ON THE MODE OF ACTION OF PREVENTIVE SERA. 83 



harmful substance. The existence of bactericidal power may thus 

 be recognized, and its nature studied in detail by means of a 

 morphological change evident on microscopic examination. 



As is well known, Pfeiffer found that when cholera vibrios are 

 injected into the peritoneal cavity of a well-immunized animal they 

 lose their motility and undergo rapid transformation. The same 

 phenomenon occurs in a normal animal provided that the emulsion 

 of vibrios is mixed with a certain amount of active preventive serum. 

 Pfeiffer thought that this phenomenon of granular transformation 

 could occur only within the animal body, and that the substance 

 which caused this metamorphosis of vibrios came from an active 

 secretion of the endothelial cells; he thought that the leucocytes 

 had no function in the elaboration or in the storing of these destruc- 

 tive substances. Metchnikoff was then able to show that Pfeiffer's 

 phenomenon could be produced in vitro, if the vibrios are mixed 

 with a small amount of preventive serum and a little peritoneal 

 lymph containing leucocytes. On the other hand, the injection 

 into a normal animal of vibrios plus preventive serum in a region 

 where there are no leucocytes (for example, a leg in which an edema 

 has been formed) is found to produce no transformation in spite of 

 the presence of the preventive serum. Some time after injection, 

 when leucocytes appear, transformation does take place, but only 

 within the cells. We found, moreover, that the metamorphosis of 

 the vibrio could be very well brought about in vitro by the action 

 of the fresh serum of an immunized guinea-pig,* even when no cells 

 are present. We found, too, that this transformation did not take 

 place if, instead of serum (that is, a fluid formed outside the body 

 that has been in contact with leucocytes and therefore liable to 

 contain products of leucocytic disintegration), we used edema fluid 

 from the same animal; in other words, a fluid separated within the 

 animal body and almost entirely free from leucocytes. 



Aqueous humor and various secretions from an animal immunized 

 against the cholera vibrio also fail to produce a transformation of 

 the organism. 



It was also shown that other bacteria (for example, B. typhosus, 

 B. coli and B. pyocyaneus) "that have been found to be suscep- 



* And also in a mixture of fresh normal serum with a small amount of pre- 

 ventive serum. 



