76 Reproduction and Life-Cycles 



there is a corresponding change in shape o£ the body. In such genera as 

 Euplotes (90), softening of the pellicle must precede this change in form. 

 Partial or complete resorption of locomotor organelles is common. As 

 traced in Woodruffia metabolica, the cilia begin to shorten as the organ- 

 ism rounds up, and shortening is completed after 22-24 hours. The endo- 

 cyst is not secreted until after the cilia have disappeared (136). Encystment 

 apparently involves some loss of water, with a corresponding increase in 

 density of the protoplasm. The resistance to desiccation, noted in stable 

 cysts but not in unstable cysts of W. tnetabolica, is attributed to a lower 

 water content of the former (136). 



The occurrence of encystment has been correlated with various envi- 

 ronmental changes. Encystment of Euplotes taylori seems to be related to 

 evaporation of the culture medium (90), while Bursaria truncatella en- 

 cysts when transferred singly or in groups to food-free spring water (16). 

 Encystment of Didinium nasutum is induced by crowding, either with or 

 without a food supply (15). Colpoda (duodennria) steinii encysts when 

 starved, and the percentage of cysts increases with the number of organ- 

 isms present. Encystment of this ciliate has been attributed to the inac- 

 tivation of essential enzyme systems by metabolic products (231). The 

 lack of materials for synthesis of such enzymes should produce the same 

 effect, and encystment of C. steinii in pure culture has followed elimina- 

 tion of thiamine, pyridoxine, nicotinamide, or pantothenic acid from the 

 standard medium, or the omission of foods known to contain several 

 B-vitamins (91). An abundance of food has been considered essential for 

 encystment of some species, but such a food supply would favor rapid 

 multiplication with subsequent crowding. Encystment of ciliates also has 

 been related to an unusually low or high pH of the medium (68), al- 

 though Didinium nasutum encysts most frequently within the range 

 favorable to growth (7). 



The varied data on encystment obviously hinder selection of any one 

 factor as the key to this process. However, such a theory as that of Taylor 

 and Strickland (231) lends itself to possible correlation with several envi- 

 ronmental changes. The inactivation of a critical enzyme system might 

 result from accumulation of metabolic poisons — induction by waste prod- 

 ucts and by crowding. Inactivation might be accelerated by a deficiency 

 of materials for synthesizing such enzymes — induction by starvation and 

 crowding. Also, the inactivation of an enzyme system might occur more 

 rapidly at one pH than at another. 



Excystment 



Excystment often includes the regeneration of peripheral organ- 

 elles as well as a certain amount of internal reorganization. Rupture of 

 the cyst membranes may involve two different mechanisms. The more 

 important seems to be the absorption of water by the protoplasm early 



