Ovum

Article By:

Tyler, Albert Formerly, Division of Biology, California Institute of Technology, Pasadena, California.

Hamilton, Howard L. Department of Biology, University of Virginia, Charlottesville, Virginia.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.479800

The egg or female sex cell. Strictly speaking, the term refers to this cell when it is ready for fertilization, but it is often applied to earlier or later stages. Confusion is avoided by using qualifying adjectives such as immature, ripe, mature, fertilized, or developing ova. The mature ova are generally spheroidal and large. In fact, the largest known single cells of a living animal are the mature ova of the ostrich and the shark Chlamydoselache, which are about 3 in. (8 cm) in diameter. Among oviparous animals, which spawn eggs at or before the time of fertilization, those that produce larvae capable of feeding at an early stage have small eggs, and their development is generally characterized by radical transitions in appearance, called metamorphosis, before the adult form is attained. The typically viviparous mammals, in which the developing embryo receives nourishment for growth through the uterine tissues of the mother, also characteristically have relatively small eggs which are about 0.1 mm (0.004 in.) in diameter. The number of ova produced at one time varies in different animals, from millions in many marine animals that spawn into the surrounding seawater to about a dozen or less in mammals in which adaptations for internal nourishment of the developing embryo and care of the young are highly developed.

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