THEORIES OF IMMUNITY 127 



finally disappeared entirely. His attention was soon directed to a 

 small, transparent crustacean, daphnia, within whose body cavity 

 could be distinguished minute wandering cells which exhibited ameboid 

 movements. The physiological significance of these ameboid cells 

 which are potentially leukocytes was not clear until it was found 

 that they engulfed and digested certain yeast spores that occasionally 

 gained entrance to the body cavity of the crustacean. If the yeast 

 spores were not too numerous the wandering cells flowed around and 

 eventually destroyed them; if, on the contrary, the number of yeast 

 spores was too great, the wandering cells could not remove the entire 



FIG. 8. Phagocytosis of gonococcus. 



number and the residual spores germinated and killed the host. It 

 was evident that the phagocytic activity of the ameboid cells played 

 a prominent part in protecting daphnia from an infection with the 

 yeast. 



Next Metchnikoff injected anthrax bacilli into the lymphatic sac 

 of frogs and found again that wandering cells leukocytes engulfed 

 and destroyed the bacteria, thus preventing infection and death of 

 the frog. This line of observation was followed through an extensive 

 series of lower animals, mammals, and finally in man, where the 

 engulfment of the meningococci, gonococci, pneumococci, and staphy- 

 lococci by .polymorphonuclear leukocytes during the course of acute 

 infections with these organisms afforded a striking demonstration 



