v PLAGUE IN THE RAT 101 



colony after about a week's incubation on gelatine, in re- 

 flected light. The colony in question looks like a limpet 

 with a heaped -up thickened central, and a thinned -out 

 filmy peripheral irregular part. In transmitted light the 

 colonies are opaque and granular. 



(b) B. pestis of another type shows, on the other 

 hand, on gelatine culture, the young colonies round and 

 distinctly translucent ; in fact, alike in its early and late 

 phases, the growth — both the separate colonies on the 

 surface, as also the surface streak culture — is markedly 

 translucent, so much so that the colonies in this respect 

 look not unlike those of the B. coli. They increase, 

 however, more slowly than the latter, and of course are 

 easily distinguished from them by the nature of the 

 growth in other media. Of course, in old cultures, i.e. 

 many weeks old, even the colonies of this (b) type become 

 opaque and thick in the centre. When examined in film 

 specimens the individual bacilli are distinctly shorter 

 (less cylindrical) than the (a) type, the majority being 

 more like oval cocci or short oval bacilli. 



Now, it is this (6) type of B. pestis, which at starting 

 is of a distinctly low virulence, that rapidly loses its 

 virulence. I have myself obtained this type from the 

 animal body in two instances, i.e. (l) from Bristol rats, 

 and (2) from an Indian native suffering from plague on 

 board the s.s. City of Perth in the port of London in 

 1902. The B. pestis last referred to I have designated in 

 my laboratory " L.P. No. II." In both instances the 

 B. pestis was from the outset of a distinctly attenuated 

 kind, and showed in a marked degree the above-mentioned 

 character of translucent colonies on gelatine. On sub- 

 culture, attenuation of its virulence proceeded so rapidly 



