RETINA OF ALLIGATOR MISSISSIPPIENSIS 225 
when investigated a divergence in the opinions of later investi- 
gators is the rule rather than the exception. 
The question as to what constitutes a rod and what a cone 
would seem to be a simple matter, but the great variety in the 
forms of the cells in different animals makes difficult a gener- 
alized classification. The contentions of Putter ('12) for a 
functional classification based on the type of connection of the 
visual cell with the bipolar, rather than a structural one, are 
good if kept within limits, but it seems to us that they are carried 
too far. As everyone knows, the foveal cones of man and mam- 
mals are cylindrical in shape, and are therefore much more like 
rods in general appearance than cones. But their known func- 
tion fits in with the general conception of the physiology of the 
apparatus for color and bright-light vision. Troland ('17) has 
pointed out that the shape of the foveal cones suggests that the 
function of the cone figure is structural rigidity rather than 
differentiation of response. 
Whether a visual cell is a rod or a cone is determined by the 
presence of one or more of three structural factors, viz., 1) the 
shape of the outer segment; 2) the shape of the inner segment, 
and, 3) the mode of connection between the visual cell and the 
bipolar cell. When we find visual cells which, from their general 
form (outer and inner segments), would be called rods, possessing 
terminal connections typical of cones (e.g., frogs, diurnal birds, 
see Ramon y Cajal, '94, pp. 31, 164, and '11, pp. 340, 327), 
there is no, or very little, reason why they should be called cones 
simply because they terminate in dendrites, and considerable 
reason for continuing to designate them as rods on functional 
as well as structrual grounds. The conditions in the geckos 
may be cited as another example. From description and illustra- 
tion it would seem as if no more typical rods could be found. 
Coupled with their structure there are functional characteristics, 
which will be referred to later, and which, from all that we know 
about rod vision, indicate that the visual cells are as functionally 
typical rods as can be found. Our work on the retina of the 
alhgator shows that rods as well as cones occur there, structurally 
as well as functionally, as exemplified in the inverse changes in 
