Igg BACTERIOLOGY. 



obtain the necessary nutrition ; potatoes and agar-agar 

 that have become a little dry offer very favorable 

 conditions, because of the limited area from which the 

 growing bacteria can draw their nutritive supplies and 

 because of the free access which they have to oxygen ; 

 for, their growth being on the surface, they are sur- 

 rounded by this gas unless means are taken to prevent 

 it. By the hanging-drop method, however, more than 

 this simple property may be determined. It is possible 

 not only to detect the stages and steps in the formation 

 of endogenous spores, but when the spores are completely 

 formed by transferring them to a fresh bouillon-drop or 

 drop of agar-agar, preserved in the same way, their ger- 

 mination into mature rods may be seen. The word 

 rods is used because as yet we have no evidence that 

 endogenous spore-formation 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 gela- 

 tin at and around the place at which the colonies are 

 growing. In some instances this liquefaction spreads 

 laterally and downward, causing a saucer-shaped exca- 

 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 liquefac- 



