Immunity against pathogenic micro-organisms 143 



condition of the micro-organisms in another way. Withdraw a drop 

 of the exudation at an advanced stage of the process when there 

 are no longer any free micro-organisms ; inside the phagocytes a few 

 scattered bacteria, more or less well preserved, can still be seen. It 

 is sufficient to keep a hanging drop of such an exudation at a tem- 

 perature of about 30 C., care being taken to keep it from drying, but 

 without adding to it any nutrient medium. Under these conditions 

 the leucocytes die more or less rapidly, but the bacteria regain vigour: 

 they begin to multiply, and at the end of a short time produce a 

 generation of bacteria within the dead leucocyte. The multiplication 

 of the bacteria goes on progressively and the hanging drop is trans- 

 formed into a real pure culture. Mesnil was able to confirm these 

 data with the exudations of frogs that had been inoculated with either 

 the bacilli of anthrax or of mouse septicaemia. 



The bacteria, ingested in the living state by phagocytes, retain [152] 

 their original virulence. Some authors think, and I was formerly of 

 this opinion, that at the end of a more or less prolonged sojourn 

 within the leucocytes, anthrax bacilli undergo an attenuation in 

 their virulence. Later, numerous researches have, however, de- 

 monstrated that this opinion is incorrect, and that the virulence is 

 maintained in the bacteria included in the phagocytes of frogs the 

 whole time that these bacteria remain alive. Dieudonn^ has in- 

 sisted on this fact as regards the anthrax bacillus. Mesnil has 

 confirmed it for this same species and for the bacillus of mouse 

 septicaemia. It is impossible, therefore, to doubt this general result, 

 that frogs which are refractory against certain bacteria resist because 

 of the phagocytosis which is exercised against living and virulent 

 micro-organisms. 



We have insisted sufficiently on the analysis of the natural 

 immunity of the frog, and need not tarry over the facts relating to 

 other amphibia which, moreover, have been much less studied. The 

 reptiles, those higher representatives of the Vertebrata called cold- 

 blooded, often present examples of really remarkable immunity. Thus 

 alligators will resist enormous doses of various bacteria, such as 

 the anthrax bacillus, that of human tuberculosis or the cocco-bacillus 

 of typhoid fever. When, some time after an injection is made, the 

 exudation at the point of inoculation is withdrawn there is found 

 a large number of leucocytes, amongst which may be recognised 

 many eosinophile microphages, though the majority are macropliages 

 with one, two or more nuclei. Really giant cells are found in the 



