The blastomeres of two in vitro derived four-cell embryos were separated and transferred individually into surrogate zonae pellucidae, then co-cultured with bovine oviductal epithelial cells for five days until blastulation. Pairs of the quarter blastocysts were co-transferred with trophoblastic vesicles into each of four synchronised Holstein heifers, three of which were diagnosed pregnant at 28 days gestation, carrying twin fetuses. Four genetically identical bull calves were delivered by elective caesarean section at term pregnancy. One pregnancy was terminated at 56 days.
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