18 P, ALEXANDER AND Z. M. BACQ 



irradiation makes it possible for the fixative to penetrate into the cells 

 and that the pyknotic changes are not caused by the irradiation, but 

 are due to access of the fixative. 



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DISCUSSION 



POWERS : What was the effect of heating in your experiments on paramagnetic 

 resonance? In other words do you admit the possibility of changes in your 

 material due to heating? 



ALEXANDER: Thcse experiments were usually carried out at a temperature of 

 — 195°C. Then the sample was warmed up to room temperature. A change in the 

 spectrum resulted. The sample was kept at room temperature for a day, and 

 showed further changes of the spectrum. We did not cool the sample again, 

 since it is quite evident that at room temperature irreversible changes could have 

 occurred as radicals acquired the possibility of meeting and recombining, which 

 does not occur at a temperature of — 195°C. 



POWERS: How do you explain these changes? 



