22 ORIENTAL PLAGUE chap. 



bacilli, the flasks are incubated at 25° C. or even down to 

 20° C. This seems to me the origin of Simpson's state- 

 ment, viz. that B. pestis grows better at lower temperatures 

 than at 37° C. I have made series of comparative experi- 

 ments with B. pestis of different races derived from 

 various sources, planting them on the surface of various 

 solid media such as are generally used for culture of 

 bacteria in the laboratory, e.g. nutrient gelatine (streak 

 and stab), nutrient agar (streak and stab), solidified blood 

 serum (streak), ascites agar (streak and stab), nutrose 

 ascites agar (streak and stab), and there never was any 

 difference in regard to the growth of the different races, 

 for all showed the quickest and most abundant growth 

 when the culture tubes were kept at 37° C. 



According to my experience, the best way to obtain 

 rapid and reliable evidence of the growth of B. pestis is to 

 plant the suspected material on the surface of the ordinary, 

 i.e. faintly alkaline agar (beef broth, peptone, agar), 

 set in a plate dish and kept at 37° C. Next day the 

 colonies are visible already to the unaided eye — better, of 

 course, with a magnifying-glass — as small, rounded, grey, 

 translucent, watery, slightly raised droplets. Examining 

 these carefully with a magnifying-glass, it is noticed that 

 the edge of the colonies is not quite rounded, showing 

 already now slight irregularities ; these become more 

 pronounced after a further twenty-four hours. At this 

 time the colony is thinner at the margin than in the 

 centre, is therefore slightly conical ; this also becomes 

 more pronounced later. In transmitted light, and viewed 

 under a glass, the substance of the colony — particularly in 

 the central portion — is finely granular. 



Now, I wish already here to state that a difference 



