ix PEOTECTIVE INOCULATION 267 



10 cc. of this filtrate on two separate occasions, November 

 21 and December 31. On February 6, i.e. thirty-seven 

 days later, they were injected with a dose each of living 

 culture which killed a control rat on the fourth day. Of 

 the four prepared rats, two died of plague on the fifth 

 day, the nature of this fatal malady being confirmed on 

 microscopic, macroscopic, and cultural evidence. The two 

 others survived. 



(b) Four half- grown rats received each 10 cc. of this 

 filtrate on March 11, and again 10 cc. on March 18. 

 On May 5 they were injected with living plague culture 

 at the same time that a control rat received an equal dose 

 of this infection. The result was instructive. The con- 

 trol rat died of typical plague on the third day ; one of 

 the prepared rats died of plague on the fourth day ; another 

 of the prepared rats died of plague on the sixth day ; a 

 third of these prepared rats died of plague on the twelfth 

 day. But the fourth survived. 



From these experiments I think it is justifiable to 

 conclude that, although the effect of the filtrate (20 cc. 

 per animal) was small, it was nevertheless of a positive 

 nature, since the prepared animals behaved somewhat 

 differently from the control rats. Some of the former 

 were found distinctly protected, while those not protected 

 died always later than the control rats. 



The 1 principle underlying the preparation of plague 

 prophylactics such as have hitherto been employed has its 

 basis in the well-known fact that immunity of an animal 

 to plague may be induced by injecting into it a certain 



1 Preliminary Report to the Local Government Board on a New Plague Prophy- 

 lactic, December 19, 1905. 



