No. 3.] VISUAL~ CELLS SIN VERTEBRATES: 573 
The advantage of size outweighed the two disadvantages 
of a small eye and a thick sclera. The small size of the 
eye renders manipulation difficult in the removal of the retina, 
and limits the amount of available eye fluids for study of the 
retina in the fresh condition, while the thickness of the sclera 
hinders the penetration of fixing fluids. 
Two general methods of study were followed; permanent 
preparations were made according to the various devices of 
microscopical technique; and fresh material was studied while 
in the eye fluids, under as nearly normal conditions as possible. 
In the technique of the rods and cones certain peculiarities 
of the tissue have to be taken into account. As they are 
such unstable and comparatively delicate bodies, special fixing 
fluids must be used. The finding of a suitable fluid was a 
matter of considerable experimentation, in which the mere 
preservation of a natural gross form of the bodies was the 
least perplexing part of the problem. It was more difficult 
to find a fluid which would permit staining, and still do little 
damage to the internal structure of the element. The tests 
for these requisites were dependent, in the main, on results 
obtained after differential staining, though tests with polarized 
light were also employed as a check on fixation. The latter 
furnished interesting data, which will be taken up in detail 
under observations upon the effects of different fixing fluids. 
The difficulty of admitting fixing fluids to the retina with- 
out violent mechanical disturbance, is a problem which pre- 
sents itself in the technique of small eyes, especially when 
they possess a heavy sclera. Immersion of the whole eye 
in the fluid does not guarantee immediate fixation, while 
cutting open the eye usually causes a wrinkling of the retina, 
even if no mechanical injury results. Wrinkling, and buck- 
ling of the retina away from the choroid, seems to be due 
to different degrees of contraction of the sclera and the retina, 
when treated with the fluids. 
I found the following method most successful in preventing 
such effects, and in preserving the eye in an apparently natural 
