Fetal membranes include the yolk sac, amnion, chorion, and allantoids. The role of these membranes is different among different species. In mammalians, extra-embryonic membranes contribute to form the placenta and the umbilical cord.

The most primitive extra-embryonic membrane is the yolk sac. It is composed of splanchnic lateral plate mesoderm and endoderm. On day after fertilization, a thin layer of hypoblast cells, known as Heuser's membrane, migrates out over the inner, cellular layer of the trophoblast. This temporary yolk sac is later replaced by another layer of hypoblast cells that becomes the definitive yolk sac. The yolk sac plays an active and crucial role during organogenesis, and is the source of the embryo's first blood cells and the primordial germ cells.

The amnion is the non-vascular and innermost membrane surrounding the embryo. It develops next in respect to the yolk sac, by...