STUDIES ON THE SERUM OF VACCINATED ANIMALS. 45 



that there are several differences of this sort in the type organisms 

 of the species "cholera vibrio." For example, the vibrio of Mas- 

 saouah does not act as do the cholera vibrios of Eastern Prussia, 

 Constantinople, or Hamburg. This does not at all mean that it 

 has not the same pathogenic properties as these latter. Immune 

 serum gives a delicate reaction, but we do not know whether the 

 distinctions it draws between vibrios are always fundamental ones. 

 It is certainly proper to collect all the organisms that react in the 

 same way to a given serum into one distinct group and to separate 

 this group from all the others, provided that the criterion of separa- 

 tion is distinctly noted. The grouping of a given species is not 

 always the same if some other criterion is used. There is no means 

 of proving that a classification based on a reaction to serum is the 

 same as a classification according to pathogenic power. There is 

 no reason for saying, for example, that the vibrio of Massaouah can- 

 not produce cases or epidemics of cholera from the simple fact that 

 it is not affected by the serum of animals immunized against the 

 cholera vibrio from Eastern Prussia. 



VI. The Nature of the Bactericidal Substance in Vacci- 

 nated Animals. Its Identity with the Bactericidal 

 Substances in Normal Animals. 



The immunized animal is remarkably well equipped for combating 

 the organism against which it has been vaccinated. In its phag- 

 ocytes it has a bactericidal power which would seem to direct itself 

 exclusively against that bacterial species against which it has been 

 immunized. But even the normal un vaccinated animal has a 

 certain amount of protective power against vibrios. When vibrios 

 are inoculated in normal serum there is generally noted in the 

 beginning a decrease in their number. Although this destructive 

 power is not distinctly marked against virulent vibrios, it is more 

 marked, as Pfeiffer has recently shown, against such vibrios as have 

 been grown for some time on artificial media and have therefore 

 become unaccustomed to the body fluids and lost their virulence. 

 It is well known, moreover, that this normal bactericidal power may 

 be increased by injecting into the animal a few cubic centimeters 

 of bouillon or of normal serum. 



There are certain properties in common between the bactericidal 



