MUTAGENS OF POTENTIAL SIGNIFICANCE 193 



were not nonpigmented melanoblasts or melanocytes, but that since in 

 the original explant of the tissue it is impossible to separate the 

 melanoblastic cells from connective tissue elements also present in the 

 iris, what you get is a replacement of melanoblasts by other cells which 

 are of a different type, and never able to produce pigment in the first 

 place — for example, fibroblastic types of cells. 



Atwood: Do you think this is what happened in the heterozygous 

 cultures that gave you the large numbers of unpigmented? 



Glass: No, in that case, when the nonpigmented cells were examined, 

 they were definitely of a melanoblastic type. 



Atwood: What Dr. Stern implied by his question was how do you 

 know those weren't the mutants? 



Auerbach: Yes. The crucial comparison would then be with homo- 

 zygous pigmented ones, to show that the white cells in your experiments 

 were mutants. 



Glass: These were untreated cells. 



Auerbach: Yes, but the homozygous ones then should never, or 

 hardly ever, give these white melanoblasts. 



Glass: I think they did, but I don't recall the data. 



Lederberg: Are you attaching some significance to this difference 

 between the survival of the melanocytes or the presumptive melano- 

 cytes, as between the heterozygous and the homozygous, or do you 

 think it is too big a chance variation of the experiment? When you say 

 in the heterozygous tissues you maintain the melanocytes, in the 

 homozygous explants, you did not maintain the homozygous melano- 

 cytes, are you attributing some significance to that? Do you think 

 those cells are less able to compete with the connective tissue elements? 



Glass: It may even be that in the iris of the albino the nonpigmented 

 melanocytes do not exist any more. 



Lederberg: There are two types of mutants in this respect, aren't 

 there? I don't know what the situation is right here, but certainly 

 in the mouse, in one case, the melanocyte does not exist; in the other 

 case, you have an extreme dilution. There are cells which do not have 

 pigment. 



Glass: Yes. It would be possible to use a different allele, perhaps, and 

 get it. 



Neel: I think those of us who have been involved with the problem 

 of the genetic effects of radiation in man know how quickly a concern 

 can gain momentum in this country, with the result that one finds 

 oneself confronted with questions from individuals and institutions 

 who have policies to determine. I don't believe it is at all a wild con- 



