188 STRONG AND TEAGUE. 



many epidemics of bubonic plague. In stained microscopic prep- 

 arations made from the organs at necropsy it appears in its 

 most characteristic form as a short bacillus, more or less ovoid 

 in form, swollen in the center, and rounded at the ends. It 

 exhibits marked bipolar staining, the central portion either 

 remaining uncolored or staining lightly. Such preparations and 

 those made from sputum often show, besides these bipolar forms, 

 great variation in the morphology of the organisms present. 

 Involution forms, consisting of longer, thicker, deeply staining 

 rods, or of organisms which have assumed a spherical or orbic- 

 ular outline, or, occasionally even appearing very much as yeast 

 cells, may be encountered. Many of these forms stain poorly, 

 or sometimes only a portion of the organism is stained, and in 

 the shorter bacilli the appearance of ring forms is thus produced. 

 In agar-cultures, and particularly in 3 per cent salt agar, these 

 large involution forms and degenerating organisms of very dif- 

 ferent shapes are very numerous and characteristic: long and 

 slender or thick bacilli and also boat-shaped, dumb-bell, ring- 

 shaped, and spherical organisms may all be observed. The or- 

 ganism generally appears in preparations from agar-cultures as 

 a short or longer rod, and does not so frequently reveal the 

 marked bipolar appearance when stained. In hanging-drop prep- 

 arations no true motility is exhibited. No flagella are visible in 

 properly-stained preparations, and no spores have been demon- 

 strated. It stains easily with all the anilin dyes, and partic- 

 ularly well with dilute carbol-fuchsin solution, and is easily and 

 completely decolorized by Gram's stain. 



CULTURAL CHARACTERISTICS. 



The cultural characteristics of the bacillus also are practically 

 identical with those of many bubonic strains. The bacillus grows 

 well upon neutral, or slightly alkaline, moist agar at a temper- 

 ature of from 25° to 35° C, and is aerobic. After twenty-four 

 to forty-eight hours on agar-cultures inoculated with the pneu- 

 monic strain, usually small, delicate, transparent, dewdrop-like 

 colonies appear, which after forty-eight to seventy-two hours 

 have increased in size, are more raised, and have become less 

 translucent. After this time many of these colonies do not 

 perceptibly increase in diameter, while others later become much 

 heavier and larger, so that the two types of colonies are frequently 

 observed in the same culture: the one smaller and more or less 

 translucent, and the other much larger (four or five times the 



