VITALITY 475 



448 generations in isolation cultures in sterile spring water with 

 HaUeria (jrandinella as food (Calkins, 1912) and SpathidiumspatJmla 

 through 218 generations with Colpidium col pod a as food (Mood}', 

 1912) the organisms finally dying in both cases. 



Further and very complete evidence that environmental condi- 

 tions are not responsible for waning vitality and death is afforded 

 by a long-continued study of the protoplasm of Uroleptiis mohilis 

 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 inter- 

 vals. The vitality history of twenty-three series averaged for sixty- 

 day periods, and the history of the double Vroleptus are shown in 

 Fig. 198. The av^erage 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-conjugants repeat the history of the parent series but 

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

 plasm of the original Vroleptus which was isolated November 17, 

 1917 is still under observation (June, 1925) although any single 

 series lives 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 is identical throughout; both are 

 subject to the same deleterious conditions if present but parents die 

 and offspring live, a history which has been repeated more than 120 

 times with as many series during the last seven years. 



From these clear-cut experimental results with Urolephts mohilis 

 the fact is obvious that under these experimental conditions a fairly 

 uniform life cycle is the rule. The 120 completed life cycles upon 

 which this conclusion is based are all characterized by the same 

 sequence of phenomena, ri~,; (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 "immaturity" lasting from the first thirty to ninety days 

 during which encystment may occur if appropriate external condi- 

 tions are provided but conjugation does not occur; (4) a period of 

 maturity beginning after the first thirty to ninety days approxi- 

 mately and lasting until the ultimate depression when conjugation, 

 under appropriate external conditions does occur; and (5) a period of 



