Extraembryonic Membranes

the membranous structures that in some invertebrate animals, sharks, all higher vertebrates, and man provide for the life activities of the embryo and ensure its protection from injury.

As distinguished from the egg membranes, extraembryonic membranes are formed not during the maturation of the ovum but during embryonic development from the cells of the embryo. Among the extraembryonic membranes are the amnion, the chorion, and the allantois. The amnion is formed from the lateral folds of the extraembryonic ectoderm and the external layer of the lateral plates, which rise and close up over the embryo, or by means of the formation of a cavity among the embryo cells that is gradually transformed into the membrane that surrounds the embryo. The amnion is filled with fluid and protects the embryo from desiccation; it shields the embryo from contact with other membranes (sometimes very hard, such as the eggshell) and from mechanical injury.

The outer wall of the amniotic folds forms the chorion. In mammals and man the chorion is in direct contact with the wall of the uterus. In reptiles and birds this extraembryonic membrane is usually called the serosa. In mammals and man the chorion provides for the exchange of substances between the body of the mother and the fetus; it forms during the early stage of development, when the embryo is still a small vesicle (the so-called blastocyst) consisting of an aggregation of small cells, the so-called embryoblast, surrounded by a layer of larger cells, the trophoblast. The chorion arises from the cells of the trophoblast, which are then lined with the cells of the extraembryonic mesoderm. It has characteristic villi that take root in the tissue of the uterus and form the fetal part of the placenta. The villi are supplied with vessels that form in connection with the development of the yolk sac and the allantois. The allantois begins as an outgrowth of the posterior section of the intestinal tube of the embryo. In reptiles and birds it resembles a pouch in shape. It grows intensively and covers the amnion and yolk sac. Supplied with a large number of blood vessels, the allantois serves as the embryonic organ of respiration. The metabolic products of the embryo, principally urine, collect in the allantoic cavity. In mammals and man the allantois is small; the vessels of the umbilical cord, which connect the embryo with the placenta, are formed in its mesenchyme. In the later stages of development the urinary bladder (in mammals and man) and the cloaca (in birds and reptiles) form from the intraembryonic portion of the allantois.