180 BACTERIOLOGY. 



vation, 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 con- 

 stantly employed as one of the means of differentiating 

 otherwise closely allied members of the same family of 

 bacteria. 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 lobu- 

 lated, 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 organs. 



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 micro- 

 scopic examination of scrapings from the surface of the 

 potato reveals an active multiplication of the organisms 

 which had been planted there. The potato is one of 

 the most important differential media which we possess 

 for this work. 



