THE ADVANCE OF SCIENCE 139 



nins,' under which name he has made most valuable 

 application of the method of injecting them into the 

 body so as to facilitate the work of the phagocytes in 

 devouring the hostile bacteria of various diseases. Each 

 kind of disease-producing microbe has its own sensitiser 

 or opsonin ; hence there has been much careful research 

 and experiment required in order to bring the discovery 

 into practical use. Metschnikoff himself holds and quotes 

 experiments to show that the ' opsonins ' are actually 

 produced by the phagocytes themselves. That this should 

 be so is in accordance with some striking zoological facts, 

 as I pointed out nearly twenty years ago. 1 For the lowest 

 multicellular animals provided with a digestive sac or gut, 

 such as the polyps, have that sac lined by digestive cells 

 which have the same amoeboid character as ' phagocytes,' 

 and actually digest to a large extent by swallowing or 

 taking into their individual protoplasm raw particles of 

 food. Such particles are enclosed in a temporary cavity, 

 or vacuole, into which the cell-protoplasm secretes diges- 

 tive ferment and other chemical agents. Now there is 

 no doubt that such digestive vacuoles may burst and so 

 pour out into the polyp's stomach a digestive juice which 

 will act on food particles outside the substance of the 

 cells, and thus by the substitution of this process of out- 

 pouring of the secretion for that of ingestion of food 

 particles into the cells we get the usual form of diges- 

 tion by juices secreted into a digestive cavity. Now 

 this being certainly the case in regard to the history of 

 the original phagocytes lining the polyp's gut, it does 

 not seem at all unlikely, but on the contrary in a high 

 degree probable, that the phagocytes of the blood and 

 tissues should behave in the same way and pour out 



1 In a review of Metschnikoffs 'Legons sur 1'Inflammation ' in 

 Nature, 1899. 



