STRUCTURE AND DEVELOPMENT OF REISSNEr’s FIBRE. 81 
already projects freely into the ventricle, these being the 
constituent fibrillae of Reissner’s fibre (Fig. 45, fh). For 
the greater part the cells are still undifferentiated; they are 
destined, however, to give rise to the cells of the sub-com- 
inissural organ, while from their cell bodies grow out the 
-constituent fibrillas of Reissner’s fibre. I am not prepared, 
of course, to say that no single cell in this group depicted by 
Sargent in his fig. 3 could form part of the future “Dachkern,” 
but that such cells, if present, send axons into the ventricle, 
I entirely disbelieve. 
(c) Reissner’s Fibre in the Brain -ventricles. 
In two or three other particulars my observations upon 
Reissner’s fibre in the lamprey are completely at variance 
with those recorded by Sargent. Thus, in no single instance 
in any specimen of the Petromyzontidae did I find any part 
of Reissner’s fibre arising from the dorsal surface of the 
posterior commissure, and I am utterly at a loss to explain 
Sargent’s statements that it arises there. If the sub- 
commissural organ extends onto that sui'face there i.s, of 
course, no reason why the fibre should not receive factors 
frotn that region, but, as I have pointed out above, Sargent’s 
figures do not convey the impression that such a dorso- 
posterior extension of the sub-commissural organ has any 
real existence. Sargent’s statements, however, imply that 
not Tuerely does the fibre arise in part from that surface, but 
that these portions of the fibre constitute its main trunks. 
Indeed, the factor from the left side of the brain, according 
to Sargent, spi-iugs wholly from the dorsal aspect of the 
posterior commissure (’04, pp. 155-6). The part of the fibre 
Avhich he finds in the diacoele arises, he claims, wholly from 
the habenular ganglion of the right side, and is collected into 
a single factor which joins the main right trunk behind the 
posterior commissure. He could find, he states, no fibre upon 
the left side traceable to the region of the left habenular 
ganglion. 
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