The estrous cycles of 34 Dorset Horn ewes were observed during the spring and summer in East Anglia. The majority of sheep (22 out of 34) came in heat fairly regularly. In 26 out of 37 estrous cycles the duration was between 14 and 25 days. Laparotomy performed soon after estrus did not prolong the subsequent estrous cycle in the majority of animals. Failure of ovulation following estrus was not observed; the observations were made either at laparotomy or at autopsy. The rate of ovulation was 1.55 per cycle as assessed by counting the number of corporalutea in the ovary. The time of ovulation was estimated at about 30 to 40 hours after the onset of estrus.
Following deposition of semen into the Fallopian tubes of nine ewes, either before or soon after ovulation, five eggs were recovered but none of these was found to be fertilized. Following intra-cervical insemination or mating 21 fertilized eggs, at different stages of division, were found in 11 out of 29 ewes. In 13 ewes out of 26 which were examined 5–17 days after mating 15 normal embryos were found. Photomicrographs of early sheep embryos are presented and the low fertility of sheep in summer is discussed.