740 HISTOLOGIC TECHNIG 



days to penetrate the tissue. Delafield's hematoxylin also gives brilliant 

 results in bulk staining. 



Regressive Staining. Staining in bulk is necessarily a regressive 

 process, viz., the tissue is first overstained and then partially decolorized. 

 With borax carmin the decolorization is accomplished by acid alcohol 

 (hydrochloric acid 1 c.c., 70 per cent, alcohol 100 c.c.). Since the 

 stain is removed more rapidly from the cytoplasm than from the nucleus, 

 a differentiation is thus produced. 



Progressive Staining. In progressive staining the dye, having been 

 once taken up by the tissue, is not removed, the differentiation of 

 nucleus and cytoplasm being accomplished by the selective affinity of 

 the dye. Thus, certain dyes are nuclear (basic), others are cytoplasmic 

 (acid). The former possess a special affinity for the nucleus, the latter 

 stain both nucleus and cytoplasm. 



Certain dyes may be used either progressively or regressively ; in 

 the former case care must be exercised that the section be not over- 

 stained; in the latter case overstaining is impossible, but decolorization 

 must be watched with care. 



Classification of Dyes. Dyes may be classified according to their 

 affinity for certain granules or other portions of the cytoplasmic struc- 

 ture. A classification of this kind was advanced by Ehrlich through 

 his pupil, G. Schwartze (Inaug. Dissert., 1880), and has been greatly 

 elaborated by Pappenheim (Grundriss der Farbechemie, 1901). Such 

 a classification is very incomplete and unsatisfactory, but in a very 

 general way serves a useful purpose. The following is sufficient for our 

 present needs: 



1. Basic dyes, those which color the chromatin of the nucleus 

 (largely nucleic acid) and the so-called basophil granules. Hematein, 

 methylene blue, methyl green, safranin, and basic fuchsin are examples. 



2. Acid dyes, those which are usually cytoplasmic dyes, and have 

 an affinity for the acidophil (i.e., basic) granules. Such are eosin, 

 Congo red, orange G, methyl blue, and acid fuchsin. 



3. Neutral dyes, which result from a due admixture of acid and 

 basic colors, and which gives a specific tint to the so-called ueutrophil 

 or azure granules. Such dyes are Ehrlich's triacid mixture, eosinated 

 methylene blue, etc. 



4. Specific dyes, which result from the due admixture of dyes with 

 certain reagents, other dyes or chemicals, and which have a selective 

 affinity for particular tissues. This is an indefinite class which includes 

 Weigert's elastic tissue stain, Mallory's connective tissue stain, Sudan III 



