PROTOPLASM 35 



but they do not tell us what life is. They also pave the way for 

 an interpretation of a characteristic phenomenon in the life- 

 history of the smallest living things. This phenomenon, known 

 as encystment or the encysted state, is a period of rest in the 

 life of a minute organism. When about to encyst, a unicellular 



Fig. 17a— CYST OF UROLEPTUS MOBILIS 



With protoplasmic membrane and cyst membrane. Encyst- 

 ment is complete and the organism may remain in this 

 dried state for months or even years 



From a photomicrograph by the author 

 Magnification, 1000 



individual becomes much condensed through loss of water; waste 

 matters in the protoplasm are eliminated and motile organs and 

 other structures characteristic of the active individual are ab- 

 sorbed or discarded, all these changes occurring before or during 

 the secretion of a firm, resistant membrane or cyst wall, within 

 which the cell is protected against adverse environmental condi- 

 tions, such as drought. Within this cyst an individual may 

 remain in a completely dried state for months or even years, but 

 when again immersed in a proper medium it emerges from the 

 cyst as a young, vigorous individual, typical of its own species 

 (Figs. 11 a and 17 b) . 



Are such encysted cells alive? They are, but not in the same 

 sense as the active, motile individuals which emerge from the 

 cysts. I am accustomed to describe such an encysted form as 

 comparable with an automobile in the garage, and the individual 

 which has emerged from the cyst, with the machine under full 

 headway. The organization of the car in the garage is perfect, 

 but static. Introduce oxygen, gasoline, and a spark, and the parts 

 of the organization work together harmoniously; the machine 

 has become dynamic. 



So it is with an encysted organism. Except for the absence 

 of water, the organization of the protoplasm of the encysted 

 individual may be considered perfect; yet without this water, 

 interaction of the constituent substances is impossible, and the 



