THE BRAIN. 



407 



the ectoderm while all other supporting tissues are 

 derived from the mesoderm. The development of 

 neuroglia cells is closely associated with the origin of 

 neurons, as both are derived from epithelial cells that 

 lie primarily near the central canal of the nervous 

 system. The embryonic neuroglia cells are called 

 spongioblasts, while those that develop into true 

 nerve elements are 

 called neuroblasts. 

 Both kinds pass out 

 very early and per- 

 manently lodge in 

 the gray matter. 



The neuroglia ele- 

 ments appear as 

 cells with many ra- 

 diating, slender pro- 

 cesses that are usu- 

 ally unbranched, 

 and because of 

 their peculiar ap- 

 pearance have been 

 called spider cells or 

 mossy cells, or astro- 

 cytes. The cell 

 bodies contain but 



very little protoplasm, and their shape is modi- 

 fied according to their surroundings, being triangu- 

 lar, or quadrangular, or polyangular, the protoplas- 

 mic processes arising from their angles. According 

 to the length of their processes attempts have been 

 made to classify them as short-rayed astrocytes, pos- 



Fig. 281. Neurogliar cells: a, from 

 spinal cord of embryo cat; b, from brain 

 of adult cat; stained in chrome-silver 

 (Huber). 



