Structure of the Amniotic Egg

Evolution of eggs with a water-impermeable amniotic membrane surrounding a fluid-filled amniotic cavity permits embryonic development on land without danger of dessication. The main diagram shows the arrangement of membranes in a typical egg-laying oviparous vertebrates. In live-bearing viviparous vertebrates, the shell and chorionic membrane are absent and the allantoic and yolk sac membranes are incorporated into the connection with the maternal circulation.