432 SAMUEL C. PALMER 
Where the retinal cells are very small and numerous the difficul- 
ties of obtaining accurate counts are greatly increased. Hess 
(05) has demonstrated in the retinas of Eledone and Sepia the 
great variation which exists in the number of visual cells in closely 
approximated regions in the same retina, and thereby implies 
the impossibility of obtaining reliable estimates of the total num- 
ber of visual cells in a retina from the number in a given ‘belt.’ 
It is commonly believed that the retinal cells in vertebrates 
are not evenly distributed over the retina. Franz (’05, ’09) 
states that the visual cells are more numerous about the fundus 
than near the periphery; Howard (’08) calls attention to the double 
layer of nuclei near the periphery in the outer nuclear layer in 
Necturus; and my own observations on the same species show a 
slight variation in the numbers of the different elements in the 
different regions. Consequently I believe that estimates based 
on counts in a restricted region, as for example, the fundus, or 
the periphery, give no fair idea of the number of elements in the 
retina as a whole, and my observations show that estimates 
based on counts of elements in a line of definite length exaggerate 
the number of cells in a given retina. Nor can I accept Piitter’s 
(02) method, viz., that of computing from the diameter of a 
rod the number of visual cells that may be found in a square 
millimeter and in the retina as a whole. Figs. 4 to 9 show that 
in Necturus the interstices between the visual cells make up a 
large portion of the area of a zone passing through the outer 
segment of the visual cells, and these spaces must be considered 
in the retinas of all species of vertebrates. Without considera- 
tion of these spaces the number of elements obtained would greatly 
exceed the actual number present. 
In order to avoid, then, what appear to me to be errors of meth- 
od, I have taken great care to secure accurate counts per unit 
area in a number of regions of the retina in several animals. 
The retina of Necturus lends itself to a count of this kind, because 
the visual cells, the external sheet of the outer nuclear layer, 
Miiller’s fibers, and the ganglion cells are each represented by 
a single layer of nuclei. 
