The larger, usually nonmotile female reproductive cell of most organisms that reproduce sexually. Eggs are haploid (they have half the number of chromosomes as the other cells in the organism's body). During fertilization, the nucleus of an egg cell fuses with the nucleus of a sperm cell (the male reproductive cell) to form a new diploid organism. In animals, eggs are spherical, covered by a membrane, and usually produced by the ovaries. In some simple aquatic animals, eggs are fertilized and develop outside the body. In some terrestrial animals, such as insects, reptiles and birds, eggs are fertilized inside the body but are incubated outside the body, protected by durable, waterproof membranes (shells) until the young hatch. In mammals, eggs produced in the ovaries are fertilized inside the body and (except in the cases of monotremes) develop in the reproductive tract until birth. The human female fetus possesses all of the eggs that she will ever have; every month after the onset of puberty, one of these eggs matures and is released from the ovary into the fallopian tube, where it is either fertilized or discarded during menstruation. In many plants (such as the bryophytes, ferns, and gymnosperms) eggs are produced by flasked-shaped structures known as archegonia. In gymnosperms and angiosperms, eggs are enclosed within ovules. In angiosperms, the ovules are enclosed within ovaries. See also oogenesis.
In many animals, a structure consisting of this reproductive cell together with nutrients and often a protective covering. The embryo develops within this structure if the reproductive cell is fertilized. The egg is often laid outside the body, but the female of ovoviviparous species may keep it inside the body until after hatching.