Schematic representation of the early mammalian embryo during gastrulation. The extraembryonic mesoderm (EM) is represented surrounding the amniotic cavity (A) and the yolk sac cavity (Y). Between the epiblast (E) and the hypoblast (H) the mesoderm internalizes (IM) by epithelial-mesenchymal transition. On top of the figure, some of the characteristics of the amniotic axis are summarized. On the other hand, on the bottom, some of the characteristics of the yolk sac axis are exposed.