Development of a placental blood circulation in rat embryos in vitro.

Rat embryos explanted with their membranes at head-fold stage (9 1/2 days gestation) formed an allantoic placenta which enlarged in culture and developed a foetal blood circulation. Embryos explanted at early somite stages (10 1/2 days) also formed a growing allantoic placenta but only after removal of most of the ectoplacental trophoblast. Assays of total protein in the embryo and placenta suggested that, in the absence of a maternal blood circulation to the placenta, embryo and placenta compete for the respiratory and nutritional resources obtained through the yolk-sac.