The amnion is a membranous sac which surrounds and protects the embryo. It is the first of the three cavities (amnion, chorion and yolk sac) in the embryo and is formed on 8 dpc. The amniotic cavity is roofed in by a single stratum of flattened, ectodermal cells, the amniotic ectoderm, and its floor consists of the ectoderm of the embryonic disc. A thin layer of mesoderm, continuous with that of the somatopleure, is located just outside the amniotic ectoderm, and is connected to the mesodermal lining of the chorion by the body-stalk.
When first formed, the amnion is in direct contact with the body of the embryo, but about the fourth or fifth week, amnionic (amniotic) fluid (liquor amnii) begins to accumulate within it. As the volume of the fluid increases, the amnion expands and ultimately adheres to the inner surface of the chorion. Increasing liquor amnii quantities allow free movements of the fetus during the later stages of pregnancy, and also protect it by diminishing the risk of injury.
The amnion forms as a layer of epiblast cells expands towards the embryonic pole and differentiates into a thin membrane constitutes the ectodermal inner lining of the amnion (appears on day 8 of human development).
At the point of constriction where the primitive digestive tube of the embryo joins the yolk-sac, the amniotic fold is formed from the upward folding of the somatopleure (a combination of ectoderm and mesoderm). The somatopleure is formed in both the amnion and chorion, where the ectodermal tissue supplies functioning epithelial cells and the mesoderm generates the essential blood supply to and from this epithelium. The fold tips eventually meet and fuse, forming the amniotic cavity. After fusion, the two layers of the fold become completely separated, where the inner layer forms the amnion, the outer forms the false amnion or serosa. The space between the amnion and the serosa constitutes the extraembryonic coelom.