ENVIRONMENTAL EFFECTS 



tive to irradiation in late metaphase I, almost 

 equally so in latest prophase I ( diakinesis ) and 

 least so in prophase stages before diakinesis. 

 Adult (haploid male) survival from unfertilized 

 eggs laid by treated females is used as the 

 criterion (A. R. Whiting, 1939a, 1940a, c). 



Hatchability percentages of unfertilized 

 Habrobracon eggs X-rayed in late prophase I with 

 doses up to 400 R units are significantly high- 

 er than those of controls, and the lethal dose 

 is about 35,000 R units (A. R. Whiting, 1941). 

 Eggs treated in late metaphase I are sensitive 

 to 50 R units, and the lethal dose is abo^it 

 1,250 R units. There is no evidence of recovery 

 •in this stage. The dose-hatchability curves of 

 the two stages differ, that of metaphase I show- 

 ing a linear relationship to dose. In 1942 (A. 

 R. Whiting, 1942a) evidence against "physiolog- 

 ical effect" as an important factor for meta- 

 phase I sensitivity was presented as well as 

 preliminary cytological observations. 



Two criteria for judging injury are used (A. 

 R. Whiting, 1945a). One of these, hatchability , 

 gives degree of lethal effect, both dominant and 

 recessive, and is a very accurate measurement in 

 Habrobracon where unfertilized haploid eggs are 

 capable of development, where every egg can be 

 accounted for, where the ideal environmental 

 conditions are known and easily controlled, and 

 where there is available a stable wild-type 

 stock consistently giving 96 per cent hatchabi 1- 

 ity or higher. The second criterion, chromosome 

 changes in successive stages of the treated oo- 

 cytes, is less satisfactory because Habrobracon 

 3:hromosomes are too small (1 u in diameter) for 

 very careful analysis. Giant salivary chromo- 



129 



