amniotic egg
A shelled, water-retaining egg that enables reptiles, birds, and egg-laying mammals to complete their life cycles on dry land; developed during the Mesozoic
The amniotic egg is a type of egg that is formed by reptiles, birds, and some species of mammals, such as monotremes. It is named after the amnion, a membrane that surrounds the developing embryo and protects it from drying out or physical shocks. The amniotic egg has several structural features that make it different from other types of eggs:
1. The eggshell: The amniotic egg has a hard, calcified shell that protects the embryo from physical damage and predators. The shell is made of calcium carbonate and is porous, allowing the exchange of gases between the inside and outside of the egg.
2. The amnion: The amnion is a fluid-filled sac that surrounds the developing embryo and provides a cushion against physical shocks. It also helps regulate the temperature and hydration of the embryo.
3. The yolk sac: The yolk sac is a membranous sac that contains the yolk, which provides the embryo with nutrients for its growth and development.
4. The allantois: The allantois is a membranous sac that collects waste materials from the embryo and stores them until they are excreted.
The amniotic egg is an important adaptation that allowed reptiles, birds, and mammals to colonize terrestrial environments. It protected the developing embryos from desiccation and allowed them to develop without relying on an external water source.
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