IF differentiated (or determined) tissue grafts are transplanted into late stage embryos, it is possible to obtain experimentally in the chick1–3 and in the mouse4 a genetic chimaerism that is limited to a tissue type or to an organ. By the induced integration of two embryos at the morula stage into a single embryo which is normally developing, it has more recently become feasible to produce in the mouse an unrestricted chimaerism, in which the genetic mosaic condition is present in nearly all the tissues of the body. This was first accomplished by Tarkowski5 and Mintz6, the latter author referring to such type of mosaic mice as “allophenic hybrids”. Other workers have since confirmed the experiments7,8. The development of animals with unrestricted genetic mosaicism offers unique opportunities for investigations of basic aspects of ontogenesis in both early embryogenesis and later life.