48 
GEORGE E. NICHOEJjS. 
whose lumina face ventro-mesially (figs. 1^ 35, 38, ^.c.o.). This 
species thus differs from both Petroiuyzou marinus and 
Geotria australis, in Avhich the lumiiia of the grooves face 
directly ventrally. It differs, also, from Geotria in that, in the 
latter, the two ependymal bands are in contact mesially. 
'I'he epithelium of the sub-commissural organ is composed 
of elongated, almost fibre-like cells, radially arranged and 
with deeply seated nuclei which stain very strongly. The 
cytoplasm of these cells usually stains comparatively lightly, 
so that, in transverse sections, the sub-commissural organ 
appears as a pair of pale ci'escentic areas whose outer bounding 
curves are defined by closely set nuclei (figs. 38, 39). The inner 
ends of the cells of the sub-commissural organ are produced 
into long neuroglia fibres which become collected into bundles 
and extend to the dorsal surface of the brain, the nerve-fibres 
of the posterior commissure passing between them. 
Laterally the sub-commissural organ is sharply marked off 
from the general ependymal epithelium, which consists of 
short columnar cells with more superficially disposed nuclei 
(fig. 38, e.e'p.). This general ventricular ependymal epithe- 
lium seems to be freely furnished with long cilia, but these 
are replaced upon the sub-commissural organ by very short, 
close-set cilia. The latter are not always preserved, but are 
occasionally quite conspicuous, especially upon that part of 
the sub-commissural organ Avhich lies in the infra-pineal 
recess. 
Mesially, beneath the posterior commissure, those cells 
which form the dorsal edges of the grooves are much less 
elongated, and finally there is a Gansition into an epithelium 
of flattened cells which alone cover the ventricular surface of 
the posterior commissure between the two halves of the sub- 
commissural organ (fig'. 38). This flattened epithelium con- 
tinues around the hinder end of the posterior commissure and 
forms the only epithelial covering of the dorsal (posterior) 
surface of that structure, for onto this surface the sub- 
commissural organ does not extend in Petromyzon fluvia- 
tilis. Thus, at the hinder end of the posterior commissure 
