May 22, 2022

An egg is an organic container containing a zygote in which the embryo develops until it can survive on its own, where the animal hatches. Eggs are produced by fertilization of the egg. Most arthropods, vertebrates (excluding currently living mammals), and molluscs lay eggs, although some, such as scorpions, do not. Reptile eggs, bird eggs, and monotreme eggs are laid out of water and surrounded by a protective shell, both flexible and inflexible. Eggs laid on land or in nests are usually kept in a warm and favorable temperature range as the embryo grows. When the embryo is sufficiently developed, it hatches, that is, comes out of the eggshell. Some embryos have temporary egg teeth which they use to crack, break, or peel off the shell or covering of the egg. The largest recorded egg came from a whale shark and measured 30 cm × 14 cm × 9 cm (11.8 inches × 5.5 inches × 3.5 inches). Whale shark eggs usually hatch inside the mother. Weighing 1.5 kg (3.3 lb) and up to 17.8 cm × 14 cm (7.0 inches × 5.5 inches), ostrich eggs are the largest eggs of all living birds, although elephant birds extinct and some non-avian dinosaurs had larger eggs. The bee hummingbird lays the smallest known bird egg, which weighs half a gram (about 0.02 oz). Some of the eggs laid by reptiles and most fish, amphibians, insects, and other invertebrates can be even smaller. Reproductive structures similar to eggs in other kingdoms are called "spores", or in spermatophytes "seeds", or in gametophytes "egg cells".