450 



NOTOCIWHI). 



Fig. 313. Horizontal section theough 

 the trunk of an embryo of scylllum con- 

 siderably younger than f in fig. 28. 



The section is taken at the level of tlie 

 notochord, and shews the separation of the 

 cells to form the vertebral bodies from the 

 muscle-plates. 



c/j. notochord; pjj. epiblast ; TV. rudiment 

 of vertebral body; mp. muscle-plate; mp'. 

 portion of muscle-plate already diilerentiated 

 into longitudinal muscles. 



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 

 notochord becomes almost en- 

 tirely formed of large vacuoles 

 separated by membranous septa 

 which form a complete sponge- 

 like reticulum (fig. 313). In the 

 Ichthyopsida most of the proto- 

 plasm with the nuclei is carried 

 to the periphery, where it forms 

 a special nucleated layer some- 

 times divided into definite epi- 

 thelial-like cells (fig. 314), while 

 in the meshes of the reticulum 

 a few nuclei surrounded by a 

 little protoplasm still remain 

 In the Amniotic Vertebrata, 

 probably owing to the early 

 atrophy of the notochord, tlie 



distribution of the nuclei in the spaces of the mesh-work remains 



fairly uniform. 



In the earJy stages of development the spaces iu 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 reti- 

 culum 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, Elas- 

 mobranchii and Teleostei an elastic 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 per- 

 sist as the sole representative of the triie vertebral 

 axis. The sheath becomes very much thick- 

 ened ; and on the membrana elastica covering it 

 the vertebral arches directly rest. In Elasmo- 

 branchii the sheath of the notochord undergoes 

 a more complicated series of changes, which re- 

 sult first of all in the formation of a definite 



Fig. 314. Section 

 through the spinal 

 column of a young 

 Salmon. (From Ge- 

 genbaur.) 



ct. sheath of noto- 

 chord ; k. neural arch ; 

 k'. haemal arch ; m. 

 spinal cord ; a. dorsal 

 aorta ; t'. cardinal 

 veins. 



