"STAINING 655 



the most useful dye for the purpose, and is used as described below, 

 except that it will require two or three days to penetrate the tissue. 



Regressive Staining, Staining in bulk is necessarily a regres- 

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

 decolorized. With borax carmin the decolorization is accomplished 

 by acid alcohol (hydrochloric acid 1 cc., 70 per cent, alcohol 100 

 cc.). 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 differentia- 

 tion of nucleus and cytoplasm being accomplished by the selective 

 affinity of the dye. Thus, certain dyes are nuclear, others are 

 cytoplasmic. 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 

 overstained ; in the latter case overstating is impossible, but de- 

 colorization 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 

 structure. A classification of this kind was advanced by Ehrlich 

 through his pupil, G. Schwartze,* and has been greatly elaborated 

 by Pappenheim.f 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 

 and the so-called basophile granules. Hematein, methylen blue, 

 methyl green, safranin, and basic fuchsin are examples. 



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

 have an affinity for the acidophile 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 give a specific tint to the so-called neutro- 

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

 eosinated methylen blue, etc. 



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

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

 affinity for particular tissues. This is an indefinite class which in- 

 cludes Weigert's elastic tissue stain, Mallory's connective tissue 



* Inaug. Dissert., 1880. f Grundriss der Farbechemie, 1901. 



