many of the amebas began to take on an opaque appearance, 

 and ceased normal movements. The substance which was 



u 



formerly a food now act^ as a poison. Child (1015) has 

 shown that susceptibility to poisons increases during 

 starvation. 



Altho these cultures v/ere not sterile, it v/as possible 

 to keep the bacteria fi-om becoming numerous, and thus pre- 

 vent their becoming an appreciable source of food for the 

 amebas. 



Having found a carbohydrate food for Ameba, a search 

 was now made for a source of nitrogen. In such a primitive 

 cell, the possibility that simple compounds of nitrogen 

 may be used, at once suggests itself. Of various compounds 

 of nitrogen which were tried, the best results were ob- 

 tained with ammonium nitrate and urea; and of the.se two, 

 urea was much the better, owing to its comparatively lov/ 

 toxicity. The effect of these substances is seen in the 

 follov;ing experiment. (The comparatively short life of the 

 animals is to be attributed to the use of an inferior 

 culture) . 



(35) 



