1232 



BIOLOGICAL EFFECTS OF RADIATION 



come from sperm treated at different stages of spermatogenesis, making 

 a comparison immediately possible. The proper tests will then show any 

 variation of effect at the different stages. Experiments of this sort 

 have been carried out by Harris (72), Hanson and Heys (62), and 

 Timofeeff-Ressovsky (178) for the X-chromosome, and by Sidorov (158) 

 and Shapiro and Neuhaus (156) for the second chromosome. The 

 experiments on the X-chromosome agree in showing a striking decrease in 

 the percentage of lethal mutations, occurring between the twelfth and 

 sixteenth days after raying (Table 10). But in the second chromosome, 

 the decrease is much slighter — scarcely statistically significant (Tables 

 11, 12). These differences, both between early and late broods, and 

 between the behavior of the X- and second chromosomes, are fully 

 explained by the assumption (Harris, Timofeeff-Ressovsky) that lethal 

 effects induced in the immature germ cells are eliminated before the 

 formation of the mature gametes. 



Table 11. — Lethals in Chromosome II and Age op Germ Cells in Treated 



cf cf OF Drosophila melanogaster 

 Data of Sidorov (158): 50 kv., 5 ma.; 1 mm. Al filter; distance 17 cm.; exposure 2 hr. 



Table 12. — Comparison of Frequency of Lethal Mutation in Ger.m Cells 



OF Different Ages in Chromosome II of Drosophila melanogaster 



Shapiro and Neuhaus (156): X-ray treatment; 120 kv., 5 ma.; 1 mm. Al filter; 17 cm. 



from anticathode; 11-min. exposure; 2134 r 



Timofeeff-Ressovsky (178) in particular has put this to the test. 

 Lethal effects of the type postulated, when they occur in the mature 

 sperm, will have no effect (Muller and Settles, 122); but they will be 



