64 
GEORGE E. NICHOLES. 
one instance. It must, liowever, be remembered that the 
material had not been preserved for the study of this structure, 
the animals, in every case, having been decapitated. Under 
the circumstances, therefore, it is surprising that the fibre 
had been preserved in the normal position in any. The 
explanation, in this case, appears to be tliat, in some pre- 
liminary exposure of the brain, blood had found its way into 
tlie ventricles, and, clotting there, firmly secured the fibre 
from recoil. The condition of Reissner’s fibie in this speci- 
men is represented in fig. 57. The clot above referred to had 
filled the fourth ventricle and extended into the mesocoel, but 
the fibre can nevertheless be readily followed. It may be made 
out arising from the epithelium of the sub-commissural 
organ by many fine branches which run together into two 
principal factors. These unite, midway along the cavity of 
the midbrain, to form a single thread which passes back- 
wards into the fourth ventricle, traversing a deep isthmic 
canal upon the ventral surface of the i hombo-mesencephalic 
fold. Behind the fourth ventricle the fibre may be followed 
to the end of that part of the can alls centralis included in 
the piece sectioned. 
It is important to note that just as the two halves of the 
sub commissural organ in Geotria are tending to become 
mei'ged into a single median structure, so also the two halves 
of the fibi-e are more completely united. Their union occurs 
well forward in the mid-brain, whereas in Petromyzou and 
Ichthyomyzon,^ in which the grooves are distinct and widely 
separated, the two factors of the fibi-e only lose their identity 
at a point far more posterior, beneath the rhombo-mesen- 
cephalic fold. The isthmic canal in these two latter forms 
may accordingly show more or less definite traces of a paired 
character, but in Geotria it is a simple median groove. 
Three other series of sections also show particularly well 
the paired origin of the fibre in Geotria. 
In one of these the whole length of the fibre, from the sub- 
commissural organ to the end of the piece of the spinal coi-d 
‘ See below. 
