IMMUNITY AND SUSCEPTIBILITY. 95 



which Metschnikoff gives the name ?nicrophages — and the 

 endothelial and connective-tissue cells — which together 

 with the large lymphocytes of the blood and occasional 

 epithelial cells constitute what he describes as macro- 

 phages, do take up bacteria as well as other particles, just 

 as the free unicellular animals do, and appear to digest 

 them. If an injection of anthrax bacteria be made 

 into the lymph-sac of a frog, it will be found upon subse- 

 quent examination that the bacteria have been consumed 

 by the leukocytes, in whose protoplasm they appear. 

 The bacteria contained in the leukocytes are shown by 

 staining (vesuvin is said to be useful for the purpose) to 

 undergo a gradual destruction, by which, though at first 

 staining uniformly, they lose this property and appear 

 pale and irregularly colored. Later, only the outline of 

 the bacillus is visible, and finally it disappears. It cer- 

 tainly seems as if the immunity of the frog depends upon 

 the activity of its leukocytes in destroying the bacteria. 



The rabbit is susceptible to anthrax and succumbs to 

 subcutaneous inoculations. If a little of the fluid from 

 the gelatinous edema surrounding the seat of inoculation 

 be examined microscopically after the death of the ani- 

 mal, it will be found that, while this fluid contains abun- 

 dant bacilli and large numbers of leukocytes, not a single 

 bacillus is contained within a leukocyte. Contrasted 

 with the other observations, one might readily conclude 

 that the death of the rabbit depended upon the failure of 

 its leukocytes to take up and destroy the bacteria. 



It appears to be the rule that when an animal is im- 

 mune phagocytosis is active and the leukocytes readily 

 take up the parasites ; when an animal is susceptible the 

 leukocytes refuse to take up the parasites. Whether or 

 not one is justified in concluding with Metschnikoff that 

 the animal is immune because its leukocytes are active 

 in operating upon the micro-organisms is questionable. 



"If One examines the exudate in erysipelas, it will be 

 found, at the extending zone of the disease, that many of 

 the streptococci are being taken up by the leukocytes, while 



