320 SEX IN MICROORGANISMS 



autogamy. Therefore, the main reaction chain is activated beyond 

 the CM block in natural autogamy through a mechanism and recep- 

 tor other than the normal sexual one. These relations are presented 

 graphically in Fig. 2. 



No systematic effort has been made to induce activation in para- 

 mecia by physical or chemical agents under conditions that would 

 exclude natural autogamy. However, one group of agents is known 

 to activate under such conditions. These agents may therefore be 

 regarded as artificial parthenogenetic agents for Faramecmm. These 

 are culture fluids from certain "killer" stocks of Faramecium. The 

 effect has been reported in P. bursaria (Chen, 1945) and in P. aurelia 

 (Freer, 1948; Jacobson, 1948). Induction of paroral cone formation 

 (Chen, 1945), normal type macronuclear breakdown (Freer, 1948; 

 Jacobson, 1948), and probably meiosis (Chen, 1945) with or without 

 pairing are reported. When pairs are formed, they appear several 

 hours after addition of the "killer" fluid. These are not normally 

 conjugating animals (see under subsequent activation phenomena). 

 They may be united by the holdfast region. An interesting relation- 

 ship exists between the animals producing the fluids and those which 

 give the parthenogenetic response. In the effective combinations the 

 fluid and animals are of different varieties. In P. bursaria, fluid from 

 certain variety 5 animals activated animals of two mating types in 

 varieties 2 and 3 and of one mating type each in varieties 4 and 6. 

 Chen (1945) states that certain stocks in these varieties failed to 

 respond, but he gives no details. In P. aurelia the relationship is 

 highly stock specific. Killer stock G, variety 2 animals activate stock 

 F variety 1, type I sensitive animals. Fluids from stock G animals of 

 both mating types (III, IV) are effective. It is clear from these obser- 

 vations that the parthenogenetic effect is unrelated to mating type. 

 The parthenogenetic agent in such fluids is probably paramecin, since 

 the effect is produced by killer lines but not sensitive fines of stock G. 

 Why this agent activates certain sensitive stocks and not others is 

 unknown, and its mode of action has been investigated to a limited 

 extent only. Faramecin-treated animals undergo macronuclear break- 

 down several hours later than conjugating controls mated simulta- 

 neously. Evidently the action of paramecin is delayed in some way. 

 Jacobson (1948) suggests that the site of paramecin action is internal 

 and that this delay represents time required for the paramecin to 

 penetrate to this site. If this interpretation is correct, it means that 



