188 BACTERIOLOGY. 



liquefaction of the gelatin at and around the place at 

 which the colonies are growing. In some instances 

 this liquefaction spreads laterally and downward, caus- 

 ing a saucer-shaped excavation, while in others the 

 colony sinks directly down into the gelatin and may be 

 seen lying at the bottom of a funnel-shaped depression. 

 These differences are constantly employed as one of 

 the means of differentiating otherwise closely allied 

 members of the same family of bacteria. (See Fig. 

 34.) Studies upon the spirillum of Asiatic cholera 

 and a number of closely allied species, for example, 

 reveal a decided difference in the form of liquefaction 

 produced by these different organisms. The slightest 

 detail in this respect must be noted, and its frequency 

 or constancy under different conditions determined. 



Cultures on Potato. — A very important feature 

 in the study of an organism is its growth on sterilized 

 potato. Many organisms present appearances under 

 this method of cultivation which alone can almost be 

 considered characteristic. In some cases coarsely lob- 

 ulated, elevated, dry or moist patches of development 

 occur after a few hours; again, the growth may be finely 

 granular and but slightly elevated above the surface of 

 the potato; at one time it will be dry and dull in ap- 

 pearance, again it may be moist and glistening. Some- 

 times there is a production of bubbles, owing to fermen- 

 tation brought about by the growth of the organisms. 



A most striking form of development on potato is 

 that possessed by the bacillus of typhoid fever and the 

 bacillus of diphtheria. After the inoculation of a potato 

 with either of these organisms there is usually no naked- 

 eye evidence of a growth in either instance, though 

 microscopic examination of scrapings from the surface 



