STEUCTURE AND DEVELOPMENT OF REISSNER’s FIBUE. 63 
missure that the characteristic epithelium appears iu the 
actual median section of series cut sagittally (Dendy, ’07, 
pi. i, fig. 2). Immediately in front of the posterior com- 
missure there is even an actual confluence for a short 
distance, the grooves presenting there the appearance of 
a single horse-shoe-shaped structure which lines the arching 
roof of the infra-pineal recess. Dendy (’07, pi. i, fig. 3) has 
figured the condition of the sub-commissural organ, as seen 
in transvei'se section, at a point immediately behind the 
infra-pineal recess. The condition of the sub-commissural 
organ in the infra-pineal recess foreshadows the condition 
which has become genei-al along the entire length of the 
organ in many of the higher vertebrates (figs. 2, 3, 6, 8). 
Ahlborn (’83), in his figures of P. planeri, has shown 
indistinctly (figs, 25, 26) what may be the paired ends of 
the grooves immediately behind the posterior commissure, 
and an unpaired median ependymal mass well developed 
beneath the posterior commissure. If this, indeed, represents 
the sub-commissural organ, we have here a further advance 
upon the incipient fusion of the grooves in Geotria 
australis. In Petromyzon raarinus, on the other hand, 
the grooves appear, judging from Sargent’s figures, to be 
even more widely separated than iu Petromyzon flu- 
V i a t i 1 i s . 
Of the eleven series of sections through the brain of the 
velasia stage of Geotria, seven were cut sagittally and four 
transversely. In three of these the 6bre could not be cer- 
tainly made out, but the sections in two of these three cases 
were transvei’se, and in such sections Eeissner’s fibre is 
always particularly^ difficult to recognise. 
In only one of the sagittally cut series of sections did I fail 
to find the fibre, and in this particular specimen the choroid 
plexus of the fourth ventricle had been dissected away before 
the sections were cut, and almost certainly the fibre was 
carried away at the same time. 
In the eiglit brains in which Reissner’s fibre was clearly to 
be seen it had retained its strictly normal position in only 
