STUUCTUUE AND DEVELOPMENT OP REISSNER’s FIBRE. 95 
tlie sinus jnesoccelicns (Text-fig. 7, s. m. ; figs. 21, 22, 
s. m.' , s. m."). This chamber, which, as already stated, forms 
by fill- tlie most considerable of tlie brain ventricles, must 
be regai'ded as equivalent to the hinder portion only of the 
iter of other vertebrates. 
Sanders has described this chamber (in Myxine), as seen 
in sagittal section, as like a pipe-bowl in shape, and this 
description would apply equally well to the sinus meso- 
coelicusof Bdellostoma. In this latter animal, however, 
it is partly separated by a marked horizontal constriction 
into upper and lower chambers (figs. 21-2-3, -v. m.', .s. m."). 
It is into the upper chamber that there opens the small 
anterior prolongation of the iter abovm i-eferred ten 
The horizontal constriction becomes more pronounced 
posteriori}^ and finally forms a distinct hoi-izontal partition 
which completely separates the two chambers (fig. 24). The 
two chambers so formed are at first of nearly equal size and 
are separated only by an epithelial partition. Further back 
the dividing lamella becomes much thicker and the two 
s})aces become reduced in size and canal-like. 
'I'he upper one (fig. 24 and Text-fig. 7, i. c.), the posterior 
continuation of the upper chamber of the sinus niesocoeli- 
c u s, is, without question, the homologue of the isthmic canal 
of the Petromyzontidm, and must be considered as part of the 
mesoccEl. It passes backwards with but slight alteration in 
size, but loses its circular outline (fig. 25, i. c.) and becomes 
flattened dorso-ventrally. Presently it opens widely, ven- 
trally, into the fourtli ventricle. 
'I’he lower canal (fig. 24 and 'I’ext-fig. 7, r. c.), whic'h I 
have called the " ventricular canal/’ diminishes in size very 
rapidly, and, after separating somewhat widely from the 
isthmic canal, again appi'oaches it (fig. 25), the two ultimately 
reuniting to constitute the fourth ventricle. 
At the point of junction of these canals there is a slight 
dilatation (Text-fig. 7), but through the greater part of the 
extent of the medulla oblongata the fourth ventricle is 
chiefly represented by the narrow posterior continuation of 
