VIII.] 



THE GREY MATTEB. 



253 



sequent stages, and extends so as gradually to cover the 

 whole cord (Fig. 83). 



Fig. 83. 



Section through the Spinal Cord of a Seven Days' 

 Chick. 



pow. dorsal white column, tew. lateral white oolumrii. acw. ven- 

 tral white column, c. dorsal tissue filling up the part where 

 the dorsal fissure will be formed, pa. dorsal grey cornu. 

 ac. anterior grey cornu. ep. epithelial cells, age. anterior 

 commissure, pf. dorsal part of spinal canal, spc. ventral 

 part of spinal canal, af. anterior fissure. 



The grey matter and the central epithelium are 

 formed by a differentiation of the main mass of the 

 walls of the medullary canal. The outer cells lose their 



