\4'2 Chapter VI 



into an additional argument in favour of phagocytic reaction being 

 a general factor in immunity. In support of this hypothesis I may 

 adduce a further example, already mentioned in a preceding chapter 

 when discussing another question. The frog is very refractory against 

 the cholera vibrio. When these vibrios are inoculated into the dorsal 

 lymphatic sac or into any other part of the body the animal retains 

 its health unimpaired. An examination of the exudation at the point 

 of inoculation demonstrates that the vibrios meet with a vigorous 

 opposition on the part of the phagocytes, which ingest and com- 

 pletely digest them. This is of special interest from the fact that 

 the frog is very sensitive to the toxin of the cholera vibrio. When 

 injected in a weak dose it kills the frog very quickly. Two small 

 frogs died in less than an hour from the effect of 0'5 c.c. of cholera 

 toxin. 



The natural immunity of the frog against the cholera vibrio affords, 

 [151] then, an example in which the organism, destroying the vibrio by 

 phagocytosis, prevents the production of the poison, \vhich, otherwise, 

 would infallibly kill it. 



Having demonstrated that phagocytic reaction manifests itself in 

 the frog in all cases of natural immunity that have been sufficiently 

 studied, we must dwell for an instant on the question of the con- 

 dition of the bacteria at the moment of their ingestion by the 

 phagocytes. It is very evident that this phagocytic defence is only 

 efficient on condition that it is exercised against bacteria which, in 

 its absence, might injure the organism by their multiplication and 

 then* virulence. For this reason the question as to whether the 

 micro-organisms, before being ingested, were living and capable of 

 producing their pathogenic action has been widely discussed. It 

 has even been suggested that the phagocytes are only capable of 

 ingesting the dead bodies of micro-organisms that have been killed 

 by other agents. Frogs are very suitable for a study of this question. 

 When a drop of the exudation is removed some time after inocu- 

 lation with a motile organism, such as the Bacillus pyocyaneus or 

 the cholera vibrio, the organism was often found moving rapidly 

 within the vacuoles inside leucocytes. The experiment will succeed 

 even more completely if a drop of frog's lymph be mixed, on a slide, 

 with a trace of a culture of these motile micro-organisms, the latter 

 being soon found in the clear vacuoles included in leucocytes and 

 executing extremely rapid movements. 



Besides this direct proof we can assure ourselves of the living 



