The mammalian yolk sac placenta

Abstract

There are three fetal membranes in mammals, namely the yolk sac, allantois and amnion, but only the first two form a placenta. In Monotremata (monotremes), Marsupialia (marsupials) and Placentalia (eutherians), the yolk sac transfers nutrients originating from uterine glandular secretion or the maternal blood. Ontogenetically, the yolk sac of most eutherians loses contact with the peripheral chorion and forms a free splanchnopleuric yolk sac that transfers substances from the exocoelomic cavity, not directly from the endometrium as in marsupials and rodents. This free yolk sac has been preserved in humans and substances originating from glandular secretions are transferred from the exocoelom..