92 
GEORGE E. NiCHOLLS. 
compared with figt. 21, 22, and Text-fig. 7), this anterior 
portion of the mid-brain ventricle has a practically circular 
outline. Its diameter for the greater part of its length 
remains nearly constant, but increases slightly near the 
junction, posteriorly, with the second and more dorsally 
directed canal. 
In that portion of the brain which immediately overlies 
the anterior extension of the mesocoel there is a commissural 
tract which has been identified by Edinger (’06) and Sterzi 
(’07) as the posterior commissure, to which conclusion my 
own observations have also led me. The anterior canal or 
chamber itself is not, however, I believe, the homologue 
of the whole of the iter in this region, the greater part of 
that passage beneath the posterior commissure having been 
obliterated. 
The portion which persists (figs. 21-23 and Text-fig. 7, 
s. c. c.) is completely invested by an extraordinarily developed 
high columnar epithelium, from which the delicate fibrillse of 
Reissner’s fibre spring. This epithelium, therefore, represents 
the sub-commissural organ, and the enclosed space is almost 
certainly nothing but the product of the complete concur- 
rence of a pair of ependymal grooves. I shall hereafter 
speak of this canal as the “ sub-commissural canal.” 
I have pointed out above that the two halves of the 
sub-commissural organ, primarily distinct (e. g. Petromyzon 
flu viatilis), have in Geotria united beneath the anterior 
portion of the posterior commissure to form a median 
structure, which appears, in transverse section, as a horse- 
shoe-shaped band. 
This tendency towards fusion in the middle line dorsally, 
seen in Geotria alone amongst the Petromyzontidae,^ 
has become the rule in higher vertebrates. Not only so, but, 
since in these higher forms the iter becomes compi’essed and 
nari'owed from side to side (cf. figs. 4, 7), the ventral 
edges of the lips of these ependymal bands often nearly meet 
below (cf. figs. 2, 6). In such cases the iter may be more 
' Petromyzon planeri (?), vide supra. 
