THE HISTOGENESIS OF THE NERVOUS TISSUES 



317 



The Development of the Neuroglia Cells and Fibers. — The spongioblasts 

 of the neural tube (see p. 311) differentiate into the supporting tissue of the central 

 nervous system. This includes the ependymal cells, which line the neural cavity, 

 forming one of the primary layers of the neural tube, neuroglia cells and their 

 fibers. 



We have described how the strands of the syncytium formed by the spongio- 

 blasts become arranged radially in the neural tube of early embryos (Fig. 298 D). 

 As the wall of the neural tube thickens, the strands elongate pari passu and form a 



Pig. 304. — Ependymal cells from the neural tube of chick embryos: A, of first day; B, of third day. 



Golgi method (Cajal). 



radiating branched framework (Fig. 304 A, B). The group of spongioblasts 

 which line the neural cavity constitutes the ependymal layer. Processes from 

 these cells radiate and extend through the whole thickness of the neural tube to 

 its periphery. The cell bodies are columnar and persist as the lining of the 

 central canal and ventricles of the spinal cord and brain (Fig. 305). 



Near the median line of the spinal cord, both dorsally and ventrally, 

 the supporting tissue retains its primitive ependymal structure in the adult. 

 Elsewhere the supporting framework is differentiated into neuroglia cells and 

 fibers. The neuroglia cells form part of the spongioblast^ syncytium and are 



