Egg (biology)

Structure and function

Eggs are designed to support development and to mediate exchange with the environment. Typical components include the nutritive yolk, protective membranes and coverings, and sometimes an albumen or analogous fluid. These parts perform different roles:

• Yolk: energy and building materials for the developing embryo.
• Membranes and fluids: physical separation, cushioning and chemical balance.
• Covers or shells: mechanical protection and, in some eggs, regulated gas exchange.

Gas exchange is essential; for example, porous shells permit exit of carbon dioxide and entry of oxygen, enabling aerobic metabolism of the embryo.

Variation among animal groups

Different animal lineages have evolved contrasting egg types and reproductive strategies. Many vertebrates, arthropods and molluscs lay eggs externally; the coverings range from hard mineral shells to soft gelatinous layers. For instance, fish and amphibians often produce numerous small, poorly protected eggs, while bird eggs are larger relative to embryo size and are well provisioned.

• Cleidoic eggs: Reptiles, birds and monotremes produce eggs with substantial food stores and protective shells that allow gas exchange—these are sometimes called cleidoic eggs.
• Arthropod eggs: Many insects and arachnids lay eggs with specialized outer layers adapted to their habitats, including desiccation resistance in dry environments (arthropods).

Developmental strategies

Species differ in how long embryos develop inside versus outside the parent. Oviparous animals lay eggs that develop and hatch externally. Some species exhibit ovoviviparity, where eggs develop inside the mother and hatch near or at birth. Others, including most mammals, evolved viviparity and support the embryo internally without an egg stage that is laid.

Uses, importance and examples

Biologically, eggs are central to reproduction and species continuity. They enable dispersal of young, permit developmental stages adapted to particular environments, and allow parents to vary the number and investment per offspring. Human cultures also exploit eggs (primarily bird eggs) as a nutrient-dense food source and study eggs as models in developmental biology and embryology.

Notable distinctions and terminology

When discussing eggs it helps to distinguish the cellular gamete (ovum) from the larger reproductive structure used in oviparous species. The nourishing structure that surrounds an embryo may be called an egg, a clutch element, or, in internal gestation systems, part of placental support. For further reading on fertilization and embryonic terms see links such as fertilization, ovum, and embryo, and taxon-specific articles on fish, arthropods and birds.

Related topics include egg physiology, the evolution of reproductive modes, and ecological trade-offs between egg number and parental investment. See resources tied to each group for more detail: fertilization, zygote, cleidoic eggs, and gas exchange concepts (CO2, O2).