ABSTRACT
Mouse embryos obtained at the presomitic stage develop in vitro up to 70 h. During the first 24 h development remains comparable in vitro and in utero. Afterwards, the chronology of embryonic development is modified and in our successful prolonged experiments (70 h culture), foetal organogenesis reaches a stage corresponding only to 
days in utero. In some instances a chorioallantoic circulation has been observed in the umbilical cord.
The three trophoblast cell types present in egg-cylinders at 8 days post coitum, develop in the same way in utero or in vitro. After 48 h culture electron microscopy shows that numerous annulate lamellae appear in the nuclei and in the cytoplasm of some of the giant cells. The syncytium does not differentiate between the trophoblast and the allantoic mesoderm. The haemo-trichorial placental barrier does not differentiate in vitro, although foetal capillaries form.
In vitro, the endoderm cells (from the splanchnopleure or from the omphalopleure) actively proliferate and form an homogeneous substance located between the giant and the polyhedric trophoblast cells.
