212 GENERAL BIOLOGY OP MICRO-ORGANISMS 



(pneumococcus). There is some indication that micro-organisms 

 may produce special chemical substances to neutralize the agencies 

 which the host employes against them. These defensive sub- 

 stances have been designated by Bail as aggressins. Ehrlich 

 has also found evidence of the acquirement of immunity to 

 chemical substances by certain pathogenic microbes, especially 

 trypanosomes and spirochetes, and he ascribes this property of 

 the parasites to an alteration of their cell-chemistry. 



