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 dippedinto 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 proto¬ 
plast. 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 
judged not only as to killing and hardening but also as to reaction 
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 (OsCL) comes in sealed glass tubes containing 
0.5 gm. or 1 gm. It has a very powerful odor and is easily affected 
by organic materials. It is used in 1 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 (Cr 0 3 ) in 0.5 to 1 per cent, aqueous solution is very 
favorable for nuclear structure but like osmic acid penetrates rather 
slowly. Picric acid C 6 H 2 (0H)(N02)3 is one of the most penetrating 
