..... Click the link for more information. . 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 parthenogenesis (pär'thənōjĕn`əsĭs) [Gr.
..... Click the link for more information. .
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
Oogenesis
Ova are produced in the ovary ovary, ductless gland of the female in which the ova (female reproductive cells) are produced. In vertebrate animals the ovary also secretes the sex hormones estrogen and progesterone , which control the development of the sexual organs and the secondary sexual
..... Click the link for more information. 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 meiosis (mīŏ`sĭs)
..... Click the link for more information. ), 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 interphase the chromosomes are dispersed in the nucleus and appear as a network of long, thin threads or filaments, called the chromatin. At some point before prophase begins, the chromosomes replicate themselves to form pairs of identical sister chromosomes, or chromatids; the
..... Click the link for more information. ) producing a multicellular embryo embryo (ĕm`brēō), name for the developing young of an animal or plant.
..... Click the link for more information. and finally a mature individual. In all sexually reproducing animals the production and maturation of the ovum, its fertilization Cross-fertilization indicates fusion of a sperm of one hermaphroditic plant or animal with an ovum of another, as distinguished from self-fertilization, in which ovum and sperm of the same individual are fused.
..... Click the link for more information. , 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 pistil (pĭs`tĭl), one of the four basic parts of a flower , the central structure around which are arranged the stamens, the petals,
..... Click the link for more information. of the flower. After fertilization by a sperm contained in a pollen grain, the zygote develops into the embryo, contained in the seed.
ovum
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.
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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
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