STUDY OF SPORE-FORMATION. 207 



sibility of error that must be guarded against. All 

 microscopic insoluble particles in suspension in fluids 

 possess a peculiar tremor or vibratory motion, the so- 

 called " Brownian motion." This is very apt to give 

 the impression that the organisms under examination 

 are motile, when in truth they are not so, their move- 

 ment in the fluid being only this molecular tremor. 



The difference between the motion of bodies under- 

 going this molecular tremor and that possessed by cer- 

 tain living bacteria is that the former particles never 

 move from their place in the field, while living bac- 

 teria alter their position in relation to the surround- 

 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 SPORE-FORMATION. 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 



