46 
GEORGE E. NICHOLES. 
stated to take the form of paired bands of high columnar 
ependymal epithelial cells, which are grouped so as to bound 
more or less deep longitudinal grooves, arising anteriorly 
immediately behind the habenular ganglia and extending 
backward beneath the posterior commissure. 
In Petromyzon fluviatilis I find a similar disposition 
of this characteristic epithelium. It appears anteriorly as a 
well-defined band in the roof of each of the narrow clefts 
which separate on either side the downwardly projecting 
Text-fig. 
A slightly cliagramiiiatic median (sagittal) section tlirongh the 
brain of Petromyzon fluviatilis. One of the two factors 
of Reissner's fibre is shown from the snb-commissnral organ 
liackwards, the other branch is indicated near their junction at 
the anterior end of the isthmic canal. (In a truly sagittal 
section neither would be seen in front of that ^loint in the mid- 
brain, for they actually lie just to the right and left of the 
median plane.) c.c. Canalis centralis, cmh. Cerebellum. 
h(j. Habenular ganglion, i.c. Isthmic canal, inf. Infra-pineal 
recess, infd. Infundibulum, iter. Iter. p.c. Posterior com- 
missure. r.f. Reissner's fibre, s.c.o. Sub-commissural organ. 
III. Third ventricle. IV. Fourth ventricle. 
habenular ganglia from the side walls of the diencephalon 
(Fig. 33, s.c.o.) . Owing to the greatei- size of the right 
habenular ganglion the cleft upon that side is almost obliter- 
ated and the corresponding ependymal groove crowded out. 
