The Retina and Optic Ganglia in Decapods, especially in Astacus. 5 
instead of appearing as continuous blue lines such as one sees in 
the fresh preparatious , are represented often by a single row of 
discouuected bluish dots. I, therefore, usually studied the fìner 
fibrillae in the freshly stained material, and employed sections only 
when it was necessary to determine the exact position of ganglion 
cells or nerve fibres. 
GoLGi's method, as well as that of Ehrlich, has the peculiarity 
of staining relatively only very few nervous elements. Of the more 
usuai methods by which ali the elements in a ganglion are stained, 
I know of none that gives such good results as the one recently 
suggested by vom Rath (93, pag. 102). According to this method, 
the tissue is fixed in a solution of osmic, acetic, and picric acids, 
and piatirne chloride, and afterwards »reduced« in crude pyroligneous 
acid (Holzessig). This method presents the double advantage of 
being unfailing in its results and of yielding preparations remarkably 
clear and trustworthy. 
For the study of the retina and retinal nerve fibres it was neces- 
sary to prepare depigmented sections, and for this purpose I used as 
a depigmenting fluid a 0.1^ aqueous solution of potassio hydrate 
in a way already described (cf. Parker, 90, pag. 3). In one respect, 
however, I bave improved this former method. Instead of using, 
as a fixative for the sections, a Sühällibaum mixture containing 
Squibb's flexible collodion, which sometimes allows the section to 
loosen in absolute alcohol, I now employ a mixture of the fixatives 
of Schällibaüm and Mayer. When small drops of each of these 
fluids are thoroughly mixed on a slide, a whitish sticky paste results, 
which, even in extremely small amounts, resists the loosening action • 
of both potash and absolute alcohol. The originai cloudiness of the 
mixture is due to the presence of oil, water, and glycerine; upon 
dehydrating the sections in absolute alcohol, this, of course, disap- 
pears, and mounted preparations are as clear as those made with 
ßither one of the fixatives. 
In the course of my work, it was necessary to make careful 
enuraerations of retinal elements and of nerve fibres. The optic 
nerve fibres can be counted in good transverse sections of the 
optic nerve, the Operation being more tedious than difficult. On account 
of the Variation in the size of the fibres, it was necessary to count 
ali the fibres in a nerve, the method of estimating by proportions 
giving rise to too great an error. The fibres were counted by means 
of a crossed-line eye-piece micrometer, such as is used for counting 
