cells. 

 In the types in which the notochord persists in the adult the mesh-work 

 becomes highly complicated, and then forms a peculiar reticulum filled with 

 gelatinous material, the spaces in which do not indicate the outlines of 

 definite cells (figs. 315 and 318). 



Around the sheath of the notochord there is formed in the 

 Cyclostomata, Ganoidei, Elasmobranchii and Teleostei an clastic 

 membrane usually known as the membrana elastica externa. 



In most Vertebrates the notochord and its sheath either 

 atrophy completely or become a relatively unimportant part of 

 the axial skeleton; but in the Cyclostomata (fig. 315) and in the 

 Selachioidean Ganoids (Acipenser, etc.) they persist as the 

 sole representative of the true vertebral axis. The sheath becomes 

 very much thickened; and on the membrana elastica covering 



