NERVorS XYXTEM OF THE VERTEBRATA. 



343 



a layer on the ventral and lateral parts of the spinal cord but not 

 extending to the dorsal surface (Elasmobranchii, rig. 185, W) ; or it 

 may form four patches, viz. an anterior and a posterior white column 

 on each side, which lie on a level with the origin of the anterior and 

 posterior nerve-roots (the Fowl, Human embryo, etc.). Jn whichever 

 of these forms the white matter appears, it is always, at first, a layer of 

 extreme tenuity, which rapidly increases in thickness in the subse- 

 quent stages, and extends so as gradually to cover the whole cord 

 (tig. 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 epi- 

 thelium of the nervous sys- 

 tem probably corresponds 

 with the so-called epidermic 

 layer of the epiblast. 



The grey matter soon 

 becomes prolonged dorsal ly 

 and ventrally into the pos- 

 terior and anterior horns. 

 Its fibres may especially be 

 traced in two directions : 



(1) round the anterior end 

 of the spinal canal, imme- 

 diately outside its epithe- 

 lium and so to the grey 

 matter on the opposite side, 

 forming in this way an 

 anterior grey commissure, 

 through which a decussa- 

 tion of the fibres from the 

 opposite sides is effected : 



(2) dorsalwards along the 

 outside of the lateral walls 



of the canal. 



There is it this neriod will be formed ; pc. dorsal grey coruu ; ac. anterior 

 mere is at mis penou n ur,Kd i^iis =/;. anterior com- 



Icw 



a c 



\f 



FIG. 246. SECTION THROUGH THE SPINAL CORD 

 OF A SEVEN DAYS' CHICK. 



pew. dorsal white column ; lew. lateral white 

 column; acw. ventral white column; c. dorsal 

 tissue filling up the part where the dorsal fissure 



no trace of the ventral or 



grey coruu; ep. epithelial cells; ai/c. anterior com- 

 missure; pf. dorsal part of spinal canal; spc. 



dorsal fissure, and the shape ventral part of spinal canal; (if. anterior fissue. 



