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- For the female reproductive cell, see Ovum.
Some animals, mostly mammals, keep the zygote inside their body where the embryo grows until it is ready to be born. Other animals (tetrapods) lay a protective egg outside the mother. The egg nourishes and protects the embryo. When the baby animal is ready, it hatches; that means it comes out of the egg.
Reptiles, birds and monotremes lay cleidoic eggs, like a chicken's egg. Other mammals form similar eggs, but without the shell. They develop internally. Fish, amphibia, insects and arachnids lay simpler eggs in greater numbers, but with much less protection and nourishment.
The ostrich has the largest eggs of all currently living animals. Eggs, mostly bird eggs, are often eaten as food. The shell of an egg is a hard calcareous (CaCO3) material. The shell of an ostrich egg can support the weight of a fully grown human.