CHARLES B. MKTZ 21 



they regularly undergo natural autogamy. Natural autogamy includes loss of 

 mating activity, formation of paroral cones (fig. i), macronuclear breakdown 

 and meiosis. It therefore includes the major activation changes of conjugation. 

 ^Accordingly, it is reasonable to suppose that the same chain of reactions operates 

 in autogamy and sexually induced activation. However, autogamy occurs 

 spontaneously in single animals. It involves no sexual association with a mate. 

 Since the CM animals can not be activated by sexual means but can nevertheless 

 undergo natural autogamy, it follows that sexually induced activation and 

 natural autogamy are initiated through separate routes or receptors. Further- 

 more, natural autogamy must be initiated at a point or site 'internal' to the 

 'CM block.' These relationships are presented schematically in figure 3. 



INTERACTION CM NATURAL AUTOGAMY 



OF SURFAC E 

 / 



(MATING TYPE?) 

 SUBSTANCES 



INITIATED HERE 



HOLDFAST SUBSTANCE 

 ^ FORMATION(?) 



/ » LOSS OF MATING 



^ ^^ ACTIVITY 



— * -> -» PARORAL CONE 

 \^ FORMATION 



\ >* MEIOSIS 



\ MACRONUCLEAR 

 BREAKDOWN 



o b c d e 



Fig. 3. Scheme for activation in Paramecium, a, Complementary surface substances, the 

 mating type substances, interact to initiate activation, h, The CM bloci< assumed to lie 'inter- 

 nal' to the activation initiating reaction, c, Natural autogamy initiated internal to the CM 

 l)lock. d, Breakup of main reaction chain into side reactions. These are assumed to arise inde- 

 pendently from the main chain. (See ref. 62 for detailed discussion.) 



These findings have interesting implications for parthenogenesis (62, 67). 

 From the physiological viewpoint, natural autogamy in Paramecium may be 

 considered analogous to natural parthenogenesis in Metazoa. The study of the 

 CM animals has shown that sexually induced activation and autogamy are 

 initiated through different routes in Paramecium. Similarly, it is possible that 

 sexually induced and natural parthenogenetic activation of the egg may also 

 be initiated through different routes in those forms with eggs that develop 

 with or without fertilization by a spermatozoan. The comparison of conditions 

 in Paramecium and ]\Ietazoa may even be extended to include artificial par- 

 thenogenesis. Certain agents (paramecins) are known to activate some stocks 

 of paramecia (see ref. 62 for review and references). Accordingly, these agents 

 may be regarded as artificial parthenogenetic agents for paramecia. The mode 

 of action of these agents has not been investigated extensively, but Jacobson 

 (43) found that the action of the agents is delayed as compared with normally 

 conjugating controls. This delay is interpreted as the time required for the 



