 He returned to Oxford in 1946 and became Peter Medawar's research student.During his PhD he worked on skin grafts in guinea pigs, demonstrating that when black skin was grafted onto white skin, the white skin became black.They proposed that the change was due to the dissemination of a self replicating agent from normal melanocytes into non-melanin-producing cells; but this hypothesis was wrong and they later showed that pigment spread was due to cell migration.