52 
GEORGE E. NICHOLLS. 
Beneath and behind the posterior commissure the right and 
left halves of the fibre pass backward and ventrally in a nearly 
parallel course, but converging slightly (fig. 37, r.f.), till, 
beneath the rhombo-mesencephalic fold, they may be seen, in 
a series of transverse sections, to enter a pair of deep grooves 
on the ventral surface of that fold (fig. 10, i.c.). These 
paired gi'ooves after a short separate course become confluent 
behind to form a single median groove, which continues back- 
wai'd to the extreme caudal end of the ventral surface of the 
fold. This groove, which I propose to call the “isthmic 
canal” (Text-fig. 5, i.c.), is lined by a distinct columar epithe- 
lium, which is much more strongly staining than the ependymal 
epithelium which covers the rest of the ventricular surface 
of the rhombo-mesencephalic fold. It is of very general 
occurrence, at any rate iu the lower vertebrates, and probably 
increases in depth during life. In the Myxinoids, as will be 
described below, this isthmic canal becomes converted into 
a tubular structure. 
At the point of confluence of the paired anterior parts of 
the isthmic canal the paired branches of Reissner’s fibre also 
unite to form a single median thread, which lies in the groove 
close against the ependymal epithelium. In the photomicro- 
graph reproduced as fig. 10, the pair of fibres [r.f.) may be seen 
in the two halves of the isthmic canal at a point about twenty^ 
micra in fiont of the confluence of the canals. In other 
sei iesof transverse sections thi ough the brain of Peti'omyzon, 
the isthmic canal does not show the paired anterior portion, 
and the two halves of the fibre appear to unite slightly 
antero-ventral to the rhombo-mesencephalic fold (fig. 1 l,r./.). 
These are probably younger specimens, for this difference 
seems to be due to a difference of age and degree of down- 
growth of the brain-tissue upon the fibre. I have found a 
similar difference in the frog, where in a young specimen the 
fibre lies freely beneath the ventral surface of this part of 
the brain, while in an older (fully grown) specimen the fibre 
has come to lie in a deep isthmic canal. 
Behind the cerebellum the fibre (Text-fig. 5, r.f.) emerges 
