22 



the infection. In the smears made on the fourth day no free streptococci 

 could be found, although diligent search was made for them. Plates 

 made with a small quantity of discharge, which was obtained high up in 

 the nares by means of a sterile cotton swab, contained, however, many 

 colonies of streptococci after twenty- four hours' incubation. The fact 

 that many colonies developed in the plates, although no free streptococci 

 could be found in the smears, seems to indicate that the organisms were 

 not dead when they were taken up by the phagocytes, but were taken 

 tip alive. It is possible, however, that there were free streptococci in this 

 purulent discharge, even though they were not found in the smears. 



Fig. 5. Photomicrograph X 1500. Streptococci in a leucocyte from nasal 

 discharge on fourth day of sickness. 



IMMUNIZATION OF RABBITS WITH STREPTOCOCCI. 



There can no longer be any doubt about the statements of many in- 

 vestigators that laboratory animals acquire a fair degree of immunity 

 after repeated injections of small doses of virulent streptococci. I have 

 injected a large number of rabbits first with several doses of heated cul- 

 tures (65 C. for 20 minutes) and then with living cultures of medium 

 virulence, and have found that some animals acquire enough resistance 

 after 4 or 5 injections to withstand injections of several times the mini- 

 mum fatal dose of the streptococcus. Thus, for instance, a full grown 

 rabbit weighing 2000 G. had received 2 large injections of heated culture 



