RETINA OF ALLIGATOR MISSISSIPPIENSIS 229 
very question, especially in the lizards, for example, the geckos 
as compared with other lizards and particularly the horned toad 
which has a retina, from all structural indications, peculiarly 
adapted for day vision only. By investigating in the gecko 
and the horned toad the relative visibility of wave lengths of 
equal energy and the relative powers of adaptation, we will 
obtain information concerning the question of the selective 
response of rods to different wave lengths as compared with 
that of cones. Visual purple absorbs all wave lengths except 
a little red and violet. The rods therefore are presumably 
sensitive to all wave lengths except the extreme red and violet. 
Since rods are in general 'color-bhnd,' there is opportunity here 
of differentiating between wave length and stimulating value. 
In connection with the question as to which kind of visual 
cell represents the more primitive condition, the histogenesis 
of the retina is worthy of investigation. But the histogenesis 
of the neuro-epithehum is a subject about which our knowledge 
is most imperfect. In general, the rods are regarded as the more 
primitive type of visual cell (Graham Kerr, '19), while the cones 
are considered as specialized rods. 
According to Leboucq ('09), the tw^o kinds of visual elements 
develop simultaneously and are distinguishable only by the fact 
that the axis of the diplosome is perpendicular to the surface in 
the case of the rod and parallel to it in the case of the cone. 
Cajal ('11, p. 356) says that the cones and rods evolve in the 
same way and that it is difficult to distinguish between them at 
the beginning (also Fiirst, '04). 
Cameron ('11) reiterates a view earlier championed by him- 
self ('05) as well as by Bernard ('03) to the effect that the cones 
represent early stages in the formation of rods. We mention 
this here because of the fact that in looking over some slides of 
early amphibian embryos, the eyes do seem to contain nothing 
but cones or conically shaped elements. Graham Kerr (p. 137) 
finds that the visual elements (rods) in the retina of Lepidosiren 
shorten in the light and elongate in the dark, which is similar to 
to the usual behavior of cones and contrary to that of rods. 
THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 32, NO. 2 
