STRUCTURE AND DEVELOPMENT OP EEISSNER’s FIBRE. 49 
the sub-commissural organ ends abruptly (see Text-fig. 5). 
Owing, however, to the great length of their component cells 
the two halves of the sub-commissural organ continue to 
appear for a short distance in transverse sections as crescentic 
epithelial structures, lying almost or quite freely in the 
rnesocoel altogether behind tbe commissure (fig. 36, s. c. o.). 
This manner of ending of the sub-comiuissural oi’gan is 
absolutely characteristic of all the members of the Petro- 
myzontidae which I have examined. In P. mar in us, 
however, Sargent states (’04, p. 152) that the sub-commissural 
organ (“ependymal grooves”) extends around the postei’o- 
ventral surface of the posterior commissure onto the dorsal 
(posterior) surface of that structure, although his figures 
(’04, pi. i, figs. 6 and 7) do not seem to bear out his state- 
ment. I shall return to this matter later. 
Reissner’s Fibre. 
Springing fi’om the epithelium of the sub-commissuinl 
organ, where this lines the infra-pineal recess on either side, 
a number of delicate -fibrillae may be made out which coalesce 
to form factors of Reissner’s fibre (fig-. 39, fh.). Mingled 
with these fibrillae are numei'ous fine strands, which I was at 
first inclined to consider as artifacts due to coagulation. 
They are, however, invariably present at this point, and 
closely resemble the constituent fibrillae of Reissner’s fibre, 
difi’ering principally in that they have a slightly irregular 
and wavy course, whereas the undoubted constituents of the 
fibre ai’e straight, as though di'awn taut under considerable 
tension. It may well be that the loose wavy fibrillae (fig. 39, 
fh.') are simply factors of Reissner’s fibre which have been 
broken and contracted under the action of the fixing reagent. 
The snb-commissural organ, as already stated, consists of 
separate paired bands, which in Petromyzon fluviatilis 
nowhere coalesce nor even closely approach, and it is an 
interesting fact that in this species the fibilllae unite to foini 
a pair of principal factors (fig. 37, r.f.) of Reissner’s fibre. 
VOL. 58, PART 1. NEW SEKIES. 4 
