CYTOPLASMIC EQUILIBRIUM 



as the existing amount has been dissipated. Thus K, or rather some 

 product of its activity, is necessary for the reproduction of kappa, 

 but it cannot initiate the formation of kappa. The relevant product 

 of K is one, but only one, of the precursors of kappa, and these 

 precursors can be welded together to give kappa only by kappa 

 itself. 



We have therefore to assume at least four substances in the 

 cytoplasm concerned v^rith the Killer behaviour, viz: (i) the dif- 

 fusible Killer substance; (ii) the reproductive particle, kappa; (iii) the 

 product of the nuclear gene K; and (iv) at least one other precursor 

 substance. These substances must have the following minimal 

 relation : — 



Precursor ^^ ^ reaction governed by kappa 



kappa > Killer 



/ 1 

 K product' 



Nuclear genes are co-ordinated in reproduction both with one 

 another and with the cell. Each gene appears, therefore, with the 

 same dose in each nucleus, and, subject to the number of nuclei 

 being constant, in each cell. For a plasmagene like kappa, whose 

 reproduction is under chemical rather than mechanical control, this 

 is no longer true. The amount of kappa is not constant from cell 

 to cell, or even within a single cell. It depends on the rate of 

 reproduction of kappa relative to cell fission. 



In variety 2 of Paramecium aurelia, by raising the food supply, 

 Freer was able to increase the rate of cell fission to 3 -4 times a day, 

 whereas kappa could double itself only i -9 times a day. In these 

 circumstances the concentration of kappa diminishes until it is finally 

 lost. When the average concentration has become small, the pro- 

 portion of cells devoid of kappa is found to be that expected from 

 the random sorting out of particles. In this way Freer was able to 

 demonstrate the particulate nature of kappa and to show that the 

 concentration necessary for the Killer behaviour to be shown was 

 200-300 particles per cell. It was also evident that, so long as one 

 particle remained in the cell, the full concentration, and with it 

 Killer behaviour, would be restored when conditions supervened 



Elements of Genetics 177 M 



