194 BACTERIOLOGY 



micrococci, and a third from a culture of the bacillus of 

 typhoid fever. In what way do they differ? 



Study of Spore-formation. — The hanging-drop method just 

 mentioned is not only employed for detecting the motility 

 of an organism, but also for the study of its mode of spore- 

 formation. 



Since with aerobic organisms spore-formation occurs, as 

 a rule, only in the presence of oxygen, and is induced more 

 by limitation of the nutrition of the organisms than by any 

 other factor, it is essential that these two points should be 

 borne in mind in preparing the drop-cultures in which the 

 process is to be studied. For this reason the drop of bouillon 

 should be small and the air-chamber relatively large. 



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

 tuted for the bouillon. It serves to retain the organisms 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, and kept under 

 continuous observation. The form of chamber best adapted 

 to the purpose is one which envelops the whole microscope. 

 It is provided with a window through which the light enters, 

 and an arrangement by which the slide may be moved 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 immediately precedes, but differs 

 from it in one respect, viz., that in this manipulation we 

 are not making a preparation which is simply to be ex- 

 amined and then thrown aside, but it is an actual pure 



