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. 



FIG. 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 



