RETINA OF ALLIGATOR MISSISSIPPIENSIS 209 
the nuclei are very close to the basal cell boundary. The pig- 
ment processes reach as far as the inner segments of the visual 
cells, the outer segments being deeply imbedded in the guanin- 
containing portion of the epithelium. 
Krause ('93) described the visual cells in the retina of Alligator 
mississippiensis and quotes from Hofmann's description of the 
retina of Crocodilus vulgaris. In the alligator, Krause considers 
that the rods could be taken for small cones, because the slender 
inner segments are slightly tapering, while the outer segments 
are almost cylindrical. The inner segments of the cones, on the 
other hand, are thick and the outer segments short and pointed. 
Hofmann describes the rods of the crocodile as numerous except 
in the fovea and the surrounding regions of the retina. They 
are very similar to the red rods of the frog, and Krause reproduces 
a figure from Hofmann of a rod and cone. The cones are single 
and doable. Krause reproduces (again from Hofmann) cones 
with very long, pointed outer segments from the fovea. There 
are no rods in the area and onty single cones, the inner segments 
of which become narrower as the fovea is approached. Accord- 
ing to Krause, the nuclei of the visual cells in Alligator mississip- 
piensis are all in one row, the cone nuclei being rounder than those 
of the rods. In the crocodile, Hofmann says that they occupy 
two rows, with the rod nuclei next to the external limiting 
membrane. 
Abelsdorff ('98) considers that very strong support is given 
by the conditions in the reptilian retina to the view first put 
forth by Max Schultze, that the rods serve for the reception of 
weak and colorless light stimuli. He calls attention to the fact 
that most reptiles have practically only cones, the exceptions 
being the geckos (in some of which the cones seem to be entirely 
lacking), the crocodiles, and the boa. These are all nocturnal 
animals. The crocodile, he says, on account of its rod-rich 
retina, is not only capable of seeing in a very weak light, but can 
find its way about in pitch darkness, this property being enhanced 
by the light reflecting tapetum in the upper portion of the eye, 
the rods being thereby doubly stimulated. Abelsdorff points 
out that it is particularly in water that the upper part of the eye 
