MAKING SECTIONS 71 



are given of individual staining methods. The purpose of this chapter is to 

 discuss only the general principles involved. 



It is assumed that the sections will be mounted in a solution of dried bal- 

 sam in xylene. The slide is removed from the xylene, drained and placed on 

 any convenient flat surface. A drop of the mountant is taken from the bottle 

 and liberally dropped over the surface of the sections. A coverslip of suitable 

 size (Fig. 52) is held at an inclined angle with a bent needle and slowly low- 

 ered so as to exclude all air bubbles. Then the edges of the slide are wiped 

 roughly, and the slide is returned to the hot table shown in Fig. 51 to evap- 

 orate the solvent used for the resin. 



Although this is the conventional method of operation, it is by no means 

 the best. In particular, there is a tendency to have a higher concentration of 

 solvent along the edges of the coverslip than is in the center, and it also takes 

 a surprisingly long space of time for all the solvent to be removed. It is much 

 better, if one can spare the time, to place a relatively thin coat of mounting 

 medium on top of the slide and then to allow the solvent to evaporate from 

 this on the surface of a hot plate. There is no risk that the slide will dry out 

 because the mountant will act as a varnish. It is needless to say that in fine 

 work it is necessary to cover the slide and hot plate with some dustproof 

 cover while this is going on. The next day the slide is examined and, if it 

 appears to be sufficiently varnished, the coverslip is placed on the surface. 

 Then the whole slide is warmed, while maintaining steady pressure, above the 

 softening point of the resin. The slide will be hardened as soon as it is cooled 

 and may be cleaned and put away. This custom of evaporating the solvents 

 from the surface of the slide rather than from the edge of the coverslip is 

 considered old fashioned nowadays, but there is no doubt that it produces a 

 better and more durable slide than does the more usual procedure. 



It must not be imagined that, just because all these directions have been 

 followed scrupulously, a perfect slide will result. There are nearly as many 

 things which can go wrong with a section after it has been cut (see Table 1) 

 as there are things that can happen in the course of cutting. Some of the more 

 important of these things are listed in Table 2, but it must be realized that 

 no amount of written instruction can take the place of experience. 



Frozen Sections 



There are two circumstances under which paraffin sections cannot be used: 



first, where it is desired to preserve in the tissues some fatty material which 

 would be dissolved out by the reagents used prior to impregnating; and, 

 second, when speed is of primary importance, as in the production of quick 

 sections from tumors for diagnostic purposes. In both these cases, recourse 



