PATHOGENIC BACTERIA. 317 



appearance. They are nearly round at first, and granular 

 as seen under the low power of the microscope; but at the 

 end of about twenty-four hours the outline is slightly irregu- 

 lar, and the surface looks as though it were covered with 

 finely-broken glass. The outline later becomes still more 

 irregular or scalloped. As liquefaction of the gelatin takes 

 place a funnel-shaped depression is formed, into which the 

 colony sinks. The plates should be kept at a temperature of 



FIG. 91. 



Involution forms of the spirillum of cholera. (Van 



from 20 to 22 C. In stab-cultures in gelatin a white 

 growth forms around the stab, and at the end of about 

 thirty-six to forty-eight hours a funnel-shaped depression 

 occurs at the surface, owing to the liquefaction of the 

 gelatin. This depression increases in size, and the surface 

 of the liquefied gelatin seems to be surmounted by an air- 

 bubble, which appears to have taken the place of the part 

 of the fluid gelatin which has evaporated. In the deeper 

 portion of the stab liquefaction is less noticeable. The 

 growths on agar are not characteristic. In bouillon a pellicle 

 forms on the surface. On potato in the incubator the 

 growth is whitish or brownish, not conspicuously elevated. 

 After growing it in Dunham's peptone solution in the incu- 



