HISTOLOGICAL ELEMENTS RETINA NECTURUS 409 
as the rods, which would make the enormous total for the outer 
nuclear layer in Phoca barbata more than 4,000,000,000 nuclei. 
In all the species mentioned in table 1, the number of nuclei in 
the inner nuclear layer approached more closely that of the rods, 
being slightly less in most cases. Equally important is the com- 
paratively small number of optic nerve fibers associated with 
this enormous number of retinal cells. Pitter estimated that 
in Balaenoptera physalus there were only 13 optic nerve fibers 
per square millimeter, giving a total of 157,000 for the entire 
nerve. In this case there would be about 5000 visual cells to 
a single optic nerve fiber. Considerable difference was found 
between the number of elements in embryonic and adult retinas. 
As a rule the nuclei were more abundant in the former. 
Investigation into the numerical relations of the retinal cells 
in other mammals, though meagre and fragmentary, indicate a 
condition similar to that in man. Chiarini (’06) expressed in a 
general statement concerning the visual cells of the dog, the un- 
satisfactory condition in which we find this problem in mammals, 
when he said, ‘‘ The cones are less numerous than the rods.” 
Franz (’09) has given a definite enumeration of the numerical 
relations of the retinal elements for the central and peripheral 
regions of the retina in birds. The nuclei were found in eight 
species to be more numerous at the center than at the periphery. 
In the fundus of the retina of Motacilla alba there were on a 
line 0.1 mm. in length 266 nuclei in the inner nuclear layer, 60 
in the outer nuclear layer, and 50 in the ganglionic layer; but 
over the same distance at the periphery there were only 40 nuclei 
in the inner, 20 in the outer, and 4 in the ganglionic layer. To 
find the total number in one square millimeter, Franz multiplied 
these numbers by 10 and squared the product. On this basis 
the fundus was stated to have in Motacilla 250,000 ganglion 
_ cells and 360,000 rods and cones, in Bubo 36,000 ganglion cells 
and 78,400 rods and cones to a square millimeter. 
I have been unable to find any enumerations of the retinal 
elements of reptiles. It is important to note, however, that the 
so-called rods are said to be wanting in some species. Thus 
Ramon y Cajal (’94) says, ‘“‘In die Retina der Eidechse die Stib- 
