CONCEPT AND CRITERIA OF RADIOLOGIC AGEING 205 



CASAKETT: This is a very good point. I did emphasize however that this test, limited as it 

 is, was essential!}^ best applied after maximal tissue regeneration, in other words when 

 the acute and sub-acute death periods have passed and there has been maximal tissue 

 regeneration. If done periodically, of course, one is testing different degrees of change 

 and also different diseases in terms of resistance to lethality. Of course we are again 

 limited when the age-dependent diseases begin to take their toll. We start then to get a 

 difference in the degree of difference between LDgo's if we measure at the same chrono- 

 logical ages, but if comparison is made at the median death times, which essentially is a 

 more comparable situation after these diseases begin to take their toll, one can find, at 

 least in some cases, still a similar degree of difference in the LDjq's. Tliis has to be taken 

 into account also. 



KOTBLAT: For the first point, the use of the LDjo's as a criteria, you are probably right 

 in saying that it is not a ver}' good tool unless you use such a large dose that you almost 

 end the life, in which case the animal died, so you can't use it in any case. On the other 

 hand, the remarkable thing is that there is a correlation between the long-term effects 

 which you express in the life-shortening, and the sensitivity to the acute effects. This is 

 quite remarkable. 



