40 PROBLEMS OF FERTILIZATION 



function of time rather than of cell generations. It 

 is obviously an expression of senescence in its descending 

 phase and of rejuvenescence in its ascending phase 

 if these terms are to have any precise biological signifi- 

 cance. These organisms may thus continue to live 

 indefinitely, as Weismann contended, but they are not 

 eternally young, though they bear with them a foun- 

 tain of youth in the process of endomixis that may cause 

 indefinite rejuvenation. 



How, now, does this determination bear on the ques- 

 tion of whether conjugation does or does not involve 

 a rejuvenation process? Obviously it involves no 

 contradiction, for in conjugation we have an equally 

 complete process of nuclear reorganization; the added 

 factor of nuclear exchange between the partners and 

 and nuclear syngamy in each may, however, act detri- 

 mentally to the life of the organism, as Jennings showed. 

 Endomixis definitely demonstrates, however, that the 

 rejuvenescence theory of conjugation went too far in 

 asserting that conjugation itself, presumably in all its 

 phases, is necessary for rejuvenescence. Calkins has 

 interpreted the phenomenon of endomixis as a kind of 

 parthenogenesis, admitting its rejuvenating influence; 

 but in so doing, it seems to the writer, he has relinquished 

 the most distinctive part of Maupas' theory. Both 

 Calkins and Jennings, moreover, have shown that 

 conjugation of even the most closely related individuals 

 has no injurious effect, and have thus removed another 

 pillar of Maupas ' edifice. 



Jennings (1912) has also ingeniously shown that if 

 conjugants are separated in the very first stage of the 

 process they may continue to multiply with undimin- 



