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

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 1/2 to 6 1/2 days. Of 78 embryos transferred, 69 (88.5%) were recovereod, 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.