308 HISTOGENESIS ' 



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 neuro- 

 lia cells and fibers. The neuroglia cells form part of the spongioblastic 

 syncytium and are scattered through the mantle and marginal layers of the 

 neural tube. By proliferation 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 processes of the neuroglia cells, and, as the latter primarily 

 form a synctium, the neuroglia fibers may extend from cell to cell. The 

 neuroglia fibers develop late in fetal life and undergo a chemical transfor- 

 mation into neurokeratin, the same substance that is found in the sheaths 

 of myelinated fibers. 



