Fig. 1. A photograph of a petri tlish on which a mixture of four types of T2 pliage 

 r+ li^, rk^, ?•+ h, and rh were plated on a mixture of E. culi B and the T2 resistant strain 

 B/2. The clear spots in the rh'^ plaques are due to phage mutations to h which occurred 

 in the plaque. 



