HISTOGENESIS OF THE NERVOUS TISSUES 



307 



mammals and man. Those found in the spinal nerves arise from the small 

 cells of the spinal ganglia. 



Development of the Supporting Cells of the Neural Tube. The 



spongioblasts of the neural tube (p. 301) differentiate into the supporting 

 tissue of the central nervous system. This includes the ependymal cells 

 which line the neural cavity, and form one of the primary layers of the 

 neural tube, and neuroglia cells and their fibers. 



FIG. 312. Ependymal cells of the lumbar cord from a human fetus of 44 mm. (Cajal). 

 A, Floor plate; B, central canal; C, line of future fusion of neural walls; E, ependymal cells; 

 *, neuroglia cells and fibers. 



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

 spongioblasts become arranged radially in the neural tube of early embryos 

 (Fig. 305 D). As the wall of the neural tube thickens, the strands elon- 

 gate pari passu and form a radiating branched framework (Fig. 311). 

 The group of spongioblasts which line the neural cavity constitute 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. 312). 



