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.
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".
The rest of this article is mainly devoted to the discussion of vertebrate eggs.
Reptile eggs, bird eggs, and monotreme eggs, are laid out of water, and are surrounded by a protective shell, either flexible or inflexible. Eggs laid on land or in nests are usually kept within a favourable temperature range (warm) while the embryo grows. When the embryo is adequately developed it hatches, i.e. breaks out of the egg's shell. Baby animals which have just hatched are hatchlings, though standard names for babies of particular species continue to apply, such as chick for a baby chicken. Some embryos have a temporary egg tooth with which to crack, pip, or break the eggshell or covering.
The largest recorded egg is from a whale shark, and was in size; whale shark eggs normally hatch within the mother. At and up to , the ostrich egg is the largest egg of any living bird, though the extinct elephant bird and some dinosaurs laid larger eggs. The bee hummingbird produces the smallest known bird egg, which weighs half of a gram (around 0.2 oz). The eggs laid by some reptiles and most fish can be even smaller, and those of insects and other invertebrates can be much smaller still. [...]