24 ORIENTAL PLAGUE chap. 



is coarsely granular, almost filamentous, and its margin 

 irregular and much fringed, such as is shown in Figs. 

 34 and 36. Seen in reflected light under a glass it may 

 truly be compared to a limpet -like mass stuck to the 

 surface of the gelatine (Fig. 28). 



Removing a particle of a colony from the gelatine 

 surface it will be found to be coherent, but less so than 

 from agar, and further, that it emulsifies better in saline 

 solution than one from agar, but there are nevertheless 

 present larger or smaller aggregations of bacilli. 



When viewing a surface gelatine plate, i.e. one con- 

 taining numerous surface colonies, it will be found that, 

 while all are more or less angular with filmy margin and 

 conically raised centre, there is a distinction to be drawn 

 if of a recent plate, e.g. one only a few days old (21° C), 

 an impression preparation is made. When after staining 

 and mounting they are examined under the 'microscope 

 with a low magnifying power, say 60 to 90, two kinds of 

 colonies will be- noticed: (a) the great majority are 

 typical colonies — angular patches composed of rod-shaped 

 bacilli fairly well separated from one another by clear 

 interstitial substance ; (b) a small number of irregular 

 colonies, which, however, are composed chiefly of more or 

 less thready or long cylindrical bacilli ; these latter 

 colonies represent what I have called atypical colonies, 

 and both sets are well shown in Figs. 41 and 42. 1 



We shall have the opportunity of showing that as 

 regards virulence two types of B. pestis may be dis- 

 tinguished : the first type (type 1) — human type — is the 

 one that is the more virulent, and is found in typical 



1 As to the filamentous modification of B. pestis in salted media, see a later 

 page. 



