266 L. H. Gray 



shape corresponding well with expectation for a two-hit type 

 of effect. When grown on minimal medium, however, log 

 survival curves were strictly linear and with the slope approxi- 

 mately double that of the asymptotic (low survival) portion 

 of the log survival curves of organisms grown on complete 

 medium (LD37 --^ 15,000 r). These relations are to be expected 

 between survival on complete and minimal medium if the 

 injury is of the nature of a dominant lethal mutation, since 

 on minimal medium inactivation of either nucleus would be 

 lethal to the spore. Atwood concluded that the injury which 

 was lethal to the spore was entirely nuclear in origin and of 

 the nature of a dominant lethal mutation. The experiment 

 was repeated using different combinations of the markers and 

 with trinucleate cells, each of which was doubly marked. All 

 were consistent with the hypothesis of dominant lethal 

 mutation as the cause of death. Whatever the precise nature 

 of the damage to the genetic material, its effects are obviously 

 very far-reaching since a nucleus so damaged is unable even 

 to synthesize the single amino acid or other growth factor 

 required by the second nucleus. It is remarkable that meta- 

 bolic activity can be so completely inhibited by the passage of 

 a single ionizing particle. 



Summary 



The results which have been discussed are summarily 

 classified in Table I. It is at once obvious that the forms of 

 damage which have been analysed are not representative of 

 radiobiology as a whole. Seven out of sixteen entries are con- 

 cerned with cell reproduction, and in these the criteria 

 adopted are tantamount to a test of the reproductive integrity 

 of the cell. Six other cases are concerned with nuclear 

 components or nuclear function. To destroy the reproductive 

 integrity of the cancer cell is the aim of radiotherapy and in 

 this connection the effects listed in Table I are of special 

 interest despite their limited range. The cell is a unit of 

 biological organization and, as might be expected, vital 



