Amniotic eggs are the specialized eggs of tetrapod vertebrates the reptiles, birds, and mammals that allowed them to reproduce on dry land without the need for water (like fishes and amphibians). There are four sacs of amniotic egg: chorion, amnion, allantois, yolk sac.
Chorion is the outermost sac. In reptiles and birds, it is located under the tough outer shell of the egg, while in mammals it fuses with the lining of the mother's uterus to form the placenta. Its function is the transport of the oxygen and carbon dioxide to and from the embryo. In mammals, chorion forms the chorionic villi that invade the endometrium, and allow the transfer of nutrients from the maternal blood to fetal blood.
Amnion is the second layer, under the chorion and they together form the amniotic sac. Its function is to keep the embryo from drying out. Amnion can be defined as a membrane that closely covers the embryo and it fills with the amniotic fluid.