STRUOTURE AND DEVEI,OPMENT OF REISSNEr’s FIBRE. 23 
and continues to receive fresh accessions of strands of united 
fibrillge along the entire length of the posterior commissure, 
until it passes candally from the hinder border of that struc- 
ture. 'I'lieiice it stretches as a taut thread lengthwise through 
the mesocoel, usually coming to lie close against the ventral 
sui-face of the rhombo-mesencephalic fold. Upon this surface 
there is almost invariably a median longitudinal groove, 
wliich I have termed the “ isthmic canal” (Te.xt-fig. 5, i.c.). 
'I'll is appears to becojne deeper with age. It is lined by an 
('pendymal epithelium that differs from the general ventricular 
investment, the cells being more elongated and staining very 
strongly. In this groove the fibi'e lies freely, and, so far as 
my observations go, never becomes embedded in the 
brain-tissue (as has been asserted). The isthmic canal is 
usually deepest at its anterior end, becoming continually more 
shallow postei'iorly till it fades out, and Reissner’s fibre, 
emerging from its hinder end, may be readily traced back- 
wards through the fourth ventricle as a tightly stretched 
thread which enters the canalis centralis of the spinal 
cord. 
The ependymal cells which line the canalis centralis are 
furni.shed somewhat spai-sely with short cilia. Here and 
there, however, at comparatively short intervals, slightly 
longer cilia may be made out, which appear to be attached to 
Reissner’s fibre (compare fig. oG). It is, of course, extremely 
diflicult, if not impossilile, to determine with absolute certainty 
that these cilia are not simply lying in contact with the fibre 
or even glued to it by a coagulum of cerebro-spinal fluid. An 
examination of a very large number of sections, however, leads 
me to believe that many, at any rate, of them are indeed 
actually fused with Reissner’s fibre and probably form an 
integral part of it. Whether, however, they are quite short 
cilia fused to the fibre only by their tips and serving merely 
as su])])orts and stays for it, or whether they are of a similar 
character to the long cilia-like processes of the cells of the 
sub-commissural organ, I am quite unable to determine, but 
1 suspect that they are of this latter class, and that they 
