272 CHROMOSOME ABERRATION PRODUCTION 



total dose of 500 r. Two control series were run, in one of which the 

 procedure just described was followed except that helium rather than 

 oxygen was introduced. In the other control a total dose of 500 r was 

 administered in 100 sec of continuous exposure in vacuum; helium was 

 then introduced and remained in the chamber for 8 niin after the x-ray 

 exposure. The data from this experiment are presented in Table 1. It 



TABLE 1 



Test of the Effect of Oxygen on the Recovery Mechanism, Using 

 Fractionated Doses 



All series received a total dose of 500 r at 300 r per min. In series A this dose was 

 delivered in five equal fractions, with irradiation occurring in vacuum, and each 

 intervening recovery period of 8 min occurring in oxygen at a positive pressure of 

 1500 mm of Hg. Series B was similar, except that the recovery periods occurred in 

 helium. Series C received one continuous exposure in vacuum, with recovery 



occurring in helium. 



Series Conditions 



A Fractioned dose; irradiation in 



vacuum; recovery in oxygen 

 B Fractioned dose; irradiation in 



vacuum; recovery in helium 

 C Continuous exposure; irradiation 



in vacuum; recovery in helium 



is evident that there is no increase in aberration frequency in series A, 

 in which oxygen was present during recovery, as compared with B, in 

 which helium was present. The somewhat higher values for C are to be 

 expected, since this was a continuous exposure with no intervening re- 

 covery periods. 



A second experiment has been performed (in cooperation with A. V. 

 Beatty) to retest the observation of Giles and Riley (12) that the ad- 

 dition of oxygen during irradiation results in an immediate increase in 

 aberration frequency. These data are presented in Table 2. In this in- 

 stance, essentially the same experimental conditions were utilized as 

 previously, except that a second exposure (series C) was made in which 

 oxygen was present during only the last 15 sec of the total x-ray exposure 

 of 1 min. The results of this experiment are in agreement \\\ih the 

 earlier one in indicating that the introduction of oxygen during irradia- 

 tion results in an immediate increase in aberration frequency. 



It is clear from these comparisons (and those reported earlier) that the 

 addition or removal of oxygen immediately after irradiation does not 



