340 S. O. MAST 



NATURE AND CAUSE OF ENCYSTMENT, VITALITY AND ACTIVATION 



OF CYSTS 



When didinia are aljoiit to encyst they become quiet, loose 

 their ciUa, mouth and seizing apparatus and secrete a heavy wall 

 about themselves. The wall usually adheres firmly to the sub- 

 stratum. Thus the cysts become fastened to various objects 

 usually near the surface of the solution in which they live. Under 

 certain conditions practically all of the didinia in a solution sud- 

 denly encyst, so that cultures having countless numbers of active 

 didinia at a given time may have practically non'e a few hours 

 later. 



Encystment can usually be induced at any time by cutting 

 ofT the food supply. But it frequently occurs when there is an 

 abundance of food present and sometimes it does not occur 

 when there is none. This is especially true for cultures in which 

 conjugation has been prevented for a considerable number 

 of generations. In fact, in lines which had passed through 

 1500 to 1600 generations it was almost impossible to induce 

 encystment. 



To induce the didinia to come out of the cysts it is usually 

 only necessary to add a vigorous culture of paramecia, or the 

 solution from such a culture. The paramecia are not absolutely 

 necessary. Active didinia are ordinarily found twenty-four 

 hours after the solution has been added; but at best only a 

 very small percentage of the cysts develop. Some can, however, 

 live a long time and drying under atmospheric conditions is not 

 fatal. In an experiment described elsewhere, cysts kept sealed 

 air-tight in a 10 cc. vial for nearly five years were still found to 

 be viable. 



Didinia are consequently very hardy and they are, as far as 

 known, destroyed by only two organisms; a small rhizopod 

 which probably" develops inside of the cysts, devours the proto- 

 plasm and then bores its way out through the wall and a hypo- 

 tricate which swallows them whole in considerable numbers. - 



Encystment clearly serves to bridge over unfavorable environ- 

 mental conditions, and it unquestionably facilitates distribu- 



