The hormonal regulation of embryo development during early pregnancy in the ewe has been examined. Ovariectomized ewes received injections of oestradiol (E2) and progesterone (P) according to schedules designed to simulate endogenous ovarian secretion during the luteal phase of the previous oestrous cycle (priming P), around the time of oestrus (oestrous E2 ) and during early pregnancy (maintenance P, maintenance E2)' Embryos were transferred to the ewes on the 4th day after induced oestrus, and ewes were killed at 6 or 13 days after transfer to assess embryo development. Cytosol concentrations of oestradiol 'and progesterone receptors and RNA and protein metabolism in the endometrium and amounts of protein in uterine flushings were examined on the day of embryo transfer and 6 days after transfer.Twenty one of 27 ewes which received maintenance P, oestrous E2 and priming P carried normal embryos at the time of killing. Omitting maintenance E2 had no effect on the proportion of ewes in which embryos developed normally. When either priming P or oestrous E2 was omitted embryos ceased to develop normally within 1-2 days of the time of transfer. Omitting oestrous E2 reduced the amount of protein in the uterine lumen and the RNA: DNA ratio, rate of synthesis of protein and amounts of oestradiol and progesterone receptors in the endometrium at the time of embryo transfer. Omitting priming P caused a small decrease in the concentration of progesterone receptor but no other significant changes in the endometrium or uterine flushings at the time of transfer. It is suggested that oestrous E2 controls embryo development by regulating endometrial sensitivity to the progesterone of pregnancy. The mechanism by which priming P affects embryo development remains unknown.

Continued development in vitro of recently fertilized ova has been recorded in a number oflaboratory species and Brinster (1965a, 1965b, 1965c, 1965d) has achieved a sufficient degree of success with mouse ova to allow critical studies of the nutritional requirements during development from two cells to blastocysts. Fertilized sheep ova have been successfully stored for short periods between collection and transfer to recipient ewes but there are no reports of continued development of sheep ova during longer periods of in vitro culture.

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In an attempt to produce identical offspring artificially, the inner cell mass of ova, collected on days 4, 6 and 7 after oestrus (day 0 = day of oestrus), was mechanically divided into two equal sections.The sections were then either cultured in vitro for 2 days and the resulting apparently normal blastocysts transferred to recipient ewes or, in the case of day-7 ova, transferred to recipient ewes without a period of in vitro culture.Half sections of day-4 ova showed little further development in culture and in most the individual cells separated and dispersed. Those of day 6 and 7 showed no dispersion and 24 apparently normal blastocysts were recovered following the culture of 82 half sections. Fourteen of the normal blastocysts developed from each of both half sections of seven day-6 ova.Nineteen normal blastocysts were transferred individually to recipients following culture, but only two developed into lambs and both lambs developed from blastocysts of different ova. A further 18 half sections of day-7 ova were transferred without an intervening period of culture but none developed into lambs.Although a substantial proportion of the half sections of day-6 and day-7 ova showed apparently normal development in culture, very few showed continued normal development in recipients. It is suggested that cellular specialization within the inner cell mass may have occurred before the inner cell mass was divided.

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