A. D. HERSHEY 



27,35). It is also agreed that the efficiency of transfer never 

 exceeds 40 or 50 per cent (18,22,65). The mechanism of 

 transfer, and therefore its significance, remains undecided. The 

 question of mechanism is conveniently discussed in terms of 

 alternative explanations for the low efficiency of transfer. 



7. The low efficiency of transfer may result from losses 

 occurring in some but not all the infected bacteria, either be- 

 cause preparations of phage always contain inviable particles 

 or because for other reasons some of the bacteria yield few or no 

 viral progeny. Although losses from both causes undoubtedly 

 occur and cannot at present be measured satisfactorily, most 

 observers have considered this explanation inadequate. Losses 

 of this type, of course, have nothing to do with the mechanism of 

 transfer. 



2. The loss may be an essential feature of viral growth, 

 but occur before multiplication starts. This would imply that 

 about half the parental DNA goes into vegetative phage, and the 

 other half serves a different, presumably nongenetic, function. 

 This possibility has been tested in two ways. Maal0e and Wat- 

 son (47) measured the efficiency of transfer of P^^ during two 

 successive cycles of growth, and found only the usual transfer of 

 40 per cent during the second cycle. This showed that the 

 transferred atoms from the parental DNA are incorporated into 

 both transferred and nontransferred parts of the DNA of the 

 progeny. Hershey et al. (29) found that the four bases of the 

 DNA of C^^-labeled T2 are transferred with equal efficiency. 

 These two results show that the loss is random, that is, if the 

 parental DNA splits into two parts that perform different func- 

 tions, and if only one part is conserved among the progeny, the 

 choice is probably made between identical parts. In this form 

 the idea of functional differentiation within the viral DNA is 

 unattractive but cannot be considered disproved. 



3. All the parental DNA may enter vegetative phage 

 where it is subject to a succession of small losses during replica- 

 tion. The losses would have to amount to 5 or 10 per cent per 

 generation to account for an observed transfer of 50 per cent 



16 



