100 OEIENTAL PLAGUE chap. 



is a natural sequence with most races of B. pestis under 

 continued subculture ; as a matter of fact, it is impossible 

 to be quite certain about the action of any race of 

 B. pestis that has been kept in artificial culture for many 

 removes. Only quite recently I have found unexpectedly 

 that gelatine culture of even a race that hitherto had not 

 failed in any way, viz. that obtained from the London 

 dock case of 1896, had abated somewhat in its virulence ; 

 so that it was necessary to pass it again, starting with a 

 big dose, through a series of animals, in order to restore it 

 to a fair degree of virulence. 



With reference to the slight alteration of the morpho- 

 logical character exhibited by Edington's bacillus, viz. its 

 being shorter and more like oval cocci, this has been 

 observed in the cultures of various other races of B. pestis 

 having attenuated virulence. For instance, I find this to 

 be the case with greatly attenuated plague culture of a 

 Yeddah case, of an Oporto case, and of a case of an 

 Indian native from s.s. City of Perth. 



As regards the different character of the appearance 

 of colonies on the surface of nutrient gelatine, it is to 

 be noted that there certainly exists a marked difference 

 between the aspect of the colonies on gelatine of the 

 B. pestis derived from different cases. It seems to me 

 that two definite types of these colonies can be recog- 

 nised : — 



(a) Typical virulent B. pestis forms on gelatine, in 

 twenty-four to forty-eight hours, angular colonies, grey, 

 translucent, heaped up in the centre, or slightly excentrical, 

 and exhibiting granulation under a glass in transmitted 

 light. All these features become more pronounced as 

 incubation proceeds. Fig. 28 shows such a typical 



