For almost a century micromanipulation of embryos has been used in invertebrates and lower vertebrates to elucidate patterns of early development, and it is largely through such investigations that cellular interactions and the role played by individual blastomeres have been established. The potential value of a similar approach in the study of early development in mammals has long been recognized, but it was only after methods of embryo collection, handling, and transfer had reached a high level of efficiency that this became technically feasible.