HISTOLOGICAL ELEMENTS RETINA NECTURUS A417 
black spots of different sizes. In a longitudinal section individual 
fibers could be traced only a short distance. At the distal end 
they spread out in bundles along the inner margin of the retina 
and disappeared radiating outward between the nuclei of the 
ganglionic layer. I was wholly unable to detect a union between 
the ganglion cells and the optic nerve fibers, and I am unable 
therefore to state the exact relation of the optic nerve fibers to 
the retinal elements. 
IV. OBSERVATIONS 
A. Measurements 
a. Retinal layers. The eyeballs of Necturus lie well forward 
on the dorso-lateral aspect of the head, where they appear as 
shghtly arched whitish bodies. They are unusually small for 
the size of the animal, and are approximately spherical in shape. 
The retina, which is closely applied to the sclera, has, therefore, 
the shape of a zone of one base whose area is only slightly less 
than that of a hemisphere. Since the area of a zone of one base 
is equal to the area of a circle whose radius is the chord of the 
generating arc, the area of the retina in Necturus is equal tothe 
area of a circle whose radius is the shortest distance from the 
periphery of the retina to the center of its fundus. To obtain 
this center I made a camera drawing ( X 101) of a median section 
of the eye, and erected a perpendicular at the middle point of 
the chord joining the ends of the retina at the ora. The point 
of intersection of the perpendicular and the retina marks the 
center required. The chord joining this center to the periphery 
is equal to the radius of a circle whose area is that of the zone 
of one base, which marks the limits of the retina. In many cases 
the center lay directly in the optic nerve, while in nearly all the 
remaining eyes, it lay very close to that nerve. By this method 
I have calculated the minimum, maximum, and average areas 
of zones of one base (which hereafter I shall refer to as zones) 
for 14 retinas, the three zones coinciding with the external lim- 
iting membrane, the middle part of the inner nuclear layer and 
the middle of the ganglionic layer (fig. 1, cd, xz, vw). The results 
