THE PHYSIOLOGY OF FERTILIZATION IN CILIATES 319 



autogamy.* This process occurs frequently and regularly only in 

 F. poly Ciiry 117/1 and P. aiirelia, and it has been studied intensively only 

 in tlic latter. Tlie conditions which induce autogamy in P. aurelia 

 arc identical with those which bring animals into mating condition, 

 namely, mild starvation. However, autogamy can be induced only 

 after a certain minimum number of fissions following the previous 

 autogamy or conjugation. This suggests that autogamy functions in 

 a "life cycle" in P. aurelht, and this is borne out by Sonneborn's 

 (1938c) demonstration that, when autogamy is prevented, the clone 

 eventually dies. 



Conjugating and autogamous animals undergo nearly identical 

 physiological and morphological changes. In both processes the ani- 

 mals lose mating reactivity, form paroral cones, and undergo macro- 

 nuclear breakdown and meiosis in identical order. This indicates that 

 the same chain of reactions is operative in conjugation and autogamy 

 (Fig. 2). However, the chain of reactions is set in motion by diflFerent 

 mechanisms in the two types of behavior. In conjugation, initiation 

 of activation requires interaction of surface substances on mating ani- 

 mals; in autogamy, no mate is present. Of greater significance is the 

 fact that different routes or receptors are clearly operative in conju- 

 gation and autogamy. This is evident from the behavior of the mu- 

 tant CM stock of P. aurelia (Metz, 1948; Metz and Foley, 1949). 

 These CM animals possess the activation-initiating mechanism, at 

 least so far as their ability to activate normal mates is concerned, yet 

 they cannot themselves be activated by sexual means (Table IV). 

 Evidently some block, the CM block (Fig, 2b), prevents sexually 

 induced activation from proceeding beyond initial stages in CM 

 animals. Nevertheless, these CM animals regularly undergo natural 



* Some may object to this use of the term parthenogenesis since self- 

 fertilization occurs in autogamy. This distinction between parthenogenesis 

 and autogamy has considerable genetic significance but no physiological im- 

 portance. The behavior of pronuclei at the critical fertilization stages is prob- 

 ably controlled by the cytoplasm (see Nanney, 1952) and follows a definite 

 pattern. Whether or not this pattern includes pronuclear fusion depends upon 

 the number of pronuclei present. Thus, when amicronucleate and normal ani- 

 mals conjugate, two haploid exconjugants are produced. These develop by 

 parthenogenesis in the genetic sense, for no zygote nucleus has been formed. 

 Yet the animals have been activated and have passed through all the other 

 fertilization changes (Chen, 1940b,c). For present purposes this is considered 

 a variant of the normal sexual process. As used here, parthogenesis means non- 

 sexual activation followed by a normal nuclear cycle with or without pro- 

 nuclear fusion. 



