378 SENESCENCE AND REJUVENESCENCE 



On the other hand, Enriques, Jennings, Woodruff, Baitsell, 

 and others have shown that conjugation may be induced experi- 

 mentally. According to Jennings, different races show great differ- 

 ences in their capacity or tendency to conjugate, some conjugating 

 every few weeks, others at intervals of a year or more, or not at all. 

 But in those races which conjugate readily conjugation occurs, 

 "not as a result of starvation, but at the beginning of a decUne in 

 nutritive conditions after a period of exceptional richness that has 

 induced rapid multiplication. At the time of conjugation the 

 animals are often in good condition, and multiplication may still 

 be in progress" (Jennings, 'lo, p. 298). As regards these points 

 Calkins is in essential agreement with Jennings. In his experi- 

 ments with a single race Zweibaum found that conjugation may be 

 induced in a great variety of ways, provided a certain nutritive 

 condition exists in the animals. This condition is brought about 

 by keeping animals which have been richly fed for some weeks in 

 a medium with less food and then removing to a medium containing 

 almost no food. Differences in the methods of Jennings and 

 Zweibaum may be due to differences in the races used for experi- 

 ment, but there is general agreement that decreased nutrition favors 

 the occurrence of conjugation. Woodruff has recently brought 

 about conjugation experimentally in the Paramecium culture which 

 has been bred agamically for nearly five thousand generations, and 

 Baitsell has also found that the occurrence of conjugation in other 

 infusoria can be experimentally controlled. 



These recent investigators agree in general that conjugation is 

 not the result of a progressive senescence, and so does not represent 

 the end of the life history. Calkins and Gregory maintain further 

 "that the progeny of an ex-conjugant is not a homogeneous race, 

 but consists of differentiated individuals which give rise to pure 

 lines, some of which conjugate, others do not. In other words, 

 some Paramecia are potential germ cells, others are not." Wood- 

 ruff, however, disputes this conclusion and holds that the occur- 

 rence or non-occurrence of conjugation depends on environmental 

 conditions. 



In chap, vi facts are cited which indicate that some degree of 

 senescence occurs during the life of each generation and some 



