During the breeding seasons 1957-58 and 1958-59, a total of 280 fertilized eggs collected from Suffolk and Welsh Mountain ewes, mated to rams of their own breeds, was transferred at rates of either two or five eggs per animal to the Fallopian tubes or uterine horns of eighty Suffolk recipient ewes. Half of the recipients had been pretreated with injections of 1500 i.u. PMS in order to induce superovulation. The survival and development of transferred eggs, however, was found to be unaffected either by superovulation or by the breed of eggs transferred.
Overall prenatal mortality was divided into two periods (I) up to 17 or 18 days and referred to throughout as embryonic mortality or loss, and (2) 17 or 18 days to term, referred to as foetal mortality or loss.
Two peaks of embryonic mortality were observed, one occurring before and the other after attachment of embryos to the endometrium. Embryonic death accounted for almost all the prenatal mortality and the majority of the loss occurred before attachment. Foetal mortality occurring later than the 17th or 18th day of pregnancy was negligible.
The proportion of recipients becoming pregnant was the same in the groups which received two eggs and the group which received five eggs, but the ovum-survival rate following the transfer of five eggs was significantly lower than that following the transfer of two eggs. However, those ewes which received five eggs and subsequently became pregnant had significantly more lambs at term and normal embryos at autopsy than those which received only two eggs.
Of all the eggs transferred to the uteri and tubes, 49% and 29%, respectively, developed into normal embryos or lambs. The smaller survival rate following tubal transfers was due to a higher rate of embryonic mortality.