The Nobel Prize in Physiology and Medicine in 1960 was awarded to two renowned scientists in Australia and Britain, Frank Macfarlane Burnet and Peter Brian Medawar. Many historical accounts have described Burnet's theoretical prediction of immunological "tolerance" and its experimental confirmation by Medawar as a momentous achievement in immunology. This book examines their distinct research pathways, especially their different ways of approaching the immunological changes of the host organism during embryogenesis and foetal development.