HYPOTHESIS OF WELCH. 333 



stimulated by its opponent to the production of 

 cytotoxins hostile to the other, and thereby en- 

 deavors to make itself immune against its an- 

 tagonist." (Welch.) 



A more reasonable hypothesis could hardly be 

 advanced, and no small number of facts known 

 at the present time are in harmony with it. 

 Walker had already performed work of a funda- 

 mental character, which showed that the typhoid 

 bacillus, when grown in the presence of its anti- 

 serum, acquires greater virulence for animals. 

 Furthermore, a greater dose of protective serum 

 was required to save guinea-pigs from infection 

 with the immunized culture than from the same 

 strain which had not been immunized. The fact 

 has been known for a long time that the typhoid 

 bacillus resists agglutination when freshly cul- 

 tivated from a patient having the disease, whereas 

 it becomes easily agglutinable after a period of 

 artificial cultivation. It may well be assumed that 

 the bacillus, when playing the part of an infecting 

 organism, gradually was immunized against the 

 agglutinating properties of the patient's serum; 

 and, on the other hand, that it lost this resistance 

 after it had been removed from the stimulating 

 influence of the infected body. This immuniza- 

 tion with agglutinins may be carried on in the 

 test glass, and bacteria which have been so treated 

 acquire the power to absorb a greater quantity of 

 agglutinin from the homologous serum (Bail). 



Another pertinent observation was that by 

 Wechsberg, who found that a strain of the diph- 

 theria bacillus when grown in a medium contain- 

 ing diphtheria antitoxin could be made to pro- 



