Oviparity

Oviparity is a mode of reproduction in which embryos develop inside eggs that are laid outside the parent's body. In most oviparous species, fertilization is internal before the egg is deposited, and the developing embryo relies on yolk and other nutrients contained in the egg as it grows until hatching.

Eggs vary in structure. Amniotic eggs with hard or leathery shells are common in birds and many reptiles, while eggs laid by most fishes and amphibians have little or no calcium shell and are surrounded by Membranes or gelatinous coverings. Nutrients are usually supplied by yolk, though some species also provide extraembryonic nutrition through specialized tissues.

Distribution includes birds, most reptiles, amphibians, and many fishes. Among mammals, monotremes such as the platypus and echidnas lay eggs. Oviparity is also widespread in invertebrates such as certain insects, mollusks, and crustaceans, as well as in some fish groups.

Incubation can occur outside the body, often in nests or warm environments, and parental care ranges from none to substantial guarding or brooding. The timing of hatching varies widely by species.

Evolutionarily, oviparity has evolved independently in many lineages and can be followed by shifts to viviparity (live birth) or to ovoviviparity, where eggs hatch internally. The term contrasts with viviparity and ovoviviparity.