for three to five days. Excess of bichromate is removed from the sections by blotting paper, and they are transferred to the freshly prepared staining mixture:—
| Phospho-molybdic acid (10 per cent.) | 1 | minim | 2 | drops. |
| Nitrate of silver (1 per cent.) | 1 | ounce | 60 | c.c. |
which must not be filtered. Stain for several days.
The sections should be cut at once after removal from the staining solution. It is claimed that the minute details of structure of the cell processes are better shewn by this method.
Corrosive sublimate method.—This method is similar in its mode of action to the last, mercury being deposited in the cell instead of silver. It is rather less certain and requires more practice. It seldom stains uniformly. One cell will be found exquisitely stained while those in its vicinity are unaffected.
Small pieces of cortex are hardened for several weeks in Müller’s fluid, or other bichromate solution, and are then transferred direct to a one-half per cent. aqueous solution of corrosive sublimate, in which they should be left from three to six weeks. Shorter periods will only give disappointing and inconstant results. Sections should be cut, if possible, in gum. They may be mounted in Farrant, or dehydrated and mounted in balsam. Tal has proposed to render the effect sharper by transforming the deposit of mercury into mercuric sulphide, by treating the sections with a solution of sulphide of sodium, which he prepares by saturating a ten per cent. solution of caustic soda with sulphuretted hydrogen and then adding an equal quantity of fresh soda solution. They are stained in this for a few minutes and then thoroughly washed.
By this method the pyramidal cells and their delicate processes appear as black opaque objects on a light ground. The neuroglia cells with their fine delicate processes are often also beautifully stained.
Nissl’s aniline method.—This method is complementary to Golgi’s method. The latter impregnates the cell rendering it opaque and shewing its form with great definiteness.
Nissl’s method stains the protoplasm without greatly reducing its transparency and allows us to study details of cell structure. Small portions of tissue, removed as soon as possible after death, are hardened in alcohol. Sections are then cut, preferably in gum, as celloidin is inconvenient owing to its staining so deeply with aniline dyes.