FERTILIZATION IN THE LIFE-HISTORY 37 



binations produce equal vigor, and one factor in many 

 cases in the production of increased vigor of offspring is 

 found in the union of contrasting germ plasms, whether 

 with reference to one or to several characters. 



These several problems appear in their simplest 

 forms in the life-histories of certain Protozoa, and they 

 have been studied most intensively in ciliates, especially 

 in Paramecium. It has long been recognized that for a 

 true comparison of the protozoan life-cycle to that of 

 Metazoa, the period between two divisions does not 

 suffice. It is true that two individuals result from such 

 a division, but these are comparable in many respects 

 to cell generations in the metazoan body, with the main 

 difference that the cells remain associated in the latter 

 case but not in the former. 



Looked at in this way, it is frequently possible to 

 distinguish periods of youth, maturity, and senescence 

 in a protozoan life-history comparable to the same 

 period of a metazoan individual. This point of view 

 goes back to Blitschli (1876), but was first placed on an 

 experimental basis by Maupas (1888-89), who regarded 

 the process of conjugation in ciliates as a rejuvenating 

 process: The ex-conjugant begins a cycle of cell gener- 

 ations with a full store of energy; under suitable nutritive 

 conditions a series of fissions follows. During the early 

 part of the series there is no regular tendency toward 

 conjugation, but later in the series (after several hun- 

 dred generations usually) the tendency to conjugate in 

 pairs becomes very strong and may result in veritable 

 epidemics of conjugation. Maupas calls this the stage 

 of sexual maturity. Individuals that do not succeed 

 in conjugating then gradually pass into a period of 



