RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1233 



likely to kill the zygote in the egg stage. Hence if the mortaUty of 

 progenies from the different types of treated gametes is compared, a 

 greater egg mortality should be found where mature sperm were treated, 

 than where immature germ cells (either from treatment in the larva, or 

 from males two weeks after treatment) are concerned. This is indeed 

 the case (Table 13) and is made particularly evident by the substantial 

 identity between the larval mortalities in all three progenies. 



Table 13.— Mortality of Eggs and Larvae in the Progeny of d' d', Whose 



Germ Cells Were Irradiated at Different Stages 



(Timofeeff-Ressovsky, 178) 



This should hold for the X, where in the treated male, only one 

 representative is present; but in the autosomes, two of each type are 

 present in the immature germ cell, and as has been shown earlier, only one 

 of each of the allelomorphs in a cell mutates. In such a case, autosomal 

 lethals should be at a lesser disadvantage. Thus, the slight decreases 

 in the lethal percentage from immature germ cells, obtained by Sidorov 

 (158) and by Shapiro and Neuhaus (156) in their work on the second 

 chromosome support the view of elimination of lethals rather than a 

 difference in the susceptibility to radiation. In the female, the picture 

 is pretty much that of the immature germ cells of the male (MuUer, 105, 

 112). Similar results have been ohtsdned ioT Drosophila pseudo-obscura 

 (Schultz, 148). 



It seems that the lethals which are eliminated probably belong to 

 the class of chromosome aberrations rather than gene mutations. Patter- 

 son and MuUer (140) noted that the frequency of chromosome aberration 

 is higher in adult males than in females treated with the same dosage 

 (Table 14). Shapiro (155) and Shapiro and Neuhaus (156) have 

 similarly shown that the frequency of translocations between the second 

 and third chromosomes is greater in mature spermatozoa than in the 



