DISCUSSION 
the use of sperm or egg banks, you must know whether or not 
the preservation of germ cells at low temperatures has an effect 
on the rate of mutation. So far we do not know anything about 
mutation rates at extremely low temperatures. ‘That is some- 
thing that must be settled before we can donate sperm to be 
kept for generations. 
In addition to the preservation of eggs or sperm, there is the 
possibility of preservation of the generative tissues—the testes 
and the ovaries—particularly, as Muller points out, in the 
embryonic stage. If we could take tissue containing embryonic 
germ cells, preserve it for long periods of time, and transplant 
it to useful recipients, we might have a source for future 
breeding which could offer a chance for survival of spermato- 
genic tissue in particular for much longer than now seems 
possible. 
Dr. Muller has not discussed in his paper the possibility of 
breeding by parthenogenesis, that is, reproduction from the 
unfertilized egg. This has numerous advantages over artificial 
insemination, particularly the possibility of attaining a purer 
strain (homozygosity) in a relatively short time, whereas if you 
are going to use selected sperm as a means of reaching any 
desired degree of homozygosity, you have a long way to go. 
Intensive application of research efforts to this problem of 
parthenogenesis should be very profitable. 
Hoagland: Hasn’t it been shown in the fruit fly, Drosophila, 
that the mutation rate increases with temperature and is lower 
at lower temperatures ? 
Haldane: Yes. In addition, somatic mutation in some plants 
has a negative temperature coefficient. 
Koprowski: ‘The effect of preservation at low temperature 
on mammalian cells may be related to the type of tissue and the 
species of origin. We have considerable data indicating that 
some human embryonic cells (fibroblasts) retain their genetic 
integrity in the frozen state but we do not have any information 
about epithelial-like cells. It is also possible that freezing 
favours selection of the fittest cells, namely, those that have the 
best growth potential. These may “throw off’? mutants at a 
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