Surgical procedures are described which permit direct experimental access to postimplantation mouse embryos. These procedures arose from our finding that development proceeds normally to term after embryos have been released from the confines of the uterus, while remaining attached to it via the placenta. Embryos continue to develop exo utero, within the abdominal cavity of the mother, and are capable of surviving a variety of different surgical manipulations. This finding opens the way for experimental analyses of mouse development in vivo.
