THE MODE OF ACTION OF VITAL DYES 291 



in the form of clumps within a vacuole. These facts suggest 

 flocculation. 



Although solution and flocculation no doubt play their parts in 

 particular cases, yet it seems certain that dyeing in the strict sense 

 also occurs. The facts are not in accordance with Fischer's ^^^ dog- 

 matic statement, 'Without fixation there is no dyeing of histo- 

 logical preparations'. When brilliant cresyl blue colours a nucleolus 

 there is no reason to doubt that the basic dye-ion is combining 

 with RXA. Metachromatic colouring points in the same direction, 

 for, as w^e have seen, the results are often the same in living and 

 fixed cells. We can scarcely doubt that dyeing also occurs when a 

 lipid globule colours strongly wdth a vital dye. The lipid globules 

 that react in this way are not droplets of triglyceride or other lipid. 

 On the contrary, they evidently contain a considerable amount of 

 water and indeed are often spoken of as 'vacuoles'. In some cases 

 the refractive index is quite low and it may even be rather hard to 

 see the vacuole without using a vital dye. If the cell bursts, how- 

 ever, and the vacuoles escape from the cytoplasm, the lower re- 

 fractive index of the saline solution in which the cell is suspended 

 makes them easily visible. *^^ That such vacuoles contain lipid 

 may be proved by histochemical tests. *^^ It is evident that the lipid 

 is dispersed in water. Now phospholipids are easily dispersed in 

 this way, and when so dispersed they present negative charges (on 

 the phosphoric acid component), which could react with the dye- 

 ion of a basic dye. It is probable that basic dyes often colour lipid 

 globules and 'vacuoles' because these cytoplasmic inclusions con- 

 tain acidic lipids dispersed in water. 



The food-vacuoles of phagocytes are of quite a different nature 

 from the vacuoles we have been considering. It has long been 

 known that food-vacuoles are strongly coloured by neutral red. 

 To prevent any possibility of misunderstanding, it must be re- 

 marked that this has no connexion with the uptake of insoluble 

 pigments or the aggregation of acid azo dyes by phagocytic cells. 

 We are concerned here with the colouring of pre-existent objects 

 by a basic dye. It was at first thought that the object coloured was 

 the protein matter of the foodstuff, in the course of digestion. "^^^ 

 It was noticed, however, that when particles of talc were taken up 

 by leucocytes, they appeared to become red, though talc itself has 

 no affinitv for the dve.'^^^ It was evident that the fluid of food- 

 vacuoles had an affinity for the dye, though the ingested food often 

 coloured as w^ell. It was remarked by Marston ^^^ that all the azine 



