HISTOLOGICAL ELEMENTS RETINA NECTURUS 437 
Because of the great variability of the inner nuclear layer, I 
cannot help wondering what the influence of such variation may 
be on the sight of the animals. Can animals with five layers 
of cells in this region see better than those with two? Or does 
the number of cells involved have no’ effect whatever on the 
clearness of vision? 
Enumerations of the optic nerve fibers in cross-sections of 
the optic nerve show that distally there are nearly double the 
number of fibers that there are in the proximal portion; a con- 
dition which suggests at once a dichotomous division of the axis- 
cylinders. Such an interpretation of the increase of the fibers 
distally would mean that a majority of the optic nerve fibers in 
Necturus have their origin in the brain, which is the reciprocal 
_ of the condition found by His (’90), Assheton (’92), and Robinson 
(96) in other vertebrates. But Robinson also states that the 
optic nerve fibers arise in the retina and are more numerous near 
the retina even when the fibers have come to occupy the entire 
length of the optic stalk. The implication is that some of the 
fibers fail at this stage to reach the central organ. 
In my treatment of the optic nerve of Necturus with Bielschow- 
sky’s fluid, I have been unable to demonstrate a morphological 
connection between the ganglion cells and the optic nerve fibers, 
but the fibers seem to pass between the nuclei of the ganglion 
cells and enter the inner reticular layer. If the fibers can in any 
way be shown to be joined to the ganglion cells, then we should 
have in the adult Necturus a condition similar to that found by 
Robinson in the embryos of higher vertebrates. 
Since a direct union, then, between the optic nerve fibers and 
the ganglion cells is not an established fact, the origin of the 
fibers is a matter of speculation. Do the fibers originate wholly 
in the brain and pass centrifugally to the retina, branching with- 
in the optic nerve and ending freely in the retina? Or do they 
have their origin chiefly in the retina, in spite of the facts that 
staining with Bielschowsky’s fluid fails to show positive connec- 
tion between the ganglion cells and the fibers, and that only a 
half of them reach their destination in the brain? In consider- 
ation of these questions the influence of the degeneracy of the 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 22, NO. 5 
