ON SECTION CUTTING. 9 
removal from the picro-carmine, and then placing in 1 per cent, 
acetic acid for an hour before transferring to alcohol. 
4, Magenta. Stains very rapidly, but diffusely: the colour 
also is not permanent. Useful for fresh specimens of infusoria, 
hydra, and some other of the lower animals. 
5. Silver nitrate. A 1 per cent. solution in water stains the 
intercellular substance, which binds together the several cells of 
a tissue, much more strongly than the cells themselves, and is 
therefore chiefly used when we wish to render prominent the 
outlines of the individual cells. The specimens should be placed 
fresh in the silver solution for from two minutes to a quarter 
of an hour, then washed thoroughly with distilled water and 
exposed to the light until stained sufficiently deeply, when they 
may be mounted in glycerine. Such preparations are rarely 
permanent, as the reduction of the silver, to which the staining 
is due, continues until the specimens ultimately become too 
dark to be of any use. 
6. Osmic Acid. A 1 per cent. solution of osmic acid in water 
forms an extremely useful staining reagent. It is especially 
useful for the detection of fat which is stained by it a dark 
brown or black colour. Specimens, which must be quite fresh, 
should only be left in it a few minutes, and may then be 
mounted in glycerine or else dehydrated and mounted in balsam. 
7. Acetic Acid. Although not strictly a staining agent 
inasmuch as it does not colour the specimens, acetic acid may 
conveniently be mentioned here as it is used for the same pur- 
pose as the true stains, z.e., for the sake of rendering certain 
parts of the cells especially distinct. Acetic acid, of which a 
1 per cent. solution is employed, causes the protoplasm of cells 
to swell up and become transparent, and brings the nuclei 
into special prominence. It is used with fresh specimens, 
VII.—ON SECTION CUTTING. 
Many tissues and organs can only be studied satisfactorily by 
cutting them into thin sections, and this method of investiga- 
tion is of such importance as to require special notice. There 
are three chief stages: Hardening, Imbedding, and Cutting, 
which will be noticed in succession. 
A. Hardening. 
Before the object can be cut into sections it is necessary to 
