420 SAMUEL C. PALMER 
of the drawing, and 7 equals the thickness of the paper. In 
sections near the chiasma allowance was necessarily made for the 
lumen of the stalk, which becomes obliterated distally. 
B. Enumerations 
a. Visual cells. The sections used in enumerating the visual 
cells were cut tangentially to the surface of the retina in the five 
regions already referred to, passing through the outer segments 
of the rods and cones, and through the paraboloids of the double- 
cones (fig. 1, ab). The visual cells, therefore, were seen in cross- 
section, and because of their structure, larée diameter, and differ- 
ential staining qualities they were easily distinguished from one 
another. I was unable to detect any plan in the arrangement 
of the rods and cones, such as Schwalbe (’87) has depicted, but 
noted a marked tendency for five or six elements of a kind to 
run in lines, a feature which I think is without real significance. 
The association of rods or cones in groups of six or eight. (figs. 
6 and 8) was of frequent occurrence. Their usual distribution 
may be described as irregularly scattered, and free from contact 
along their outer segments with neighboring elements (figs. 5 to 9). 
With a magnification of 615 diameters the cross-sections of the 
rods were bright red, circular to oval in outline, and with the 
characteristic vacuolated structure described by Howard (’08). 
They were larger than the cones and somewhat more numerous. 
The cones appeared as small round, blue-black, homogeneous 
bodies scattered irregularly over the field; the double-cones were 
far less numerous and much larger than either the rods or the 
cones. As one should expect, the paraboloids were fused along 
one side (figs. 5 to 9), giving the appearance of double-elements. 
Their structure was reticular and took the stain only slightly. In 
addition to the elements already mentioned, there were a few 
whose identity could not be established with certainty, due, I 
think, to disintegration having set in before they were reached 
by the fixative fluids. They constituted only a very small per- 
centage of the total number of visual cells. 
Studies of the cross-sections of the cells in the five regions 
described, showing the close relation between the number of 
rods, cones, and double-cones, in the different positions, are 
