The embryos of reptiles, birds, and mammals produce 4 extraembryonic membranes, the
(1)amnion
(2)yolk sac
(3)chorion, and
(4)allantois
In birds and most reptiles, the embryo with its extraembryonic membranes develops within a shelled egg.
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 amnio
