THE CAUSE AND NATURE OF EXCYSTMENT IN 

 POLYTOMELLA CITRI. 



J. McA. KATER AND R. D. BURROUGHS, 

 PRINCETON UNIVERSITY. 



Although encystment has long been known to play an im- 

 portant part in the life of most protozoa the cause and nature of 

 the phenomenon has been the subject of relatively little careful 

 experimental work. The evidence derived from the earlier work, 

 slight as it was, together with the fact that cysts of protozoa 

 are very resistant to drying, toxic substances, etc., has led the 

 writers of text-books to speak of encystment as a passive response 

 to adverse environmental conditions (Calkins 1909, Minchin 

 1912, Doflein 1916). In spite of the fact that such generaliza- 

 tions are quite common, there is little evidence that, on critical 

 examination, cannot be interpreted otherwise than as mentioned 

 above. The general confusion that has arisen from the con- 

 flicting data of almost every piece of work on this subject makes 

 it desirable to give a brief historical review. 



From the time that encysted protozoa were first observed in 

 1769 by Saussure until 1855 the few observations made on this 

 subject did not deal at all with the cause of the process, but 

 merely with establishing the relationship between cysts and the 

 active forms to which they give rise. In 1855 Cienkowsky not 

 only called attention to the fact that encystment is probably 

 not universal among protozoa (as was supposed before that time) 

 but also succeeded in producing encystment by a lengthy drying 

 up of the medium. 



After this date dessication was considered to be the principal, 

 if not the sole cause of encystment, until the subject was attacked 

 by Maupas and Fabre-Doumergue in 1888. The former pro- 

 duced encystment in predaceous ciliates by the deprivation of 

 food, while the latter showed that drying up of the medium was 

 not of paramount importance. By growing several ciliates in 

 cultures placed in a moist chamber, he discovered that they 



38 



