THE ORGANS OF THE OUTER GERM-LAYER. 



419 



canal and the spongiosa (spongioblasts of His), and (2) into elements 

 which are transformed into ganglionic cells and nerve-fibres (neuro- 

 blasts of His). The thickening of the lateral walls depends partly 

 upon the multiplication of cells, but mainly upon the fact that nerve- 

 fibres apply themselves to the cell-mass from the outside. In time 

 these fibres are separated into the anterior, lateral, and posterior 

 columns of the spinal cord (fig. 233 pew, lew, acw). At their first 

 appearance the nerve- 

 fibres are non-medul- 

 lated (fig. 232 nf), 

 and only subse- 

 quently, sometimes 

 earlier, sometimes 

 later, acquire a me- 

 dullary sheath. In 

 this manner the al- 

 ready considerably 

 thickened halves of 

 the spinal cord be- 

 come differentiated 

 into the central gray 

 substance containing 

 the ganglionic cells, 

 and into the white 

 substance, which en- 

 velops the surface of 

 the former like a 

 mantle. 



Since, meanwhile, 

 the roof- and floor- 

 plates grow only a 

 little and are not 

 differentiated into 

 ganglionic cells, they come to lie deeper and deeper at the bottom 

 of anterior and posterior longitudinal furrows (c and af). Finally, 

 the completely formed spinal cord is composed of large lateral halves, 

 which are separated from each other by deep anterior and posterior 

 longitudinal fissures, being united only deep down by a thin trans- 

 verse bridge. The latter is derived from the roof- and floor-plates, 

 which have been retarded in their growth, and encloses in its middle 

 the central canal, which has also remained small. 



Fig. 233. Cross section through the spinal cord of an embryo 

 Chick of seven days, after BALFOUR. 



pew, Posterior white column ; lew, lateral white column ; 

 acw, anterior white column ; c, dorsal tissue filling up the 

 place where the dorsal fissure will be formed ; pc, posterior 

 horn of the gray substance ; ac, anterior horn ; ep, epithelial 

 cells ; age, anterior gray commissure ; pf, posterior [dorsal] 

 part of the spinal canal ; spc, anterior [ventral] part of the 

 spinal canal ; af, anterior fissure. 



