MICROSCOPIC EXAMINATION OF PREPARATIONS 211 



The difference between the motion of bodies undergoing 

 this molecular tremor and that possessed by certain living 

 bacteria is that the former particles never move from their 

 place in the field, while living motile bacteria alter their 

 position in relation to the surrounding 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 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 



