412 SAMUEL C. PALMER 
bury (’95), likewise, describes the optic nerve of Necturus as 
hollow for a portion of its length and states that its fibers are 
entirely amyelinic. This latter condition is said by Edinger 
(92) to be the case in the young of certain other amphibians. 
After a careful survey of all the literature at hand bearing on 
the numerical relations of the histological elements of the optic 
nerve and retina, I have failed to find for any vertebrate a con- 
sistent enumeration of these elements which has been carried 
out in such a way as to give reasonably safe grounds for compar- 
isons. I have, therefore, undertaken this task in reference to 
Necturus. 
Ill. MATERIAL AND TECHNIQUE 
‘ A. Material 
It has long been known that the retinal elements in amphib- 
ians are very large as compared with those in other vertebrates. 
Howard (’08) gives the dimensions of the outer and inner segments 
of the rods in several species of amphibians as follows: In the 
outer segment the length varied from 244 in Triton to 76 in 
Bufo, and the width from 6y in the frog to 12u in both Triton 
and Salamandra. The outer segments of the rods in Necturus 
are said to be 36 to 40u long and 124 wide. Measurements 
made by myself on the diameter of the cones of Necturus just 
distal to the ellipsoids averaged 4.8u, and through the ellipsoids, - 
where visual cells first come in contact with neighboring visual 
cells, they averaged 10u. The diameters of the rods through 
the ellipsoids was slightly larger than that of the cones. Slonaker 
(97) states that ‘‘“Amphibia have not only long rods, but the 
thickest found in vertebrates.’”? In speaking of the cones, he 
says, they have the ‘greatest diameter in mammals.’’ Table 
2 shows the relative sizes in micra of the rods and cones in rep- 
resentatives from all classes of vertebrates. The length of the 
elements is non-essential for my purpose, but the diameter is 
of especial significance because it is one of the most important 
factors in determining the number of elements which can occupy 
a given space, when they are crowded together as are the visual 
cells in the vertebrate retina. From the measurements quoted 
