STUDY OF GELATIN CULTURES. 187 



times days, and coutaiuiuation must, therefore, be care- 

 fully guarded against. The study should be begun with 

 the vegetative form of the organisms ; the hanging-drop 

 preparation should, for this reason, always be made 

 from a perfectly fresh culture of the organism under 

 consideration before time has elapsed for spores to form. 



The simple detection of the presence or absence of 

 spore-formation can in many cases be made by other 

 methods. For example, many species of bacteria which 

 possess this property form spores most readily upon 

 media from which it is somewhat difficult for them to 

 obtain the necessary' nutrition; potatoes and agar-agar 

 that have become a little dry offer very favorable con- 

 ditions, 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 com- 

 pletely formed by transferring them to a fresh bouillon- 

 drop or drop of agar-agar, preserved in the same way, 

 their germination into mature rods may be seen. The 

 word rods is used because as yet we have no evidence 

 tliat 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 

 diffei-s — some of them producing apparently no altera- 

 tion in the medium, while the growth of others is 

 accompanied by an enzymotic action that results in 



