Conjugation 27 



nuclei, so that each has after mating a new reserve nucleus, 

 composed half of reserve nuclear material from its own 

 body, half of reserve nuclear material from the body of its 

 mate (Figure 8, G.). Later this new reserve nucleus divides 

 into parts, some of which become new, large, active macro- 

 nuclei, while the others remain as minute micronuclei (see 

 Figure 30, Lecture 5). 



So there are two main things in conjugation: (1) The 

 old active nucleus is replaced by parts of the reserve nuclei ; 

 (2) the two mating individuals exchange parts of their 

 nuclear material. 



Now, evidently the replacement of the old active nucleus 

 by part of the reserve nucleus is just the sort of thing that 

 one would expect if there is to be rejuvenescence; indeed, it 

 is rejuvenescence of the macronucleus. It looks very much 

 as if the old active nucleus might have gotten worn out or 

 used up in its activity, so that it has to be replaced by 

 reserve material which has not been used. 



But what has all this to do with mating? If the point 

 is merely the replacement of worn nuclear material by fresh 

 material from the reserve store that each animal carries, 

 why need there be this complicated process of exchange of 

 nuclei? Rejuvenescence should occur just as well without 

 this exchange, without conjugation, as with it. 



The occurrence of these two distinct things at conjuga- 

 tion, replacement from a reserve, and exchange, has al- 

 ways kept the theory of rejuvenescence ambiguous. Is it 

 the replacement from reserve material, or the exchange, that 

 makes the organism young again? Authors have as a rule 

 either not ventured to answer this question, or have not 

 clearly analyzed the process into its two elements, speaking 

 merely of conjugation as a whole. Arguments based on the 

 replacement process have been used or accepted as argu- 



