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 Figure 21, with the cooling attachment devised 
by Dr. Land, will cut even ribbons, 1 n 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. Usu¬ 
ally 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, and other 
material for a study of mitotic figures, it is necessary that killing 
be very prompt. In a weak solution of chromo-acetic acid, nuclei 
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