NERVOUS TISSUE. 



Ill 



FIG. 84. 



medullary tube shrink, so that spaces are present between 

 them ; the proximal ends break up into irregular branches, 

 which anastomose to form a spongy network, the neuro- 

 spongium of His (Fig. 84). At the outside of the medullary 

 tube this network forms a dense 

 spongy structure known as the 

 peripheral or marginal veil. 



Soon after the formation of the 

 medullary plate large spherical cells 

 appear between the distal ends of the 

 epithelial cells (Figs. 82 and 83). 

 These are the germinal cells (Keim- 

 zellen of His). The nature of these 

 cells is a matter of dispute. From 

 the proximal end of each of the 

 germinal cells there grows out a 

 process, which, together with the cell, 

 forms a pear-shaped structure known 

 as the neuroblast. This becomes con- 

 verted afterward into a nerve cell, 

 and the process becomes its axone. 

 The dendrites develop later on. The 



Section through the wall of a 



neuroblasts show a tendency to move neural tube of a rabbit O i derthan 

 outward to the marginal veil, where that in Fig. 83. Differentiation of 



-r i ''. i i the two ends of the epithelial cells. 



they are stopped. In the spinal cord (Barker, after His.) 



these arrange themselves parallel to 



the surface of the marginal veil, and on the ventral part of 



the cord send out processes through the marginal veil to form 



the ventral roots of the spinal nerves. The cell bodies become 



the ventral horn cells. The other neuroblasts do not send 



processes out of the cord. 



The marginal veil itself later becomes a part of the epen- 

 dyma which is present in the white matter of the whole 

 central nervous system. 



The origin of the peripheral sensory neurones is still a sub- 

 ject of much dispute. It is agreed generally that these neu- 

 rones are derived from the ectoblast at the edge of the 



