Also found in: Dictionary, Thesaurus, Medical, Legal, Financial, Acronyms, Idioms, Wikipedia.


ovum (ōˈvəm), in biology, specialized plant or animal sex cell, also called the egg, or egg cell. It is the female sex cell, or female gamete; the male gamete is the sperm. The study of the ovum is included in the science of embryology. The development of a new individual from an unfertilized ovum is called parthenogenesis.

In higher animals the ovum differs from the sperm in that it is larger and is nonmotile, a smooth sphere or oval lacking the flagellum of the sperm. Like that of the sperm, its nucleus contains the chromosomes, which bear the hereditary material of the parent. A gamete, ovum or sperm, contains half the number of chromosomes found in the body cells of the parent, i.e., the gamete is haploid. In animals, ova contain stored food called the yolk, the amount of which varies in different species, depending on the length of time required for the embryo to become self-sufficient in obtaining nourishment outside the egg.

The term ovum is usually restricted to the single female sex cell, but the term egg, in its common use to indicate a bird's egg, refers to a more complex structure, only part of which is produced in the ovary of the bird. The ovum of such an egg is a cell swollen with yolk material. The rest of the egg—e.g., the jelly mass surrounding amphibian eggs, and the shell, membranes, and egg white, or albumen, of bird eggs—is not cellular and is secreted around the ovum as it passes down the oviduct.

Animal Ova


Ova are produced in the ovary of the female; they are formed from reproductive cells (called primordial germ cells) in a process called oogenesis. In this maturation process a germ cell builds up its food supply and then undergoes a series of cell divisions (called meiosis), by which the number of chromosomes in the mature ovum is reduced by half. In oogenesis in animals only one of the four cells formed by meiotic division is functional. In this ovum all the yolk from the original cell is collected; the three other, yolkless, cells are called polar bodies and never develop further. Maturation also occurs in the formation of sperm (spermatogenesis), but in spermatogenesis, in contrast to oogenesis, all four of the cells formed by meiotic division are functional.

Fertilization and Maturation

The union of mature sperm and ovum, each bearing half the normal number of chromosomes, results in a single cell (the zygote) with a full number of chromosomes. The zygote undergoes a series of cell divisions (see mitosis) producing a multicellular embryo and finally a mature individual. In all sexually reproducing animals the production and maturation of the ovum, its fertilization, and its early embryonic development are essentially identical.

Plant Ova

In plants that reproduce sexually the pattern is similar to that of animals. In the mosses and ferns, however, the egg cells are formed in special organs called archegonia and are fertilized by sperm that are commonly flagellated and motile like those of animals. In flowering plants the ovary is situated in the base of the pistil of the flower. After fertilization by a sperm contained in a pollen grain, the zygote develops into the embryo, contained in the seed.
The Columbia Electronic Encyclopedia™ Copyright © 2022, Columbia University Press. Licensed from Columbia University Press. All rights reserved.

Egg (fowl)

A single, large, living, female sex cell enclosed in a porous, calcareous shell through which gases may pass. Although they vary in size, shape, and color, the eggs of chickens, ducks, geese, and turkeys are essentially the same in structure and content (see illustration). Inward from the shell are the outer and inner shell membranes which are also permeable to gases. The membranes are constructed to prevent rapid evaporation of moisture from the egg but to allow free entry of oxygen, which is necessary for life. Air begins to penetrate the shell soon after the egg is laid, and it tends to accumulate in a space between the two membranes at the large end of the egg. See Cell (biology)

Egg of a birdenlarge picture
Egg of a bird

The inner shell membrane surrounds a mass of fluid albumin which, in turn, encloses a body of dense albumin; these two types of protoplasm constitute the so-called egg white. The central part of the egg is occupied by the yolk, which contains the vital egg nucleus and its associated parts. The yolk consists of alternating layers of yellow and white yolk. The yolk, enclosed by the vitelline membrane, is held in place by the chalaza which is anchored at each end of the egg and prevents undue mechanical disturbance. See Cell nucleus, Yolk sac

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the female sex cell of animals, including humans, and plants from which a new organism arises as a result of fertilization or parthenogenesis. The animal egg is a highly specialized cell that contains the stored nutrients essential for embryonic development. It usually is surrounded by membranes. Egg formation, or oogenesis, generally takes place in the ovaries. The eggs of most animals are round or oval; in insects and cephalopod mollusks they are elongated. The eggs of the most primitive multicellular animals—sponges and some coelenterates—do not have any distinctive shape and are capable of amoeboid movement.

The size of an egg varies with the amount of yolk in the cytoplasm. For example, the yolk-poor eggs of placental mammals range in diameter (not counting membranes) from 50 micrometers (μm) in voles (or 70 μm in rats and mice) to 140–150 μm in bats, guinea pigs, cows, and dogs. Among placental mammals, sheep have the largest eggs, measuring 180 μm in diameter. Human eggs measure 89–91 μm in diameter. The eggs of many different invertebrates, including coelenterates, worms, echinoderms, and mollusks, are similar in size.

By storing fairly large amounts of yolk, eggs may attain large, sometimes gigantic, dimensions. The egg diameter is about 1.4 mm in some invertebrates (some mollusks, echinoderms, and crustaceans), 4.3 mm in the platypus, 7–9 mm in salmon, and 50–70 mm in elasmobranchs. Avian eggs are also very large. The chicken egg measures more than 30 mm in diameter (not counting the surrounding albuminous membrane), and the ostrich egg has a diameter of 80 mm (the total length of the egg in the shell is 155 mm).

There is no clear-cut relationship between the size of the egg and the size of the adult organism. The eggs of humans and rabbits are the same size. The egg of an aquarium guppy, the female of which grows to a length of only 5–6 cm, is twice the size of the egg of a blue tuna, which reaches an adult length of 1.5 m. (The diameters of the eggs are 2–2.1 mm and 1–1.1 mm, respectively.)

There is usually an inverse correlation between the number of eggs produced and egg size. Specifically, animals that protect their offspring generally produce a few large eggs. Birds, for example, lay one (penguins and some birds of prey) to 22 (partridges) large eggs. Catfish of the family Ariidae, whose males carry the eggs in their mouths, lay 20 to 40 eggs measuring 17–21 mm in diameter (the largest eggs among bony fishes). Fishes that deposit their eggs in the water and show no concern for their young are extremely fecund. Many cyprinids (for example, the Caspian roach, bream, and crucian carp) deposit about 300,000 eggs, the wild carp about 1.5 million, and the cod about 10 million. The diameter of the eggs is less than 2 mm.

Polymorphism is characteristic of the eggs of some animals. Specifically, seasonal polymorphism is found in animals having a complex reproductive cycle, for example, rotifers, water fleas, and aphids. In water fleas, for example, several generations of females succeed one another in summer, producing yolk-poor eggs with a thin membrane that develop without fertilization. With the onset of unfavorable conditions, the females deposit yolk-rich fertilized “winter” eggs enclosed in a protective membrane; these eggs develop only after a long period of dormancy. Rotifers produce three types of eggs: diploid eggs, from which females are produced by parthenogenesis; small haploid eggs, which develop into males; and dormant, or “winter,” eggs, which develop into females after fertilization. Sexual dimorphism of eggs also exists. In butterflies, caddis flies, reptiles, and birds, the eggs that develop into males and females differ with respect to the sex chromosomes (female heterogamety). Rotifers and some worms are marked by external sexual dimorphism: the large eggs produce females, and the small ones males.

The mature egg has a definite organization, primarily manifested by polarity. The nucleus is frequently found near the yolk-poor animal pole, whereas the yolk inclusions are concentrated fairly close to the opposite vegetal pole. Moreover, the eggs of many animals exhibit local differences. Such differences are especially pronounced in mollusks, worms, and ascidians. In these animals the fate of the various regions of the egg has already been determined, and if the egg is divided into parts, deformed embryos lacking one or more organs develop (mosaic eggs). In other animals, echinoderms in particular, the organization of the egg is more unstable, so if the egg is divided into parts, several small but essentially normal embryos develop (regulative eggs). Mosaic and regulative eggs formerly were regarded as opposed to each other, but the differences between them are no longer believed to be basic. They appear to be caused by more or less early differentiation of the cytoplasm.

The surface of an egg is invested with a semipermeable elastic plasma membrane capable of contracting. Numerous projections, or microvilli, form on the surface (cortical) layer of the cytoplasm. In many animals the surface layer contains bodies that are rich in mucopolysaccharides and that vary in diameter from several tenths of a micrometer (cortical granules of bivalve mollusks, echinoderms, and mammals) to 5–40 μ.m (cortical alveoli of bony fishes). The bodies are arranged in one or more rows near the plasma membrane. Their contents are released from the cytoplasm near the membrane when the egg is activated. Pigment granules are usually found near the surface as well. Mitochondria, Golgi complexes (concentrated at the periphery of the egg in many animals), endoplasmic reticulum, and ribosomes are regular constituents of the egg, as they are in other cells. The cell center of the egg disappears in most animals toward the end of oogenesis. It occasionally remains, for example, in some leeches and planarians, during maturation divisions. In other cases, a new cell center is formed after fertilization from centrioles introduced by sperm or, infrequently, by new formation. Yolk inclusions are a specific constituent of the cytoplasm. The differences in amount and distribution of yolk in various animals are responsible for the different kinds of cleavage. The cytoplasm contains a large store of macromolecules, including the entire protein-synthesizing system (messenger RNA and essential enzymes), DNA and RNA polymerases, and nonhistone nuclear proteins (tubulin and others). The macromolecules are used only after the egg is activated and during the early stages of embryonic development.

Eggs in plants are discussed in EGO CELL.


Wilson, E. Kletka i ee rol’ v razvitii i nasledstvennosti, vols. 1–2. Moscow-Leningrad, 1936–40. (Translated from English.)
Sokolov, I. I. “Tsitologicheskie osnovy polovogo razmnozheniia mnogokletochnykh zhivotnykh.” In Rukovodstvo po tsitologii, vol. 2. Moscow-Leningrad, 1966. Pages 390–460.
Bodemer, C. Modern Embryology. Moscow, 1971. (Translated from English.)
Boyd, J. D., and W. J. Hamilton, “Cleavage, Early Development and Implantation of the Egg.” In Marshall’s Physiology of Reproduction, vol. 2. London, 1952.
Austin, C. R. The Mammalian Egg. Oxford, 1961.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

What does it mean when you dream about an egg?

Eggs are universal symbols of wholeness, fertility, new birth, and rebirth (or resurrection; hence, their association with Easter). Broken eggs may present a state of brokenness in one’s life or “breaking out of a shell.”

The Dream Encyclopedia, Second Edition © 2009 Visible Ink Press®. All rights reserved.


(cell and molecular biology)
A large, female sex cell enclosed in a porous, calcareous or leathery shell, produced by birds and reptiles.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


colored eggs as symbol of new life, adopted to reflect Resurrection. [Christian Tradition: Brewer Dictionary, 361]
See: Easter


symbol of Christ’s resurrection. [Art: Hall, 110]
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.


1. the oval or round reproductive body laid by the females of birds, reptiles, fishes, insects, and some other animals, consisting of a developing embryo, its food store, and sometimes jelly or albumen, all surrounded by an outer shell or membrane
2. any female gamete; ovum
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

Easter Egg

An undocumented function hidden in software that may or may not be sanctioned by management. Easter Eggs are secret "goodies" found by word of mouth or accident. They are also used in video games, movies, TV commercials, DVDs, CDs, CD-ROMs and every so often in hardware.

In Applications
Very often, pressing some key combination when the About box is open (Help/About) displays the names of the developers who worked on the software. A video game might display the game designer's name when a certain maneuver is completed.

The Media
A movie or TV Easter Egg could be a date on a calendar of a famous event that provides a clue to the movie's ending. The headline in a newspaper casually lying on the table might offer a clue or perhaps some tongue-in-cheek comment.

The right menu sequence on a DVD might produce extra trailers, and extra bonus tracks (extra goodies) may also be called Easter Eggs.

In Hardware
A hardware Easter Egg might play a song if a certain key combination is pressed. For more information, visit See back door, cheat code and cuckoo egg.
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.


Eggs are symbolic of something new and fragile. They represent life and development in its earliest forms and, as such, the possibilities are limitless. At times, eggs can represent captivity or entrapment. Carl Jung said that eggs represent our captive souls. Therefore, the egg in your dream may very well represent you in the most profound sense. Are you trapped in a shell or did you break out of it and are you now free to soar?
Bedside Dream Dictionary by Silvana Amar Copyright © 2007 by Skyhorse Publishing, Inc.
References in periodicals archive ?
This is the first programme in India that we have made different kinds of egg products."
'We visited traditional battery-cage egg farms and we could smell the stinking odor from far away.
This has, however, not kept Christians from sending and receiving Easter egg gifts.
PS1.49 | LION EASTER EGG (190G) Another mediumsize hollow milk chocolate egg, but this time with a selection of Lion favourites.
* Make patterns or other designs on the eggs. Use a white crayon to draw on the egg directly.
The tradition of decorating trees with elaborately made eggs for Easter is centuries old.
He said: "Free-range eggs are a staple of British diets but shoppers have become obsessed with getting the biggest egg they can.
Freezing a raw egg morphs the creamy golden yolk into a firm, gelatinous ball that will--alas--never be creamy again.
Quail eggs treat many conditions and are effective in treating cough and asthma.
The consumer in general and egg breaking industry, in particular, are very much concerned regarding egg components shares.
This was briefed to Chairperson Sindh Board of Investment (SBI) Naheed Memon in a meeting regarding Barkat Frisian Pasteurized egg company plant here, said a statement on Wednesday.
Chocolate Avocado Egg (250g), PS8, Waitrose A realistic looking, supersize avocado shape that looks like it should be mashed on toast, but is made entirely of dark and green coloured chocolate.