Gravar-mail: Studies of the Origin of Bacterial Viruses : VII. The effect of various mutagens (urethane, ethyl urethane, hydrogen peroxide, desoxycholate, maleic hydrazide, butadiene dioxide, triethylene melamine, versene, and acriflavine) on the proportion of virus-producing and streptomycin-resistant cells in cultures of B. megatherium 20δ