Reproduction and Life-Cycles 79 



The physiological aspects of excystment are probably no less compli- 

 cated than the morphological changes. Excystment of Colpodo steinii 

 involves several stages. In an initial phase, the length of which is in- 

 fluenced by temperature but not by oxygen tension, essential organic 

 substances are absorbed from the medium. The activities of three later 

 periods, distinguishable by varying susceptibility of the cysts to X-rays, 

 are influenced by oxygen tension but not by organic components of the 

 medium (25). Weyer (241) suggested that excystment of Gastrostyla 

 steinii is induced solely by organic substances elaborated by bacteria in 

 the medium. Excystment of DicUniiim nasutum, in various media, de- 

 pends upon the presence of living bacteria. Previously bacterized culture 

 fluids are inactive after being heated or filtered to remove the bacteria 

 (13). Entamoeba histolytica will excyst in the absence of living bacteria, 

 but only at a low oxidation-reduction potential (216). 



Barker and Taylor (5) apparently were the first to show that excyst- 

 ment can be induced specifically by adding certain animal or plant ex- 

 tracts to basal media. Some substance or group of substances was active 

 for Colpoda steinii in dilutions as high as 1: 100,000,000. In attempts to 

 isolate these factors, two concentrates from hay extract were found to be 

 active separately, and also to show complementary effects in combinations 

 (232). The activity of hay extracts was next related to salts of organic 

 acids (acetic, citric, fumaric, malic, and tartaric), the effectiveness of 

 which was quadrupled by a co-factor prepared from hay and replaceable 

 by certain sugars in dilute solutions (105). Tavo crystalline substances, 

 prepared from corn leaves, proved active at concentrations of 2.0-4.0 x 

 10-^ gm/ml in the presence of suitable co-factors. The co-factors, pre- 

 pared from corn extract and essentially inactive themselves, could be 

 replaced in part by a sugar solution and certain combinations of thiamine, 

 nicotinic acid, nicotinamide, adenylic acid, citrate, and malate (206). A 

 later report (226) indicates that potassium ions, not replaceable by so- 

 dium ions, are essential to excystment of C. steinii. Several vitamins pro- 

 duced no significant effect, although certain carbon sources (citrate, 

 glutamate, malate, and propionate) and adenosine triphosphate showed 

 some activity. 



Requirements for excystment are less complex in certain other ciliates. 

 Distilled water induces excystment of TilUna magna (11) and Colpoda 

 cucullus (147), and dilution of the original medium is effective for 

 Euplotes taylori (89). 



SEXUAL PHENOMENA 



Varieties of sexual phenomena 



Although sexual processes are not necessarily a prerequisite to 

 reproduction as they so commonly are in Metazoa, and although many 



