Reproduction and Life-Cycles 97 



other sources. Glaucoma scintillans showed no senescence after 2,700 

 generations (84). Lines of Paramecium caudatum lived for ten years 

 without conjugation or decrease in vitality (178, 179). The colonial flagel- 

 late, Eudorina elegans, was maintained for eight years without syngamy 

 or indications of senescence (110, 111). Actinophrys sol passed more than 

 1,200 generations without syngamy (19). Spathidium spathula, previously 

 credited with a cycle, survived for a thousand generations without con- 

 jugation or endomixis (252). Didinium nasutum was maintained by Beers 

 without conjugation or a decrease in vitality so long as the food supply 

 was adequate (8). However, depressions were readily induced by an 

 inadequate diet (9). 



In contrast to various other ciliates, Uroleptus mobilis failed to follow 

 the prevailing pattern. Instead, a physiological cycle was reported, with 

 the strains living an average of 350 generations (34, 35). Attempts to 

 prolong the cycle by varying the environmental conditions were unsuc- 

 cessful (2), and this species remains one in which the cycle has not been 

 eliminated. More recently, Jennings (127, 131) concluded that his experi- 

 ence with Paramecium bursaria also supports the concept of a physiologi- 

 cal cycle, although some clones were maintained for eight years before 

 their health began to decline. 



In spite of the fact that strains of various ciliates could be grown in 

 the laboratory for long periods without conjugation — and perhaps they 

 could be maintained indefinitely — one question remained unanswered. 

 Does conjugation really have any stimulatory or rejuvenating effect on 

 ciliates? 



The early investigations had produced little information. A few ob- 

 servations by Hertwig (114) on Dileptus gigas and some inconclusive data 

 cited by Calkins (33) represented the available evidence. Some years later, 

 the first convincing experiments were reported by Calkins (34). Several 

 strains of Uroleptus mobilis, which were entering depressions, showed a 

 higher fission-rate and greater longevity after conjugation than the non- 

 conjugant parental stocks. Comparable effects of conjugation were re- 

 ported subsequently in Spathidium spathula (253) and Paramecium 

 bursaria (131). 



At present it seems clear that conjugation, whether or not it is essential, 

 can produce a physiological stimulation in at least certain strains. How- 

 ever, it is equally evident that conjugation is no universal remedy for 

 senescent ciliates. In fact, the odds are slightly against survival after con- 

 jugation in Paramecium bursaria. Records kept for 20,478 exconjugants 

 show that under conditions in which all non-conjugant lines remained 

 vigorous, 29.7 per cent of the conjugating ciliates died before the first 

 post-conjugant fission, and only 47.3 per cent survived for more than four 

 fissions (127). Conjugation between inbred lines is even more dangerous, 

 and mortality often reaches 90-100 per cent in such cases in P. bursaria 



