1 82 BACTERIOPHAGES 



(3) a stage may be reached after which the intrabacterial DNA is 

 dispensable, presumably owing to a transfer of DNA function to 

 other substances not containing phosphorus. 



The tliird alternative was also suggested by Tomizawa and 

 Sunakawa (1956) on the basis of experiments showing that 

 chloramphenicol, added to cultures infected with phage T2 

 under conditions permitting synthesis of DNA but not protein, 

 blocked those processes resulting in stabilization of the infected 

 bacteria toward the destructive effects of ultraviolet light. 



At the present time no decisive choice can be made among the 

 three hypotheses suggested by Stent. It should be recalled, 

 however, that the experiments with T7 do not show the same 

 phenomenon, and one can ask which result reflects basic features 

 of viral growth least complicated by side eff'ects. These and 

 other related experiments are discussed in somewhat different 

 contexts by Hershey (1957) and Stent (1958). 



6. Morphological Stages in Phage Development 



Objects the size of bacteriophages can be studied by means of 

 the electron microscope, which has revealed structures that may 

 be intermediate stages in the maturation of infective particles. 

 One of these is known by the colloquial namie "doughnut." It is 

 a crumpled disk with a central depression which makes it look 

 like a torus in shadowed electron micrographs. Beautiful pic- 

 tures of doughnuts were published by Wyckoff' (1949b). They 

 are also illustrated in papers by Hercik (1955), Heden (1951), 

 Levinthal and Fisher (1952), DeMars, Luria, Fisher, and Levin- 

 thai (1953), and T. F. Anderson, Rappaport, and Muscatine 

 (1953). The latter authors prepared specimens by the critical 

 point method and show that doughnuts are really empty mem- 

 branes of the same size and shape as the phage head but without 

 any tail. They have been found in association with the large, 

 tailed phage particles T2, T4, T6, and T5. 



The size, shape, and occurrence of these particles suggested 

 that they might be an intermediate stage in reproduction. 

 Evidence in support of this view was obtained by Levinthal and 



