MICROSCOPIC EXAMINATION OF PREPARATIONS 195 



culture, and must be kept as such, otherwise the observa- 

 tion 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 sometimes days, and contamina- 

 tion must, therefore, be carefully 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 

 nourishment; potatoes and agar-agar that have become 

 a Uttle 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 

 specific property may be determijied. It is possible not only 

 to detect the stages and steps in the formation of endogenous 

 spores, but when the spores are completely formed their 

 germination into mature rods may be seen by transferring 

 them to a fresh bouillon-drop or drop of agar-agar preserved 

 in the same way. The word rods is used because we have as 

 yet no evidence that endogenous spore-formation occurs 

 in any of the other morphological groups or bacteria. 



