676 MANUAL OF PHYSIOLOGY. 



into the anterior gray columns, in connection with which arise 

 the anterior roots of the spinal nerves ; while at the upper part 

 the posterior gray columns are formed in connection with the 

 posterior roots of the spinal nerves and their ganglia. 



The white columns are thought by some authors to be derived 



ac 



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. Anterior gray commissure, a/. Anterior fissure, not yet 

 well formed, c. Tissue filling in the upper part of the posterior fissure, pc. Cells form- 

 ing the posterior gray matter, pew. Posterior white column, ct. Mesoblast surround- 

 ing the spinal cord. lew. Lateral white column, acw. Anterior white column, ac. 

 Cells forming the anterior gray matter. 



from the mesoblast surrounding the canal, but by others they are 

 assigned to the epiblast. 



The upper or dorsal canal becomes converted into a fissure by 

 the absorption of its root, and is thus changed into the posterior 

 fissure of the spinal cord. 





