THE CAUSE AND NATURE OF ENCYSTMENT. 39 



regularly encysted. This author attributed the causal factors to 

 putrefaction of the medium, resulting in the release of toxic 

 chemical substances. 



The contention of Maupas has received support from other 

 workers. Among them Root (1914) concludes that lack of food 

 is the cause of encystment in the suctorian Podophrya collini. 

 Mast (1917) obtained similar results with Didinium nasutum. 

 He says: "Encystment in Didinia can usually be induced by 

 cutting off 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." However, Mast and Ibara (1923) after 

 some carefully conducted experiments decide that "They encyst 

 more freely in cultures supplied with food than in those without 

 food, and this probably is due to greater increase in numbers, 

 resulting in greater accumulation of waste material in the one 

 than in the other." Hogue (1915) observed that the Umax 

 amoeba upon which she was working encysted in the presence of 

 abundant bacteria, upon which it feeds, and that the cell body of 

 the encysting amoeba always contained numerous bacteria. In 

 both this paper and a later one (1917) she expresses the opinion 

 that accumulation of byproducts of metabolism and oxygen 

 deficiency are the principal causal factors. She thinks that they 

 operate by causing the amoebae to lose the power of assimilation. 

 Carter (1919) concludes that abundant food is necessary for 

 encystment of Amceba proteus. 



Kofoid and Swezy (1921) consider that encystment in the 

 marine dinoflagellates is due to one of two causes, the need for 

 protection during the period of multiple and binary fission, and 

 the need for a quiescent period for the assimilation of a large food 

 body that has been ingested. Hall (1925) assigns Oxyrrhis 

 marina to the second group given by Kofoid and Swezy. In 

 addition, referring to drop cultures sealed with vaseline, he says: 

 "After six weeks, exclusively non-motile forms were observed; 

 in these, no flagella could be detected in observation under the 

 oil immersion objective, and in some cases a 'cyst wall' seemed 

 to surround the flagellates." Since no food vacuoles were 

 present, he thinks that these non-motile forms may represent 

 cysts formed in response to unfavorable conditions. From his 



