152 GENERAL DISCUSSION 



irradiation from laboratory sources in addition to background irradiation; and it is 

 also true, as you saw from the first slide, that we get about 7% death and resorp- 

 tion of embroys in the controls. This may be due to cosmic radiation or natural 

 background radiation, or even may be due to genetic causes. We do not know. 

 We have been dealing with exencephaly as an anomaly that was definitely pro- 

 duced by irradiation. We have never seen this anomaly except following irradiation, 

 and we have produced this at the low level of 15 r in the early embryo. The second 

 question dealt with genetic effects. I did not state that the three generations of 

 exencephaly found following irradiation of the ovary or the testes were due to 15 r. 

 It was due to a much larger dose as for instance, 100 r to the ovary of the great- 

 grandmother. The important point was that the effect was carried through three 

 generations. Obviously, it would be necessary to determine the statistical frequency. 

 However, that it occurs at all following irradiation of the ovary is of concern to 

 every potential mother. The point to emphasize is that this anomaly was produced 

 by irradiation of the germ cells and was found in three successive generations. 

 It therefore had a genetic origin. Snell showed about 1935 that x-irradiating the 

 testes and having the male mate with a normal female produced in the second 

 generation something like 35% of such anomalies. We have carried it from both 

 the sperm and the egg through several generations, and we were simply empha- 

 sizing that this anomaly appears to be similar, whether it is derived from an effect 

 on the chromosome or an effect directly on neurogenesis. How it is produced 

 genetically we do not know, but having had considerable experience in experimental 

 embryology with amphibia and chicks, it seems to me it is probable that the 

 damage was to chromosomes which at the time of gastrulation caused such dis- 

 ruption in organogenesis that any gross change could follow. This happens to be 

 one that is simply produced due to failure of closure of the cranial roof and loss 

 of neuroblasts and probably osteoblasts during development. The third question 

 related to the dangers of cosmic radiation. Like taxes, we are all faced with 

 cosmic and natural radiation, and there is nothing much we can do about it. This 

 may actually be good for evaluation! I think, however, in line with the last paper 

 and those of Drs. Gentry, Wesley, and others, it may be proven statistically that 

 there is some correlation between the amount of background radiation and the 

 incidence of congenital anomaHes. If our thesis is correct, at this early stage of 

 development the embryo is so extremely radiosensitive that 5 r causes a 10% 

 increase of resorptions and 15 r causes exencephaly in the embryos which develop 

 later. 



Percfval Bailey (University of Illinois): This afternoon I am handicapped by 

 lack of intimate knowledge of the embryonic cerebral cortex and by the quantity 

 of the material presented. You cannot really judge histologic material by a few 

 projections. The material presented here seemed inadequate for any fine cytologic 

 study. I suppose I should be happy that it is so, because that leaves an oppor- 

 tunity for somebody else to make a good cytologic study of the effects of radiation 

 on the cells of the cerebral cortex, with more adequate cytologic technic. 



Orville Bailey (University of Illinois): Dr Brownson, were you radiating the 

 whole animal or the head only? And did you conclude that fractional doses of 

 radiation produced less or more effect than the same total dose at one time? How 

 long did the animals live? 



