THE SPINAL COED 



321 



ventral portion of the layer, which is thus narrower than the dorsal portion in 

 10 to 20 mm. embryos (Figs. 307 and 308). Consequently, the ventral portion 

 of the mantle layer is differentiated first. The neural cavity is at first somewhat 

 rhomboidal in transverse section, wider dorsally than ventrally. Its lateral angle 

 forms the sulcus limitans which marks the subdivision of the lateral walls of the 

 neural tube into the dorsal alar plate and ventral basal plate. When the ependy- 

 mal layer ceases to contribute new cells to the mantle layer its walls are approxi- 

 mated dorsally. As a result, in 20 mm. embryos the neural cavity is wider ven- 

 trally (Fig. 208). In the next stage, 34 mm., these walls fuse and the dorsal 

 portion of the neural cavity is obliterated (Fig. 309). In a 65 mm. embryo the 



Roof plate 



Post .funiculus 



Neural cavity 



Marginal layer 



EpendymaL Layer 



Ani. median fissure 



Floor plate 



Fig. 308. — Transverse section of the spinal cord from a 20 mm. embryo. X 44. 



persisting cavity is becoming rounded (Fig. 310). 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 ventrally to 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 ependymal 

 cells of the roof plate no longer radiate, but form a medium septum (Fig. 309). 

 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 septum of the adult spinal cord (Fig. 310). 



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



