SUSCEPTIBILITY AND IMMUNITY. 257 



The observations which first led Metschnikoff to adopt this view 

 were made upon a species of daphnia which is subject to fatal infec- 

 tion by a torula resembling the yeast fungus. Entering with the 

 food, this fungus penetrates the walls of the intestine and invades the 

 tissues. In certain cases the infection does not prove fatal, owing, as 

 Metschnikoff asserts, to the fact that the fungus cells are seized upon 

 by the leucocytes, which appear to accumulate around the invading 

 parasite (chemiotaxis) for this special purpose. If they are success- 

 ful in overpowering and destroying the parasite the animal recovers ; 

 if not, it succumbs to the general infection which results. In a simi- 

 lar manner, Metschnikoff supposes, pathogenic bacteria are destroyed 

 when introduced into the body of an immune animal. The colorless 

 blood corpuscles, which he designates phagocytes, accumulate at the 

 point of invasion and pick up the living bacteria, as they are known 

 to pick up inorganic particles injected into the circulation. So far 

 there can be no doubt that Metschnikoff is right. The presence of 

 bacteria in the leucocytes in considerable numbers, both at the point 

 of inoculation and in the general circulation, has been repeatedly 

 demonstrated in animals inoculated with various pathogenic bacteria. 

 The writer observed this in his experiments, made in 1881, in which 

 rabbits were inoculated with cultures of his Micrococcus Pasteuri ; 

 and it was this observation which led him to suggest the theory 

 which has since been so vigorously supported by Metschnikoff. But 

 the presence of a certain number of bacteria within the leucocytes 

 does not prove the destructive power of these cells for living patho- 

 genic organisms. As urged by Weigert, Baumgarten, and others, 

 it may be that the bacteria were already dead when they were picked 

 up, having been destroyed by some agency outside of the blood cells. 

 As heretofore stated, we have now experimental evidence that blood 

 serum, quite independently of the cellular elements contained in it 

 in the circulation, has decided germicidal power for certain patho- 

 genic bacteria, and that the blood serum of the rat and other animals 

 which have a natural immunity against anthrax is especially fatal 

 to the anthrax bacillus. 



Numerous experiments have been made during the past two or 

 three years with a view to determining whether pathogenic bacteria 

 are, in fact, destroyed within the leucocytes after being picked up, 

 and different experimenters have arrived at different conclusions. 

 In the case of mouse septicaemia, already alluded to, and in gonor- 

 rhoea, one would be disposed to decide, from the appearance and ar- 

 rangement of the pathogenic bacteria in the leucocytes, that they are 

 not destroyed, but that, on the other hand, they multiply in the in- 

 terior of these cells, which in the end succumb to this parasitic in- 

 vasion. In both of the diseases mentioned we find leucocytes so 

 17 



