60 
GEORGE E. NICHOLES. 
3, and p. 17) do not form part of this “roof nucleus,” but 
belong to the nucleus of the posterior commissure. It is in 
these lateral]}’ placed cells that Sargent finds the second 
source of axons for the fibre of Reissner. The cells of the 
“Dachkern” in Geotria lie comparatively near to the middle 
line, scattered among the fibres of the posterior commissure 
immediately dorsal to the sub-commissural organ. 
It is principally iu Petromyzon f 1 uviatilis, however, 
that I have observed the “Dachkern” in this family. In 
this species it is moderately well developed, and is found to 
consist of a somewhat variable number of large cells which 
lie in the course of the posterior commissure. They are 
never very remote from the sub-commissural organ, against 
the inner border of which they mostly lie, either just above 
or to the outer side. Tlie paired character of the nucleus is 
somewhat obscured, tlie cells lying sometimes closely adjacent 
in the middle line, sometimes trailing out in uneven lines with 
usually more cells upon one side than the other. Their 
number, all told, does not appear to exceed two dozen, but it 
was somewhat difficult to determine accurately their precise 
number, for, owing to their large size, each may appear in 
several consecutive sections. Some of the largest cells have 
a maximum long diameter of about 30 micra with a short 
diameter of approximately 12 micra. Their size and shape 
ai e very variable, but iu all there is a large clear nucleus with 
well-marked chromatin network and one or two nucleoli. The 
diameter of the nucleus is usually about 10 or 11 micra. 
That these cells, in the various Petromyzoittidie referred 
to, represent the “ Dachkern ” of higher vertebrates admits, 
I think, of no doubt, but that their axons constitute a part 
or the whole of Reissner’s fibre I emphatically deny. It is, 
of course, possible that they are related to the cells of the 
sub-commissural organ, although 1 have not been able to 
establish this relation ; their principal axons appear to pass 
with the fibi’es of the posterior commissure to that side of 
the brain remote from the cells from which they arise, but 
beyond that point I have not attempted to trace them. 
