Amniotic egg

Amniotic egg refers to the type of egg produced by amniotes, a group of tetrapods (four-legged animals) that includes reptiles, birds, and mammals. The amniotic egg is characterized by its structure, which includes several layers not present in the eggs of other animals. These layers provide protection and nourishment to the developing embryo.

Structure

The amniotic egg is composed of several layers. The outermost layer is the shell, which provides physical protection to the embryo. The shell is permeable, allowing for the exchange of gases between the embryo and the outside environment.

Inside the shell is the amnion, a membrane that surrounds the embryo and creates a fluid-filled cavity. This amniotic fluid acts as a shock absorber, protecting the embryo from physical harm.

The yolk sac provides nourishment to the embryo, while the allantois serves as a repository for waste products. The chorion surrounds all of these structures and facilitates gas exchange between the embryo and the outside environment.

Evolution

The evolution of the amniotic egg was a key factor in the success of amniotes in colonizing terrestrial environments. The amniotic egg allows for the complete development of the embryo within a protected environment, reducing the need for a water-based life cycle stage. This adaptation allowed amniotes to become fully terrestrial, unlike their amphibian ancestors.

Amniotes

Amniotes are a group of tetrapods that includes reptiles, birds, and mammals. The name "amniote" comes from the presence of the amnion in their eggs. While mammals do not lay eggs, they are included in this group because their embryos develop an amnion during gestation.

See also

• Egg (biology)
• Reptile
• Bird
• Mammal