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 days) 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. 
Fic. 145. — Transverse section through the spinal cord, and the eighteenth 
spinal ganglion of an eight-day chick. 
Centr., Centrum of vertebra. d.R., Dorsalroot. 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. Up to 
the sixth day it is elongated dorso-ventrally, usually narrowest 
in the middle with both dorsal and ventral enlargements. About 
