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The fertility of DDK mice was investigated in reciprocal crosses between DDK and C57BL/6J strains of mice. When mated to C57BL males, DDK females were semi-sterile. By contrast, when C57BL females were mated to DDK males, they showed better fertility than in the intra-strain mating. Embryological study has revealed that the low fertility of DDK females is caused by embryonic death at the morula-blastocyst or pre-egg cylinder stage, 3 to 5 days after copulation. A defect in trophoblast formation was a common feature of the dead embryos.

Experiments involving ovary transplantation have shown that the cause of embryonic death in DDK females mated to C57BL males is not to be found in the interrelationship between the F1 embryos and the uterine environment of DDK females, but in the fertilized eggs themselves. Possible mechanisms leading to the embryonic death are discussed.

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A low fertility of DDK females mated to males from other inbred strains of mice, revealed in preliminary inter-strain crosses of several strains, was confirmed in a detailed study of strains DDK, KK and NC, the nine possible matings being arranged. The mean litter sizes of four types of crosses, DDK females × KK males, DDK females × NC males, KK females × DDK males, and NC females × DDK males were 1·1, 2·6, 7·2 and 7·7 respectively. The mean litter sizes of the three inbred strains, DDK, KK and NC, were 5·8, 5·6 and 7·0 respectively.

The lowered fertility of DDK females mated to KK and NC males was accounted for by embryonic losses which occur about 3 to 4 days after copulation. The embryos die just before or in the process of implantation, i.e. before or during the process of blastocyst formation.

Probable causes of the embryonic losses in DDK females mated to KK and NC males are discussed.