No. 3.] VISUAL  GELLS “INVERTEBRATES. 507 
light, between the rods and the sheath of a medullated nerve 
(Pl. 5, Figs. 43, 44). When seen in optical section the sheath 
reaction is opposite in character to that of the axis cylinder, 
being yellow when that is blue and vice versa. It will be seen 
from the figure (Pl. 5, Fig. 44) that the sheath substance is 
neutral where the light comes from a direction at right 
angles to a surface tangent. Thus a single nerve fiber with 
its highly refracting medullary sheath appears most conspic- 
uously as two bright yellow or blue (depending on the angle 
of the microscope stage) parallel lines with a faintly colored 
axis cylinder between. The faintness of color in the latter 
is probably due to the predominance of the sheath reaction. 
The rod outer segments are chemically similar to the 
medullary sheath according to researches of Kuhne (see 
Schwalbe, 87, p. 104). The evidences for this are from 
such micro-chemical tests as the blackening with osmic acid, 
etc. If the rod were made up of plates of medullary substance 
with transverse nerve fibrils between, the relation of medullary 
substance would be as in the nerve trunks. These fine trans- 
verse nerve fibrils of the rod would not affect the light reac- 
tion perceptibly (in polarized light) unless in quantity great 
in proportion to the total mass of the rod. The longitudinal 
superficial fibrils of the rod are probably not sufficient by them- 
selves to account for the very distinct anisotropic reaction of 
the rod, because their mass is much less than that of the 
comparatively faint axis cylinders of nerves. The quality of 
the rod reaction in its brilliancy is more like that of the medul- 
lary sheath. 
The outer segments of the cones in Necturus show aniso- 
tropism, but not to so pronounced a dergee as in the rods. 
Outer segments from an animal kept in ordinary daylight 
and examined while in situ and in good condition gave the 
same reaction as rods; 1. e., positive with respect to the long 
axis. Cones from an animal kept in total darkness three days 
gave the long axis as the negative optical direction. This 
reaction is complicated in some way, for when the outer seg- 
