242 GARY N. CALKINS. 



and myself during the last year, while proving that conjugation 

 -does bring about renewed vigor, also show that the interpretation 

 must be trimmed of some of its generalizations. 



It has also been generally assumed in cases of conjugation 

 where, as in Paramecium, both individuals are similar in size, 

 and where conjugation is only temporary, that both individuals 

 are fertilized, but according to these experiments which I have 

 cited Miss Cull has shown that, in the majority of cases, while 

 one individual of the original pair is markedly vigorous after 

 conjugation, the other one either forms a weak strain or dies off 

 at an early period. 



It would seem from these results that in cases like this of 

 isogamous conjugation we can catch a glimpse at least of the 

 same principle that operates in the fertilization of an egg by a 

 spermatozoon, where one cell loses its identity and continues to 

 exist only in conjunction with another cell. It appears to be a 

 case of incipient fertilization and indicates some physiological 

 difference between the conjugating individuals analogous to that 

 between spermatozoon and egg. 



The analysis of the conditions governing conjugation has not 

 yet been carried very far. My own experiments show that 

 Maupas's three conditions can not hold. Hunger apparently 

 has nothing to do with it, and diverse ancestry is not essential, 

 for I have obtained as large a percentage of successful endogam- 

 ous as exogamous pairings and have carried one endogamous 

 exconjugant through 379 generations. Maturity, however, the 

 third " condition " postulated by Maupas, seems to be necessary, 

 understanding by this term the peculiar state of cytoplasm and 

 nucleus when conjugation is possible and a condition which can 

 be induced by artificial means such as change in temperature or 

 of density in the surrounding medium. 



There is yet another matter that I wish to speak of in connec- 

 tion with conjugation and rejuvenescence, and that is the question of 

 artificial rejuvenescence, a matter which has an important bearing, 

 it seems to me, in all protozoan life histories. In a series of 

 experiments which I carried on for twenty-three months with one 

 race of Paninicciniii, it was found that periodic reductions of 

 vitality occurred at intervals of about six months. At such 



