24 Animal Micrology 



to clearing, before mounting them on the slide. In such cases 

 the sections need not be fixed to the slide. With serial sections, 

 however, the sections must be held in place some way during 

 their transition through the reagents (see memorandum 12, 

 chap, vii) . Unlike paraffin, the celloidin is not ordinarily removed 

 from the tissues. 



DECOLORIZING 



Not infrequently in staining the tissue becomes overstained 

 and requires that some of the color be extracted from certain of 

 the elements to bring about a proper differentiation. The fact 

 that certain tissue elements retain stain more tenaciously than 

 others is sometimes taken advantage of and overstaining followed 

 by decolorization is practiced intentionally. Alcohol slightly 

 acidulated with hydrochloric acid (0.1 to 1 per cent.) is commonly 

 used for the extraction of surplus color. In special cases other 

 decolorizers are used: for example, iron-alum in the iron-hema- 

 toxylin method (reagent 18, chap. i). 



BLEACHING 



In some cases, tissues are obscured because of the presence of 

 natural pigments or on account of blackening caused by the fixing 

 reagent. Such tissues must be bleached. Chlorine, peroxide of 

 hydrogen, or sulphurous acid are commonly employed. A method 

 is given in chap, v, memorandum 12. 



CORROSION 



To obtain skeletal structures, as for example the spicules of 

 sponges or the hard parts of insects, various methods of corrosion 

 are employed. Nitric acid, caustic potash, caustic soda, eau de 

 Javelle are reagents often used for this purpose. Corrosive prep- 

 arations of injected vessels and cavities may also be made. 



DECALCIFICATION AND DESILICIDATION 



Tissues impregnated with lime salts or with silica must have 

 such hard parts removed usually before they can be sectioned. 

 For decalcification, one of several acids may be used. The details 

 are given in the chapter on bone, tooth, etc. (chap. xi). For de- 

 calcifying reagents, see Appendix B, v. 



