276 DYEING 



It is necessary to distinguish between two quite different ways 

 in which coloured substances can be used in the study of Hving 

 cells. On one hand certain particular cells actively take up coloured 

 particles in the course of their ordinary function of eliminating 

 foreign bodies. On the other hand cells of all kinds allow certain 

 dyes to infiltrate into them and to colour certain pre-existent 

 cellular constituents. For the clear distinction between these two 

 kinds of vital colouring w^e are largely indebted to von Mollen- 

 dorff.^^^' ^^* It is obvious that the second kind of vital colouring 

 has much more general interest than the other. It will be con- 

 venient to consider the more special kind first, in order to leave the 

 field clear for a detailed study of the wider subject. 



The cells that actively take up coloured particles are those of 

 the excretory and phagocytic systems, especially the reticulo- 

 endothelial system of vertebrates, and particular coloured sub- 

 stances may indeed be injected into the blood-stream of an animal 

 with the deliberate intention of finding out which cells have the 

 function of excreting or storing foreign particles. The coloured 

 substances used for this purpose need not be dyes: it is only 

 necessary that the particles should be sufficiently small for uptake 

 by single cells. The carbon particles of Indian ink are suitable. 

 Carmine may be used in the form of minute, insoluble particles. 

 The only dyes that are suitable for use in solution are those that 

 have a strong tendency to flocculate into particles of colloidal 

 dimensions. Certain acid disazo dyes, especially trypan blue, are 

 particularly suitable. A considerable number of such dyes has been 

 used, and some have even been synthesized especially for the 

 purpose, ^^^ but it would not appear that any of them are superior 

 to trypan blue for general use. 



This kind of vital colouring was invented by Baron von 

 Gleichen ^^* nearly two centuries ago, in the course of his investi- 

 gation of the way in which ciliates nourish themselves. He knew 

 that madder coloured the bone of animals fed on it, and sought to 

 apply a similar process to microscopical organisms. Particles of 

 carmine were eaten by the animals and seen in the food-vacuoles. 

 The method w^as copied by other protozoologists, especially 

 Ehrenberg,^^^ but appears not to have been noticed by histologists 

 until Ranvier ^^^ began injecting various insoluble colouring 

 agents (aniline blue SS, vermilion, sepia, carmine, and others) into 

 the dorsal lymph- sack of the frog and subsequently examining the 



