1 



226 



Barbara E. Holmes 



mitosis was not seen until 12 hours later. The DNA synthesis 

 could be inhibited by irradiation (800 r whole body) at 12 

 hours (not later than 24 hours) after poisoning, but even 

 2000 r could not inhibit the synthesis while it was actively in 

 progress. Another sensitive period for this inhibition was 



t 



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c 



E 

 Q.800 



Q. 



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£ 



O4OO 



o. 



c 



o 

 U 



200 — 



Few 



4 A Irradiated before operotion 



X X Irradiated at 12 hours 



Control 



/ 



I 



3.2(25«/^/x 



Figures represent mitotic , 

 count /lOOO 



] /o Mitos«s showing 



chromosome fragments 



± 



19 6(66%! 



^ y A 6 [some] 



Xr-^' 



J \ L_ 



6 12 15 18 21 24 27 30 33 36 39 42 



HOURS AFTER KERATECTOMY. 

 Fig. 1. Rate of DNA synthesis and mitosis in regenerating liver. 



found by Mrs. Kelly to occur later than 60 hours after poison- 

 ing (Kelly et al, 1955). 



The curve of regeneration we obtained after hepatectomy 

 (Fig. 1) has some different time relationships. The peak of the 

 DNA synthesis rate seems to be shortly before the first large 

 outburst of mitosis. As soon as large numbers of mitoses are 

 present the rate is diminished. 



A dose of 2000 r inhibits DNA synthesis by about 50 per 

 cent at any time during the cycle and the effect is immediately 

 apparent. Here our results differ from those obtained by 



