334 



NORMAL HISTOLOGY. 



cesses, whose ramifications later present such striking pictures, are 

 subsequently acquired, after the lapse of considerable time. 



FIG. 360. 



Portion of wall of neural tube, exhibiting germ-cells (g) among the differentiating 

 spongioblasts (s). (After His.) 



The development of the neuroglia depends upon the special- 

 ization of the columnar elements, the 

 spongioblasts. The epithelial cells 

 elongate, their protoplasm at the same 

 time undergoing vacuolation and 

 partial absorption, resulting in the 

 production of an elongated framework 

 of connected slender trabeculae. The 

 extremities of the changed epithelial 

 elements, or spongioblasts, greatly dif- 

 fer ; the inner ends of the cells extend 

 to the inner boundary, where they are 

 united to form a continuous sheet, the 

 membrana limitans interna, and the 

 outer processes break up into irregular 

 branches, which ultimately form a close 

 reticulum. The early spongioblasts ex- 

 tend the entire thickness of the neural 

 wall, but with the subsequent increase in 

 this structure their attachments become 

 broken, the spongioblasts then lying free 

 among the surrounding nervous elements. 

 The general growth of the tissues is ac- 

 companied by great extension and sub- 

 division of the neuroglia fibres, which 

 eventually become the nucleated masses of fine, bristle-like processes 



Spongioblasts from neural tube ; 

 their expanded upper ends unite to 

 constitute the internal limiting mem- 

 brane next the brain-cavity ; their 

 outer ends break up into reticulum. 

 (After His.) 



