41 6 SPINAL CORD. 



longitudinal nerve-fibres, which remain for a long period without 

 a medullary sheath. These fibres appear in transverse sections 

 as small dots. The white matter forms a transparent investment 

 of the grey matter and would seem to contain neither nuclei nor 

 cells 1 . The white matter may from the first form only two 

 masses, one on each side, forming a layer on the ventral and 

 lateral parts of the spinal cord but not extending to the dorsal 

 surface (Elasmobranchii, fig. 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 



c 



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 will be formed ; pc. 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. 



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 



1 This holds true at first for Elasmobranchii, but at a later stage there are present 

 numerous nerve-cells in the white matter, so that the distinction between the white 

 and grey matter becomes much less marked than in higher types; in this respect Elas- 

 mobranchii present an approximation to Amphioxus. 



