The development of mouse embryos in culture from the implantation to the head-fold stage was sequentially examined. Our goal was to compare the morphology of embryos grown in vitro to those developed in vivo, published in standard texts, and to delineate the stages involved in the process of tissue differentiation and organization. Mouse blastocysts (stage 6) were collected at 3.5 days p.c. and cultured. Attachment of the blastocysts occurred on the second day of culture (stage 8). Following the collapse of the blastocyst endoderm cells began to migrate and to encircle the inner cell mass. At 2 days in culture the embryonic and extra-embryonic ectoderm became distinguishable and the proamniotic cavity appeared (stage 9). Egg cylinders began to project above the substrate at 2.5 days in culture (stage 10) and to progress through the stages observed in vivo. At 4 days a posterior amniotic fold began to form (stage 11) and was followed at 5 days by the formation of the chorion, the appearance of mesoderm, exocoelom, and head fold (stage 12). At 6 days in culture the embryo had differentiated longitudinally and developed an allantois, blood islands, Reichert's membrane, head process, and primitive streak. At 7 days somites as well as the neural fold and heart were observed (stage 14) and were followed by further differentiation at 8 days (stage 15). These observations indicate that apparently normal embryo development can be maintained in vitro through the early stages of organogenesis, thus providing a unique opportunity for investigating the regulation of early mammalian development.