772 



RADIATION BIOLOGY 



Stages arranged in successively decreasing order of sensitivity were: 

 entrance of sperm head into egg cytoplasm and its approach to the egg 

 nucleus, early prophase, and late prophase. No delay resulted from 

 treatment at metaphase. 



Yamashita, Mori, and Miwa (1939) also determined the effect of 

 irradiating different stages of one- and two-celled Pseudocentrotus eggs on 

 the length of the second cleavage division. They found that it was 

 delayed only slightly by treatment of the one-celled stage at metaphase or 

 earlier, markedly by treatment at anaphase and telophase, and maximally 

 by treatment at the beginning of the two-celled stage. 



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20 60 100 140 180 



MINUTES AFTER IRRADIATION BEGAN WHEN EGGS WERE INSEMINATED 

 Fig. 11-4. Relation of time intervening between beginning of X irradiation and 

 insemination of Arbacia eggs to the cleavage delay, showing recovery from effect; 

 dosage rate, 520 r/min (after Henshaw, 1932). 



The rate of recovery in the Arbacia egg has been shown by Henshaw 

 (1932) to be exponential. Eggs exposed to different doses of X rays 

 at the same dosage rate were inseminated at different times after the end 

 of treatment. For a series of exposures ranging from 5-60 minutes at 

 520 r/minute he found that the longer the delay between irradiation and 

 insemination, the less the first cleavage was delayed (Fig. 11-4). When 

 minutes delay on a logarithmic scale was plotted against minutes inter- 

 vening between irradiation and insemination, a straight line was obtained. 



The doses and dosage rates used in these experiments on the sea urchin 

 are large, ranging from 2600 to 249,600 r delivered at rates of 120 to 

 7800 r/minute. These doses are of a much greater order of magnitude 

 than those used with most other kinds of material. Cleavage delays 

 recorded are from a few minutes to somewhat less than 3 hours. 



Both Chortophaga neuroblast and sea urchin egg experiments indicate 



