HISTOLOGICAL ELEMENTS RETINA NECTURUS 431 
V. DISCUSSION 
My intention in this investigation has been, as far as possible, 
to make a consistent enumeration of all the nervous elements 
in the retina and optic nerve in a single species of animal, and I 
have believed the logical choice of species to be some animal 
whose retinal cells are very large and few in number. Such an 
animal was naturally sought among the amphibians, and espe- 
cially among those species whose eyes and optic nerves have under- 
gone reduction in size, probably through diminished functioning 
powers, and whose histological elements were well known to be 
of unusual size. In Necturus I have found an animal in which 
both eyeballs and optic nerves are reduced in size; the retinal 
elements are extremely large, if not the largest known; and the 
optic nerve fibers are entirely non-medullated. 
The nearly spherical shape of the eyeball in Necturus and the 
close application of the retina to the sclera have made the meas- 
urements of the retina comparatively easy and have led, I believe, 
to accurate results. The method employed has been described 
on page 417. The individual retinal cells are so large and free 
from contact with one another that I have been able to count 
and number the individual cells in each unit area. Following 
this the application of the unit area to the area of the retina has 
been a matter of simple mathematical calculation. 
An attempt to estimate the total number of visual cells in the 
retina of Necturus by counting the rods, cones, and double-cones 
in a median section of the eye extending from periphery to 
periphery gave a total of 148,000 visual cells for an average sized 
retina. This number exceeds that found by the method finally 
adopted by about 31,000 cells or 28 per cent. Since in the latter 
method every element per unit area was counted and averages 
obtained as already stated, and the number of areas in a given 
retina is not subject to variation, I am convinced that the number 
of retinal cells in Necturus is close to the number obtained by 
this means, and that the method of counting elements in a line 
as a basis of enumeration of the visual cells should be rejected 
on the ground that it gives too large a number of cells. 
