136 Chapter VI 



injection of a considerable number of bacilli into the abdomen. 

 When kept at temperatures of 15°— 20° C. or even 23° C, a tem- 

 perature at which the bacilli are able to develop very abundantly, 

 these fishes destroy a large number of the bacteria in their bodies. 

 Soon after the introduction of the bacilli into the peritoneal cavity, the 

 numerous leucocytes accumulate around them and ingest them by the 

 same mechanism that is observed in the Invertebrata or in the same 

 fishes when absorbing the red blood corpuscles of alien species. In 

 the gudgeon, at as early as six and a half hours, a very marked, nay, 

 an almost complete phagocytosis is set up. 



It is impossible to doubt the fundamental fact that the bacilli, 

 at the moment of their ingestion, are in a perfect condition of 

 vitality and virulence. The fluid of the peritoneal exudation, when 

 withdrawn from the animal, is of itself incapable of preventing the 

 development of the anthrax bacilli. The peritoneal lymph of the 

 above-mentioned fishes is, in vitro, even a good culture medium for 

 these bacilli. 



When, long after the completion of the phagocytosis by the 

 leucocytes of the peritoneal exudation, a drop of the exudation is 

 withdrawn and kept outside the organism under suitable conditions 

 of temperature and moisture, a number of the ingested bacilli begin to 

 multiply and give an abundant culture. This experiment proves, in- 

 disputably, that the bacilli are devoured in the living state. If a little 

 of the peritoneal exudation, withdrawn several (up to nine) days after 

 the injection of the bacilli, be injected below the skin of guinea-pigs 

 these animals die from generalised anthrax, a fact which demonstrates 

 that the bacilli, which have been ingested alive, have retained their 

 virulence a long time after they have been devoured by the leuco- 

 cytes. But, if the peritoneal exudations that have been withdrawn at 

 still longer periods after injection be examined, it is found that they 

 [145] no longer contain bacilli capable of developing in culture media or 

 of setting up the disease in the most susceptible animal. Hence it 

 follows that in the organism of the refractory fish, the bacteria are 

 not destroyed by the fluids but by the phagocytes, which take a long 

 time to bring about the complete intracellular digestion of ingested 

 micro-organisms. 



The phagocytes which assure immunity to the osseous fishes that 

 were studied by Mesnil belong principally to the group of haemo- 

 macrophages. These are leucocytes with abundant protoplasm 

 which stain readily by basic aniline dyes, mononuclear cells whose 



