THE NERVOUS SYSTEM. 



515 



At four weeks there are blood vessels in the mesodermal tissue surrounding 

 the neural tube. Branches of these soon penetrate the tube itself. 



From its first appearance in the cord as an oval bundle, during the fourth 

 week, the dorsal funiculus steadily increases in size, forming a "root zone" in 

 the marginal layer of the dorsal half, but not reaching the roof plate (Fig. 443). 

 This increase in size is probably produced in part by the addition on its 

 inner side of overlapping ascending arms of dorsal root fibers from lower 



Partly differentiated mantle layer 

 Mantle layer 



Dorsal funiculus 

 (post, white column) * 



Lateral gray 

 column (lat. horn) 



Ventral root 



FIG. 443. Half of a transverse section of the spinal cord of a 4^ weeks (10.9 mm.) human embryo. His. 

 A.s., Artery in ventral longitudinal sulcus; A.sp. a., ventral (anterior) spinal artery; Bp, floor plate; 

 Dp, roof plate; 1. 1., inner layer. The faint inner outline is the outline of the cord proper. 



cord segments. The mantle layer of this part contains an increasing number 

 of cells forming curved or arcuate fibers. (Fig. 443.) The increase in the 

 mantle cells of the dorsal part marks the beginning of the dorsal (posterior) 

 gray column or horn (terminal nucleus of the dorsal root fibers). Later, other 

 cells become differentiated from the inner layer which do not apparently form 

 arcuate fibers (Fig. 443) and which subsequently become part of the posterior 

 horn. It is possible that the axones of some of these cells form the compara- 



