EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



399 



are occurring in the efferent part of the neuroblast, other processes, 

 called dendrites, are given off by the cell-body. They belong to 

 the afferent side of the cell. There are usually several dendrites. 

 They branch richly close to the cell-body and terminate, as a 

 rule, within the mantle layer. The spongioblasts are the smaller 

 of the indifferent cells. They are primitive neuroglia cells (Fig. 

 53). The spongioblasts develop many very slender and richly 

 branched processes of ray-like or arborescent form, which by their 

 interlacement constitute a delicate supporting reticulum. The 

 neurones are held in the meshes of this reticulum, and both the 

 neurones and neuroglia cells are still further supported by a coarse 

 mesoblastic framework produced by the ingrowth of blood vessels. 

 Zones (Figs. 117 and 127). While the neurones and neuroglia 

 cells are forming, there occurs in each lateral wall of the neural 



Fig. 117. Transverse section of the cervical part of the spinal cord of a human 

 embryo of six weeks. (Gordinier after Kolliker, from Quain.) 



c. Central canal, e. Its epithelial lining, e. (Superiorly) The original place of closure of 

 the canal, a. The white substance of the anterior columns, g. Gray substance of anterior 

 columna. p. Posterior column, ar. Anterior roots, pr. Posterior roots. 



canal a longitudinal evagination. This evagination produces a 

 longitudinal groove on the ventricular surface of the neural tube, 

 which divides it into a ventral and a dorsal zone. The ventral 

 zone is largely efferent in function, hence from it issue the out- 

 going tracts (Figs. 127 and 128). It is composed of a median 

 floor plate and, on either side, of a basal, or ventral lamina. The 

 dorsal zone is made of up two alar, or dorsal laminae joined by 



