380 F. W, STAHL 



fraction of the genetic material of the population is prevented from entering 

 into the acts leading to multiplicity or cross reactivation (Weigle and 

 Bertani, 1956; Harm, 1958a). This probably reflects a loss of ability of such 

 phage to inject its DNA (Hershey et al., 1954). 



Genetic recombination increases when X-rays are applied to infected 

 cells (Harm, 1958a). Qualitatively, multiplicity reactivation with X-rays 

 applied to infected cells looks similar indeed to multiplicity reactivation 

 with ultraviolet light. A quantitative comparison is presently not profitable 

 since, necessarily, the X-ray experiments involve delivering to the host 

 cell the same dose of X-rays which one delivers to the phage. It has been 

 shown (Weigle and Bertani, 1956; Harm, 1958a) that the efficiency of multi- 

 plicity reactivation with X-rayed free phage is increased by X-ray treatment 

 of the host cells. Indeed, Harm has shown that X-rays delivered to the host 

 cells increases the cross reactivation of ultraviolet-inactivated phage! A 

 quantitative comparison of multiplicity reactivation with ultraviolet light 

 and X-rays applied to infected cells must await a systematic study of the 

 effects of X-rays delivered to the host cell on the outcome of multiplicity 

 reactivation experiments with ultraviolet-inactivated phage. 



C. Luria-Latarjet Experiments 



Luria-Latarjet experiments performed with X-rays show similarities with 

 the analogous experiments performed with ultraviolet light and P^^. As with 

 P32, X-rays delivered to T2-infected cells fail to reveal the slight change in 

 sensitivity characteristic of T2 irradiated with ultraviolet light during the 

 first 5 to 6 minutes (Symonds and McCloy, 1958). Thereafter, the sensitivity 

 of the complexes decreases rapidly, but the complexes never achieve the 

 high degree of resistance found with P^^ and ultraviolet light (Latarjet, 

 1948; Stent, 1955). Instead they become "multihit" with a target number 

 aromid 10-20 (Latarjet, 1948) and a slope at high dose which is about one- 

 half that characteristic of the complexes irradiated immediately following 

 infection. 



At present, the similarities between the Luria-Latarjet experiments with 

 the three agents appear more significant than their differencep. All may be 

 interpretable on the same basis. As with multiphcity reactivation, more 

 careful comparative studies are needed to ascertain whether the differences 

 which do exist reflect anything more significant than differing side effects on 

 the host cells. 



V. A Survey of Other Phages 

 A. Introductory Remarks 

 The T-even phages stand in contrast to most of the other known phages 

 in the degree to which their development is independent of the nuclear 



