68 STUDIES IN IMMUNITY. 



remained both bactericidal and preventive. The persistence of the 

 preventive power under these conditions has already been noted by 

 Metchnikoff. The blood of guinea-pigs immunized against the 

 vibrio Metchnikovi or the Massaouah vibrio and incapable of 

 producing the phenomenon alone, produces it very well when added 

 to the preventive cholera serum. 



The bactericidal substance, then, is to be found generally in ex- 

 perimental animals and persists even after fatal infections. It 

 occurs both in the blood of vaccinated animals and in normal 

 animals. 



Third. The effect of heat on the fresh defibrinated blood (or serum) 

 of a normal guinea-pig. 



Fresh blood serum from a normal guinea-pig was placed in 

 several tubes. One of these tubes was left unheated ; the others 

 were heated for 5 minutes to temperatures of 50 degrees, 55 degrees, 

 60 degrees, and 64 degrees respectively. 



Hanging drops. Phen m - 



Cholera + preventive goat serum 4- unheated normal serum . . 



+ normal serum 50 

 + normal serum 55 

 + normal serum 60 

 + normal serum 64 



enon 



From this table we may conclude that the bactericidal substance 

 of normal serum necessary to produce Pfeiffer's phenomenon is 

 destroyed if the serum is heated for 5 minutes to from 50 degrees 

 to 55 degrees. It may be noted that, even in those tubes where 

 there was no transformation, the vibrios were motionless and 

 clumped owing to the continued presence of the preventive 

 serum. 



Fourth. The effect of heat on the preventive serum. Pfeiffer has 

 recently shown that preventive serum heated to 65 degrees still 

 produces metamorphosis of the vibrio when placed in the peritoneal 

 cavity with the cholera organism. The same fact is true in vitro. 

 It is interesting to consider the effect of heat on quite fresh serum 

 from an immunized guinea-pig; as we have already seen, this serum 



