ENCYSTMENT 51 



the idea that cyst formation may be due to an 

 increase of saline content, caused by the concentra- 

 tion during drought, resulting in osmotic-pressure 

 effects, is contra-indicated by the fact that both 

 free amcebae and cysts may exist together either 

 in distilled water or in salt solutions of considerable 

 strength. 



Effect of Starvation. This condition can be 

 experimentally brought about by washing away 

 the bacteria (which constitute the natural food of 

 the amoebae) from a culture with these organisms on 

 a jelly film. Cysts are first planted on the jelly, 

 and the amoebae which escape are allowed to 

 multiply for a few days. The moist-chamber 

 method is employed, using the modification in 

 which the jelly film is made on a microscope slide, 

 which is then inverted, resting on corks over water. 

 Full-grown amoebae will then be found at a con- 

 siderable distance from the original site of inocula- 

 tion. After washing off most of the bacteria with 

 a gentle stream of water, the amcebae, which adhere 

 firmly to the jelly, gradually lose their characteristic 

 appearance, the granular contents accumulate at one 

 end and the clear cytoplasm at the other. The 

 movements become sluggish, the food vacuoles 

 scanty, and the contractile vacuole becomes slow 

 (fig. 22). This condition is maintained for days un- 

 changed, and, on the addition of bacteria, the normal 

 type is gradually assumed. There is no attempt at 

 encystment, as bacterial products are absent. The 

 amcebae which have been obtained in this way may 

 be said to exhibit one of the " depression periods " 

 to which protozoa are at times subject. 



Quiescent Type of Amoeba. Cysts were placed 



