No. 3.] VISUAL CELES* IN- VERTEBRATES: 501 
case, so far as can be judged from all the evidence obtainable. 
Cross sections from osmic material appear very much the same 
as the optical sections of fresh rods. There is no reason to 
doubt that the small refractive circles which in both cases 
produce the crenation are identical. 
When the cross sections of osmic material are stained, 
after bleaching, a fibril is differentiated corresponding in 
position with the small circles or the ridges in the crenate 
outline and not to the grooves of this outline. The slight 
difference in size between the fibrils as seen in the fresh 
condition and when stained can be explained as due to the 
fact that a sharply outlined, stained fibril appears smaller 
than an unstained one, whose optical differentiation from the 
surrounding substance depends simply on a different index 
of refraction. Of course it is possible that the stained fibril 
might represent only the center of the small circles, analogous 
to the axis cylinder in the center of a medullated nerve. The - 
minuteness of the objects, however, forbids the certain deter- 
mination of this point with any apparatus one can command 
at present. 
The intermediate plate in the fresh condition, when discern- 
ible, appears as a disk at the proximal end of the outer seg- 
ment. It is apparently of a different refractive index from 
the latter, or at least is more transparent. 
The ellipsoid usually has the form of a convex-concave 
segment ofa cylinder lying within the substance of the inner 
limb; its diameter is only slightly less than the limb itself. 
The convex surface is distal. In the fresh condition the 
ellipsoid is more flexible than the paraboloid. From the con- 
dition presented by the fixed visual cells, however, the reverse 
would appear to be true. The probable explanation of this 
apparent contradiction is that the reagents used harden the 
ellipsoid more quickly than the paraboloid, the latter remaining 
for a longer time comparatively plastic. Immediately after 
death the ellipsoid is comparatively clear and highly refrac- 
tive, perhaps possessing the latter characteristic to a greater 
