A considerable amount of information bearing upon mouse chimeras obtained experimentally by fusion of cleaving eggs has already been accumulated (Tarkowski, 1961, 1963, 1964a, b, 1965; Mintz, 1962a, b, 1964a, b, 1965a, b, 1967). Direct and indisputable proofs of the chimerism of the individuals produced were obtained by establishing that two types of pigment cells were present in the outer layer of the retina (Tarkowski, 1963,1964a) and in the coat (Mintz, 1965a, b, 1967). The cell composition of other organs and tissues has not as yet been investigated. True hermaphroditism in some of the animals obtained was also accepted as an indirect proof of chimerism (Tarkowski, 1961, 1963, 1964b; Mintz, 1965b) but the postulate of these individuals being six-chromosome chimeras has not as yet been directly confirmed by karyological research.

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