15.1 Extraembryonic Membranes

15.1 Extraembryonic Membranes

Why amniotes build four membranes

An embryo must obtain oxygen and food, store waste, and stay moist while its organs form. A fish embryo and a frog embryo can meet those needs in water. Dissolved oxygen, water, and wastes move between the embryo and the pond, and a jelly coat protects many amphibian eggs. Reptiles, birds, and mammals develop on land, away from that open water. They are amniotes. Each amniote embryo is surrounded by four extraembryonic membranes: the amnion, the chorion, the yolk sac, and the allantois. The membranes grow from the conceptus, yet they are not organs that remain as part of the adult body. Fish and amphibians are not amniotes. They do not assemble this four-membrane set around a shelled land egg.

What each membrane does

Amnion

The amnion forms a sac and fills it with amniotic fluid. The fluid cushions the embryo, gives it room to move, and keeps the body surface from drying or sticking to the surrounding membrane. That protective job is the same in a chicken egg and in a human pregnancy. Gas exchange is a different job. The amnion is not the surface that trades oxygen and carbon dioxide with the outside air, and it does not become the placenta. If a question assigns breathing to the amnion, it has named the cushion instead of the outer membrane.

Chorion

The chorion is the outer membrane. In a bird or a reptile it lies just inside the shell and exchanges gases with the environment. Oxygen diffuses in from the air, and carbon dioxide diffuses out. The shell is porous enough to allow that traffic, and the chorion is the living sheet on the embryo's side of the shell. The chorion does not enclose the yolk, and it does not hold the cushioning fluid.

Mammals reuse the chorion for pregnancy. Chorionic villi grow from it into the uterine lining and become the fetal part of the placenta. The next section follows that organ. Here, keep the split clear: in a shelled egg the chorion is the outer gas-exchange membrane, and in a mammal it helps build the placenta.

A bird embryo also sends blood vessels of the allantois out beneath the chorion. Those vessels pick up the gases that cross the outer membrane and carry them to the embryo. The chorion remains the exchange surface. The vessels are the transport route.

Yolk sac

The yolk sac surrounds the yolk. Yolk is the lipid-rich and protein-rich food stored in a bird or reptile egg, and it is the embryo's main food source until hatching. Vessels on the yolk sac absorb those nutrients and deliver them to the embryo. The yolk sac is a food membrane. It is not a waste sac, and it is not a placenta. A bird egg never forms a placenta, so a choice that renames the yolk sac as a placenta has imported a mammalian organ into a shelled egg.

A human egg is tiny and contains almost no yolk, so the human yolk sac cannot feed the fetus through the whole pregnancy. It remains small. Its early contribution is blood: the first blood cells form in the yolk sac, before the liver and later the bone marrow take over blood formation. Nutrition then comes from the placenta. A stem that calls the human yolk sac the main food supply for the entire pregnancy has copied the bird-egg job onto the wrong animal.

Allantois

The allantois grows out from the hindgut. A shelled egg has no pond to carry waste away, so the allantois stores metabolic waste inside the shell. Birds and reptiles deposit much of their nitrogenous waste as uric acid, a paste that can remain in the allantois without washing the embryo in toxic ammonia. Vessels in the allantoic wall also carry gases between the embryo and the chorion.

Mammals do not need a large storage sac. The mother's blood removes carbon dioxide and urea. The mammalian allantois stays small, but its blood vessels help form the umbilical circulation in the cord that joins the fetus to the placenta. Waste leaves through the mother. The vessels stay in use.

Bird egg and mammal side by side

Reading the animal in the stem

Name the animal before you name the membrane. A bird, a reptile, a chicken egg, or a shelled egg calls for the bird-egg column. A human or another placental mammal calls for the changed jobs. A frog, a tadpole, or a fish calls for neither column. Those vertebrates are not amniotes, and they do not build this set. An aquatic egg can still be large and yolky, as many fish eggs are, without ever forming an amnion.

When two answers both mention exchange, separate the cargo. Food from yolk belongs to the yolk sac. Gases crossing the shell belong to the chorion, with allantoic vessels as the blood route. Stored waste belongs to the allantois. Cushioning belongs to the amnion. Mammals move the chorion toward the placenta, limit the yolk sac to early blood formation, and use allantoic vessels in the umbilical circulation. The amnion keeps cushioning the embryo.