MUTAGENS OF POTENTIAL SIGNIFICANCE 185 



of the mouse. This is not specifically related to the germinal tissue, but 

 one can at least see that what one thinks of as a little implausible can 

 really happen. 



They injected C^^-adenine into a pregnant mouse in order to label 

 the DNA of the fetuses and then sampled the young mice after birth 

 at various intervals up to one year, half of the life expectancy of the 

 mouse. Throughout this time there was turnover of RNA but not of 

 DNA; that is to say, there was growth, there was addition of new DNA 

 in the liver, for example, but the label which had been introduced was 

 entirely conserved. This would indicate not only that the cells formed 

 in fetal life remained essentially intact, but there was also no significant 

 turnover of their DNA, which would have been detected as a decrease 

 in total radioactivity in those two organs. The same result was obtained 

 in the brain, an organ that for a long time has been supposed not to 

 undergo further mitosis and cell division after birth. In that sense it is 

 comparable to the germ line of the female. These data therefore show 

 the plausibility of a very long dormant period for the female germ cells 

 and their DNA. 



Let us return now to a concrete discussion about drugs. If one asked 

 which kinds of chemical compounds have ever been shown to be muta- 

 genic, and then examined each drug that might be similar to such a 

 compound, the task would be almost hopeless. It seems to me more 

 sensible to place primary emphasis upon the way drugs are actually 

 used, the extent to which the population is exposed to various agents, 

 keeping in mind the differing importance of the various periods of life 

 for males and females. Such a classification of drugs has never been 

 made because the issue we are dealing with here has not arisen before. 

 Category I* would contain drugs to which people are exposed only 

 occasionally and briefly, and which can therefore probably be dis- 

 missed from consideration as possible genetic hazards on that ground 

 alone. Category II would contain drugs to which special groups of 

 people may be exposed for long periods of time. Here mutagenic 

 potentialities would definitely present a problem for the exposed in- 

 dividuals, but since large population groups are not involved, the 

 genetic hazard to the race would be minimal. Finally, Category III, 

 upon which I want to dwell, contains drugs and other chemicals to 

 which a large fraction of the population is exposed for long periods of 

 time, even for an entire lifetime. 



Magni: Excuse me, but this classification is probably done on a 

 logical basis. Could it be reviewed on another basis, that is, not the 



* The long lists of drug names have been omitted here. 



