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The viability of early cleavage stages containing half the normal number of blastomeres in the sheep

Summary. The developmental capacity of embryos produced by injection of single blastomeres of 2-cell embryos, pairs of blastomeres of 4-cell embryos or 4 blastomeres of 8-cell embryos into foreign zonae pellucidae was studied. The micromanipulated embryos were embedded in tiny agar cylinders, transferred to ligated oviducts of ewes on Days 2–8 of their oestrous cycle and recovered when the total age of the embryos was 5–6 days. Of 78 embryos transferred, 69 (88·5%) were recovered, and of the latter 65 (94·2%) had developed at a normal rate. Thirty of these 'half' embryos were transferred to ewes on Day 6 of the oestrous cycle: 24 (80%) developed into lambs. There were no differences apparent between embryos derived from the 2-, 4- or 8-cell embryos with respect to developmental capacity. A further 12 'half' embryos were stored by deep-freezing for 1 or 2 months. After thawing, 9 of these were selected for transfer to ewes on Day 6: 3 developed into lambs, each of which was the monozygotic twin of a lamb resulting from the first series of transfers.

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