VARIATION AND HEREDITY 239 



are different; that is, one is a killer and the other is a sensitive 

 culture. F 2 progeny obtained by selfing show no segregation. There- 

 fore, the difference between the killer and the sensitive is due to a 

 cytoplasmic difference and not to a genie difference. 



The same observer noted that the thin cytoplasmic paroral strand 

 which appears between conjugating pair that ordinarily breaks off 

 within a minute, occasionally may remain for a long time, and if the 

 strand persists as long as 30 minutes, there occurs an interchange of 

 cytoplasm between the pair (Fig. 101). When this happens, both 

 exconjugants produce killer clones. In F 2 no segregation takes place. 

 Thus killers can introduce the killer trait to sensitives through a 

 cytoplasmic connection between them. Sonneborn supposed that the 

 killers contain a cytoplasmic genie factor or a plasmagene which de- 



Fig. 100. Paramecium aurelia. The changes leading up to death when the 

 sensitives are exposed to the killer stock 51 (variety 4) (Sonneborn). 



termines the killer trait and called it kappa. Preer (1948) demon- 

 strated that this kappa is a particle which can be recognized in 

 Giemsa-stained specimens (Fig. 102). It was further found that kill- 

 ers can be irreversibly transformed into hereditary sensitives by 

 eliminating kappa particles by exposure to high temperature (Sonne- 

 born, 1946), x-irradiation (Preer, 1948b) or nitrogen mustard (Geck- 

 ler, 1949) and that sensitives can be transformed to hereditary killers 

 by placing them in concentrated suspensions of broken bodies of 

 killers (Sonneborn, 1948a). Therefore, it became clear that kappa is a 

 self-multiplying cytoplasmic body which is produced when some are 

 already present. 



Killer races of variety 2 differ from each other and from that of 

 variety 4 mentioned above, in the effects produced on sensitives be- 

 fore the latter are killed. These sensitives possess a gene different 

 from that of the killers and cannot be changed into killers by im- 

 mersing it to kappa suspensions of broken bodies of killers. When 

 this sensitive is mated with a killer, F 2 generation produced by self- 



