THE NERVOUS SYSTEM 230 
A narrow non-nucleated margin, known as the marginal 
velum, appears in the lateral walls of the neural tube external 
to the nuclei (Fig. 188). This is occupied by the outer ends of 
the epithelial cells. At this time, therefore, three zones may 
be distinctly recognized in the walls of the neural tube, viz., 
(1) the zone of the germinal cells, including also the inner ends 
of the epithelial cells, (2) the zone of the nuclei of the epithelial 
cells, (8) the marginal velum. No distinctly nervous elements 
are yet differentiated. 
Such elements, however, soon begin to appear: Fig. 139 repre- 
sents a section through the 
cord of a chick embryo of 
about the end of the third day; 
it is from a Golgi preparation 
in which the distinctly nervous 
elements are stained black, and 
the epithelial and germinal 
cells are seen only very indis- 
tinctly. The stained elements 
are the neuroblasts, and it will 
be observed that they form a 
layer roughly intermediate in 
position between the marginal 
Fia. 139.— Transverse section through 
the spinal cord and ganglion of a 
velum and the nuclei of the chick about the end of the third 
epithelial cells. They are day; prepared by the method of 
usually regarded as derived Golgi. (After Ramon y Cajal.) 
i C., Cones of growth. Nbl.1, 2, 3, 4, 
from germinal cells that have  Neuroblasts of the lateral wall’ (1 and 
migrated from their central 2); of the spinal ganglion (3); of the 
es es ventral horn (motor neuroblasts) (4). 
position outwards; but it is 
possible that some of them may have been derived from epithelial 
cells. However this may be in such an early stage, it is certain 
that the neuroblasts formed later are derived from germinal cells. 
It will be observed that each neuroblast consists of a cell- 
body and a process ending in an enlargement. The process 
arises as an outgrowth of the cell-body, and forms the axis cylin- 
der or axone of a nerve-fiber; the terminal enlargement is known 
as the cone of growth, because the growth processes by which 
the axone increases in length are presumably located here. It 
may be stated as an invariablerule that each axone process of a 
medullary neuroblast arises as an outgrowth, and grows to its 
