The time of intrauterine migration and spacing of embryos was determined in 36 gilts with one oviduct occluded, and in 48 gilts with one ovary. Embryos could enter the uterus from one oviduct only. Uteri of seven pregnant gilts were examined on each of days 6, 7, 8, 9, 10, 11, 12, 13, snd 15 of gestation. Migration of embryos from one horn to the other usually occurred first on day 8 or 9. The uterus was occupied completely by day 15. Rate of migration and dlstribution of embryos was not affected by number of embryos, number of corpora lutea, or by uterine length. Uterinc length did not change betwccn days 6 and 15. Bilateral ovariectomy after breeding was associated with arrested growth of embryos, a decreased rate of embryo migration and a reduction in uterine length.Intrauterine migration of embryos is known to occur in many mammals (Boyd, Hamilton and Hammond, '44; Hafez, '64) and has been regarded as a means of equalizing the distribution of embryos in cases of disparity in the number of fertilized eggs from each ovary. Internal transuterine migration of embryos was first reported in the pig by Corner ('21a) and was confirmed by Warwick ('26), Kelly ('28), and Dxiuk, Polge, and Rowson ('64). Preliminary reports (Dhindsa and Dziuk, '65; Waite and Day, '65) have given some evidence of the stage of gestation at which migration occurs. A more detailed and definitive study would be useful in understanding the mechanisms of migration and distribution of embryos. The following study was conducted to determine the time of migration of embryos in the pig and the effect of varying their numbers on rate of migration and distribution.
MATERIAIS AND METHODSNinety-one gilts of' Yorkshire, Duroc, or Yorkshire X Duroc breeding were used for the study. Gilts were tested for estrus daily in the presence of a boar, and were bred on the first day of estrus either by natural mating or by artificial insemination. The day after breeding was designated as day one. Gilts were identified individually by tattoo and were assigned arbitrarily to treatments on the day of breeding.Uterine surgery was done aseptically via midventral laparotomy, using general AXAT. REC , 159 323-330.