v PLAGUE IN THE EAT ill 



An important point noticeable in the B. pestis of 

 the pneumonic plague in the above rats was that its 

 virulence was distinctly of a subnormal character ; this 

 appeared to follow from the experimental fact that when 

 tested on the guinea-pig by subcutaneous injection of 

 even considerable doses (of a twenty-four hours agar 

 culture an emulsion having been made, and several drops 

 of the emulsion being used for injection) it was found 

 that in no case did it cause the acute form of plague, 

 but always the subacute form leading to death in five 

 to nine days. The same applies also to other instances 

 in which natural or artificial attenuation of a strain 

 of B. pestis had been produced, viz. that even fairly 

 large doses injected subcutaneously into the guinea-pig 

 cause the subacute form of the disease (see later). 



In connection with the subject of attenuated type of 

 B. pestis, I now proceed to record experiments which 

 indicate that some strains of B. pestis, however virulent 

 originally, may in the laboratory, i.e. artificially, be made 

 to breed true as the attenuated or rat type, and these 

 experiments would suggest that occasions may arise also 

 in nature when a strain of B. pestis, originally virulent, 

 may become attenuated, and may then persist in this 

 form through many generations. I have above already 

 indicated two such occasions — 



(1) By passing through a succession of rats by 

 which the B. pestis becomes of the nature of the rat 

 type (2) ; and 



(2) By the bacillus being allowed to breed in a rat of 

 lesser or subnormal susceptibility. 



I proceed now to record a number of experiments in 

 further support of these propositions. 



