Figure 32.19 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 chicken egg is shown here. (B) In most mammals, the developing embryo is retained inside the mother’s body, with which it exchanges nutrients and wastes via the placenta.