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.Mammals fall into three groups that differ in the way they use the amniotic egg.
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 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.
Humans are placental mammals.
|Link to an illustrated discussion of pregnancy in humans.|