166 Chapter VI 



several observers. A second advantage of the streptococcus, however, 

 is the high degree of natural immunity manifested against it by 

 [176] a laboratory animal so convenient as the guinea-pig. Dr Jules 

 Bordet 1 studied this subject in my laboratory. He observed that the 

 injection of streptococci into the peritoneal cavity sets up a marked 

 leucocytosis which ends in a complete destruction of the micro- 

 organisms. The leucocytes rapidly ingest the great majority of the 

 streptococci and destroy them ; there remain only a few isolated and 

 free individuals which are protected by a clear zone (aureola) which 

 develops around them, but in the end they also become the victims 

 of the voracity of the phagocytes. When we increase the dose of 

 streptococci injected, phagocytosis still goes on, but some of the 

 streptococci succeed in escaping, and we see a new generation 

 produced which is distinguished by the thickness of the protective 

 aureola. In spite of the afflux of a large number of leucocytes, 

 they no longer ingest the streptococci and generalisation of the 

 infection results, followed by the death of the animal. Natural 

 immunity, then, can be suppressed under certain definite conditions. 

 Dr Jules Bordet 2 wished to satisfy himself whether the leucocytes 

 failed to fulfil their phagocytic function because of the paralysis of 

 their movements, or as the result of some other weakness. With 

 this object he injected into the peritoneal cavity of guinea-pigs, at 



the moment when the 

 streptococci begin to get 

 the upper hand, of the 

 leucocytes, a definite quan- 

 tity of a culture of Proteus 

 vulgaris. These small 

 bacilli in a short time 

 become the prey of phago- 

 cytes which, however, still 

 refuse to ingest strepto- 

 cocci(fig.36). Thereisthus 



Fio. 36. Peritoneal exudation from guinea-pig 



showing free streptococci and microphages m the Peritoneal Cavity a 



that have ingested Proteus bacilli. kind of Selective prOCCSS 



as regards the ingestion 



of these microbes. The Proteus disappears as the result of phago- 

 cytosis, whilst the streptococci thrive in the fluid of the exudation 



1 Ann. de Vlnst. Pasteur, Paris, 1897, t. xi, p. 177. 



2 Ann. de VInst. Pasleur, Paris, 1896, t. x, p. 104. 



