CONDITIONS OF GAMETE FORMATION 379 



degree of rejuvenescence, at least in the cytoplasm, in each agamic 

 reproduction, and the periodic process of endomixis and the repro- 

 ductive rhythms associated with it were interpreted as periods of 

 senescence, death, and replacement of the meganucleus. Since 

 Woodruff and Erdmann ('14) have demonstrated, not only that 

 endomixis occurs periodically, but that it has no relation to the 

 occurrence of conjugation, we must conclude that the progressive 

 senescence of the meganucleus which results in endomixis is not 

 the essential factor concerned in bringing about conjugation. 

 Moreover, since conjugation is not a feature of an internally 

 determined invariable life cycle, but is rather associated with and 

 dependent upon certain environmental conditions, at least to a 

 high degree, it seems probable that the physiological conditions of 

 conjugation are primarily cytoplasmic rather than nuclear, for the 

 cytoplasm is more affected than the nucleus by the environmental 

 conditions. 



Since cytoplasmic rejuvenescence occurs with each agamic 

 reproduction, it is evident that the physiological age of the cyto- 

 plasm attained in each generation may depend, at least in part, 

 upon the frequency of reproduction. With abundant food and 

 favorable medium the reconstitution associated with one repro- 

 duction is scarcely completed before another reproduction occurs. 

 Under such conditions the degree of physiological senescence be- 

 tween two successive fissions must be less than when the interval 

 between reproductions is longer. Consequently certain conditions 

 which retard growth and agamic reproduction, but which are not 

 so extreme as to bring about either complete quiescence or star- 

 vation and reduction, favor the attainment of a more advanced 

 cytoplasmic age by the individuals of each generation. Under 

 these conditions the parts continue to exercise their special func- 

 tions for a longer period before undergoing regressive changes in 

 connection with reproduction, and the advance of senescence in 

 each generation may not be balanced by the rejuvenescence 

 occurring in each reproduction, so that progressive cytoplasmic 

 senescence of the race may occur. We need not expect, however, 

 to find conspicuous morphological differences between such ani- 

 mals and those which are growing and reproducing rapidly. The 



