The evolution of the watertight amniotic egg with its series of membranes allowed reptiles to replace amphibians and become the dominant terrestrial vertebrate. Starting with the outermost membrane, the lies just beneath the porous shell of the egg. The encases the developing embryo within a fluid filled cavity. The provides food for the embryo via blood vessels connecting to the embryo's gut. The surrounds a cavity into which waste products from the embryo are excreted.