THE NERVOUS SYSTEM 239 



Fig. 142 represents six central processes of ganglionic neuroblasts 

 entering the cord and branching as described. 



After this preliminary account of the neuroblasts we may 

 take up the development of the spinal cord, brain, and peripheral 

 nervous system. 



II. The Development of the Spinal Cord 



We have seen that the epithelial cells of the neural tube 

 stretch from the lumen of the central canal to the exterior, and 

 that the nuclei are arranged so as to leave the outer ends free, 

 thus forming the marginal velum. 



In the roof and floor the epithelial cells are relatively low, 

 and in the lateral zones much elongated. The epithelial cells 

 are added to at first by transformation of some of the germinal 

 cells; but they do not appear to multiply by division, and as 

 development proceeds they become more and more wideh^ sep- 

 arated, the interstices being filled up by neuroblasts, embryonic 

 glia cells, and fiber tracts. As the wall of the neural tube grows 

 in thickness, the epithelial cells become more and more elongated, 

 seeing that both external and internal connections are retained; 

 and, as the growth takes place mainly external to their nuclear 

 layer, the latter becomes reduced, relative to the entire thickness 

 of the neural tube, to a comparatively narrow zone surrounding 

 the central canal, and is now known as the ependyma (Fig. 143). 

 Cilia develop on the central ends of the ependymal cells in the 

 central canal, and from the outer end of each a branching process 

 extends to the periphery anastomosing with neighboring epen- 

 dymal processes so as to form a skeleton or framework enclosing 

 the other cellular elements and fibers of the central system. 



Beginning with the third day a new layer appears between 

 the nuclei of the epithelial cells and the marginal velum. This 

 layer, known as the mantle layer, is composed of neuroblasts 

 and embryonic glia cells, and represents the gray matter (Figs. 

 140 and 144). The white matter of the cord is laid down in 

 the marginal velum. The sources of the cells composing the 

 mantle layer may be twofold, viz., from the young epithelial 

 cells or from the germinal cells. According to some authors 

 young epithelial cells may be transformed into either neuroblasts 

 or neuroglia cells. Thus the form of the youngest neuroblasts 

 in Fig. 139 indicates derivation from epithelial cells, but this 



