In most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum.

It nourishes and protects the embryo.

Oviparous animals are animals that lay eggs, with little or no other development within the mother.

This is the reproductive method of many fish, amphibians and reptiles, all birds, the monotremes, and most insects and arachnids.

Reptile eggs, bird eggs, and monotreme eggs, which are laid out of water, are surrounded by a protective shell, either flexible or inflexible.

The 1.5 kg ostrich egg contains the largest existing single cell currently known, though the extinct Aepyornis and some dinosaurs had larger eggs.

The bee hummingbird produces the smallest known bird egg, which weighs half a gram.

The eggs laid by some reptiles and most fish are even smaller, and those of insects and other invertebrates are much smaller still.

The study or collecting of eggs, particularly bird eggs, is called oology.