126 OMENTAL PLAGUE chap. 



dark spleen. Bubo, spleen, and heart's blood yielded 

 numerous B. pestis of type 2. 



There is distinct evidence, then, that the B. pestis 

 taken from mouse 1 was also for mice 5 and 6 of a 

 distinctly minor virulent character, since death ensued in 

 one case on the fifth day, in the other on the seventh. 

 This is altogether different from what took place in the 

 case of mice inoculated cutaneously with the B. pestis of 

 the Cardiff bubo type (experiment 15). 



It is justifiable, therefore, to conclude that the B. pestis 

 secured through the medium of the Haffkine protected rat 

 is of a different type ("rat " type) to the B. pestis derived 

 from the Cardiff bubo ("human" type), seeing that 

 B. pestis in this phase maintained its particular and 

 minor virulence, as also its morphological and cultural 

 characters, unimpaired, however many times it was 

 subcultured or was passed through the animal body. 

 Further, there is in these experiments confirmation of 

 previous observations as to type No. 1 (or human type) 

 being of a more virulent character than type No. 2 (rat 

 type) in the fact that type No. 2 has been now derived 

 from the swollen gland of a protected rat — a rat, namely, 

 that had by previous injection with plague prophylactic 

 been furnished with a high degree of resistance. 



A strain of B. pestis which possesses and which 

 maintains in its passage through successive rats its 

 attenuated virulence might obviously very well be bred 

 in nature. If, for instance, in a ship sailing from an 

 infected port plague brought on board by rats spread 

 epidemically amongst the ship rats in the course of a 

 long voyage, the most susceptible of these ship rats 

 would of course be the first to die off; the last to remain 



