114 STUDIES IN IMMUNITY. 



inoculated into the peritoneum, which contained many leucocytes. 

 As we have already shown, these old cultures are much more easily 

 phagocyted than are young cultures. The exudate examined 3 

 or 4 hours after injection showed no free bacteria; all the cocci had 

 been taken up by the cells. Animals similarly inoculated, but 

 without serum, usually die in 24 hours. The animal that has re- 

 ceived the serum resists infection and, on the following day, the 

 exudate contains many leucocytes, some of them containing more 

 or less altered streptococci; there are no free bacteria. Nor are 

 there any bacteria present the following day, and it would seem as 

 if they had been entirely taken up. The animal, however, although 

 remaining well for several days, at the end of the week succumbs to 

 a generalized streptococcus infection. These instances of a late 

 outgrowth of streptococci are frequently met with in experiments ; 

 they occur, irrespective of the point of inoculation, in animals that 

 have received either too small a dose of serum or too large a dose 

 of bacteria. In such cases death may not occur for two weeks and, 

 exceptionally, not even until the third week. To sum up, the viru- 

 lent streptococcus has two qualities that render it dangerous, and 

 particularly so for the rabbit: it repels leucocytes, and it can remain 

 living for a long time in an animal that is apparently cured. 



A COMPARISON OF STREPTOCOCCUS INFECTION IN THE GUINEA- 

 PIG AND IN THE RABBIT. 



We must now compare the appearance of the streptococcus in- 

 fection in the guinea-pig and in the rabbit in order to understand 

 why the first of these animals resists the infection so much better 

 than the latter. 



We have already shown in a previous article that the normal 

 serum of rabbits or of guinea-pigs is a favorable culture medium for 

 the streptococcus and has no bactericidal power against it. 



We have just shown, moreover, that streptococci grow very well 

 and rapidly in the peritoneal exudate of infected guinea-pigs; the 

 only condition necessary for a uniform development is that they 

 remain free in the fluid and protect themselves from the phagocytes 

 by their negative chemiotactic power. The guinea-pig, then, has 

 no more antiseptic properties in its fluids than has the rabbit. But 

 there is a very striking difference between the phagocytes of these 



