THE NERVOUS SYSTEM 



235 



A narrow non-nucleated margin, known as the marginal 

 velum, appears in the lateral walls of the neural tube external 

 to the nuclei (Fig. 138). 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, (3) 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 

 velum and the nuclei of the 

 epithelial cells. They are 

 usually regarded as derived 

 from germinal cells that have 

 migrated from their central 

 position outwards; but it is 



m/3^ 



MM^- 



mi.4. 



Fig. 139. — Transverse section through 



the spinal cord and ganglion of a 



chick about the end of the third 



day; prepared by the method of 



Golgi. (After Ramon y Cajal.) 



C, Cones of growth. Nhl. 1, 2, 3, 4, 



Neuroblasts of the lateral wall (1 and 



2); of the spinal ganglion (3); of the 



ventral horn (motor neuroblasts) (4). 



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 invariable rule that each axone process of a 

 medullary neuroblast arises as an outgrowth, and grows to its 



