332 BIOLOGY OF THE PROTOZOA 



multiplication. For another series, however {Stylonychia mytilus), 

 data were given for a different purpose and from these the graph 

 shown in Fig. 165 (below) was constructed. From this graph it 

 is apparent that his conclusions regarding multiplication and 

 vitality do not agree with his records. Maupas' experimental 

 evidence in connection with vitality after conjugation thus counts 

 for very little either for or against rejuvenescence. 



A much more carefully planned and executed series of experiments 

 to test the effect of conjugation on the division-rate were carried 

 out on Paramecium by Jennings (1913) and later by his students 

 (Stocking, 1915; Raffel, 1930, et al). He found: (1) That ex- 

 conjugants in only a few exceptional cases have a higher division- 

 rate than do non-conjugants of the same strain; (2) that conjugation 

 causes a decrease in division-rate of the great majority of ex-con- 

 jugants; (3) that conjugation causes a high mortality among 

 ex-conjugants; (4) that it causes a marked increase of weak, sickly, 

 and abnormal individuals. From these results it would appear 

 that conjugation is a highly unprofitable habit of the Infusoria 

 which, if freely indulged in by Paramecium, would soon lead to 

 the extermination of the race. The annual crop of Paramecium, 

 however, remains about the same and we are forced to interpret 

 Jennings' results as due more probably to the conditions under 

 which the experiments were carried on than to the effects of con- 

 jugation (see infra p. 350, and Calkins, 1923). 



The question of increased vitality after conjugation receives a 

 definitely affirmative answer with Woodruff and Spencer's experi- 

 ments with Spathidium spathula (1924). Conjugation tests fur- 

 nished material from pure lines for conjugation and ex-conjugants 

 were isolated and followed out in isolation cultures. The daily 

 division-rates for parent and offspring series were compared with 

 great exactness. Ninety-four different ex-conjugant series were 

 thus available for comparison with their respective parental series. 

 Of these the parent series died in 15 cases during the first fifteen 

 days of life of the ex-conjugants but the latter "all actually divided 

 more rapidly than their respective parents" (p. 187) during the 

 periods in which the parents were alive. In 67 cases both parents 

 and offspring continued to live and divide for more than fifteen 

 days, the offspring in all cases dividing more frequently than the 

 parents. Eighty-two cases therefore out of 94 ex-conjugant series 

 showed a definitely marked increase in vitality as measured by 

 the division-rate, as a result of conjugation: "it is evident that 

 conjugation directly induces an immediate acceleration of the 

 reproductive activity" (1924, p. 188). The same conclusion is 

 reached for the full life history of ex-conjugants in comparison 

 with the remaining life of the parental series after conjugations 

 have occurred. "Since conjugation is the sole variable involved 



