498 SCIENCE PROGRESS 



may be described as the ingestion of micro-organisms by 

 leucocytes. Wright has shown that this ingestion is more 

 rapid when it takes place in serum from an immune animal, than 

 in that from a non-immune. This fact can be quantitatively 

 determined by the method of measuring the degree of 

 phagocytosis devised by Leishman. The experiment as per- 

 formed in the laboratory is as follows : An intimate mixture of 

 leucocytes and micro-organisms is placed in an appropriate 

 vessel and kept at blood heat. After about fifteen minutes, a 

 sample drop of the mixture is taken out, placed on a microscope 

 slide, and spread out into a thin film. The film is fixed and 

 stained by a method which causes leucocytes and organisms to 

 assume different colours. The number of micro-organisms in- 

 gested by (say) ioo leucocytes is counted — and this divided by 

 ioo gives the average content. If two experiments are performed 

 in this manner, one with an unknown serum and the other with 

 a serum which is known to be normal, and if the average 

 content with unknown serum is divided by the average content 

 with the normal serum, a value is obtained which is called the 

 opsonic index. 



The phenomenon which has taken place between the two 

 types of cell, leucocyte and micro-organism, is a complex one. 

 Leaving the mode of action of the immune serum out of account, 

 each ingestion may be considered as having taken place in two 

 stages : firstly, collision between a leucocyte and an organism ; 

 and secondly, the inclusion of that organism in the protoplasm 

 of the leucocyte. 



The stage of collision, and the conditions which lead up to 

 it, are obviously capable of statistical treatment — just as the 

 kinetic theory of gases can be treated from a statistical point of 

 view. It may be argued, however, that no comparison can be 

 drawn between the conduct of a molecule of a gas and that 

 of a living cell ; that whilst, where there is no life, particles 

 may follow random paths, such will not be the case with living 

 cells which are apparently capable of voluntary movement and 

 effort. (Such movements are no doubt chemio-tactic and only 

 simulate voluntary movements.) But on the other hand it must 

 be remembered that the leucocyte is in an environment of 

 particles of food of an equally tempting nature which are 

 scattered at random in its vicinity. It need only browse at 

 random as a cow browses over a fat pasture. 



