Mouse embryos were grown successfully in vitro from the blastocyst stage to the limb bud stage. Mouse blastocysts grown in vitro for 10 days showed blood circulation in the yilk sac, forelimb buds, and the primordia of liver, pancreas, and lungs. These characteristics are indicative of a developmental stage equivalent to one-half of the total gestation period in utero. Improvements in culture conditions from days 7 to 9 have made it feasible to culture mouse blastocysts beyond the early somite stage.