According to evolution, amniotic eggs were the feature that allowed reptiles to live on land, similar to how seeds allowed plants to live on land. A fluid-filled sac (the amnion) in the egg is similar to an aquatic environment; this characteristic made the egg like a portable pond that helped reptiles move from the water to the land, according to evolution. Amniotes are the organisms that have amniotic eggs. This category including reptiles and mammals (birds are included in the reptile clade). Amphibian eggs do not have shells, dissimilar to most reptilian and mammalian eggs. Bird eggs, such as the eggs shown in the photo, are less flexible than the leathery and flexible reptile eggs.
The amniotic egg has four membranes (extraembryonic membranes): the amnion, allantois, yolk sac, and chorion. The amnion is what actually creates the fluid-filled cavity where the embryo develops. The chorion membrane protects the egg and is close to the allantois, a membrane that exchanges gases and stores metabolic wastes. The yolk sac has a plethora of nutrients and blood vessels to transport the nutrients from the sac to the embryo.
A P Biology 2013 - 2014 