NERVOUS SYSTEM OF THE VERTEBRATA. 417 



in thickness in the subsequent stages, and extends so as 

 gradually to cover the whole cord (fig. 246). 



The anterior white commissure is formed very shortly after 

 the first appearance of the white matter. The grey matter and 

 the central epithelium are formed by a differentiation of the 

 main mass of the spinal cord. The outer cells lose their 

 epithelial-like arrangement, and, becoming prolonged into fibres, 

 give rise to the grey matter, while the innermost cells retain 

 their primitive arrangement, and constitute the epithelium of the 

 canal. The process of formation of the grey matter would 

 appear to proceed from without inwards, so that some of the 

 cells, which have, on the formation of the grey matter, an 

 epithelial-like arrangement, subsequently become converted into 

 true nerve-cells. 



As has already been mentioned, the central epithelium of 

 the nervous system probably corresponds with the so-called 

 epidermic layer of the epiblast. 



The grey matter soon becomes prolonged dorsally and 

 ventrally into the posterior and anterior horns. Its fibres may 

 especially be traced in two directions: (i) round the anterior 

 end of the spinal canal, immediately outside its epithelium and 

 so to the grey matter on the opposite side, forming in this way 

 an anterior grey commissure, through which a decussation of 

 the fibres from the opposite sides is effected : (2) dorsalwards 

 along the outside of the lateral walls of the canal. 



There is at this period no trace of the ventral or dorsal 

 fissure, and the shape of the central canal is not very different to 

 what it was at an earlier period. This condition of the spinal 

 cord is especially instructive, as it is very nearly that which is 

 permanent in Amphioxus. 



The next event of importance is the formation of the ventral 

 or anterior fissure. This owes its origin to a downgrowth of the 

 anterior horns of the cord on each side of the middle line. The 

 two downgrowths enclose between them a somewhat linear 

 space the anterior fissure which increases in depth in the 

 succeeding stages (fig. 246, af}. 



The dorsal or posterior fissure is formed at a later period 

 than the anterior, and accompanies the atrophy of the dorsal 

 section of the embryonically large canal of the spinal cord. 

 B. III. 2 7 



