STUDIES ON THE ORIGIN OF BACTERIAL VIRUSES : I. THE INCIDENCE OF PHAGE-PRODUCING CELLS IN VARIOUS B. MEGATHERIUM CULTURES. II. THE EFFECT OF ULTRAVIOLET LIGHT ON THE INCIDENCE OF PHAGE-PRODUCING CELLS AND OF TERRAMYCIN-RESISTANT CELLS. III. THE EFFECT OF HYDROGEN PEROXIDE ON THE INCIDENCE OF PHAGE-PRODUCING CELLS AND OF TERRAMYCIN-RESISTANT CELLS. IV. CALCULATION OF THE INCIDENCE OF PHAGE-PRODUCING CELLS