|An egg is the organic vessel containing the zygote, resulting from fertilization of the ovum, in which an animal embryo develops until it can survive on its own, at which point the animal hatches. Most arthropods, vertebrates, and mollusks lay eggs, although some do not, e.g. scorpions and most mammals.
Oviparous animals are those that lay eggs, with little or no other development within the mother. The study or collecting of eggs, particularly bird eggs, is called oology.
The term "egg" is used differently outside the animal kingdom. Reproductive structures similar to the egg in other kingdoms are termed "spores," or in spermatophytes "seeds," or in gametophytes "egg cells".