254 BIOLOGY OF THE PROTOZOA 



isms are introduced with the experimental individual. Again in 

 the majority of cases the ultimate result has been the same as with 

 bacteria eaters. Thus Actinobolina radians was followed through 

 448 generations in isolation cultures in sterile spring water with 

 Halteria grandinella as food (Calkins, 1912) and Spathidium spathula 

 through 218 generations with Colpidium colpoda as food (Moody, 

 1912), the organisms finally dying in both cases. 



Further and very complete evidence that environmental condi- 

 tions are not responsible in any direct way for waning vitality and 

 death is afforded by a long-continued study of the protoplasm of 

 Uroleptus mobilis, an hypotrichous ciliate (Calkins, 1918, 1919, 1920, 

 etc.). This rare organism found and isolated in 1917 is a bacteria 

 eater and was cultivated on a medium consisting of flour and 

 timothy hay boiled in spring water and allowed to stand for twenty- 

 four hours before using. Individuals were transferred daily to such 

 fresh medium in order to avoid an excess of bacteria. For each 

 series of five lines the division rates were figured in ten-day unit 

 periods which were then averaged for sixty-day periods at ten-day 

 intervals. The vitality history of twenty-three series averaged for 

 sixty-day periods and the history of the double Uroleptus are shown 

 in Fig. 131. The average division-rate here for the first sixty days 

 was 15.4 divisions per ten days from which it descended regularly 

 in successive sixty-day periods at ten-day intervals until death. A 

 single series by itself would be no evidence that slow killing had not 

 occurred. But when two of the progeny of a series are allowed to 

 conjugate with one another at any time after the first 75 genera- 

 tions, the ex-con jugants repeat the historv of the parent series but 

 do not die when the parent series dies. In this maimer the proto- 

 plasm of the original Uroleptus which was isolated November 17, 

 1917 was still under observation twelve years later, although any 

 single series lived from ten months to a year only. The life of the 

 progeny overlaps that of the parent; its progeny overlaps it, etc.; 

 the daily treatment of parents and offspring was identical through- 

 out; both were subject to the same deleterious conditions if present 

 but parents died and offspring lived, a history which was repeated 

 more than 140 times with as many series during a period of twelve 

 years. 



From these clear-cut experimental results with Uroleptus mobilis 

 the fact is obvious that under these experimental conditions a fairly 

 uniform life cycle is the rule. The 140 completed life cycles upon 

 which this conclusion is based were all characterized by the same 

 phenomena, viz.: (1) A high initial vitality of the ex-conjugant 

 lasting for a limited period; (2) gradually waning vitality ending 

 in complete exhaustion and death; (3) a period of sexual "immatur- 

 ity" lasting from the first thirty to ninety days during which 

 encystment occurred if appropriate external conditions were pro- 



