ii CHAKACTEKS OF THE B. PESTIS 35 



series of animals. Nor have I seen anything of a decrease 

 in virulence either in the animals of the same species or 

 when plague material (bubo or spleen of the dead animal) 

 is taken from one species (guinea-pig or rat) and is used 

 for infection of the other species (rat or guinea-pig). 



What I found in a very large number of experiments 

 made for this object is that it does not matter whether 

 the animal which yields or receives the material is a 

 guinea-pig or a rat, so long as the bubo, or spleen, or 

 blood of the plague giver contains the typical B. pestis in 

 considerable numbers, the result of cutaneous inoculation 

 of the rat or of the guinea-pig is followed by fatal plague, 

 acute in the rat, subacute in the guinea-pig. A difference, 

 however, which I have noticed to occur and which denotes 

 some kind of difference in virulence, refers to an altogether 

 different circumstance, viz. to the race of the B. pestis 

 itself. With this we shall have to deal in a future chapter. 

 Another difference which is to be mentioned in connection 

 with the above is the fact that B. pestis when passed 

 through the mouse (dead of plague after inoculation) is as 

 a rule of lesser virulence towards the other rodents than 

 when the same race of B. pestis is passed through the rat 

 or the guinea-pig. All races of B. pestis act virulently on 

 the mouse, this animal being highly susceptible and easily 

 affected with acute fatal effect by cutaneous inoculation of 

 almost any B. pestis. 



