Figure 23.47: The Amniote Egg (A) The evolution of the amniote egg, with its water-retaining shell, four extraembryonic membranes, and embryo-nourishing yolk, was a major step in adaptation to the terrestrial environment. A chick egg is shown here. (B) In mammals, the developing embryo is retained inside the mother’s body, with which it exchanges nutrients and wastes via the placenta. Note the correspondence between the various membranes in (A) and (B).