No. 3.] VISUAL CHELS \IN* VERTEBRALES: 500 
ditions determine the light reaction, it follows that if a fluid 
were found which preserved the same reaction as that of the 
fresh rod, one might infer that little violence had been done to 
that particular structural condition. Subsequent experiments 
showed that perhaps the test was too delicate for practical 
application to fixing fluids, since none were found which ful- 
filled that requirement. However, although none were found 
which preserved the reaction of normal rods, yet the rate of 
change and character of the final result furnished some index 
as to the relative merits of the fluids. 
1. Examination Media. 
For examination media the normal eye fluids would natur- 
ally be expected to be the best. In Necturus, however, the 
small volume of the fluid obtainable from an eye presents 
practical difficulties, there being scarcely enough in one eye 
after the necessary manipulation, to fill out one cover-glass. 
The eye fluids from the frog can be used for Necturus and 
make a satisfactory substitute. That the substitute is not 
perfect, however, is to be seen in the fact that the frog rods 
transferred with the fluid show better preservation after a 
given period, than do those of Necturus. Outer segments 
of rods of both species after twenty-four hours show in 
polarized light normal (positive) reactions but reduced bril- 
liancy. 
Glycerine (concentrated). Outer segments in one hour or 
less become granular and disintegrate somewhat, at the same 
time bending into horseshoe. shapes. After seven days the 
reaction to polarized light is normal except for reduced bril- 
liancy. The reaction of ligamentum nuchz was normal even 
after it had been two years in glycerine. 
Glycerme, 1 vol.; Water, 2 vols.; Alcohol, r vol. This 
mixture gave better preservation than glycerine and water 
alone, or concentrated glycerine, or in fact than any other 
purely artificial mixture tried. The reason for this, I suppose, 
