3o8 



HISTOGENESIS 



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. 312). 



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

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

 where the supporting framework is differentiated into neuroglia cells and fibers. 

 The neurogha cells form part of the spongioblastic syncytium and are scattered 

 through the mantle and marginal layers of the neural tube. By proliferation 



Fig. 312. — Ependymal cells of the lumbar cord from a human fetus of 44 mm. (Gol^ method, 

 Cajal). A, Floor plate; B, central canal; C, line of future fusion of walls of neural cavity; E, ependy- 

 mal cells; *, neuroglia cells and fibers. 



they increase in number and their form depends upon the pressure of the nerve 

 cells and fibers which develop around them. 



Neuroglia fibers are differentiated (in a manner comparable to the formation 

 of connective tissue fibers, Fig. 291) from the cytoplasm and cytoplasmic proc- 

 esses of the neuroglia cells, and, as the latter primarily form a syncytium, the 

 neuroglia fibers may extend from cell to cell. The neuroglia fibers develop late 

 in fetal life and undergo a chemical transformation into neurokeratin, the same 

 myelinated substance which is found in the sheaths of medullated fibers. 



