THE CYTOPLASM 



less favourable to fission, so that kappa reproduced more quickly 

 than the cell. The concentration of kappa increases most rapidly 

 when the cell is not undergoing any fission at all. This diminution 

 of kappa by rapid fission proved impossible in variety 4, where the 

 reproduction of cells and of kappa remained in step even when 

 each doubled itself six times a day. 



The relation between cell multiplication and the concentration 

 of kappa can be observed in another way. When the cells are heated 

 up to 38 '5° C. kappa is destroyed before the cell is killed. Thus, by 

 adjusting the time of exposure, the concentration of kappa can be 

 reduced to a very few particles and the Killer behaviour lost. If the 

 cell is restored to conditions favouring rapid fission, kappa will stay 

 at this low level. But if the rate of fission is reduced by reduction 

 of food, old age, or sexual processes kappa increases in concen- 

 tration until it is back to the 200-300 particles per cell required for 

 Killer behaviour. 



When all the kappa particles are destroyed by heat treatment the 

 ability even to develop Killer behaviour is irretrievably lost, just 

 as it is when kappa is totally lost by sorting out in variety 2. That 

 the loss is due merely to the absence of kappa can be shown in 

 another way. Normally conjugating cells separate in less than 3^ 

 minutes and no cytoplasm is exchanged. If, however, the cells remain 

 connected for 4 or more minutes (and this connection can be 

 encouraged by heat treatment), cytoplasm may be exchanged. When 

 this occurs between a Killer and a Sensitive, the Sensitive can develop 

 into a Killer if its nucleus contains K. Its cytoplasm must have 

 received kappa by exchange from the Killer, and once infected (as 

 we may say) with kappa. Killer behaviour can ensue. The time 

 taken for it to develop depends on the relative reproduction rates 

 of kappa and cell, in the way we have already seen. It also depends 

 on the length of time the conjugants have remained connected. 

 When this is for only 6 or 9 minutes. Killer behaviour develops 

 more slowly than when the connexion is maintained for a longer 

 period. Evidently, as one might expect, fewer particles of kappa 

 pass into the formerly Sensitive cell in the shorter space of time. 



The extra-nuclear component in the heredity of the Killer- 

 Sensitive difference can, as we have seen, be understood in terms 

 of a single type of plasmagene, kappa, characteristically associated 



178 



