Amniotic egg

An egg whose membranes enclose an embryo in a protected, fluid-filled environment. It enabled vertebrates to reproduce on land without returning to water to lay eggs.

Amnion: A membrane that surrounds the embryo in fluid within an amniotic egg. It creates the embryo’s fluid-filled protective compartment.
Gas exchange: The movement of oxygen and carbon dioxide between an organism and its surroundings. An embryo must obtain oxygen and release carbon dioxide through the egg’s coverings.
Amniota: The vertebrate clade comprising mammals and reptiles, including birds, and their extinct descendants. Amniotes share the developmental membranes associated with the amniotic egg.
Terrestrialization: The evolutionary transition of organisms from aquatic life to life on land. The amniotic egg helped vertebrates reproduce in terrestrial environments.
Chorion: The outermost extraembryonic membrane, involved in gas exchange in amniotes. It helps exchange gases across the egg’s shell.
Water balance: The regulation of water gains and losses in an organism or biological system. The shell reduces dehydration while retaining enough moisture for development.
Carboniferous: A geological period from about 359 to 299 million years ago, marked by extensive coal-forming forests. The earliest known amniotes lived during this period.
Reptile: An amniote lineage that includes turtles, lizards, snakes, crocodilians, and birds. Reptiles reproduce using amniotic development, whether eggs are laid or retained.
Allantois: An extraembryonic membrane that stores nitrogenous waste and supports gas exchange in many amniotes. It provides waste storage and joins the chorion in gas exchange.
Embryonic development: The process by which an embryo grows and differentiates from a fertilized egg. The membranes sustain development outside the aquatic environment used by many embryos.

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