In one experiment Lederberg used Gal and Gal– strains of E. coli. One half of a plate that contained medium with galactose as the only carbon source was inoculated with phages that came from lysed cells of the Gal strain. The other half of the plate was left uninoculated. A Gal- strain was then placed on the entire plate. Numerous colonies appeared on the half of the plate inoculated with the phage, whereas only a few appeared on the half that was not inoculated with phage. Explain what happened using appropriate terminology.