Extraembryonic Membranes

• amnion
• yolk sac
• chorion, and
• allantois

• The amnion protects the embryo in a sac filled with amniotic fluid.
• The yolk sac contains yolk — the sole source of food until hatching. Yolk is a mixture of proteins and lipoproteins.
• The chorion lines the inner surface of the shell (which is permeable to gases) and participates in the exchange of O₂ and CO₂ between the embryo and the outside air.
• The allantois stores metabolic wastes (chiefly uric acid) of the embryo and, as it grows larger, also participates in gas exchange.

With these four membranes, the developing embryo is able to carry on essential metabolism while sealed within the egg. Surrounded by amniotic fluid, the embryo is kept as moist as a fish embryo in a pond.

Although (most) mammals do not make a shelled egg, they do also enclose their embryo in an amnion. For this reason, the reptiles, birds, and mammals are collectively referred to as the amniota.

• Monotremes
  

  These primitive mammals produce a shelled egg like their reptilian ancestors. Only four species exist today: three species of spiny anteater (echidna) and the duckbill platypus. [More]

• Marsupials
  

  Marsupials do not produce a shelled egg. The egg, which is poorly supplied with yolk, is retained for a time within the reproductive tract of the mother. The embryo penetrates the wall of the uterus. The yolk sac provides a rudimentary connection to the mother's blood supply from which it receives food, oxygen, and other essentials. However, this interface between the tissues of the uterus and the extraembryonic membranes never becomes elaborately developed, and the young are born in a very immature state.

  The photo (courtesy of Dr. Carl G. Hartman) shows 18 newborn baby opossums fitting easily into a teaspoon.

  Despite their tiny size, they are able to crawl into a pouch on the mother's abdomen, attach themselves to nipples, and drink milk from her mammary glands.

  Marsupials are still abundant in Australia, but only the opossum is found in North America.

• Placental mammals
  In placental mammals, the extraembryonic membranes form a placenta and umbilical cord, which connect the embryo to the mother's uterus in a more elaborate and efficient way. The blood supply of the developing fetus is continuous with that of the placenta. The placenta extracts food and oxygen from the uterus. Carbon dioxide and other wastes (e.g., urea) are transferred to the mother for disposal by her excretory organs.

  Humans are placental mammals.

  Link to an illustrated discussion of pregnancy in humans.