770 



RADIATION BIOLOGY 



egg and untreated sperm and from untreated egg and irradiated sperm. 

 An additive effect in this sense would hardly be expected, unless recovery 

 from the irradiation effects took place successively in the maternal and 

 paternal parts of the zygote. It seems more likely that it would occur 

 concurrently in both. If such is the case, cleavage of the fertilized egg 

 would always have to await the recovery of the gamete with the greater 

 radiation-induced effect. The combining at random, therefore, of irradi- 

 ated eggs and sperm, which show individual variation with respect to 

 their radiation-induced damage, would in itself result in a greater average 

 delay than if eggs or sperm alone had been treated. 4 Lea (1938b, 1946), 

 concluded that one would expect the cumulative doses, but not the effects 



Q 100- 



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Q 

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60 



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MIN EXPO. (CONTROLS) 



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100 140 180 220 



MINUTES AFTER FERTILIZATION 

 Fig. 11-2. Relation of time after insemination to per cent of Arbacia eggs cleaved for 

 several doses of X rays administered at 7800 r/min to sperm before fertilization {after 

 Henshaw, 1940a). 



as measured by delay in cell division, to be additive. He found that the 

 curve calculated on this basis fitted the experimental observations 

 satisfactorily. 



In order to determine whether cleavage delay is due to a retardation of 

 one stage or is spread over the whole period from insemination to cleavage, 

 Henshaw (1938, 1940b) fixed samples of fertilized eggs, in which sperm or 



4 1 am indebted to Mr. Jack Moshman for some calculations bearing on this point. 

 Using the variation in cleavage time after treatment of the sperm (Henshaw, 1940a. 

 Table I, 4-minute exposure), which shows a normal distribution, and assuming that 

 the range and delay would be similar in the egg, we find that a random combination 

 of the gametes would increase the time for 50 per cent cleavage by about 3J^ minutes. 

 This is approximately half the increase in delay produced by treatment of both 

 gametes over sperm alone, as indicated in another paper (Henshaw and Francis, 1936, 

 Fig. 1) for a comparable dose. It should be pointed out that this test, as set up, gives 

 a maximum increase in delay for irradiation of both gametes over either alone. If one 

 gamete is more sensitive than the other, the effect for irradiation of both will be closer 

 to that resulting from irradiation of the more sensitive one. 



