CHAPTER IX 

 THE PARAFFIN METHOD 



The paraffin method is still the most important of all histological 

 methods now in use. The results obtained by this method would 

 have been regarded as almost miraculous by the histologists of one 

 hundred years ago. At that time it was customary to observe 

 things dry, and no cover-glasses were used. Section-cutting with 

 sharp knives or razors did not become general until about 1830. The 

 need for an instrument which would cut sections without demanding 

 an extreme degree of manual dexterity was soon felt, but a successful 

 microtome did not appear until much later. The latest microtomes, 

 while rather complicated, give wonderful results. The Spencer 

 microtome, shown in Fig. 19, with the cooling attachment devised 

 by Dr. Land, will cut even ribbons, 1 p in thickness, from such 

 material as the antheridial receptacles of Marchantia. This means 

 that a series of sections can be cut from pollen grains or spores too 

 small to be seen by the naked eye. Many of the principles involved 

 in this method are general in their application, and some of the pro- 

 cesses are common to other methods. Before attempting the free- 

 hand sectioning, the beginner should read the following paragraphs 

 on killing and fixing, washing, hardening and dehydrating, and on 

 clearing. 



KILLING AND FIXING 



As stated in the chapter on " Reagents" (chap, ii), the 

 purpose of a killing agent is to bring the life-processes to a sudden 

 termination, while a fixing agent is used to fix the cells and their 

 contents in as nearly the living condition as possible. The fixing 

 consists in so hardening the material that the various elements may 

 retain their natural condition during all the processes which are to 

 follow. Usually the same reagent is used for both killing and fixing. 

 Zoologists, from humane motives, may use chloroform for killing, 

 while other reagents are used for fixing. In fixing root-tips, anthers, 



102 



