FERTILIZATION 617 



In other ciliates, however, conjugation apparently reduces vitahty. 

 Thus in Blepharisma undulans Calkins (1912) found that all excon- 

 jugants died, although Woodruff (1927) concluded from his investiga- 

 tions that conjugation in this species accelerates the division rate. Jen- 

 nings ( 1913) concluded that conjugation reduced vitality in Paramecium 

 as indicated by a reduction in the average rate of fission in exconjugants. 



At the present time the problem as regards ciliates seems to be: does 

 increased vitality following conjugation mean that conjugation is a nor- 

 mal and essential part of the life cycle, or is it merely an emergency 

 measure called into play when unfavorable environmental conditions 

 have resulted in physiological degeneration? This problem is not easy 

 to solve, because it is difficult to know what optimum or even "normal" 

 environmental conditions are, and the two are probably not identical. 

 Another complication is that in some species endomixis may be substi- 

 tuted for conjugation as a revitalizing process. 



Another angle from which this problem may be approached is that 

 of comparison with the plant kingdom. Many plants are able to repro- 

 duce themselves indefinitely by asexual methods, but at the same time 

 sexual stages occur which, though not indispensable to their continued 

 existence, are nevertheless certainly an integral part of their normal 

 life cycle and valuable to the organism in other ways. In other plants, 

 sexual processes must occur at regular intervals under "normal" condi- 

 tions, or the species will die out. 



Further investigation may disclose a similar situation in the ciliates, 

 wherein some ciliates cannot continue to exist without periods of con- 

 jugation, while in others endomixis may be substituted for conjuga- 

 tion, and in still others asexual reproduction will carry on the line in- 

 definitely. The final answer to this problem will come only through con- 

 tinued investigation. 



Conjugation 



Conjugation has been defined as the temporary union of two proto- 

 zoan cells for the exchange of nuclear elements. It is a sexual process, 

 differing from ordinary sexual union in that it is not directly related 

 to reproduction. Two organisms enter into the relationship and the same 

 two functional units leave the relationship; no third party — no progeny 

 — has come into being. The two conjugants have been genetically 



