Fig. 313. Horizontal section through the trunk of an embryo of Scyllium considerably younger than F in fig. 28.
The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates.
ch. notochord; ep. epiblast; Vr. rudiment of vertebral body; mp. muscle-plate; mp´. portion of muscle-plate already differentiated into longitudinal muscles.
The Notochord. The early formation of the notochord has already been described in detail (pp. [292]-300). It is developed, in most if not all cases, as an axial differentiation of the hypoblast, and forms at first a solid cord of cells, without a sheath, placed between the nervous system and the dorsal wall of the alimentary tract, and extending from the base of the front of the mid-brain to the end of the tail. The section in the region of the brain will be dealt with by itself. That 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 notochord 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]), 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, the distribution of the nuclei in the spaces of the mesh-work remains fairly uniform.
Fig. 314. Section through the spinal column of a young Salmon. (From Gegenbaur.)
cs. sheath of notochord; k. neural arch; k´. hæmal arch; m. spinal cord; a. dorsal aorta; v. 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 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 persist as the sole representative of the true vertebral axis. The sheath becomes very much thickened; and on the membrana elastica covering it the vertebral arches directly rest. In Elasmobranchii the sheath of the notochord undergoes a more complicated series of changes, which result first of all in the formation of a definite unsegmented cartilaginous tube[199] round the notochord, and subsequently (in most forms) in the formation of true vertebral bodies.