82 
GEORGE E. NICHOLES. 
Tlius, ignoring even the vital differences existing between 
Sargent’s account of the origin of the factors of Reissner’s 
fibre from nerve-cells and iny own description of its origin 
from ependymnl cells, I am still unable to reconcile Sargent's 
description of the course of Reissner’s fibre in Petromyzon 
marinus with my own observations upon its path in other 
lampreys, and, indeed, in vertebrates of other groups. 
In the three species of lamprey of which I have examined 
the brain, Reissner’s fibi-e invariably takes its origin in the 
infrapineal recess by the union of delicate fihrillae into a 
pair of fine threads, which receive constant accessions in 
their nearly parallel course beneath the posterior commissure. 
Of constituent fibrillse from the dorsal surface of the com- 
missure there are none. The paired fibres converge in the 
mesocoel, and coalesce into a median structui'e either beneath 
the rhombo-mesencephalic fold (P. fluviatilis) or at some 
point in front of that ( I c h thy o m y zo n and Geo t r ia ) . 
Again, in not one of more than thirty lamprey bi-ains 
(larval and adult) which I have studied have I fouml the 
fibre embedded in the brain-tissue of the rhombo-mesen- 
cephalic fold as described by Sargent (’04, }). 1.56). He states : 
“ 'J'he fibre passes through this portion of the brain and the 
basal part of the cerebellum in the median plane a little dorsal 
to the passage connecting the third and fourth ventricles,” and 
“in both the ti-ansverse and sagittal sections that I have studied 
Reissner’s fibre has been found always to enter the right 
tuberculum acusticum (fig. a).” 
In the former statement Sargent has confirmed the descrip- 
tion })resented by Studuicka (’99) of this part of the course of 
the fibre in P. planeri, the latter author having stated that 
he found Reissner’s fibre surrounded by the brain-substance 
in his two series of sagittally cut sections of the adult brain. 
That in the lampreys (as, indeed, in many vertebrates) 
Reissner’s fibre appears in sagittal sections to penetrate the 
ependymal tissue of the rhombo-mesencephalic fold, I am 
perfectly" willing to admit. That it actually does not do so 
I am firmly persuaded. Sections cut absolutely truly in the 
