THE DIFFERENTIAL ACTION OF DYES 23 1 



as a basic dye it has general affinity for acidic tissue-constituents. 

 Some objects that do not contain DNA have a very marked 

 affinity for this dye. Certain globules in the 'vitelline' glands of the 

 liver-fluke, Fasciola, are an example.*^* Attempts to raise this dye 

 to the status of a histochemical reagent are misplaced. For a full 

 discussion of this subject, see Sandritter.^^^ 



It is claimed ^^^' ^"'^ that pyronine has a strong affinity for nucleic 

 acids that are depolymerized and methyl green for those that are 

 highly polymerized, and that the distinctive reactions are due to 

 the fact that RNA occurs in the tissues in a feebly polymerized 

 and DNA in a highly polymerized form. It is not known why the 

 two dyes should differ in this respect, if in fact they do. It is to be 

 remarked that in the absence of pyronine, methyl green will 

 colour the chromatin in tissues that have been subjected to a 

 sufficient degree of acidity to depolymerize DNA.^^^ 



Certain dyes are metachromatic ; that is to say, they are capable 

 of imparting one colour to certain objects and another to others. 

 This forms an important distinction between one dye and another; 

 but the difference is not exactly one of affinity, and metachromasy 

 will therefore be considered separately (p. 243). 



Density 



Authors often say that the cytoplasm of a particular cell is 'dense', 

 but in fact it is very difficult to find out how much matter 

 there is in a microscopical object. We can tell that a nucleolus is 

 denser than the nuclear sap if we see it fall under the influence of 

 gravity in a living cell,^^^ but this kind of opportunity seldom 

 presents itself. Phase-contrast may help us, but there are plenty of 

 traps for the unwary.*^ A study of the Becke line effect followed 

 by the use of the interference microscope is the surest guide, but 

 the actual measurement of the refractive index of microscopical 

 objects is not in any circumstances easy. No information on the 

 subject of density can be obtained by simply noticing the depth 

 of colouring with a dye. 



In life, the greater part of the cell is made up of protein chains 



intimately associated with water through -C. , -OH, -C — O, 



-NH2, and other hydrophil groups. ^^° The substance resulting 

 from this association usually has a refractive index not very far 



