NERVOUS SYSTEM OF THE VERTEBRATA. 



343 



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

 extending to the dorsal surface (Elasmobranchii, tig. 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.). In 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 tiie main mass of the 

 spinal cord. The outer cells lose their epithelial-like arrangement, 

 and, becoming prolonged into filjres, 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 dorsally 

 and ventrally into the pos- 

 terior and anterior horns. 

 Its fibres may especially be 

 traced in two directions : — 



(1) round the anterior end 

 of tlie 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) dorsal wards 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 



c/ 



lew 



acw 

 Fig. 246. Section through the spinal cord 



OF A SEVEN bays' ChICK. 



l^ew. dorsal white column ; lew. lateral white 

 column; acw. ventral white column; c. dorsal 

 tissue filling up the part where the dorsal fissure 

 will be formed ; pc. dorsal grey cornu ; ac. anterior 

 grey cornu; <?p. epithelial cells; age. anterior com- 

 missure; ff. dorsal part of spinal canal; s'pc. 

 ventral part of spinal canal; af. anterior fissue. 



