ovum

ovum

an unfertilized female gamete; egg cell

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

Ovum

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. The number of ova produced at one time varies in different animals, from millions in many marine animals that spawn into the surrounding sea water 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.

Section of a mammalian ovary

In the ovary the immature ovum is associated with follicle cells through which it receives material for growth. In mammals, as the egg matures, these cells arrange themselves into a structure known as the Graafian, or vesicular, follicle, consisting of a large fluid-filled cavity into which the ovum, surrounded by several layers of cells, projects from the layer of follicle cells that constitutes the inner wall (see illustration). The fluid contains estrogenic female sex hormone secreted by cells in an intermediate layer of the follicular wall.

Yolk, or deutoplasm, is essentially a food reserve in the form of small spherules, present to a greater or lesser extent in all eggs. It accounts largely for the differences in size of eggs. Eggs are classified according to the distribution of yolk. In the isolecithal type there is a nearly uniform distribution through the cytoplasm, as in most small eggs. The yolk in telolecithal eggs is increasingly concentrated toward one pole, as in the large eggs of fish, amphibians, reptiles, and birds. Centrolecithal, or centrally located, yolk occurs in eggs of insects and cephalopod mollusks. See Gametogenesis, Oogenesis

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

Ovum

An egg-shaped ornamental motif, used in ornamental bands in found in Classical architecture and Classical Revival styles.

Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved

ovum

[′ō·vəm]

(cell and molecular biology)

A female gamete. Also known as egg.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

ovum

In classical architecture and derivatives, an egg-shaped ornamental motif.

McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.