THE NERVOUS SYSTEM 243 
the seventh day the dorsal portion begins to be obliterated by 
fusion of the ependymal cells, and is thus reduced to an epen- 
dymal septum. On the eighth day this process has involved the 
upper third of the canal; the form of the canal is roughly wedge- 
shaped, pointed dorsally and broad ventrally (Fig. 145). The 
continuation of this process leaves only the ventral division as 
the permanent canal. 
At the extreme hind end of the cord the central canal becomes 
dilated to form a relatively large pear-shaped chamber with thin 
undifferentiated walls (Fig. 148); the terminal wall is still fused 
with the ectoderm at eight days, and the chamber appears to 
have a maximum size at this time. At eleven days the fusion 
with the ectoderm still exists, and the cavity is smaller. 
Fic. 146. — Transverse section through the 
cervical swelling of the spinal cord of a 
12-day chick. (After v. Kupffer.) 
C., Central canal. d. H., Dorsal horn of 
the gray matter. Ep., Ependyma. N. H., 
Nucleus of Hoffmann. - s. d., Dorsal fissure. 
s. v., Ventral fissure. v. H., Ventral horn 
of the gray matter. 
The development of the so-called dorsal and ventral fissures 
is essentially different. The entire ventral longitudinal fissure 
of the cord owes its origin to growth of the ventral columns of 
gray and white matter which protrude below the level of the 
original floor (Figs. 145 and 146), and the latter is thus left be- 
tween the inner end of the fissure and the central canal. The 
dorsal longitudinal fissure on the other hand is for the most part 
