126 STUDIES IN IMMUNITY. 



No growth had as yet occurred and no free bacteria were seen. 

 At ihis stage one of the rabbits was given 3 c.c. of preventive serum 

 intraperitoneally and 5 c.c. subcutaneously. 



The other rabbit that received no serum died of a generalized 

 infection in 18 hours, the delay being due to the slow growth of the 

 culture. Twenty-four hours after beginning the experiment the 

 peritoneal cavity of the rabbit that had received serum contained 

 numerous extracellular bacteria scattered in the midst of abun- 

 dant leucocytes. Growth, then, had taken place. But a few hours 

 later the phagocytic crisis occurred and all the free bacteria were 

 engulfed. 



In this experiment an extracellular growth occurred and was 

 followed by a delayed phagocytosis. And yet the serum had been 

 given some time after the injection of the culture. When the serum 

 is injected the day before, and old cultures are used it is generally 

 found that a generalized phagocytosis occurs and is not followed by 

 the appearance of extracellular bacteria. 



Engulfing may also take place in a normal rabbit and often no 

 invasion of the bacteria is noted for a number of hours ; the strepto- 

 cocci have remained in a latent condition. But the growth soon 

 starts up again and new cocci are developed and invade the exudate. 

 Do these new organisms come from the few streptococci that have 

 remained free in spite of the generalized phagocytosis? Or are 

 they derived from bacteria that were taken up, but have resisted 

 the phagocytes? Either explanation is possible. We have already 

 seen that the streptococci may remain alive for a long time in the 

 animal body even after they have been taken up by cells. 



Rabbits treated with preventive serum may resist an infection 

 with very large amounts of an old culture.* In a general way 

 these are the phenomena in rabbits vaccinated with serum and 

 subsequently inoculated intraperitoneally with the streptococcus. 

 We must now consider delayed phagocytosis in some detail. 



* The activity of several-day-old cultures naturally depends much on the nature 

 of the culture medium. When this medium is better than usual the streptococci 

 remain "younger" for a longer time, if this expression may be permitted; in other 

 words, they retain better their property of growing rapidly on a new soil. 



