FERTILIZATION IN THE LIFE-HISTORY 39 



These problems have been studied very intensely 

 in Paramecium in recent years by Calkins, Woodruff, 

 Woodruff and Erdmann, Jennings, and others. Most 

 observers have detected cycles in the life-history 

 consisting of ''a more or less periodic alternation of 

 high and low vitahty as measured by the division rate" 

 (Calkins, 191 5). The period of depression or low vitality 

 may lead to conjugation in certain strains exactly as 

 Maupas observed; on the other hand, high vitality may 

 be restored by certain changes in the culture medium, as 

 Calkins showed; or high vitality may be resumed appar- 

 ently spontaneously without conjugation over and over 

 again for periods of years and through thousands of 

 generations, as Woodruff first observed in a certain 

 strain of Paramecium aurelia. The analysis of the last 

 case showed that each such period of depression and 

 restoration was accompanied by a process of nuclear 

 reorganization comparable in many respects to those 

 taking place in conjugation: the macronucleus breaks 

 up and is finally resorbed, the micronuclei divide twice, 

 but do not carry out the third division, which in conjuga- 

 tion produces the gametic nuclei; a new macronucleus 

 is formed from the micronuclei, and the normal nuclear 

 organization is gradually restored. 



To this spontaneously recurring process of reorgani- 

 zation Woodruff has applied the name ''endomixis," 

 thus emphasizing its resemblance in many respects 

 to amphimixis. He finds that this process is essential to 

 the continuation of the life of the culture (Woodruft*, 

 191 7), for its discontinuance is invariably followed, 

 within the time of one or two rhythms, by death of the 

 the culture. Moreover its regular periodicity is a 



