CHAPTER XL 127 



blue and other thiazines are specific vital stains for nervous tissue, 

 certain safranin azo-dyes diazin-green, for example which have 

 no chemical relationship to the former, are also vital nerve stains ; 

 while similar compounds of the safranin series itself have no such 

 property. See MICHAELIS, Chemie der Farbstoffe, 1902, p. 104. We 

 must remember also that the conception of large protoplasmic 

 molecules with side-chains in the chemical sense, the so-called 

 ' biogens," is becoming more and more discredited. See HOPKINS, 

 Address to Physiology Section, Brit. Assoc., 1913. We have already 

 seen how complex are the physical factors that intervene in such a 

 simple case as the staining of paper, and to these may be added 

 questions of solid solution, distribution between phases, diffusibility, 

 and so forth. Indeed, it would seem that each individual case of 

 specific staining requires investigation by itself. EVANS (Amer. 

 Journ. ofPhysioL, vol. xxxvii., p. 255) in an investigation of " macro- 

 phages ' comes to the conclusion that chemo-receptors are not 

 responsible for vital staining with the benzidine and related series 

 of dyes. 



ALFRED FISCHER (op. tit., pp. 107- -150) gives interesting cases of 

 staining differentially particles of the same substance by different 

 dyes. Particles can be made to take up either dye, according to 

 their size, the order in which the different dyes are applied and the 

 degree of differentiation by removal of stain. 



205. Some Applications of the Theory of Staining. It is of interest 

 to see how some histological facts are explained in the theory 

 sketched above. Most of the structures in the living cell have 

 negative electric charges, probably on account of the slightly alkaline 

 nature of the surrounding fluids. This fact accounts for the ease 

 with which tissues in general are stained by basic dyes. It is remark- 

 able that haemoglobin is one of the few constituents that have a 

 positive charge. Accordingly, it is stained by acidic dyes, such as 

 eosin or acid fuchsin. The effect of electrolytes is shown in the 

 experiments of MAYR (Hofmeister's Beitrage, vol. vii., p. 560). 

 He finds that the affinity of Nissl bodies for basic dyes is abolished 

 by previous treatment with neutral salts. 



206. Objects of Staining. Most constituents of cells are, in their 

 natural state, either colourless or only faintly coloured. Thus they 

 are only visible if their refractive indices differ from those of the 

 media in which they are immersed. Such, for example, are fatty 

 globules and the granules of many secreting, cells. But, as seen 

 thus, it is not an easy matter to judge of their true forms. This is 



