142 RESPONSES OF CELLS ON THE BIOLOGIC AL LEVEL. 



genes, or not exclusively by nuclear genes, but also by 

 genetically active bodies in the cytoplasm, or plasma- 

 genes. Thus, even in the cases where it is suspected tht 

 mutation has occurred in sex cells, it is not always possi- 

 ble to test for this by the normal procedures which caa 

 be used for studying mutation of nuclear genes. 



Haddow has suggested that mutations may rather 

 commonly take place under the action of growth inhib- 

 itors and that this mutation may take the form of per- 

 mitting the escape of a cell from the action of an inhib- 

 itor. In the normal animal growth in most organs in the 

 adult is restrained to just that degree which is necessary 

 to permit replacement of cells which have died. The proc- 

 ess by which this control is established is very far from 

 fully understood. But it is known that there are some 

 substances present in animal tissues which promote cell 

 growth and cell division, and others which inhibit this 

 process. It seems likely that the growth-promoting sub- 

 stances are sometimes in some sense used up by the 

 cells upon which they act, so that in the case of the sub- 

 stances which promote cell division there tends to be an 

 equilibrium established between the concentration of the 

 substance and the number of cells acting upon it. The 

 action of inhibitors upon this process is likely to be in the 

 direction of shifting the equilibrium position so that 

 fewer cells are in equilibrium with a given concentration 

 of growth promoter. There are a great many possible ways 

 in which the inhibitory substances might produce such 



