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FUNDAMENTAL CONSIDERATIONS 23
sufficiently long to reach to the bottom of the bottle can be inserted
in the cork and used for transferring the material to the slide.
7. Ring mount with zinc white or asphaltum at the edge of cover
slip. If the cover slip is circular, this can best be done by means
of a centering turn-table. A camel's hairbrush is dipped into the zinc
white or asphaltum and held to the margin of the cover slip while
the slide fastened with clips to the turn table, is rotated with it.
8. Label slide.
If the objects or sections are such as not to be liable to shrink they can
be transferred from water directly to glycerin-gelatin.
TECHNIQUE OF FIXING, DEHYDRATING, HARDENING AND IMBED-
DING IN PARAFFIN
When the intention is to study the protoplasts in their natural
form or the processes of cell division, the fresh material must be
put through the various stages of fixation, hardening and imbedding
before it is sectioned. The steps will now be considered in the order
in which they must be carried out.
Fixation. This is the process of killing and coagulating the protoplast.
The essence of good fixation is in rapid killing. It should be
simultaneous with coagulation or hardening so that the protoplast
will not be modified by later treatment. Fixing fluids are always
substances unknown to protoplasm e.g. poisons. The coagulation
of protoplasmic structures is due to the fact that these are alkaline
in reaction whereas the fixing fluid is acid. Fixing fluids must be
but also as to reaction
judged not only as to killing and hardening
of tissues to stains afterward. Fluids that are mixtures make the
best fixing agents. Among the fixing agents employed are the fol-
lowing: Osmic acid (OsO 4) comes in sealed glass tubes containing
0.5 gm. or i gm. It has a very powerful odor and is easily affected
by organic materials. It is used in i to 2 per cent, solutions and
should be made up in distilled water. It fixes cytoplasm well but
the nucleus not as good. Its disadvantage lies in its inability to
penetrate rapidly.
Chromic acid (CrOs) in 0.5 to i per cent, aqueous solution is very
favorable for nuclear structure but like osmic acid penetrates rather
slowly.
Picric acid C6H 2 (OH)(NO 2)3 is one of the most penetrating