STUDIES ON THE SERUM OF VACCINATED ANIMALS. 57 



the metamorphosis of vibrios is quite apart from any leucocytic inter- 

 vention and due to the activity of the endothelial cells of the per- 

 itoneum. When a normal guinea-pig is given a mixture of vibrios 

 and preventive serum the endothelial cells are stimulated and react 

 by rapidly secreting harmful substances which produce morpholog- 

 ical changes in the vibrios and then destroy them. 



The transformation of the vibrio, which precedes its destruction, 

 gives evidence of the harm that the bactericidal substances in the 

 animal have done. But these substances as we have already seen be- 

 long to the leucocytes and it is quite reasonable to imagine, contrary 

 to the opinion of Pfeiffer, that the bacteria are modified because in 

 the presence of vibrios the leucocytes of the peritoneal cavity liber- 

 ate into the surrounding fluid the substances they have formed and 

 which they normally retain. Leucocytes indeed show the effects 

 of this sudden introduction of the culture fluid ; many cells become 

 motionless and swollen or broken up.* Some of them collect in 

 clumps. 



Metchnikoff has just proved that this latter reasonable hypothesis 

 represents the true state of affairs. He was able to produce Pfeiffer's 

 phenomenon in vitro by adding to a mixture of preventive serum 

 and culture, leucocytic extract from the peritoneal cavity of a nor- 

 mal guinea-pig. This experiment evidently rules out the function 

 which Pfeiffer has attributed to the endothelial cells. 



We have pointed out in our experiments on phagocytosis in 

 vitro | that vibrios that have been taken up by normal guinea-pig 

 phagocytes may undergo a granular transformation identical with 

 the one described by Pfeiffer. Since the normal guinea-pig has 

 only a faint bactericidal power the transformation of vibrios goes 

 on only where the destructive substance is most concentrated, that 

 is within the phagocyte. But if the preventive serum is added, 

 as Metchnikoff has done, the bactericidal power becomes much 

 more marked and metamorphosis of the organism may be noted 

 not only in the phagocyte but also in the surrounding fluid. 



Whatever may be the ultimate conclusions, Peiffer has made a 

 great contribution to the study of immunity. By this visible and 

 microscopically detectable alteration the vibrio shows the effect 



* Elie Metchnikoff, Annales de 1'Institut Pasteur, June, 1895. 

 t See p. 33. 



