290 IDYEING 



nerve-cells, axons, and dendrites is much the most familiar 

 example of the latter sort of vital dyeing. The special suitability of 

 methylene blue in this kind of work is unexplained. 



Cell-inclusions are diverse chemically, and quite different 

 processes are probably involved in the colouring of different 

 inclusions. The subject is very complicated, and any attempt at 

 facile presentation would be misleading. 



When Nile blue colours a lipid inclusion pink or pinkish, there 

 is every reason to suppose that the red oxazone present in solutions 

 of this dye simply dissolves in the lipid, so that in this particular 

 case there is no question of dyeing but only of colouring by 

 solution (p. 302). Bismarck brown is exceptional among dyes in 

 being fairly soluble in olive oil,*^^ and it possibly colours certain 

 lipid globules by dissolving in them. Neutral red is very slightly 

 soluble in olive oil, with a pale yellow colour, but the other vital 

 dyes are insoluble. However, since most vital dyes are somewhat 

 soluble in mixtures of olive oil with lecithin or oleic acid, it might 

 be thought that they colour lipid globules by solution. This is not 

 so, for when microscopical droplets of such solutions are examined, 

 there is not enough dye in them for the colour to be visible. ^^^ 



Somehow or other most vital dyes are aggregated by certain 

 lipid globules to a very high concentration. Seki relates this to 

 their tendency towards fiocculation. Many basic dyes are electro- 

 positive through a wide range of pH, but certain of them colour 

 collodion (electronegative) less strongly in alkaline than in acid 

 solution, which is the opposite of what one would expect. These 

 exceptional basic dyes are dahlia, Nile blue, thionine, azure II (a 

 mixture of azures with methylene blue), methylene blue, toluidine 

 blue, neutral red, safranine, Janus green, and Bismarck brown. *^^ 

 It is a striking fact that every one of these except safranine is a vital 

 dye. Seki noted that most of these dyes flocculate in alkaline solu- 

 tion, and he relates easy fiocculation to the capacity to colour 

 vitally. Vital dyes are often easily flocculated or indeed precipi- 

 tated by salts, especially calcium chloride. *^^ Four of the dyes 

 mentioned above (Nile blue, neutral red, Janus green, and Bis- 

 marck brown) tend to become negatively charged in alkaline 

 solution. ^^^ 



When a vital dye colours a vacuole or other cytoplasmic in- 

 clusion, it is often seen first in the form of a speck on the surface 

 of the inclusion. ^^^ Satellites of neutral red are sometimes formed 

 round the food-vacuoles of Protozoa. ^^^ The dye may also be seen 



