22 ACTION OF AUXETICS AND KINETICS 



air-chamber method " is as follows : A jelly film 

 is allowed to set in the bottom of a small two-inch 

 Petri dish. Cysts are then placed on one or more 

 spots on the surface and the dish is inverted over 

 water in a larger vessel (PI. III., A). Thus there is a 

 moist air space between the water and the jelly, and 

 evaporation is prevented. By this means a more 

 abundant growth is obtained than if a Petri dish in 

 the ordinary upright position is used, and it is the 

 best way of maintaining a constant stock of the 

 culture. By the continuous supply of moisture 

 and air combined with emigration of the amoebae 

 from the original site of the inoculation, growth 

 continues almost indefinitely until the whole surface 

 of the jelly is nearly a solid mass of cysts. Again, 

 the amoebae which have wandered from the central 

 spot tend to run together into clusters (fig. 2), and 

 by this means groups of cysts, which may contain 

 any number up to a hundred or so, can be picked 

 off almost bacteria free in a capillary tube and 

 employed in the preparation of pure cultures 

 (fig. 3). 



A modification of this method, which has been 

 found useful for various purposes, and which can be 

 kept sterile for some time, is to use a jelly film on 

 a microscope slide which is then inverted, resting on 

 corks, over water in a large Petri dish (PI. III., B). 

 A cover-slip may then be employed, and does not 

 press on the amoebae. For continuous observations 

 extending over a few days, a live cell made of 

 molten paraffin wax has occasionally been used in 

 the following way : A thin film of jelly on a cover- 

 glass is smeared with cysts and inverted quickly 

 over a layer of soft wax (which has a hole in it), a 



