STUDIES ON THE SERUM OF VACCINATED ANIMALS. 75 



out, a great number of granules will be found with the normal 

 vibrios. 



After twenty passages through bactericidal serum, the vibrio 

 is quite as apt to be transformed as in the beginning. It is quite 

 evident, then, that the vibrio acquires no tolerance and becomes no 

 less susceptible to the bactericidal effect of the fluid. It continues 

 to show a granular change which is not strictly a disintegration, 

 but an active contraction of the organism which would appear to 

 serve some purpose. In this new form it presents in a given volume 

 a smaller surface of contact to the surrounding fluid, and, therefore, 

 better avoids its harmful effect. 



The fact that the vibrio does not become accustomed to contact 

 with the bactericidal substances from leucocytes explains the diffi- 

 culty in increasing its virulence. After a few passages through the 

 animal body a definite virulence is reached which is not easily in- 

 creased. The cholera vibrio does not seem to possess the characters 

 necessary for a dangerous general parasite and does not give gener- 

 alized infections. It is not necessary for the organism to acquire 

 a great resistance to the bactericidal properties of the animal body 

 in order to live in the intestinal contents and form toxins there, 

 for, as we have already shown, no bactericidal substance is present 

 in the intestinal secretion. It is, therefore, by no means proved that 

 a vibrio that is only slightly pathogenic in the peritoneal cavity 

 may not be very pathogenic when present in the digestive tract. 



REMARKS ON THE MECHANISM OF IMMUNITY. 



I. ACTIVE IMMUNITY. BACTERICIDAL PROPERTIES OF LEUCOCYTES. THE 

 CONCEPTION OF A " UNITY OF THE BACTERICIDAL SUBSTANCE." 



Active immunity is the immunity shown by animals vaccinated 

 by repeated injections of living or sterilized cultures of bacteria or 

 of bacterial products. It is established slowly, but it is much more 

 enduring than immunity brought about by preventive sera. This 

 form of immunity is accompanied by cellular changes that are 

 sufficiently lasting to create temporarily in the immunized animals 

 properties that are not to be found in normal animals. 



The study of immunity has convinced us that the resistance of 

 animals to pathogenic organisms is regularly due to the activity of 

 phagocytes. In a previous article we have considered the more 



