148 RESPONSES OF CELLS ON THE BIOLOGICAL LEVEL 



ally as a foreign body. Alternatively, cell division may 

 proceed until a cell is produced which lacks the gene or 

 prirner which cannot reproduce itself. Both possibilities 

 result in the appearance of a cell which is lacking the 

 gene upon which the hormone acts, i.e. deletion of the 

 gene has occurred. 



Mutation or deletion of a gene need not become ob- 

 vious immediately. For example, let us consider the 

 case of a chromosome gene, which exercises its physio- 

 logical effect by producing a product which is itself com- 

 petent to reproduce, e.g. the product is a plasmagene. 

 Then, destruction of the chromosome gene may only 

 appear if, i . later circumstances also prevent self- repro- 

 duction of the plasmagene, or 2. later circumstances re- 

 sult in mutation of the plasmagene. In fact damage to 

 a gene caused by one circumstance may become appar- 

 ent only very much later, as the result of some other 

 circumstance quite unrelated to the first, and perhaps 

 many years may elapse before the second circumstance 

 occurs. It may be that it is phenomena such as these 

 which account for the appearance of a tumour after the 

 elapse of many years from the time of exposure to a car- 

 cinogenic agent. And phenomena of this type may be 

 involved in results such as have been described above 

 with croton oil and dimethyl benzanthracene. I.e. croton 

 oil may perhaps reveal the damage caused to a gene 

 by dimethyl benzanthracene, through an action upon 

 a plasmagene which is the normal product of the 



