XIV 
ORIGIN OF PIGMENTS 
2 93 
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 Metschnikoff 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 
