Pig embryos after 33 days of development. At this stage the embryos measure about 16 millimetres in length. Embryo piglets (Sus scrofa domestica) are surrounded by several membranous sacs, including the amnion (closest to the embryo) and the chorion (furthest from the embryo), which has been removed from the uppermost piglet. 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 uterus wall to form a diffuse placenta for the exchange of nutrients and metabolic waste products between maternal and foetal blood. Gestation in pigs is 112 to 115 days. Developing ribs are seen.