Studies on the Transfer of Fertilized Mouse Eggs to Uterine Foster-Mothers

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Abstract
1. Post mortem examinations were made at 16½ days post coitum of the uterine contents of female mice belonging to the following five groups: untreated control females; females receiving ‘dummy’ transfers of saline without eggs; females receiving five to ten fertilized eggs (‘low’ group); females receiving fifteen to twenty eggs (‘middle’ group); females receiving twenty-five to thirty eggs (‘high’ group). All transfers were made into the left uterine horn 2½ days after mating the recipient to a fertile male. Genetic markers enabled embryos of donor and recipient origin to be distinguished by eye colour. 2. The transfer operation did not affect the pregnancy rate, nor the implantation rate in the uninjected horn. 3. The yield of live embryos of donor origin showed a systematic improvement in all three groups throughout the 18 weeks of the experiment, rising from about 7% of eggs transferred at the beginning to about 36% at the end. 4. The percentage yield was not affected by the number of eggs transferred. 5. The implantation of transferred eggs was found to inhibit the implantation of native eggs in the same horn, but not in the opposite horn. 6. Embryonic mortality in the injected horn was approximately doubled by the transfer operation, but was unaffected by the number of eggs transferred. 7. These findings are discussed and compared with results reported in an earlier paper (McLaren & Michie, 1956).