ii CHAKACTEES OF THE B. PESTIS 21 



on Plague, that the plague bacillus grows better at the 

 lower than at the higher temperature. I am confident it 

 would considerably delay diagnosis in a given analysis of 

 suspected material if the cultures were kept at a tem- 

 perature lower than 37° C. Plague bacilli planted, for 

 instance, on agar kept at 37° C. show their colonies well 

 developed to the aided and unaided eye already after 

 twenty-four hours, whereas when kept at the temperature 

 of 20° to 25° C. for forty -eight hours would be only just 

 discernible with a glass. I believe the statement of 

 Simpson may be explained in the following manner : — As 

 will presently be shown, plague bacilli grow well, but 

 somewhat slow, in broth even at 37° C. In preparing 

 Haffkine's prophylactic (see later) a broth is used, of 

 which the top is more or less covered with clarified butter 

 (ghee) ; now this was introduced by Haffkine in order to 

 obtain copious masses of plague bacilli, for he made the 

 observation that in connection with the ghee drops of the 

 surface a rich crop of plague bacilli is continuously 

 reproduced (stalactites), and the culture flask being 

 shaken every twenty-four hours or so, these masses fall to 

 the bottom, forming and accumulating here as a copious 

 whitish, powdery, flaky sediment. Now, the most 

 abundant crops of bacillary masses sprouting downwards 

 from the surface or ghee layer are developed if the ghee 

 layer is in a solid state — they are far less abundant if the 

 ghee layer is in a fluid state. From this it follows that 

 such a ghee broth flask shows far less abundance of masses 

 (granules and flakes) of plague bacilli if incubated at 37° C. 

 — at which temperature the ghee is fluid — than at 25° C. 

 — at which temperature it is solidified. In order to obtain 

 in such ghee broth the most abundant masses of plague 



