STRUCTUEE AND DEVELOPMENT OF REISSNER’s FIBRE. 93 
or less reduced to a tube partly enclosed by the sub-coui- 
missurul organ. In Myxinoids an actual ven tral fusion of 
the right and left halves of the sub-commissural organ has 
clearly taken place, and the two grooves have been merged 
in the single tubular sub-commissural canal. The lower 
portion of the i te r, which existed originally ventral to the 
sub-commissural oi'gan, has been obliterated. In one of my 
specimens, however, a trace of this lower portion has per- 
sisted as a small canal which runs forwards from the antero- 
dorsal region of the sinus inesocoelicus. Its course is 
parallel to, and but slightly ventral to, the sub-commissural 
canal. At first cylmdrical, it gradually tapers away and is 
lost at a point considerably posterior to the anterior end of 
the sub-commissural canal. It is lined by ordinary columnar 
epithelium, and at its posterior end is separated from the 
sub-commissural organ by but a slight thickness of nervous 
tissue. 
It was in this same specimen, too, that the third ventricle 
was least reduced, a small canal lined by flattened epithelium 
continuing forward the lumen of the sub-commissural canal 
for some distance into the ’tween brain. A small isolated 
chamber lying dorsal and anterior to the sub-commissural 
canal apparently represents a reduced infra-pineal or a 
diacoelic recess. 
The second division (figs. 21-23, 'I’ext-fig. 7, ojn) of the short 
wide canal leading from the sinus inesocoelicus, which I 
have spoken of as more dorsally directed than the sub-com- 
missural canal, is apparently much more variable. The 
epithelium, too, which lines it, is, over the greater part of its 
extent, much less developed, and is markedly elongated only 
in the region immediately adjacent to its junction with the 
sub-commissural canal. There can be little doubt that this 
dorsal or postero-dorsal canal in the upper portion of the 
mid-brain represents the cavity of the optic lobes of the 
liigher vertebrates, and is equivalent, therefore, to that large 
space which, in the Petromy/.ontidae, lies above and behind 
the posterior commissure. In the Myxinoidei, however, the 
