254 THE FIFTH DAY. [CHAP. 



epithelial-like arrangement, and, becoming prolonged 

 into fibres, give rise to the grey matter, while the inner- 

 most cells retain their primitive arrangement, and con- 

 stitute 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. 



The central epithelium of the nervous system pro- 

 bably 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 : (1) 

 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) dorsal wards 

 along the outside of the lateral walls of the canal. 



There is at this period (fourth day) no trace of the 

 ventral or dorsal fissure, and the shape of the central 

 canal is not very different from 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 begins on the fifth 

 day and owes its origin to a downgrowth of the an- 

 terior horns of the cord on each side of the middle line. 

 The two downgrowths enclose between them a some- 



