HOWARD B. NEWCOMBE 



Somewhat similar evidence was obtained from the changes which occurred 

 under more nearly normal conditions. In most of the experiments where 

 the spores were incubated in medium prior to irradiation, the sensitivity fell 

 off rapidly as the average number of nuclei increased, to four or eight per 

 strand. In one experiment, however, the decline was much more gradual 

 in relation to the nuclear increase, and this was found to be associated with a 

 more poorly synchronized growth, due apparently to an unusual degree of 

 crowding ; the spores passed through the first nuclear division together, but 

 a proportion of them lingered much longer than usual in the sensitive 

 binucleate state. 



Thus all of the evidence so far, emphasizes a close association between 

 nuclear number and response to radiation, which can be observed over a 

 wide range of conditions of growth. The sensitivity changes do not seem 



Table V. — Sensitivity to X-ray induced mutation 

 Effect of pre-incubation in the presence of traces of nutrient 

 Effect of removal of germinated spores by filtration 

 (Spores harvested from nutrient agar culture by washing with sahne ; incubated without 



dilution. Dose, 8,000 r) 



* Same incubated suspension. 



t Same irradiated suspension. 



Unirradiated controls : 0-hour, 0-7 per cent mutation (15/2,015) ; 28-hour, 0/9 per cent 

 (13/1,509). 



Survival : in unirradiated control, 75 per cent after 28 hours' incubation ; in O-hour 

 irradiated 0T2 per cent; 28-hour irradiated 0-5 per cent; irradiated-filtered and 

 filtered-irradiated 0-05 per cent each. 



to be influenced by the particular stage in the nuclear division cycle, since 

 the period of maximum sensitivity apparently extends over the whole of the 

 binucleate state and not just a part of it, and the nuclear divisions subse- 

 quent to the first are almost completely unsynchronized. Bvit it is still not 

 certain that nuclear number directly determines the response, since changes 

 in both nuclear number and response might have a common physiological 

 cause. 



SENSITIVITY DIFFERENCES WHICH ARE NOT DUE TO 

 NUCLEAR NUMBER 



It also appeared that other differences in the state of the cell at the time 

 of irradiation could influence the proportions of induced mutant colonies, 

 even where the nuclear number was constant. This was indicated by 

 minor irregularities in certain of the experimental results, such as those 



333 



