STUDY OF CULTURES ON POTATO. 113 



mination into mature rods may be seen. The word 

 rods is used because as yet we have no evidence that 

 endogenous spore-formatiou occurs in any of the other 

 morphological groups of bacteria. 



Study of Gelatin Cultures. — As has been pre- 

 viously stated, the behavior of bacteria toward gelatin 

 differs — some of them producing apparently no altera- 

 tion in the medium, while others bring about a form 

 of peptonization which results in liquefaction of the 

 gelatin at aud around the place at which the colonies 

 are growing. In some instances this liquefaction spreads 

 laterally, in others it sinks directly down into the gelatin. 

 These differences have been conspicuously shown and em- 

 ployed as one of the means of differentiation of otherwise 

 closely allied members of the same family of bacteria. 

 Studies upon the organism of Asiatic cholera and a 

 number of closely allied forms reveal a decided differ- 

 ence in the manner 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 appcaianccs under 

 this method of cultivation which alone can almost be 

 considered characteristic. In some eases 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 

 appearance, again it may be moist and glistening. 

 Sometimes there is a production of bubbles, owing to 

 fermeptation brought about by the growth of the organs. 



