436 SAMUEL C. PALMER 
V1. THEORETICAL CONSIDERATIONS 
Theoretical consideration of the optic functions is not the pri- 
mary object of my investigation, and I shall enter into it only so 
far as my research indicates exceptional conditions in Necturus. 
I have found in the visual cells that rods and cones alike develop 
at the extreme margin of the retina, the initial. element being 
indifferently a rod or a cone. Since this is so, whatever differ- 
ences there may be in the light perceiving potentialities between 
central and peripheral regions, they must depend largely on the 
degree of the functional development of the visual cells in these 
regions. The presence of rods and cones alike over the whole 
retina—if the theory that rods are organs for light and shadow 
perception and cones for color preception is true—indicates 
power to distinguish colored lights as well as light and shadow 
at the periphery as well as in the fundus. In this connection 
Reese (’06) found that Necturus responded to both red and blue 
lights, but since no analysis of the lights for purity of color or 
intensity was made, it is not evident to what the reactions were 
due. Pearse (’10) has shown that Necturus reacts readily to 
ordinary light stimuli through both the eye and the skin. From 
these investigations it appears that Necturus is capable of dis- 
tinguishing colored as well as white light, but on this point there 
is need of further investigation. 
I know of no positive evidence that horizontal cells exist in 
the retina of Necturus, but I know of no other explanation for 
the group of cells referred to on page 423 (figs. 2 and 3, ). The 
arrangement of the nuclei in the inner nuclear layer seems to 
me to preclude the existence of such cells in that layer. Since 
I have already shown that the nuclei of Miiller’s fibers may mi- 
grate or be pushed outward in some way from the inner nuclear 
layer into the outer nuclear layer, it is possible that a similar 
change in position has taken place with the horizontal cells, and 
that in time they have become permanent constituents of the 
outer nuclear layer. Investigation into the identity of these 
cells by Golgi’s, or some other stain equally valuable for nerve 
processes, is very desirable. 
