The program for in vitro fertilization at Bourn Hall began in October 1980. Various types of infertility have been treated during this time using the natural menstrual cycle or stimulation of follicular growth with antiestrogens and gonadotrophins. Follicular growth and maturation are assayed by urinary estrogens and LH, monitored regularly during the later follicular stage. Many patients had an endogenous LH surge; others needed an injection of HCG to induce ovulation. All oocytes were recovered by laparoscopy. Wide variations occurred in the time interval between the start of the LH surge and oocyte recovery and between oocyte recovery and insemination. Embryos taken between the one- and the eight-cell stage were replaced into their mother, no standard procedure being adopted for all patients. The results of all treatments including patient's responses during the follicular and luteal phases, oocyte recovery, fertilization, cleavage, replacement, implantation, abortion, and birth and the effect of factors such as replacing two or more embryos, maternal age, and previous obstetric history are described in detail. The incidence of implantation after embryo replacement improved from 16.5% initially to 30% currently. More than 118 babies have been born, and many pregnancies are continuing.

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In rodents, chimaeric blastocysts produced by combining embryonic cells of two different species have been used in investigations of cell lineage and interaction during development (Mus musculus-Rattus norvegicus, M. musculus-Clethrionomys glareolus, M. musculus-Mus caroli). However, interspecific chimaerism also offers new approaches to the study of reproductive incompatibilities between species and may even allow such incompatibilities to be neutralized, thus improving the chances of successful hybridization and interspecific embryo transplantation. We report here the production of sheep-goat chimaeras by embryo manipulation and the use of interspecific chimaerism to allow successful interspecific embryo transplantation in sheep and goats.

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