FROZEN SECTIONS 499 



The sections are floated into water, in which they unroll. Select a 

 good section and spread it smoothly on a slide which has been coated 

 with a thin, even layer of albumen-glycerin. Superfluous water is 

 drained off and the section pressed upon the slide with a piece of smooth 

 blotting paper, by exerting an even but not great pressure with the ball 

 of the thumb. The section will adhere to the slide. 



Now quickly cover the section with a small quantity of 95 per cent, 

 alcohol, followed in a few seconds by absolute. Pour from a drop bottle 

 quickly and evenly over the section and adjacent surface of the slide a 

 very thin solution of celloidin dissolved in equal parts of ether and abso- 

 lute alcohol. Drain off immediately, blow the breath once or twice on 

 the surface of the section and immerse the slide at once in water for a 

 few seconds. The thin film of celloidin thus formed fastens the section 

 to the slide. The solution of celloidin should be almost watery in con- 

 sistence, and so thin that it will form drops readily without stringing. 

 If it is too thin, it will not hold the section on the slide, while if it is too 

 thick, the layer on the slide will become white when it is immersed in 

 water. The film of celloidin should be so thin as to be almost invisible. 



The section may now be stained by any of the usual methods applied 

 to sections affixed to the slide. The thin layer of celloidin offers no ob- 

 struction to the staining. After staining, the section is dehydrated by 

 covering with 95 per cent, alcohol for a few seconds. Absolute is now 

 poured on and allowed to remain for a few seconds. This removes most 

 of the celloidin, but, unless the action is unduly prolonged, the section 

 will not be loosened. Clear in xylol. 



The microtomes and knives mentioned in this section are described 

 and directions for their use are given in Mallory and Wright's "Patholog- 

 ical Technique." Their use, however, is seldom learned except by personal 

 demonstration in the laboratory. 



4. Staining. 



The purpose of staining is to differentiate the tissue elements. The 

 staining of tissues is in a measure a micro-chemical color reaction, the 

 differential staining being due to the fact that certain elements take up 

 more of the stain than others. 



Stains used in microscopic work may be divided into two general 

 classes according to their chemical properties (i) basic stains, which 

 show especial affinity for the nuclei of cells and are called nuclear stains, 

 and (2) acid stains, which affect the cytoplasm more readily and are called 

 cytoplasmic stains. Certain so-called selective stains (either acid or basic) 

 affect one tissue element especially, or even exclusively. Preparations 

 may therefore be stained with several dyes, each affecting certain tissue 



