What characteristic defines an amniotic egg?

An amniotic egg is defined primarily by its structure, which includes a protective shell made of hard or leathery materials. This protective feature is crucial as it allows the embryo to develop in a terrestrial environment, preventing desiccation and offering a stable environment for development. In contrast to eggs that require a watery environment for protection and development, such as those of amphibians, the amniotic egg is designed to be self-contained, minimizing the risks associated with the external environment.

The amniotic egg contains specialized membranes (the amnion, allantois, and chorion) that help in nutrient transfer, waste management, and respiration, further enhancing the survival of the embryo as it develops. This adaptation is particularly significant for reptiles, birds, and mammals, allowing them to reproduce away from aquatic environments.