242 



THE DEVELOPMENT OF THE CHICK 



and gray matter (Fig. 145, eight days), but the dorsal columns 

 of the two sides are still separated in the median line by a broad 

 septum of ependymal cells. Later (Fig. 146, twelve da\^s) this 

 septum becomes very narrow, and the accumulation of fibers in 

 the dorsal columns causes the latter to project on each side of 

 the middle line, thus forming an actual fissure between them. 



Fig. 145. — Transverse section through the spinal cord, and the eighteenth 

 spinal ganglion of an eight-day chick. 

 Centr., Centrum of vertebra, d. R., Dorsal root. Ep., Ependyma. Gn., 

 Spinal Ganglion. Gn. symp., Sympathetic ganglion. Gr. M., Gray matter, 

 m. N., Motor nucleus. R. com., Ramus communicans. R. d., Ramus dor- 

 salis. R. v., Ramus ventralis. Sp., Spinous process of vertebra, v. R., 

 Ventral root. Wh. M., White matter. 



Central Canal and Fissures of the Cord. The central canal 

 passes through a series of changes of form in becoming the prac- 

 tically circular central canal of the fully formed cord. L'p to 

 the sixth day it is elongated dorso-ventrally, usually narrowest 

 in the middle with both dorsal and ventral enlargements. About 



