138 BENZER ET AL. 



sitive index to development in the earliest stages of the latent period, 

 at a time before the completion of viable new phage particles, and the 

 technique is being applied to several problems. 



For example, the nutritional requirements for synthesis of phage 

 can be studied by starting in buffer and adding well-defined nutrients. 

 If the added materials are inadequate, no change in resistance occurs. 

 If completely adequate, the rate of increase should correspond to that 

 observed with broth. In cases where chemical transformation and 

 adaptive enzyme formation are necessary, these will be revealed by 

 the shape of the curve. 



IX. Cytology 



36. Observations on E. coli bacteria have been made with the stain- 

 ing method of Piekarski-Robinow. Before the infection one sees usu- 

 ally two or four colored bodies, "nuclei," inside the cell. Five minutes 

 after infection with T2 the nuclei seem to break down into masses that 

 line the periphery of the cell. Later, a granular material appears and 

 progressively fills the cell (Luria and Human, 1950). 



In the dark field microscope the live uninfected bacterium appears 

 as a cigar shaped body whose outline only is visible. The interior does 

 •not scatter enough light and thus appears completely dark. This is 

 still true of infected bacteria; however, they are marked by the presence 

 on their surface of strongly scattering dots and the number of these 

 dots seems to be similar to the multiplicity of infection. Later, the 

 inside of the bacterium fills with small particles in violent motion. 

 Bursting of the bacteria may take place in various manners. The small 

 particles inside may congregate near the middle of the bacterium which 

 swells to a spherical shape and then explodes (the whole process tak- 

 ing less than a second) scattering hundreds of particles around itself, 

 these phages slowly diffusing through the preparation. A long bac- 

 terium may break in two, one half hinged to the other, slowly fold- 

 ing back against the other while the phages quickly ooze out of the 

 opening. Sometimes the infected bacterium will swell to spherical 

 shape failing to explode, never liberating phage. 



There is no doubt (see 4) that the particles which are to be identi- 

 fied with the phages emerge from the inside of the bursting bacterium 

 ( Weigle, unpublished) . 



X. Lysogenesis 



37. The term "lysogenic strain" has been applied to those bac- 

 terial strains in cultures of which the presence of bacteriophage can 

 be detected. If a culture, or the filtrate of a culture of a strain originat- 



