694 Dysentery 



rally, not pure; they contain various amebas and numerous 

 bacteria. 



To isolate and cultivate a single kind of ameba Musgrave 

 and Clegg have recommended an ingenious technic. A plate 

 is selected upon which the desired amebas are so widely sep- 

 arated from one another that not more than one is in a 

 microscopic field of a low-power objective. The microscope 

 used should have a double or triple nose-piece. With a low- 

 power (Zeiss A A) objective a well isolated organism is brought 

 to the center of the field. The lens is then swung out and a 

 perfectly clean higher power lens (Zeiss DD) swung in and 

 racked down until it touches the surface of the agar-agar, when 

 it is quickly elevated again. In three out of five cases the 

 ameba adheres to the objective and is so picked up. Whether 

 it has done so or not can be determined by swinging in the 

 low-power lens again and looking for the organism. If it 

 has disappeared, it is attached to the objective. It is now 

 planted upon a fresh plate by depressing the high-power lens 

 until it touches the surf ace of the culture-medium, when, upon 

 elevating it again, it usually leaves the ameba behind. Ob- 

 servation with the low power will enable one to determine 

 whether it be successfully planted or not. 



Naturally the organisms cannot be thus transplanted 

 without some bacteria falling upon the plate, but this is not 

 very material, for in the first place they do not grow very 

 rapidly upon the medium used for culture, and in the 

 second, they are essential to the nourishment of the ameba, 

 which is holophagous, and cannot live by the absorption 

 of nutritious fluids. Later it was by Tsugitani* shown 

 that killed cultures of bacteria could supply the necessary 

 nourishment. All cultures of amebas must contain the 

 symbiotic organisms upon which the amebas live. It can- 

 not always be foretold what symbiotic organisms are needed. 

 When planted as above suggested a variety of organisms 

 grow, and as the amebas multiply and gradually extend 

 over the plate, their preference for one or other of the 

 associated bacteria may be determined in part by placing a 

 drop of the ameba culture upon a plate of sterile media, and 

 then with the platinum wire, dipped in a culture of the bac- 

 teria, drawing concentric circles about the drop further and 

 further apart. As the amebas move about over the plate, 

 passing through the growing circles of bacteria, they soon 

 * "Centralbl. f. Bakt. u. Parasitenk.," Abt. 5, xxiv, 666. 



