STEUCTUEE AND DEVELOPMENT OF EEISSNEE’s FIBEE, 47 
It extends forward, tlierefore, for a much shorter distance 
than does that of the opposite side, a condition which Deudy 
(’02, p. 489) noted in the ammocoete of Petromyzon and, later, 
in the velasia stage of Geotria (’07, p. 5). Sargent states that 
he found in Petromyzon marinus the right g-roove better 
developed anteriorly, “ corresponding in this with the greater 
size of the habenula of that side ” (’04, p. 152). 
Immediately behind the habenular ganglia these lateral 
clefts (in the roofs of which lie the forward extensions of 
the sub-commissural organ) widen out into and become 
merged with the infra-pineal recess. The bands of columnar 
epithelium also expand, spreading out upon the roof and side 
walls of that recess (fig. 34). The upper limit of the infra- 
pineal recess lies at a slightly higher level than the dorsal 
suface of the posterior commissure, and at that level there are 
always found a pair of shallow pockets or “ diacoelic recesses ” 
bulging very slightly laterally and backward, thus appearing- 
above the posterior commissure. Owing to the spreading out 
of the paired epithelial bands of the sub-commissnral organ 
upon the roof and side walls of the infra-pineal recess 
almost all of the upper region of the recess comes to be lined 
by this epithelium, and the pair of diacoelic recesses are com- 
pletely lined by it. Being paired laterally placed structures 
they do not appear in the diagrammatic repre.sentation (Text- 
fig. 5) of a truly sagittal section, but the recess^ of the right 
side is shown in fig. 35 (r.d., on the left in the figure), only one 
being shown on account of the slight obliquity of the plane of 
the section. From the infra-pineal recess the paired tracts 
of specialised ependymal epithelium extend downward and 
backward beneath the posterior commissure, sharply marked 
off from the general ependymal epithelium. They bound 
distinct and, for the most part, widely separate grooves, 
‘ These recesses appear to be identical with the “ recessi postabenu- 
lari ” described by Sterzi (’U7) in P. marinus. These latter are 
apparently much larger than the diacoelic recesses I have described in 
P. flu viatilis, and Sterzi seems to have overlooked them in the latte r 
pecies. 
