17 



used in the Tropics only where an incubator with cooling device is 

 at hand. If an impression preparation is made of one of the young 

 colonies with a homogeneous, transparent, marginal zone, it is seen 

 that this consists of bacilli arranged in curved filaments. If the 

 inoculation from the organs has been quite rich in plague bacilli 

 the surface between the colonies has a ground-glass appearance, 

 which becomes more marked after forty-eight hours, and is generally 

 well seen in cultures a few days old. After three or more days the 

 erstwhile very delicate colonies become larger, more granular, and 

 less transparent. The marginal zone likewise has become thickened, 

 is less homogeneous, and its edges become more irregular and are 

 often finely serrated at the periphery. The colonies now have a 

 more decidedly grayish-white appearance and occasionally show a 

 slight tinge of yellow. In young cultures of the first generation 

 colonies are usually small, grow slowly and remain comparatively 

 small, occasionally^ however, rapidly attaining a diameter of several 

 millimeters. We have several times seen such large colonies where 

 the material from which an inoculation was made contained few 

 bacilli and where subsequently only a few colonies developed. How- 

 ever, this is probably not the only condition under which the large 

 ones make their appearance. Older cultures of the plague bacillus, 

 particularly when they have in consequence of evaporation become 

 dry and quite granular, are somewhat iridescent. If a young 

 plague culture is touched with a platinum loop it is found to be 

 viscous and sticky. It is, however, easily removed from the surface 

 on which it grows. 



The plague bacillus, as stated before, early in its growth has a 

 marked tendency to develop involution forms. As first shown by 

 Hankin, this tendency is most pronounced in cultures on a 3 to 4 

 per cent salt agar. Hence we have in this medium one of the 

 most valuable means for the bacteriologic diagnosis of plague. It 

 is prepared and standardized like any ordinary agar, only that it 

 does not merely contain one-half but 3 to 4 per cent of common 

 salt. Hence, in case of an emergency, salt agar can be prepared 

 from the ordinary media by the addition of the proper amount 

 of NaCl to bring it up to the desired concentration. We have found 

 it best to use a 3.5 to 4 per cent salt agar and not to diminish this 

 percentage. On a medium of this concentration, particularly if it 

 is fairly dry and contains very little water of condensation, involu- 

 25483 2 



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