178 BACTERIOLOGY. 



the small air-tight chamber which exists between the 

 depression in the slide and the cover-slip. (See Fig. 32, 

 page 157.) 



A very thin drop of sterilized agar-agar may be sub- 

 stituted for the bouillon. It serves to retain the organ- 

 isms in a fixed position, and the process may be more 

 easily followed. 



As soon as finished, the preparation is to be examined 

 microscopically, and the condition of the organisms 

 noted. It is then to be retained in a warm chamber 

 especially devised for the purpose, and kept under con- 

 tinuous observation. The form of chamber best adapted 

 for the purpose is one which envelops the whole micro- 

 scope. It is provided with a window through which 

 the light enters, and an arrangement for moving the 

 slide about from the outside. The formation of spores 

 requires a much longer time than the germination of 

 spores into bacilli, but with patience both processes may 

 be satisfactorily observed. 



It will be noticed that the description of this process 

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

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

 lation 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, otherwise 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 sometimes 

 days, and contamination 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 



