and Laboratory Methods. 



2403 



They repeated this experiment with red and with violet bacteria, and obtained 

 characteristic red and violet colonies. 



To prove that the germs from which these colonies grew came from the 

 infected material in the first compartment, and not from accidental sources, con- 

 trol experiments were made with other groups of flies, but with no infected 

 material in the first compartment. In no case, however, did the Petri dishes 

 used in these control experiments develop yellow, red, or violet colonies. 



To prove that the flies were the only means of transmitting these bacteria 

 from the first compartment to the second, experiments were made with infected 

 material in the first compartment, but with no flies in the apparatus. The Petri 



Fig. 1. 



dishes from these experiments, also, developed no colonies. " The germs that 

 grew into colonies in the first experiments, therefore, (i) came from the infected 

 material in the first compartment, and (ii) were carried to the Petri dishes by 

 flies." From these results, they drew the more general conclusion, that, ''flies 

 are capable of carrying bacteria from one place to another, if they have a chance to 

 come in contact with material containing these orgaiiisms.'" 



They next undertook to determine " whether flies in nature actually do carry 

 bacteria with them." " To determine this, test tubes of sterile agar were melted 

 and then cooled to 40° C, a temperature a little above that of the human body. 

 At this temperature the agar still remains liquid, but is not hot enough to kill 

 bacteria. Flies were caught with sterile forceps and washed in this melted agar. 



