174 Chapter VI 



of the peritoneal exudation of their guinea-pigs, to detect leucocytes 

 in the act of devouring Trypanosomae which showed, by their active 

 movements, that they were still alive. Once the parasites were com- 

 pletely enclosed within the macrophages, their final disappearance 

 took place with extraordinary rapidity. 



In this chapter we have attempted to place before the reader a 

 complete series of the phenomena observed in natural immunity 

 in animals. We have passed in review the resistance of the animal 

 organism against the principal groups of bacteria, and we have dwelt 

 on certain of them which are most capable of adapting themselves 

 to various media, and on others which present examples of micro- 

 organisms more exacting and more delicate. We have examined the 

 immunity against Blastomycetes and parasitic animalcules. Above 

 all, in the lower animals, just as in the Vertebrata of all classes, we 

 have always observed this general phenomenon : phagocytic resistance 

 as the principal and constant factor in natural immunity. 



