188 



THE FIFTH DAY. 



[chap. 



Fig. 



59- 



Section through the Spinal Cord of a Seven Days' Chick. 



p.c. w. posterior white column. I. c. w. lateral white column, a. c. w. anterior 

 white column, p c. posterior cornu, of grey matter, consisting of very 

 small cells, a. c. anterior cornu of grey matter, with a peculiar mass of 

 very large cells, ep. epithelium Iming the original medullary canal, p.f. pos- 

 terior fissure. The posterior fissure is chiefly formed by the upper portion 

 of the original medullary canal which becomes open above. The upper 

 portion of it is now filled with tissue (c) which is probably derived from 

 the epithelium of the medullary canal. The lower portion of the medullary 

 canal becomes the spinal canal {sp.c.) and is eventually entirely shut off 

 from posterior fissure. The communication between the spinal canal and the 

 posterior fissure is already narrowed, and if the section had beeu made 

 further forwards, the two would have been entirely separated from each 

 other. 



a. f. anterior fissure. This is formed in an entirely different manner from 

 the posterior fissure. It is produced by the anterior column of white, 

 and the anterior cornu of grey matter, growing downwards and leaving 

 between them a fissure. It is at this time filled up with connective 

 tissue. 



a.g. c. anterior grey commissure, c. tissue filling up the end of the posterior 

 fissure, sp. c. spinal canal. Only the right half of the cord is represented 

 in the figure. The section passes through the cord between the entrance 

 of two spinal nerves. The angular form of the cells of the cord has not 

 been done justice to by the engraver. 



