Fertilized sheep and cattle ova have not been reported to develop readily during culture in vitro. Up to 60% of sheep morulae develop normally during culture (Moor & Cragle, 1971) but earlier cleavage stages undergo limited development (Hancock, 1963; Kraemer, 1966; Tervit & McDonald, 1969; Moore, 1970) and it has been suggested that there is a block to development in vitro at the eightto twelve-cell stage (Wintenberger, Dauzier & Thibault, 1953). Only the early cleavage stages of cattle ova have been cultured and these have not been reported to develop beyond the twenty-four-cell stage in vitro (Thibault, 1966; Brinster, 1968; Sreenan, 1968; Sreenan, Scanlon & Gordon, 1968). This communication describes the successful culture of one-cell to eight-cell sheep ova and one-cell and eight-cell cattle ova to the morula and blastocyst stages and reports a high embryo survival after transfer of cultured ova to recipient animals.

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Following the daily intra-uterine infusion of a homogenate prepared from frozen and thawed tissue of 14-or 15-day sheep embryos the length of the oestrous cycle was significantly prolonged. A single intra-uterine infusion, however, had only a slight effect on oestrous cycle length.The substance derived from the embryonic tissue, which presumably must be of a chemical nature, probably acts on the endometrium in aǹ anti-luteolytic' manner, that is by counteracting the action of the endometrium which otherwise would have resulted in the regression of the corpus luteum.Unlike homogenate prepared from 14-or 15-day sheep embryos similar material obtained from 25-day sheep embryos did not increase cycle length. This indicates that the effect is dependent on the stage of development of the embryo in relation to the uterus.Homogenate prepared from 14-day pig embryos infused into sheep uteri was unable to maintain the corpus luteum, which suggests that the embryonic factor may be species-specific. Repeated intra-uterine infusions of control material consisting of frozen and thawed saline, serum or serum plus white blood cells had no effect on cycle length. Transfer into the sheep uterus of a whole but heat-inactivated 12\p=m-\14-days sheep embryo had no effect on the cycle length.

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