152 RADIATION GENETICS 



In female germ cells, dominant lethal incidence was shown to be strikingly depend- 

 ent on the stage of oogenesis during irradiation; 950, 951, 1123 a dose of 70 r will induce 

 about 50 per cent dominant lethality in oocytes in the first meiotic metaphase. When 

 Bateman 57, 59 computed theoretically the relationship beween the postulated 

 number of chromosomal breaks and time of death of the conceptus due to dominant 

 lethals in the female germ cells, he found that single lethals per egg permit survival to 

 implantation but multiple lethals usually cause loss of the egg before implantation. 



These facts should encourage cooperation among geneticists, radiologists, 

 embryologists, cytogeneticists, and biochemists to focus their attention on the direct 

 relationship between chromosomal breaks and abnormal embryonic development. 



Regarding post-implantation losses, I wish to draw special attention to seven-day- 

 old embryos in mice. We have information, gained from direct irradiation effects 

 (130 r) upon embryonic differentiation of somatic tissue in successive stages of gestation, 

 that day seven seems to be most critical in early morphogenesis (figure 20). In 

 comparison to embryonic development in human beings, this would be the third week 

 of gestation (day 17 to day 21), a stage of development in which most mothers are not 

 aware of their pregnancy. This conclusion agrees with those given by Russell and 

 Russell, 1121, 1122 and they recommend that, whenever possible, pelvic irradiation of 

 women of child-bearing age should be restricted to the two weeks following the first 

 day of menstruation. Radiation hazards to the embryo are being avoided to a large 

 degree by following this advice. 



Dr. Burdette: Fairly recently we have been able to demonstrate in Drosophila, 

 although we have not tried it in mice, that actinomycin D, which is often used in con- 

 junction with irradiation, will reduce the frequency of mutations induced by X irradia- 

 tion by approximately 50 per cent, although we do not have sufficient data to 

 demonstrate the effect on natural rates of mutation. I am wondering whether you 

 will comment on possible pressures in nature which may reduce the numbers of muta- 

 tions. Photoreactivation is an example of the type of phenomenon I had in mind. 



Dr. Grahn : For mammalian systems or in man, for example, I do not know what 

 particular pressures there would be to reduce the hazard of radiation. Such things as 

 photoreactivation are probably not effective in mammalian systems. Generally it is 

 very much easier to find all the different environmental stresses that will increase the 

 mutation rate rather than to find anything that will act to reduce the potential hazard. 



Dr. Burdette: More attention directed toward reversal and inhibitory effects 

 may be very profitable in view of the current disproportion between this type of study 

 and those dealing with enhancement. Dr. Crow, do you have a comment ? 



Dr. Crow: There was a suggestion a few years ago by Carter and Haldane 170, 517 

 that one might study the over-all mutation rate in mice by finding lethal mutants 

 linked with known visibles. I wonder how widely this is being applied ? 



Dr. Grahn: Essentially not at all. Both the use of independent markers and 

 linked markers in the location of a lethal in between the markers should be mentioned. 

 Haldane 517 has worked out most of the statistical problems and has even been able to 



