STEUCTURE AND DEVELOPMENT OF REISSNER’s FIBRE. 61 
The second source, according to Sargent, of the constituent 
tibrill^ of Eeissner’s fibre is the group of large cells lying- 
somewhat lateral to the two halves of the sub-coinmissural 
organ. These, as already stated, are cleai-ly part of the 
nucleus of the posterior commissure (Cf . Sargent, ’04, pi. i, 
fig. 7, and Johnston, ’07, fig. 132). 
I have been quite unable to recognise the multipolar cells 
in the right habenular ganglion from which Sargent supposed 
a pre-commissural unpaired part of Reissner’s fibre to be 
derived. As already repeatedly stated, I find the fibre 
arising as a structure, paired even in front of the posterior 
commissure, from the paired grooves of the sub-commissural 
organ, the two halves of the fibre remaining distinct in 
Petroinyzon fluviatilis practically for the entire length 
of the mid-brain. 
Although perhaps not directly related to the subject of 
this paper, I propose to make brief mention of certain other 
large and conspicuous nerve-cells in the roof of the brain. 
These are situated, in Petromyzon fluviatilis, high up 
on the walls of the hind brain some little way behind the 
cerebellum. They occur usually just beneath the ependymal 
epithelium, through which they often appear to bulge so as 
to lie exposed, practically, in the fourth venti icle. Traced 
backsvards, these are seen to form a paired longitudinal 
series in which the two lines approach each other till that 
point is reached where the walls of the hind brain overarch 
and meet above the fourth ventricle. At this point the two 
converging lines of large cells become pai-allel and pass back- 
ward as that well-known paired tract of giant-cells which 
are such a conspicuous feature of the spinal cord of many of 
the lower vertebrates. 
Thus in the common river lamprey these giant-cells do not 
end, as is said by Johnston (’02, p. 5) to be the case in the 
brook lamprey (P. wilder!), at the commissura infima, 
but continue forwards in the hind biain to about that point 
which coincides with the forward limit of the neural crests of 
the embryo. 
