v PLAGUE IN THE EAT 99 



greater transparency of the colonies on gelatine, in the 

 shorter size of the individual bacilli (causing them to 

 resemble oval cocci), and in rapid loss by the microbe of 

 infective power when carried on in subculture. In this 

 instance, as in others, I have regularly subcultured the 

 bacillus on gelatine once a month. This Cape rat 

 B. pestis, after repeated subculture for some months — six 

 months at the least — has practically lost all, or almost all, 

 pathogenic action ; a whole agar surface culture is not 

 now able to kill a guinea-pig when injected intraperi- 

 toneally. Similarly, I have injected subcutaneously into 

 the groins of guinea-pigs and rats as much in each 

 instance as 1 c.c. of a strongly turbid emulsion of these 

 later subcultures, with the result that the animals 

 remained alive, although they developed temporary 

 tumour in the groin. Also, I have inoculated cutaneously 

 — which is the most effective method (see later) of infec- 

 tion — rats by rubbing into an abrasion of their skin a 

 solid particle of the growth of this later subculture, but 

 apparently without result. But there was a striking and 

 twofold result observed in connection with rats and 

 guinea-pigs which had been repeatedly injected sub- 

 cutaneously with large amounts of these later subcultures : 

 (a) the blood serum of animals thus " protected " against 

 B. pestis was proved to be capable of agglutinating an 

 emulsion of Edington bacillus ; (b) rats " prepared " with 

 Edington subcultures were found to be immunised against 

 infection with virulent B. pestis. These two observations 

 place it, I think, beyond doubt that the Edington cultures 

 were really those of B. pestis. 



As to the minor virulence exhibited by it, this is an 

 occurrence which has been noted by many observers, and 



