ix PKOTECTIVE INOCULATION 249 



notwithstanding that the broth itself, the addition of 

 clarified butter, the temperature, and all other conditions 

 are as far as possible the same — show after a like period of 

 incubation different amounts of bacterial growth in the 

 form of floccular and granular sediment. My experience 

 fully bears this out. Thus of a group of flasks (8-12) 

 treated seemingly, and indeed intentionally, in exactly 

 the same manner — i.e. same make of peptone broth, same 

 amount of ghee added, same stock culture of plague used 

 for infection, same platinum needle used in this process, 

 same temperature of incubation, same duration of incuba- 

 tion — not all show the same amount of sediment of solid 

 growth. While some flasks of this brew contain a 

 comparatively large amount of the floccular and granular 

 sediment, others show this to a conspicuously less degree. 

 I have found that, cceteris paribus, the incubation of the 

 flasks at 25° C. for the last fortnight or three weeks of 

 preparation — the first fortnight having been passed by 

 the flasks in an incubator at 37° C. — yields the greatest 

 amount of sediment, certainly greater than if the flasks 

 are kept for the whole five weeks at 37° C. 



Another point in which I fully confirm Haffkine's 

 statement, and one which I think of importance, is this : 

 The presence of a thin layer of droplets of ghee on the 

 surface of the broth is an excellent and sure means of 

 increasing the amount of bacterial growth. This is 

 particularly well shown after the inoculated flasks are 

 transferred to a temperature of 25° C. The ghee drops 

 at this temperature become solid flat platelets ; and in 

 connection with them, and extending underneath them 

 rapidly, the growth appears in the form of a whitish 

 scum (with stalactites), which, on shaking, becomes 



