678 MANUAL OF PHYSIOLOGY. 



The upper and lower parts of the canal do not, however, become 

 thickened. The lateral walls approximate to the centre, decreas- 

 ing laterally the lumen of the canal, which becomes narrow in 

 the middle with a dilatation above and below. The lateral walls 



lew 



uc 



Transverse section of the spinal cord of a chick of seven days. (Foster 

 and Balfour.) ep. Epithelium lining the medullary canal, pf. Part of the 

 cavity of the medullary canal which becomes the posterior fissure, spc. 

 Permanent medullary tube or central canal of the spinal cord. age. Ante- 

 rior gray commissure. /. Anterior fissure, not yet well formed, c. Tissue 

 filling in the upper part of the posterior fissure, pc. Cells forming the pos- 

 terior gray matter pew. Posterior white column, ct. Mesoblast surround- 

 ing the spinal cord. lew. Lateral white column, a&v. Anterior white col- 

 umn, ac. Cells forming the anterior gray matter. 



of the canal, thus approximated, unite in their centre, and con- 

 vert the medullary canal into two separate tubes, a dorsal and a 

 ventral. 



The lower or ventral tube of the divided canal becomes the 



