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Definition: ovum (plural: ova) from Processing Water, Wastewater, Residuals, and Excreta for Health and Environmental Protection: An Encyclopedic Dictionary

An egg; the female reproductive cell or gamete of plants and animals.

Summary Article: ovum
from The Columbia Encyclopedia

(ō'vəm), 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.

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 Encyclopedia, © Columbia University Press 2017

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