112 BACTERIOLOGY. 



is very much like that which has just been given, but 

 differs from it in one respect, viz., that in this manip- 

 ulation we are not making a preparation which is simply 

 to be examined and then thrown aside, but it is an 

 actual pure culture, and must be kept as such, other- 

 wise the observation will be worthless. For this reason 

 the greatest care must be observed in the sterilization of 

 all objects employed. Studies upon spore-formation by 

 this method frequently continue over hours, and some- 

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

 fully guarded against. The study should be begun with 

 the vegetative form of the organism ; the hanging-drop 

 preparation should, for this reason, always be made from 

 a perfectly fresh culture of the organism under consider- 

 ation, 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 

 which 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 surrounded 

 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 en- 

 dogenous 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- 



