136 THE KINGDOM OF MAN 



The first eighteen years of Metschnikoffs career, after 

 his undergraduate course, were devoted to zoological and 

 embryological investigations. He discovered many im- 

 portant facts, such as the alternation of generations in the 

 parasitic worm of the frog's lung Ascaris nigrovenosa 

 and the history of the growth from the egg of sponges 

 and medusae. In these latter researches he came into 

 contact with the wonderfully active cells, or living cor- 

 puscles, which in many low forms of life can be seen by 

 transparency in the living animal. He saw that these 

 corpuscles (as was indeed already known) resemble the 



FIG. 37. FIG. 38. FIG. 39. 



Fig. 37. Phagocyte or colourless corpuscle of a guinea-pig in the act of 

 engulphing two Spirilla or parasitic vegetable microbes of a spiral shape. 

 Fig. 38. The same half an hour later, one of the Spirilla is nearly com- 

 pletely engulphed. Fig. 39. The same ten minutes later still one of the 

 Spirilla is completely absorbed into the substance (protoplasm) of the 

 phagocyte. (From Metschnikoffs book, "Immunity," kindly supplied by 

 the Cambridge University Press.) 



well-known amoeba, and can take into their soft sub- 

 stance (protoplasm) at all parts of their surface any 

 minute particles and digest them, thus destroying them. 

 In a transparent water-flea Metschnikoff saw these amoeba- 

 like, colourless, floating blood-corpuscles swallowing and 

 digesting the spores of a parasitic fungus which had 

 attacked the water-fleas and was causing their death. 



