STRUCTUEE AND DEVELOPMENT OF REISSNEr’s FIBRE. 101 
in sliape, as seen in transverse sections, through triangular 
to oval and oblong. Finally, it becomes circular, narrowing 
continually until it has a diameter scarcely greater than that 
of the canalis centralis, into which it presently passes. 
At that point, however, where the upper discontinuous 
remnants of the fourth ventricle cease, I found in two of my 
specimens a considerable space (fig. 30, x.), which appears to 
be without epithelial lining. It is, nevertheless, lined, I 
believe, by an extremely flattened epithelium, the sparse 
nuclei of which are dotted at irregular intervals upon its 
surface. In one of the two specimens in which the cavity 
is present, greatly attenuated epithelial cells separate the 
chamber from the underlying fourth ventricle. In the other 
example the two cavities are in open communication. . 
At the posterior end of the medulla the fourth ventricle 
passes into a double canalis centralis (text-fig. 8, c. c.), but 
whereas in Bdellostoma (fig. 31) the upper canal is con- 
siderably the larger, in Myxine (fig. 28) the two canals are 
of much the same size. 
Reissner’s fibre (text-fig. 8, r.f.) has a course practically 
identical with that of the fibre in Bdellostoma. It arises 
in precisely the same way from a brush-like mass of delicate 
fibrillae near the anterior end of the sub-commissural oi’gan. 
These, however, appear to unite into a single thread which 
lies closely against the epithelium of the sub-commissural 
organ (fig. 26, r.f.). It traverses the upper part of the 
sinus mesocoelicus, and passes into the isthmic canal, lying 
clusely against the upper wall of that passage (fig. 27, r.f.). 
Emerging from the isthmic canal it extends backwards 
through the fourth ventricle (fig. 30, r.f), and where that 
gives place to the double canal of the spinal cord it passes 
into the lower of the two canals (figs. 28, 29, r.f), exactly 
as in Bdellostoma. 
It apparently extends through this lower canal practically 
to the extreme posterior end of the body. Slightly in front 
of the actual extremity, however, the two canals reunite to 
form a single canal which appears in transverse section as a 
