PHAGOCYTOSIS 295 



place in the artificial conditions of opsonin estimations, where the 

 bacteria are uniformly distributed throughout the fluid, and there 

 is no reason why the leucocyte should be attracted in one direc- 

 tion rather than in another ; and movement of leucocytes either 

 does not occur at all or does so only to a very minute extent in 

 saline solution. It takes place much more actively in unheated 

 serum a fact which gives some support to the theory of stimulins, 

 previously mentioned, but not discussed. It is quite possible that 

 all the facts related concerning opsonic action may be due to one 

 or more substances which occur in the serum, and which have 

 the power of stimulating the leucocyte, or of altering it in a 

 manner to be discussed subsequently. The phenomena of the 

 phagocytosis of sensitized bacteria in normal saline solution 

 would, of course, be due to a liberation of this stimulin from its 

 combination with the bacteria. This is known to occur, for 

 sensitized bacteria will yield some opsonin on prolonged soaking 

 in normal saline or heated serum ; the fluid acquires opsonic 

 properties, and the bacteria becomes insensitive to phagocytosis. 

 As Sellards points out, the fact that unorganized bodies, such as 

 carmine, particles of carbon, melanin, etc., are taken up more 

 readily in the presence of fresh serum is somewhat in favour of 

 this view. It is difficult to think that these substances are affected 

 in a way similar to bacteria or other antigens when combined 

 with their specific antibodies. There appears to be no crucial 

 test for determining the point. 



And there is some reason for thinking that the actual process 

 of phagocytosis may be a physical one, akin to agglutination, and 

 entirely independent of any movements or other vital processes 

 on the part of the leucocytes. The chief evidence in favour of 

 this view arises from the fact that phagocytosis may occur under 

 conditions in which no movements of any sort take place. This 

 was first pointed out by Ledingham in a series of important 

 researches on the relation between temperature and opsonization. 

 He showed that when a series of opsonin mixtures were incu- 

 bated at temperatures varying between 18 and 37 C., the 

 latter temperature brought about much more phagocytosis than 

 the former ; and, further, that at the latter point there was very 

 little difference in the index between preparations incubated for 

 fifteen or thirty minutes, while in the former there was a long latent 

 period in which but little phagocytosis occurred. This he showed 

 to be due to the fact that opsonin combines with bacteria but very 



