Pig embryo after 33 days of development. At this stage the embryo measures about 16 millimetres in length. The embryo piglet (Sus scrofa domestica) is surrounded by several membranous sacs, including the amnion (closest to the embryo) and the chorion (furthest from the embryo). The amnion is filled with fluid, which cushions and protects the developing embryo. In pigs, the chorion is covered with microscopic folds, which connect with the wall of the uterus to form a diffuse placenta for the exchange of nutrients and metabolic waste products between maternal and foetal blood. Length of gestation in pigs is 112 to 115 days. Developing ribs are seen.