2s8 



PREPARATION OF SECTIONS 



and tissues can be laid bare in their finest details. These methods 

 are intended first of all to preserve the structure of the proto- 

 plasts in its normal form, and then to cut a single cell into several 

 sections while keeping these in their natural sequence, and 

 finally to stain the sections so that different structures will take 

 on different colors. 



The preservation of the structure of the protoplasts is ac- 

 complished by plunging the material into a solution, known 



Fig. 145. — Illustrating how the section knife or razor is drawn across the strop. 



as the fixative,' which instantly kills the protoplasts so that de- 

 composition incident to slow dying is prevented, and then harden- 

 ing the protoplasts by transferring the material to alcohol, be- 

 ginning with weak alcohol and gradually increasing its strength 

 until absolute alcohol is reached, so as to avoid undue shrinkage. 



The material is next imbedded in paraffin, and sections ad- 

 hering in ribbons are cut usually .005 mm. to .010 mm. thick, 

 and these after mounting on a slide and being freed from 

 paraffin are stained with two or three different stains and then 

 sealed in balsam in the form of permanent mounts. 



The processes thus briefly outlined will now be given in 

 detail. 



The Fixing Process. — For the study of the finer structures 



