AGAMIC REPRODUCTION AND REJUVENESCENCE 143 



These changes, however, are apparently not the only factors 

 concerned in preventing progressive race senescence. In a recent 

 paper Woodruff and Erdmann ('14) have described periodic changes 

 of another sort which they call "endomixis" and which they believe 

 to be the essential factors in preventing race senescence. These 

 changes consist in the gradual fragmentation, degeneration, and 

 disappearance of the meganucleus, at least two divisions of the 

 micronuclei, degeneration of some of the micronuclei thus produced, 

 and the formation of new meganuclei from others. This process 

 of endomixis resembles the nuclear changes in conjugation, except 

 that the third micronuclear division of conjugation which gives 

 rise to the migratory and stationary micronuclei apparently does not 

 occur here, and there is no union of micronuclei at any time. Wood- 

 ruff and Erdmann point out that endomixis is in certain respects 

 similar to parthenogenesis, but not directly comparable with the 

 usual forms of it. The occurrence of rhythms of growth and rate 

 of division in protozoan cultures has been recognized by Calkins, 

 Woodruff, and various other investigators. Periods of more rapid 

 and less rapid growth and division alternate more or less regularly 

 in the history of cultures. Woodruff and Erdmann find that the 

 process of endomixis which extends over some nine cell generations 

 is coincident with the period of lowest rate of growth and division 

 in the rhythms, that at the climax of the process division is greatly 

 delayed, and that with the beginning of dift'erentiation of the new 

 meganuclei recovery is rapid. They conclude that a causal rela- 

 tion exists between the reorganization process and the rhythms. 



This process of endomixis occurs in diff'erent races of Paramecium 

 aurelia and in P. caudatum also, and probably in other ciliate 

 infusoria. Many of the observations of earUer authors on degenera- 

 tive changes and abnormal nuclear conditions undoubtedly concern 

 stages of endomixis. 



While further inv-estigation is necessary to determine how gen- 

 erally this process occurs and to what extent its occurrence may be 

 experimentally controlled, it is evident that the rhythms and the 

 process of endomixis represent a senescence-rejuvenescence periotl, 

 and we must inquire what factors are primarily or chieily concerned 

 in this periodicity. I believe that we must look, to the meganucleus 



