166 BACTERIOLOGY. 



Study op Spore-formation. — The hanging-drop 

 method just mentioned is not only employed for the 

 detection of the motility of an organism, but also for 

 the study of its spore-forming properties. 



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. 



The cover-slip and hollow-ground slide should be 

 carefully sterilized, and with a sterilized platinum loop 

 a very small drop of bouillon is placed in the centre of 

 the cover-slip. The slip is then inverted over the hollow 

 depression in the sterilized object-glass and sealed with 

 vaselin. The most convenient method of performing 

 this last step in the process is to paint a ring of vaselin 

 around the edges of the hollow in the slide, and then, 

 without taking the cover-slip up from the table upon 

 which it rests, invert the hollow over the drop and 

 press it gently down upon the cover-slip. The vaselin 

 causes the slip to adhere to the slide, so that it can be 

 easily taken up. The drop now hangs in the centre of 

 the small air-tight chamber which exists between the 

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

 32, page 164.) 



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 exam- 



