206 BACTERIOLOGY. 



ing organisms, and may dart from one position in the 

 field to another. In some cases the true movement of 

 bacteria is very slow and undulating, while in others it 

 is rapid and darting. The molecular tremor may be 

 seen with non-motile and with dead organisms. 



Note. — Prepare three hanging-drop preparations — 

 one from a drop of dilute India-ink, a second from a 

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

 bacillus of typhoid fever. In what way do they differ ? 



Study of Spoee-foemation. — The hanging-drop 

 method just mentioned is not only employed for detect- 

 ing 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. 



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 from the 

 table upon which it rests, invert the hollow over the 



