An egg is a specialized cell produced by females in many animals, including birds, reptiles, amphibians, fish, and some invertebrates. In biology, it is called an ovum or oocyte.

The egg has several important parts. The outer covering (shell in birds) protects the inside from physical damage and dehydration. Beneath the shell are protective membranes that help block bacteria and control gas exchange.

Inside the egg, there is the yolk, which is rich in nutrients such as lipids and proteins. The yolk provides energy for the developing embryo. Surrounding it is the egg white (albumen), which mainly contains water and proteins that protect the embryo and provide additional nutrients.

At the center is the germinal disc, where fertilization occurs and where the embryo begins to develop. If sperm successfully fertilizes the egg, cell division starts and the embryo gradually forms.

In birds, the egg is laid outside the body and development continues externally. In mammals, however, the egg develops inside the body (except in egg-laying mammals like the platypus).

Eggs are also important in reproduction biology because they carry half of the genetic material needed to form a new individual.

In summary, the egg is a complete biological system designed to protect, nourish, and support early life development.