ABSTRACT
Many exciting opportunities have been created by the development of techniques associated with embryo transfer, for example, storage for prolonged periods by freezing, the production of identical offspring, and "chimaeras". Embryo transfer has been employed commercially in sheep and pigs as well as cattle, but the majority of the work has been with cattle because of the greater value of each animal. Embryo transfer and its ancillary techniques have already contributed to animal breeding and more particularly to research, and their impact will undoubtedly increase. In animal breeding, the simpler forms of cloning do offer the possibility of a few extra young from selected donors, and it may be that methods of incorporating nuclei into cells with the aid of viruses will be developed to simplify the procedure. In rabbits, sheep, and cattle, pregnancy can be maintained in animals ovariectomized soon after mating by treatment with progesterone alone.
