35 



In some cases the differential staining may be accomplished in 

 the fixing (fats, impregnations). Differential staining, histo-chemical 

 methods, micro-chemical methods (as applied to the animal organism) 

 thus belong in the same category and rest upon a physico-chemical 

 basis. As in the case of fixation, a great deal remains to be done 

 in the perfection of this side of histological technique. 



78. Classification of Stains. Stains may be grouped: (a) 

 according to their chemical composition as (1) organic; hematoxylin 

 stains, carmine stains, anilin stains, (coal-tar dyes; benzene deriva- 

 tives), and (2) inorganic, (b) From another chemical aspect as (1) 

 basic or (2) acid, depending upon the chemical reaction of the staining 

 principle or group, (3) neutral ( 85). (c) Histologically, stains are: 

 (1) nuclear (chromatin stains), (2) plasma or general stains, (3) special 

 stains, (4) impregnations. The first are usually basic, the second 

 acid, the fourth inorganic. 



79. In toto staining. When in toto or bulk staining is em- 

 ployed, the piece of tissue is stained entire and imbedded and sec- 

 tioned afterwards. In this case the tissue should be stained before 

 the process of embedding has begun, after the washing out of the 

 fixer has been completed ( 7, 9). But a single stain may be given 

 and the one chosen is generally a nuclear one, hematoxylin, cochi- 

 neal, or carmine ( 91, 98 ). A counter stain may be given sub- 

 sequently after sectioning, orange G after hematoxylin or cochineal, 

 Lyon's blue after carmine being most satisfactory. In toto staining 

 is particularly useful in serial sections of embryological material and 

 in morphological work in general, as it saves time and manipulation. 



80. Section Staining. The application of the staining proc- 

 esses after the tissue is imbedded and sectioned. This is more 

 serviceable, especially if highly differential results are desired. 



81. Progressive and Regressive Staining. According to the 

 mode of application, staining is either progressive or regressive. 

 Progressive staining consists in permitting the staining to proceed 

 to the right degree of intensity and then stopping it. In regressive 

 staining the tissue is over-stained and the excess of stain removed 

 by the application of a Differentiator. Uusally in the regressive 

 method there is employed a Mordant which is a chemical solution 

 serving to make the stain "take." It possesses a double affinity, 

 to the tissue and to the stain, which is usually made to operate in the 

 interest of differential or selective staining. The mordant is not 



