324 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



(motor) . When the ependymal layer ceases to contribute new cells to the mantle 

 layer its walls are approximated dorsally. As a result, in 20 mm. embryos the 

 neural cavity is wider ventrally (Fig. 326). In the next stage, 34 mm., these walls 

 fuse and the dorsal portion of the neural cavity is obhterated (Fig. 327). In a 

 65 mm. (C R) 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 ventrad, reaching the surface of the cord in the depres- 

 sion of the ventral median fissure. 



When the right and left waUs of the ependymal layer fuse, the ependymal 

 cells of the roof plate no longer radiate, but form a median septum (Fig. 327). 



Dorsal median septum Fasciculus gracilis 

 Dorsal root--^ , ^>^ / —^ ^Fasciculus cuneatus 



Dorsal column ~ 



Lat. funiculus 





'Substantia gelatinosa 





■Central canal 







'Lot. column 



Ventral column' 



Ventral funiculus Ventral median fissure 



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



Later, as the marginal layers of either side thicken and are approximated, the 

 median septum is extended dorsally. Thus the roof plate is converted into part 

 of the dorsal median sepPiim of the adult spinal cord (Fig. 328). 



The Mantle Layer, as we have seen, is contributed to by the proliferating 

 cells of the ependymal layer. A ventro-lateral thickening first becomes promi- 

 nent in embryos of 10 to 15 mm. (Fig. 325). This is the ventral {anterior) gray- 

 column, or horn, which in later stages is subdivided, forming also a lateral gray 

 column (Fig. 328). It is a derivative of the basal plate. In embryos of 20 mm. 

 a dorso-Iateral 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 ceUs 

 the collaterals of the dorsal root fibers end. The cells of the dorsal gray column 



