The Life History of Tillina Magna 397 



nent cysts are formed. If fresh medium is not given, and if many 

 individuals are allowed to remain together in a small space, as 

 is often the case with the oldest reserve stock, then the individuals 

 round up and secrete a definite thick gelatinous membrane. The 

 cysts are usually smaller than the division cysts, both because 

 the former contain few if any food vacuoles, also because the very 

 young individuals may encyst as well as the mature individuals, if 

 the conditions are favorable for such a result. The protoplasm 

 within the cyst is dense, the nucleus is the only part remaining of 

 the normal body, the mouth, pharynx, contractile vacuole, all 

 having disappeared. As the cysts become older, the membrane 

 becomes thicker and harder; a second membrane, however, is not 

 secreted. 



Individuals may be induced artificially to form cysts at any 

 period. If they are placed in tap water, or in a sugar solution, 

 encystment will occur within a short time. Changes in tempera- 

 ture will bring about the same result. Cysts will be formed within 

 twenty-four hours if the individuals are subjected to a tempera- 

 ature of 38° C. The same conditions that cause encystment will 

 often result in the disappearance of the membrane, and the renewal 

 of active life. If encysted forms are subjected to the same tem- 

 perature (38°) that caused the encystment of the free individuals, 

 the majority will lose their membrane, and again lead a free life. 

 Similar results are obtained if the cysts are put in tap water, or in 

 fresh medium rich in food. The age of the cyst is important for 

 this point, for the older the cyst, the more difficult it is to bring the 

 organism out. Twenty-four cysts that were ten days old, were 

 put in fresh medium and twelve were kept at room temperature, 

 the other twelve were put in the water bath at a temperature of 

 38°. Not one came out of the cysts. 



April 6 12 cysts (8 days old) were put into a water bath at a temperature of 38° C. 

 8 none came out. They were transferred to normal environment. 



10 all came out. 



11 12 cysts were kept in room temperature. 



12 II came out of cysts. 



April II 12 cysts were put in water bath 38° C. 

 12 8 came out. 



