66 



S. BENZER 



[vol. 63 



preferable to work with T2r because it produces larger plaques, thereby obviating 

 in some measure the difficulty created by the decrease in plaque size at large 

 doses. 



In figure 5 a complete set of curves for T2r (latent period 19 minutes) is 

 given. These qualitatively confirm the observations of Luria and Latarjet with 

 T2. The free phage does not have a strictly exponential survival curve, but the 

 changes in resistance during the latent period are so large that this may be 

 ignored for our purposes. At t = (i.e., for a sample irradiated before the addi- 

 tion of nutrient) the resistance of monocomplexes is slightly higher (by 20 per 

 cent) than that of free phage. This may be partially due to a small amount of 

 development of the phage which can take place in buffer. 



400 500 

 SECONDS 



Figure 5. Survival curves for T2r complexes irradiated at different times during the 

 latent period. Each curve is marked with the time in minutes after adding broth to the 

 complexes formed in buffer. All data are from the same growth experiment at 37 C (latent 

 period = 19 minutes). Average multiplicity of infection = 2 X 10"'. 



As development proceeds, the resistance of all complexes increases. An es- 

 pecially rapid rise occurs between 6 and 8 minutes, the curves still remaining 

 essentially exponential. The smallness of the bend (at large doses) in the survival 

 curve taken at 7 minutes is very significant, since it implies that, at least up to 

 this point (one-third of the way into the latent period), development has pro- 

 gressed rather uniformly in all infected cells. If, at 7 minutes, 10 per cent of the 

 infective centers had progressed to a stage corresponding to 8 minutes (by 

 virtue of 15 per cent more rapid development), the bend in the 7 minute curve 

 would have been greater than observed. 



At 9 or 10 minutes the resistance reaches a maximum phase, and the curve 

 has now become "multiple-target" in character. In figure 6 data are given which 

 were obtained at 10 minutes. The points fit very well a theoretical curve for a 



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