

CHROMOSOME ABERRATIONS IN ANIMALS 689 



irradiation are in first meiotic metaphase, or in prophase. Eggs are 

 permitted to develop parthenogenetically so that the effect of the treat- 

 ment can be detected directly by its influence on hatchability (Whiting, 

 1940, 1945a). The results obtained in such experiments indicate that 

 eggs whose chromosomes are in metaphase of the first meiotic division 

 are much more sensitive than those in prophase stages of this division. 

 Embryonic abortion in these cases is presumably due in large measure to 

 single-break dominant-lethal chromosomal aberrations (Whiting, 1945b). 

 Germinal selection cannot explain the observed differences, since the 

 eggs continue meiosis and initiate the cleavage mitoses at rates not 

 noticeably different from the controls, after doses greatly exceeding those 

 necessary to inhibit embryonic development. 



Apparently the situation in Drosophila is similar, although the stage of 

 development of the egg at the time of treatment cannot be determined 

 with as great precision as in Habrobracon. Patterson, Brewster, and 

 Winchester (1932) found a higher proportion of dominant lethals among 

 eggs laid within 24 hours after irradiation than among those laid on the 

 third day, which presumably were in an earlier stage of oogenesis at the 

 time of treatment. Sonnenblick (1940) also noted that the most mature 

 oocytes are especially sensitive to irradiation. 



In Sciara the sensitivity of the oocyte increases as it passes from late 

 prophase to metaphase and anaphase, as determined by the percentage 

 of rearrangements detected and by egg hatchability (Reynolds, 1941; 

 Metz and Bozeman, 1942). In these fungus flies all oocytes develop 

 synchronously, so that the meiotic stage at the time of irradiation can 

 readily be distinguished. More detailed information regarding sensi- 

 tivity in *S. ocellaris is furnished in a later paper (Bozeman and Metz, 

 1949). Sensitivity is almost zero before the breakdown of the nuclear 

 membrane at the beginning of the first meiotic division. It then rises 

 rapidly to a peak in anaphase, and drops off in late anaphase, when 

 mitotic activity is arrested pending fertilization. The types of induced 

 rearrangements include inversions, duplications, deletions, and trans- 

 positions, but almost no translocations. Summarized data based on 

 salivary-gland-chromosome analyses are presented in Fig. 9-16. 



The most exhaustive studies of differences in sensitivity during the 

 nuclear cycle have been made by Sparrow (1944, 1948, 1951) on Trillium. 

 Meiotic divisions are generally well synchronized in the anthers of the 

 buds of this plant. The stage during which irradiation was carried out 

 could thus be determined with a high degree of accuracy, and the effects 

 of the treatment could be studied during the course of meiosis or in either 

 of the two postmeiotic mitoses. Determination of fragment number in 

 successive mitoses gave a more adequate estimate of the frequency of 

 induced aberrations than had been possible with methods previously used. 

 Since other investigators did not use this double scoring technique, their 



