THE TRANSFERRAL of tissue from one animal to another animal of the same species is a homologous transplantation, or homotransplantation. Transferral of tissue from one part of an animal to another part of the same animal is known as autotransplantation. Transplantation of tissue from an animal of one species to an animal of another species is heterologous transplantation. The problem of homologous transplantation of tissue is not one of laboratory research alone, for there are increasing demands for its solution and clinical application. A reasonable prolongation in time of survival of homotransplants would solve temporary problems of many types; permanent survival could be, in the words of Dempster and Lennox,1revolutionary. The technique of transplantation of biologic material is many years old. A form of skin graft for cosmetic reasons was practiced in India in ancient times. In early modern times the practice of blood transfusion is an example