In July 1944, Oppenheimer reorganized the laboratory to focus on the problem of building an implosion-type nuclear weapon, which was necessary because the gun-type design would not work with plutonium. P Division was broken up, and Bacher became the head of the G (for Gadget) Division. This much larger division contained eleven groups headed by leaders who included William Higinbotham, Seth Neddermeyer, Edwin McMillan, Luis Alvarez and Otto Frisch.