134 GAKY N. CALKINS 



the protoplasm of each series represented by the curve to the left 

 of this line was in a condition of sexual immaturity. Sexual 

 maturity, or ability to conjugate, persists after the first sixty 

 days practically to the end of the cycle. This is well illustrated 

 by the A series where conjugations occurred regularly up to one 

 month prior to the death of the race. Usually, however, the 

 conjugation tests in the later periods of the cycle are inconclu- 

 sive, owing to the low division rate and the resulting scarcity 

 of material. 



The cysts of Uroleptus mobilis are small and spherical and the 

 walls are smooth. When once encysted, the organisms do not 

 emerge from their cyst membranes until weeks afterwards, and 

 then only after an intervening period in a dried condition. Such 

 dried cysts, when placed in fresh culture medium, will give up 

 their contained organisms in from five to twenty days. During 

 such encystment, or prior to it, the protoplasm of the individual 

 undergoes asexual reorganization ('endomixis') and, after emerg- 

 ing, begins its life cycle with an initial vigor similar to that of 

 an ex-conjugant. The B and M series, for example, started 

 from cysts. 



Encystments have never occurred in the isolation cultures and 

 the uniformity of the curves is sufficient evidence to show that 

 no other process of parthenogenesis takes place in these cultures. 

 In conjugation tests, however, encystment seems to be due to 

 the same conditions under which conjugation is possible. In 

 such tests the date of the appearance of cysts is always recorded. 

 Such dates are indicated on diagram I by circles above the symbols 

 for the conjugation tests. 



As shown in the diagram, encystment usually takes place 

 among some individuals of a conjugation test before the first con- 

 jugating individuals appear (series D, F, G, H, and J). In some 

 cases (G and J) encystment occurs in such tests made during the 

 first ten days of the cycle. In other cases (C and I) encystment 

 and conjugation both appeared for the first time in the same test. 

 I have no comment to offer as to the significance of these results, 

 but I hope to get further light on the subject with continued 

 observations. 



