26o DYEING 



These alkaloids were tried as chlorides, in experiments carried out 

 in solution. It would be of interest to make a full study of micro- 

 scopical preparations of the parts of those plants and animals that 

 contain these substances in suitable form. The alkaloids themselves, 

 as they occur in the bark or seeds of plants, might be unreactive, 

 but sections of the testes of the appropriate fishes would pre- 

 sumably be suitable research material. It might be possible to ex- 

 tend the list of acid dyes known to be metachromatic, and to find 

 some indication of chemical similarity between them. 



Haematein, used as an acid dye without lake-formation, can in 

 certain circumstances show a metachromatic effect. ^^^' ^^^ In solu- 

 tion in 40% alcohol it colours the cytoplasm of certain nerve-cells 

 red, but the nucleus homogeneously blue. The orthochromatic 

 colour of this dye, used substantively, is reddish, and the meta- 

 chromatic shift is thus bathochrome. It would appear that the 

 nuclear sap, not the chromatin, is coloured. The facts have not 

 been explained, but it is tempting to suppose that the histone of 

 the nuclear sap may be the chromotrope. 



Certain acid dyes, particularly those that are disulphonates, 

 show a strange form of bathochrome metachromasy. The two best 

 examples are Congo rubin (not Congo red) and Bordeaux red, both 

 azo dyes. If one of these is injected into the abdominal cavity of a 

 mouse, coloured particles are later found in the phagocytic cells 

 of various parts of the body, that is, in the histiocytes and cells of 

 the reticulo-endothelial system. The uptake of certain dyes by 

 these cells will be considered in a general way in a later chapter 

 (p. 276); here it is only necessary to say that the dyes do not colour 

 pre-existing objects, but are segregated in the form of granules. 

 The interesting fact about the particular dyes with which we are 

 concerned here is that many of the granules show the metachro- 

 matic (blue) colour. 



It might be thought that the colour-change was connected with 

 pH, since Congo rubin, like Congo red, is an indicator; but this is 

 not so, for the metachromatic change is not dependent on pH, and 

 Bordeaux red and other metachromatic acid dyes are not indica- 

 tors. It seems almost certain that this kind of metachromasy is due 

 to polymerization of the dye.*^^ In concentrated solutions the dyes 

 concerned show the beginnings of a bathochrome colour-shift. 

 There are marked differences, however, from the polymerization 

 of basic metachromatic dyes. Not only is the shift in the opposite 

 direction, but it is aided by the addition of neutral salts such as 



