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ovum[o´vum] (pl. o´va) (L.)
the female reproductive or germ cell which after fertilization is capable of developing into a new member of the same species; called also egg. The term is sometimes applied to any stage of the fertilized germ cell during cleavage and even until hatching or birth of the new individual. The human ovum consists of protoplasm that contains some yolk, enclosed by a cell wall consisting of two layers, an outer one (zona pellucida) and an inner, thin one (vitelline membrane). There is a large nucleus (germinal vesicle) within which is a nucleolus (germinal spot). adj., adj o´vular.
centrolecithal ovum one with the yolk concentrated at the center of the egg, surrounded by a peripheral shell of cytoplasm, and with an island of cytoplasm surrounding the nucleus, such as that of an arthropod.
holoblastic ovum one that undergoes total cleavage.
isolecithal ovum one with a small amount of yolk evenly distributed throughout the cytoplasm.
meroblastic ovum one that undergoes partial cleavage.
primitive ovum (primordial ovum) any oocyte very early in its development.
telolecithal ovum one with a comparatively large amount of yolk massed at one pole, such as that of a reptile or bird.
A germ cell that arises very early in the development of the embryo, usually in the yolk sac endoderm, migrates into the urogenital ridge, and is the precursor for the functional gamete.
See also: ovum