210 HENRY LAURENS AND S. R. DETWILER 
needs an increase in intensity of the light impression more than 
does the lower part of the eye, because the upper portion receives 
only what little light may be reflected from the bottom. 
He figures the tapetum in a sagittal section where it can be 
seen in the upper portion of the eye between the choroid and 
retina proper, going over in the lower portion of the eye, with 
a gradually increasingly thick black border, into the guanin-free, 
melanin-containing portion of the pigmented epithelium. At- 
tempts to demonstrate a change in position of this pigment in 
light and darkness were unsuccessful. It is interesting to note 
in this connection that Garten ('07, p. 89) considered it worth 
while to have this experiment repeated, which he did, with 
results (p. 108) substantiating those of Abelsdorff. Abelsdorff 
describes the rods of the alligator as being similar to those of 
frogs, but of narrower diameter. He found that the visual 
purple, investigated by direct observation of the opened eye 
as well as ophthalmologically (assisted therein by the presence 
of the tapetum), was not confined to the upper portion of the 
eye, but, by turning the retina over and looking at it from the 
visual cell side, could be seen as well in the lower portion. He 
investigated the bleaching of the purple in dayhght as well as 
the relative amount of bleaching and the time relations upon 
exposure to hght of various wave length. The fact that the 
purple was seen throughout the eye would seem to indicate that 
the rods occurred throughout the retina, although, if one chose 
to follow Edridge-Green, it might be assumed that the purple 
diffused into the regions where the rods were not present. 
Finally, Garten ('07, p. 109) describes the visual cells in 
Alligator lucius. In the upper part of the retina (guanin portion) 
there are large cyhndrical rods only, which are surrounded in 
the light as well as in the dark eye by a mantle of guanin. This 
part of the retina is absolutely cone free. In the lower portion 
of the retina the visual cells are relatively small, possessing a 
very thin tapering outer segment, which in hght as well as in 
dark eyes is buried in pigment. Garten considers these to be 
all cones. He refers to Abelsdorff as having described visual 
purple in the lower portion of the retina, and thinks, since he 
