690 



RADIATION BIOLOGY 



data do not represent total sensitivity, but only a portion of the induced 

 effect. Breaks that are not perceptible in the condensed chromosome 

 immediately after treatment may appear after an intervening interphase 

 (as Reynolds, 1941, also showed in Sciara). Results of Sparrow's studies 

 showed that sensitivity increases during prophases of the first meiotic 

 division, reaching a peak in the interval between diplotene and first 



50 



40 - 



UJ 



o 

 < 



o 



a: 



UJ 



a. 



30 



20 



10 



PROPHASE META- EARI_r MID LATE 



PHASE ANA- ANA- ANA- 



PHASE PHASE PHASE 



Fig. 9-16. X-ray sensitivity during various stages of first meiotic division, as deter- 

 mined by salivary-gland-chromosome analysis of Fi progeny of irradiated Sciara 

 females. Left column (diagonally striped) represents calculations based on wild 

 stock; open column at right, based on yellow stock; solid black column, based on 

 totals of both stocks. (From Bozeman and Metz, 1949.) 



metaphase, and then falls off through the stages of the second meiotic 

 division, to reach a low in the postmeiotic interphase (Fig. 9-17). More 

 recent studies by Sparrow and Maldawer (1950) indicate that part of the 

 observed difference in sensitivity is due to a difference in the frequency of 

 recombination of breakage ends, which is significantly higher in cells 

 irradiated in early interphase than among those irradiated in first meiotic 

 metaphase. However, these authors state that this difference in recom- 

 bination is not sufficient to account for the aforementioned differences in 

 frequency of fragments. The recent results thus indicate that chromo- 

 somes in the condensed form have a high proportion of "potential" 

 breaks, which subsequently produce chromosomal aberrations. These 

 results stand in vivid contrast to many of the earlier plant studies, from 

 which it was inferred that chromosomes are most sensitive to breakage in 

 resting or early prophase stages. 



The factors responsible for changes in sensitivity during the cycle of 



