Experiments are described which assess the relative developmental capacities of morulae, blastocysts and ectoplacental cones transplanted to the mouse testis. The pre-implantation stages developed less successfully, yet frequently gave rise to embryonic or extra-embryonic elements as well as trophoblast. The ectoplacental cones almost invariably developed and produced a luxuriant growth of trophoblast; this trophoblast caused a measurable increase in the weight of the testis which was shown histologically to be a direct indication of the degree of trophoblast invasion into the testis tissue. Ectoplacental cones were transplanted within and between inbred strains of mice. Those transplanted to genetically dissimilar hosts produced a greater increase in testis weight; thus trophoblast invasion appears to be more extensive when there is antigenic difference between the trophoblast and the host. These findings are discussed with reference to immunological factors in placentation.
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