A. D. HERSHEY 



is a genetic concept. This concept emerges from the following 

 facts. 



Doermann (10) showed that infected cells broken open dur- 

 ing the first 10 minutes following infection do not yield any infec- 

 tive virus. This result has since been accounted for by the act of 

 injection: what goes into the cell is not virus. Doermann's 

 experiments went much further than this, however. He proved 

 that genetic determinants, that is, enumerable structures possess- 

 ing viral specificity, multiplied during the period in which the 

 cell remained free of infective virus. This proof is summarized 

 in a recent review (11). The term vegetative phage refers to 

 these noninfective structures that multiply and produce new 

 genetic combinations, and must be thought of as the connecting 

 links between parent and offspring virus. The central problem 

 of phage growth, therefore, is the problem of structure and func- 

 tion of vegetative phage. 



MATURATION 



The process by which vegetative phage is converted into rest- 

 ing phage is called maturation. From the end result we know 

 that it involves the reformation of envelope and tail, but we do 

 not know where it starts ; to answer this it is necessary to know 

 what vegetative phage is like. It seems possible, as will be dis- 

 cussed below, that vegetative phage may consist of replicas of the 

 injected DNA-containing fibrils. If so, maturation calls for 

 the complete synthesis of envelopes. 



Probable intermediate stages in maturation have been identi- 

 fied. These consist of empty, tailless envelopes (40), accom- 

 panied by detached tails, or of tailed, but probably DNA-less 

 envelopes (46). They are found only 2 or 3 minutes before 

 the phage particles of which they are the presumed precursors, 

 and so are probably not themselves vegetative phage particles, 

 which must be formed appreciably earlier. The absence of 

 DNA is puzzling, and it seems preferable to assume that the 

 observed structures lose their DNA during isolation, rather than 

 to imagine that they receive their charge of DNA as a final act 



