The amniotic embryo. The extraembryonic mesenchyma constitutes the walls of the amnion (A) and the yolk sac (Y). During gastrulation it could be considered that the extraembryonic mesenchyma internalizes with its functions: hydroelectrolitic control and neurogenesis in the amniotic side and angiogenesis and hematopoiesis in the yolk sac side. The internalization of the extraembryonic mesenchymal functions could integrate the amnion and the yolk sac original functions into the intraembryonic mesenchyma. Nonetheless, during the inflammatory response, the dedifferentiation process suffered by the tissues could re-express with clarity the primitive axes of the amnion and yolk sac.