An ovum is an egg that exists in the ovary of the female. This egg is called the female "gamete" or sex cell. It combines with the male gamete, called a sperm, to form a zygote. This formation process is called "fertilization." (see sperm, zygote).
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The female sex cell. When fertilized by a spermatozoon, an o. is capable of developing into a new individual of the same species; during maturation, the o., like the spermatozoon, undergoes a halving of its chromosomal complement so that, at its union with the male gamete, the species number of chromosomes (46 in humans) is maintained; yolk contained in the ova of different species varies greatly in amount and distribution, which influences the pattern of the cleavage divisions. [L. egg]
- alecithal o. an o. in which the yolk is nearly absent, consisting of only a few particles.
- blighted o. a fertilized o. whose development has ceased at an early stage.
- centrolecithal o. one in which the yolk is mostly located near the center of the egg, as in arthropods.
- fertilized o. an o. impregnated by a spermatozoon.
- isolecithal o. an o. in which the yolk is evenly distributed throughout the cytoplasm.
- Peters o. an o. with a presumptive fertilization age of about 13 days; for many years, it was one of very few young human embryos recovered in good condition and its study furnished many facts regarding early embryonic changes.
- telolecithal o. an o. in which there is a large amount of yolk massed at the vegetative pole, as in the eggs of birds and reptiles.

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ovum 'ō-vəm n, pl ova -və a female gamete: MACROGAMETE esp a mature egg that has undergone reduction, is ready for fertilization, and takes the form of a relatively large inactive gamete providing a comparatively great amount of reserve material and contributing most of the cytoplasm of the zygote

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the mature female sex cell (see gamete). The term is often applied to the secondary oocyte although this is technically incorrect. The final stage of meiosis occurs only when the oocyte has been activated by fertilization.

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(oґvəm) gen. oґvi pl. oґva [L.] 1. the female reproductive cell which, after fertilization, becomes a zygote that develops into a new member of the same species. 2. imprecise term for oocyte. 3. formerly, any of various stages from the primary oocyte to the implanting blastocyst. 4. in some species (e.g., insects, birds, reptiles), any stage from the unfertilized female reproductive cell, through the developing embryo surrounded by nutrient material and protective covering, up to the point when the young emerge.

Medical dictionary. 2011.

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