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. THrt 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 widely 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. 
139 and 140). 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 
