THE NERVOUS SYSTEM. 



477 



as if due to the greater thickening of the ventral walls (basal plates). The 

 increase of the mantle layer (gray) of the basal plate marks the beginning of 

 the ventral (anterior) gray column or horn. The increase in the basal plate 

 may be partly due to neuroblasts migrating from the alar plate. These 

 would be intermediate neurones. The development of the mantle layer at 

 the expense of the inner layer, due to differentiation and migration of the cells 

 of the latter, is well shown, but is more marked in the following stages. 



As already mentioned, the axones of the heteromeric cells, many of which 

 lie in the dorsal half of the lateral walls, after decussating (anterior commit- 



Beginning of 

 dorsa. > : uniculus \/ 



Dorsal root5/f 

 Mantle layer'' 



Meningeal 

 ' membrane 



Ventral root 

 r.eurcblasts 

 of mantle layer) 



FIG. 404. Half of a transverse section of the spinal cord of a 4 weeks, (6.9 mm.) human embryo. 

 Dp, Roof plate; Bp, floor plate. His. 



sure), form longitudinal fibers in the marginal layer along the ventral surface 

 of the opposite side, mostly mesial to the emerging ventral roots (Fig. 44) 

 These longitudinal fibers are the beginning of the ventral (anterior) white columns 

 or funiculi of the cord. The sides of the tube between the dorsal and ventral 

 roots contain at first only a few longitudinal fibers the beginning of the ventro- 

 lateral juniculi. Their number soon rapidly increases, the fibers apparently 

 coming from ventrally located tautomeric cells. The dorsal root fibers, as 

 stated before (p. 460), form small round bundles in the marginal layer of the 

 dorsal halves (Fig. 404). This is the beginning of the dorsal (posterior) white 

 columns or funiculi,. 



