258 G. S. STENT 



between assimilation from the medium and incorporation into antigenic 

 phage precursor protein, and a further development time of 8 minutes until 

 their incorporation into mature virus particles (Maaloe and Symonds, 1953; 

 Hershey et al., 1954, 1955). Thus, there also exists a pool of phage precursor 

 protein from which material begms to be withdrawn after the termination of 

 the eclipse for the formation of mature virus particles. The size of this 

 precursor protein pool appears to amount to about 15 phage protein equiva- 

 lents at the termination of the eclipse (Hershey and Melechen, 1957), although 

 it will be recalled that about two to three times as many incomplete phage 

 antigens and electron optically visible doughnuts have been observed to be 

 present in each cell at that time. This apparent discrepancy in intracellular 

 amount of different types of protein precursors could be either due to 

 possible variations in physiological conditions of growth employed by the 

 different investigators whose work is being compared here, or due to the fact 

 that some of the incomplete proteinaceous materials, in particular the SBP 

 structures and the doughnuts, are not reaUy precursors of the mature progeny 

 particles after all. In any case, the pool of phage precursor protein appears to 

 be smaller than the pool of phage precursor nucleic acid; this difference in 

 pool sizes can be exaggerated further by chloramphenicol treatment, which, 

 as shall be mentioned soon, permits the synthesis of phage precursor nucleic 

 acid in the absence of any phage precursor protein. Hence, the mechanism of 

 synthesis of the phage precursor nucleic acid camiot be such that the phage 

 DNA is necessarily formed inside particles possessing a phage precursor 

 protein membrane (Hershey and Melechen, 1957). 



C. Maturation 



The enclosure of the phage precursor nucleic acid into a precursor protein 

 head membrane and endowment with a precursor protein phage tail thus 

 appears to represent one of the last acts of intracellular virus growth, i.e., 

 the maturation of the infective unit. "Even when forced into this possibly 

 oversimplified scheme," wrote Hershey (1957a), "the maturation process 

 presents a more complicated problem, conceptually at least, than vegetative 

 reproduction. In this respect it resembles, of course, morphogenetic problems 

 in general. So far, this aspect of phage growth has been attacked only as a 

 side issue to what have seemed to be more promising opportunities." 



1. Phenotypic Mixing 



A series of genetic experiments relevant to the maturation process had 

 their origin in an observation by Delbriick and Bailey (1946) of anomalous 

 host range behavior of phages hberated by bacteria mixedly infected with T4 

 and T2. Novick and Szilard (1951) investigated this anomaly further and 

 found that among the progeny of T4 X T2 crosses there exists particles which 



