324 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



(Fig. 327). In a 65 mm. (CR) fetus the persisting cavity is becoming 

 rounded (Fig. 328). It forms the central canal of the adult spinal cord. 

 The cells lining the central canal are ependymal cells proper. Those in the 

 floor of the canal form the persistent floor plate. Their fibers extend ven- 

 trad, reaching the surface of the cord in the depression of the ventral median 

 fissure. 



When the right and left walls of the ependymal layer fuse, the epen- 

 dymal cells of the roof plate no longer radiate, but form a median septum 

 (Fig. 327). Later, as the marginal layers of either side thicken and are 



Dorsal median septum Fasciculus gracilis 



Dorsal root^^_. ^ XL^-r- ^_ .Fasciculus cuneatus 



Dorsal column 



-Subslantia gelatinosa 



Lat. funiculu 



Lat. column 



Ventral column 



Ventral funiculus Ventral median jissure 



FIG. 328. Transverse section of the spinal cord from a 65 mm. human fetus. X 44. 



approximated, the median septum is extended dorsally. Thus the roof 

 plate is converted into part of the dorsal median septum of the adult spinal 

 cord (Fig. 328). 



The Mantle Layer, as we have seen, receives contributions from the 

 proliferating cells of the ependymal layer. A ventro-lateral thickening 

 first becomes prominent in embryos of 10 to 15 mm. (Fig. 325). This is 

 the ventral (anterior) gray column, or horn, which in later stages is sub- 

 divided, forming also a lateral gray column (Fig. 328). It is a derivative of 

 the basal plate. In embryos of 20 mm. a dorso-lateral thickening of the 

 mantle layer is seen, the cells of which constitute the dorsal (posterior) 

 gray column, or horn (Figs. 327 and 328) ; about these cells the collaterals 

 of the dorsal root fibers end. The cells of the dorsal gray column, deriva- 

 tives of the alar plate of the cord, thus form terminal nuclei for the afferent 

 spinal nerve fibers. Dorsal and ventral to the central canal, the mantle 

 layer forms the dorsal and ventral gray commissures. In the ventral floor 



