XIV ORIGIN OF PIGMENTS 293 



tion with physiological experiments on leucocytes. 

 The modern doctrine of the physiological and patho- 

 logical importance of leucocytes and phagocytes, 

 with which the name of Metschnikofif is so honour- 

 ably associated, has been largely -founded on results 

 obtained from the injection of foreign substances in 

 suspension or solution into the body. The injected 

 substances are usually colouring - matters for con- 

 venience of observation, and the result has been to 

 prove that they are systematically removed by 

 leucocytes from the general cells of the body ; and 

 either eliminated through the excretory organs or 

 stored up in various parts of the body, where they 

 may give rise to artificial coloration. Now we have 

 already frequently seen that natural pigmentation 

 may result from the emigration of pigmented con- 

 nective tissue cells from the deeper tissues outwards 

 to the skin. This occurs, for example, in the leech, 

 and, according to Kolliker, is true for all Vertebrates. 

 This being so, it is eminently natural that the 

 physiologists should draw a parallel between these 

 natural pigmented " wandering cells " and the pig- 

 mented leucocytes found after injection of colour- 

 ing-matter, and regard the former as active agents in 

 eliminating the normal waste products. The necessity 

 for finding a physiological justification for the con- 

 tinued production of pigment being so obvious, the 

 suggestion once made has been eagerly adopted by 

 many. 



The simplest case is that in which the introduced 

 pigment is injected into the alimentary canal, and 

 its subsequent fate compared with that of the 

 pigments normally occurring in the cells connected 



