578 HOWARD. [Vov. XIX. 
so thin as not to be distinguishable from the nuclear membrane. 
This condition is evident in sections, but still more apparent 
in isolated elements (Pl. 4, Fig. 33). In such cases, ensheath- 
ing cytoplasmic material can be seen passing around the 
nucleus into the root-like process, the rod foot (Pl. 4, Fig. 
33, pd. bac.) with dentritic extensions into the outer reticular 
layer. 
The cone elements (Fig. 33) are of about the same length 
as the rods. The outer segments take a darker color than 
the other parts, but not as deep a shade as the rods. The 
osmic acid evidently (especially after longer fixation) has 
a destructive effect here, as the outer segment of the cone 
shows irregular fissures and has a broken appearance. ‘The 
other parts, corresponding with those of the rod, agree with 
these in color and appearance, indicating similar chemical 
behavior as regards osmic acid. However, there are some 
differences in form beside those of the conical outer segment ; 
thus the ellipsoid is longer and the paraboloid shorter, than the 
corresponding parts of the rod. 
Cross sections of the outer segments of the rod appear 
roughly circular with corrugated outline (Pl. 4, Fig. 36). 
At a magnification of 1,450 diameters the dark circle appears 
to be bordered usually by a light line, outside of which is 
an intensely black corrugated edge, which evidently repre- 
sents an enveloping sheath on the outer segment. In longi- 
tudinal thin superficial sections of the outer segment striations 
can be seen which correspond in number with the corruga- 
tions of the cross sections, and demonstrate that the outer 
segments of the rod are fluted (Pl. 4, Fig. 35). 
At a much higher magnification (3,800 diameters) the 
contrast between center and rim is more marked, the latter 
being blacker while the white band between the two appears 
to be interrupted by radial extensions of the central substance 
(Pl. 4, Fig. 34) corresponding with the depressions of the 
corrugations. This appearance would indicate that the light 
band is not simply the result of the optical effect of the sheath, 
