STllUCTQRE AND DEVELOPMENT OE REISSNEk’s FIBRE. 69 
even at this eai-ly age, attained its fall forward displacement 
relatively to other structures. All trace of the communica- 
tion of its lumen with the third ventricle appears to be 
already obliterated, and as the posterior commissure lies 
immediately behind the habenular ganglia and at the same 
level, the recessus infrapineal is (Text-fig. 6, inf.) exists 
as a distinct space principally in that crevice which extends 
forwards from the posterior commissure for a short distance 
to the left of the habenular ganglion. Its anterior boundary 
is already defined by the left Meynert’s bundle. 
'I'lie ventral surface of the posterior commissure is covered 
by a conspicuous columnar epithelium (figs. 41, 45, s. c. o.), 
plainly disposed in a pair of longitudinal bands which meet 
in the mid-dorsal line and thus together form a single 
inverted trough upon the roof of the brain. 
The cells of these epithelial bands attain their greatest 
length laterally, and their nuclei are there deep-seated. To- 
wards the middle line the cells become shorter and have a 
length scarcely greater than that of their nuclei. 
We have thus, at this early stage, a well-developed sub- 
commissural organ, the two halves of which are not, as yet, 
sharply marked off mesially from the ordinaiy ependymal 
epithelium. The half of the sub-commissural organ on the 
left side extends forwards into the infra-pineal recess slightly 
in advance of that upon the right side. Posteriorly the sub- 
commissural organ stretches, on both sides, the entire length 
of the posterior commissure, which, at this age, consists of a 
very thin band, indeed, of transversely coursing nerve-fibres 
(figs. 41, 45, 2 ). c.). 
lleissner’s fibre maybe made out, in sections cut sagittally, 
arising from the sub-commissural organ by several exceed- 
ingly delicate threads, which unite to form a single fibre in the 
aqufeductus Sylvii. Thence it passes backwards as a 
fine thread freely through the fourth ventricle. It seems to 
lie ngainst the ventral surface • of the plica rhombo- 
mesencephalica, but nowhere penetrates the brain tissue, 
nor is there, at this stage, any trace of an isthmic groove for 
