412 MANUAL OF BACTERIOLOGY. 



The water which squeezes out of the agar when the latter 

 soUdifies is sufficient if it is not allowed to evaporate. A 

 glass jar provided with a cover and containing a vessel 

 holding water may be employed for slant cultures. For 

 Petri dish cultures a battery jar inverted in a shallow dish 

 of water will serve to keep the cultures moist. Oxygen is 

 essential to the growth of the amebae. They grow best at about 

 20° C. to 25"^ C. but they will also grow at 37.5° C. 



ISOLATION OF PURE CULTURES OF AMEB^. 



The amebas grow only upon the surface of the solid 

 medium, and they do not remain fixed as do the bacteria by 

 the setting of the agar, but creep about from place to place. 

 It is therefore necessary to resort to special methods to secure 

 cultures consisting of one kind of ameba only. If an agar 

 Petri plate previously poured and allowed to harden is stroked 

 with a platinum needle in several parallel lines with material 

 containing ameba the lines made last will show scattered 

 individual ameba cells. The needle is dipped only once into 

 the material to be studied. The plate is examined with the 

 low power of the microscope, and a single ameba separated 

 from all the rest is located. A high power lens is then turned 

 into position and run down until it comes in contact with the 

 ameba selected. In this way the ameba is picked up on the 

 end of the lens and may be transplanted to fresh media to 

 grow out and form a culture consisting of the progeny of this 

 one cell. Another method of obtaining a pure culture of 

 amebag is to locate a cell as just described with the low power 

 of the microscope but with the Petri dish inverted. The 

 location is marked around on the bottom of the inverted dish 

 with a wax pencil or with ink. The dish is now turned right 

 side up, and the place marked out as the location of the cell 

 is inclosed by drawing a ring of vaseline around it. This 

 prevents the ameba from wandering out, and also prevents 



