No. 3.] VISUAL CEELS IN VERTEBRATES. 589 
The outer segments of the rods in Necturus are cylindrical, 
transparent, highly refractive bodies, measuring on the aver- 
age I2 micra in diameter by 36-40 micra in length. The 
distal end is rounded, but not enough to make the end hemi- 
spherical (a form usually assumed in fixed material). 
The most conspicuous finer details or markings in outer 
segments just removed from the eye, are visible under a mag- 
nification of 1,000 diameters at a low focus, but not at the 
lowest (PI. 2, Fig. 13). These are slightly oblique length- 
wise striations. They appear as dark lines with a lighter 
ground substance between them, and usually about eight to 
ten such lines are visible at once, evidently in focus on, cr 
very close to, the lower surface. On the upper surface, 
usually, only a single line or possibly two or three are visible 
at one time (Pl. 2, Fig. 12). The difference is probably 
due to the optical effect of the cylinder converging the rays 
from the lower curved surface, so as to make them visible in 
one plane, thus bringing them into the same focus. At a 
focus about the lowest at which anything is visible, the dark 
lines appear bright, while the substance of the rod is dark 
(Pl. 2, Fig. 14). These lines evidently represent less refrac- 
tive material separated by a more refractive substance. 
The outer segments, soon after the preparation is made, 
show a slight bulging of the detached proximal end, later 
this is more pronounced, and lines of transverse cleavage 
become visible (PI. 2, Fig. 15). The rod shown in Figure 
15 had been under a cover-glass three hours. In this case 
the longitudinal lines were plainly visible from the upper 
surface. The lines were not noticeably spiral in this instance 
and illustrate an individual variation in this respect. In this 
rod (PI. 2, Fig. 16) distinct plates could be seen separated 
from each other by small spaces. Occasional individuals in 
a field of detached outer segments, perfectly fresh, show 
dark transverse zones or bands, usually only one to three in 
a rod, as if some of the disks were darker than others. 
Transverse cleavage as thus described is not always found; 
