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 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 (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 



