HISTOLOGICAL ELEMENTS RETINA NECTURUS 419 
based on an area obtained for the zones at the external limiting 
membrane; Miiller’s fibers and ganglion cells for the zone through 
the ganglion layer; and nuclei for the inner nuclear layer from 
the area of the zone passing through the middle of this layer. 
The number of unit areas in each zone was found by dividing 
the number of square millimeters in the zone by the number of 
square millimeters in the unit area. The total number of elements 
in a layer obviously equaled the product of the number of areas 
into the number of elements per unit area. The methods em- 
ployed for the other layers were evidently unsuited for enumer- 
ations of the inner nuclear layer, for in this layer we have to do 
with solid content rather than surfaces. For this layer I have 
resorted to counting the nuclei in a cylinder with a head of 0.0078 
sq. mm. and with its long axis placed radially. The length of 
the cylinders was determined by the number of microtome sec- 
tions necessary to pass completely through the layer, the number 
of sections being eight in each of the two cases:counted. The 
number of such cylinders was found by dividing the area of the 
zone of the inner nuclear layer by the area of the cylinder head. 
In all cases where elements touch the boundaries of the unit 
areas (figs. 5 to 9) they were rated at one-half their value within 
the field. 
b. Cross-sections of the optic nerve. The areas of the cross- 
sections of the optic nerve varied according to their location 
(figs. 10 and 11). In all cases examined the nerve was very much 
smaller proximally than distally, in one instance the former 
amounting to little more than one-third the latter (table 15). 
My method of calculating the cross-sectional areas of the nerves 
was modelled after that employed by Parker (’95) on the optic 
nerve of Astacus. Camera drawings of all the sections to be 
measured were made on paper of uniform thickness and weight; 
these were then cut out and weighed. A standard unit 0.0015 
sq. mm. of the same magnification was also cut out and weighed. 
I could then easily obtain the required areas from the arithmet- 
ical formula: 
X « Y when T is constant 
where X equals the area of the cross-section, Y equals the weight 
