144 RESPONSES OF CELLS ON THE BIOLOGICAL LEVEL 



to be no intrinsic probability that genes are by nature 

 very stable. On the contrary both the proteins and the 

 deoxypentose nucleic acids are very unstable towards 

 suitable reagents. Early attempts to produce mutations 

 by the action of chemicals were a failure, and these 

 failures tended to reinforce the view that genes were re- 

 markably stable. But consideration of the chemicals 

 which were studied shows that practically without ex- 

 ception they were substances which either would never 

 get into a cell without first killing it by destroying the 

 permeability of the plasma membrane, or else were sub- 

 stances which were certain to undergo almost instan- 

 taneous reaction with the cytoplasm after entering the 

 cell. I.e. the substances studied were almost all singularly 

 unlikely to make contact with the nuclear genes. More 

 recent experiments, starting with those of Robson, 

 AuERBACH and Roller on mustard gas, have shown that 

 chemical substances which have physical properties 

 which will both enable them to penetrate into the cell 

 nucleus and to react with the components of genes are 

 remarkably effective in producing mutations, thus con- 

 firming the evidence obtained by the study of radiations, 

 which also have the property of being able to penetrate 

 into the nucleus and secure a reaction with gene com- 

 ponents. We may thus conclude that the stability nor- 

 mally exhibited by genes is not an intrinsic refractor- 

 iness towards change, but is attributable to the genes 

 being present in a very stable environment. Towards 



