ii CHARACTERS OF THE B. PESTIS 23 



can be noticed between the colonies of B. pestis derived 

 from a virulent source (human or rat) and those derived 

 from a slightly virulent source (human or rat), this 

 difference being that the former, as incubation proceeds, 

 show a gradually more pronounced angular margin, and 

 are more conically raised in the centre than the latter. 



What, however, is very characteristic of colonies of 

 B. pestis growing on the surface of an agar plate is the 

 fact that when attempting to remove a particle of the 

 growth with a platinum needle it is found that the sub- 

 stance of the colony is very viscid, so much so that either 

 the whole colony adheres to the point of the needle or 

 that it is difficult to take up any part of the colony ; 

 further, when the growth is deposited in a drop of fluid 

 — e.g. saline solution, water, or broth — it is difficult to 

 emulsify it, the growth remains in coherent threads and 

 flakes ; under the microscope, therefore, connected masses 

 of bacilli — small and large, according to the amount of 

 mechanical force used in the attempt to emulsify — are 

 met with, some of these masses drawn out into longer or 

 shorter strings. 



On the surface of gelatine the colonies of the typical 

 B. pestis as soon as they are discernible, i.e. soon after 

 forty-eight hours, show themselves as grey, in transmitted 

 light as somewhat opaque, distinctly granular, rounded or 

 slightly angular points. Later on the distinction between 

 an angular thin marginal and a conically raised thick 

 middle part becomes very marked ; so also the granular 

 character of the colony, so much so that when examining 

 in transmitted light, with a magnifying - glass, a well- 

 developed colony, i.e. after four to six days incubation at 

 20° to 21° C, its substance, particularly in the middle parts, 



