278 DYEING 



for which they are used is active aggregation by the cells into new, 

 microscopically-visible droplets or irregular particles. 



The rest of this chapter and the whole of the next will be de- 

 voted to what may be called general vital colouring, that is to 

 say the kind of vital colouring that is applicable to cells of all kinds. 



The number of colouring agents that have been shown to be 

 really useful in this kind of work is small. We scarcely need more 

 than the following, though a few others have limited uses and will 

 be mentioned below : — 



dahlia (triarylmethane) 

 brilliant cresyl blue (oxazine) 

 Nile blue A (oxazine) 

 azures A and B (thiazine) 

 methylene blue (thiazine) 

 neutral red (azine) 

 Janus green (azine-azo) 

 Bismarck brown (azo) 



Fluorescent dyes are sometimes used vitally,^^^ but it would 

 not appear that they are superior to ordinary vital dyes in most 

 cytological studies, and ultra-violet light does not commend itself 

 as an illuminant for living cells. 



The dyes used may be presented in very different ways. Aquatic 

 animals may simply be placed in a solution of the dye. This is in- 

 deed the only practical way of colouring most Protozoa, and it was 

 employed by Brandt ^^ when he introduced the use of dyes as vital 

 colouring agents. Ehrlich ^^^ used the same method with tadpoles 

 when he introduced neutral red as a vital dye. Low concentrations 

 must necessarily be used, even with the least toxic dyes. Ehrlich 

 used neutral red at concentrations from o-oi% down to one-tenth 

 of this. If the organism is small, it may be examined intact under 

 the microscope. If it is not small, loose cells or minute pieces of 

 tissue must be removed, still living, for examination in a suitable 

 saline medium. 



An alternative method, applicable to all animals that are big 

 enough, is to inject a solution of the dye either subcutaneously or 

 into a blood-vessel or else into one of the cavities of the body. For 

 this purpose it is best to use a freshly-made solution at high con- 

 centration (about 1% or i%). Saline solutions of vital dyes should 

 not be kept for a long time, since these dyes have a special tendency 



