The yolk sac contains yolk - the sole source of food until hatching. Yolk is a mixture of proteins and lipoproteins. 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. 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.
Monotremes These primitive mammals produce a shelled egg like their reptilian ancestors. 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.