574 HOWARD. [Vora xi 
condition. The animals were anesthetized, their hearts 
exposed, and the fixing fluid injected into the arteries as in 
ordinary injections for the demonstration of the arterial 
system. 
Some of the fluids used were aqueous mercuric chloride 
containing 5 per cent. acetic acid, Perenyi’s fluid, 11% per cent. 
osmic acid and vom Rath’s (’95)_ picro-platino-osmo-acetic 
mixture. The first two penetrated most successfully. The 
osmic preparations were only partially successful, for, owing 
apparently to the rapid constriction of the blood vessels, a 
smaller amount of the fluid reached the interior of the eye 
than by the other. methods. iter injection, the eyes were 
removed and placed in the fixing fluid, or else the whole head 
was immersed and the eye not opened until they were thor- 
oughly hardened. Eyes thus prepared were imbedded in 
melted paraffine and cut for longitudinal or transverse sections 
of the rods and cones. 
For a satisfactory study of the rods and cones it is usually 
necessary to free them from surrounding pigment. This may 
be done by bleaching on the slide, or, while the animal is 
alive, by keeping it in a dark box for a few hours, or con- 
veniently over night, and then fixing the eye without exposure 
to the light, I got best results in Necturus by aneesthetizing 
with chloretone in the dark box. Chloretone was used to 
avoid the irritation and consequent advance of pigment which 
ether and chloroform seem to produce. The method described 
causes the retinal pigment to withdraw completely from the 
region between the rods, into the bodies of the retinal pigment 
cells. : 
Bleaching can be resorted to conveniently, if there is no 
reason for avoiding the necessary chemical treatment. This 
is obviously the most practical method of studying the rods 
and cones in their “light” phase. It also has the advantage 
that the rods and cones are protected, and prevented from 
breaking by the slipping of the retina over the pigment layer, 
when portions of the eye are removed for sectioning. Bleach- 
