EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



397 



to the axis of the tube. They are the ependymal cells, which 

 form a supporting framework for the more important elements. 

 They reach from the lumen of the tube to its peripheral surface 

 and possess nuclei which are situated largely in the central half 

 of the cell. The peripheral parts of the ependymal cells, by 

 irregular branching and vacuolation, form a protoplasmic net- 

 work destitute of nuclei, called the marginal velum. This marginal 

 velum forms a peripheral zone which comprises the outer one- 



Fig. 115. Two histologic layers in the embryonic spinal cord, embryo 4.25 mm. 

 long. (McMurrich after His.) 



nw. Marginal velum formed by peripheral ends of ependymal cells, indifferent cells among 



their bases. 



sixth of the neural tube. The inner three-fourths or five-sixths 

 of the tube constitutes a thick central zone in which are the bodies 

 of the ependymal cells and, among their bases, a variety of cells 

 possessing large nuclei and small protoplasmic bodies (Fig. 115). 

 The destination of the latter cells cannot yet be determined, so 

 they are named the indifferent cells (Fig. 116). In the nuclei 



