﻿STUDIES IN PLAGUE IMMUNITY. 295 



been stated, has been passed successively for over 247 passages from 

 guinea pig to guinea pig without growth on artificial media. (See table 

 of Series 58, p. 296.) These passages have occupied more than a year 

 and a half. * The method of infection was carried on as follows : 



The first guinea pig of the series was inoculated subcutaneously with one oese 

 of the strain "Pest Virulent" isolated from a human case of plague. Upon the 

 death of the animal two days later, its spleen was removed and a portion of it 

 rubbed over a shaved area on the abdomen of a healthy guinea pig (the next 

 animal of the series). The skin was sometimes slightly scarified with a knife. 

 This method was employed on the third animal, when the second succumbed, and 

 so on without a break throughout the series,. In a few instances the autopsy 

 upon the animal and the removal of the spleen did not take place until nearly 

 forty-eight hours after the death of the animal. In a few others, the spleen was 

 removed shortly after the death of the guinea pig and placed on ice and employed 

 for the next inoculation twenty-four hours later. These instances are all noted in 

 the table. They caused no change in the results of the inoculations. 



The accompanying table (58) shows that the virulence of the organism 

 during this whole time has not varied. Moreover, its virulence not only 

 for guinea pigs, but for rats and monkeys as well, has remained the same 

 at the end of the series as it was at the beginning ; this may be seen from 

 the numerous control animals inoculated from time to time with this 

 strain throughout the series of animal experiments detailed on pages 187 

 to 288. A glance at Table 58 shows that moderate differences in sus- 

 ceptibility exist in the different guinea pigs, the most susceptible dying 

 in from two to four days after inoculation and the more resistant usually 

 in from seven to eight days. The animals were not always of the same 

 age and weight and for this reason some slight variation in susceptibility 

 also probably occurred. 



Only one animal survived for twelve days and but one other for 

 eleven days after the infection. These animals represent the thirty-sixth 

 and one hundred and twentieth passages respectively. The fact that in 

 the thirty-fifth and thirty-seventh passages, the animals died six and four 

 days respectively after infection, and in the one hundred and nineteenth 

 and one hundred and twenty-first passages, four and three days respec- 

 tively after inoculation, demonstrates that not even a temporary change in 

 virulence in the organisms had occurred at these periods. The length of 

 time the animals lived ditring the thirty-sixth and one hundred and 

 twentieth passages therefore was probably due to their natural and slightly 

 increased relative resistance to pest infection. Animal number 198 died 

 twenty-four hours after its inoculation. Tbe animal was doubtless sick 

 at the time of the infection. 



