HANGING-DROP PREPARATIONS 63 



when it is required to get the bacteria free from admixture of 

 medium, as in the case of staining flagella. 



MISCELLANEOUS METHODS. 



Hanging-drop Cultures. It is often necessary to observe 

 micro-organisms alive, either to watch the method and rate of 

 their multiplication, or to investigate whether or not they are 

 motile. This is effected by making hanging-drop cultures. The 



FIG. 27. 



A. Hollow-ground slide for hanging-drop cultures shown in plan and section. 

 B. Another form of slide for similar cultures. 



method in the form to be described is only suitable for aerobes. 

 For this special slides are necessary. Two forms are in use and 

 are shown in Fig. 27. In A there is ground out on one surface 

 a hollow having a diameter of about half an inch. That shown 

 in B explains itself. The slide to be used and a cover-glass are 

 sterilised by hot air in a Petri's dish, or simply by being heated 

 in a Bunsen and laid in a sterile Petri to cool. In the case of 

 A, one or other of two manipulation methods may be employed. 

 (1) If the organism be growing in a liquid culture, a loop of the 

 liquid is placed on the middle of the under surface of the sterile 

 cover-glass, which is held in forceps, the points of which have 

 been sterilised in a Bunsen flame. If the organism be growing 

 in a solid medium, a loopful of sterile bouillon is placed on the 

 cover-glass in the same position, and a very small quantity of 

 the culture (picked up with a platinum needle) is rubbed up in 



