oogenesis The production and growth of the ova (egg cells) in the animal ovary. Special cells (oogonia) within the ovary divide repeatedly by mitosis to produce large numbers of prospective egg cells (primary oocytes). When mature, these undergo meiosis, which halves the number of chromosomes. During the first meiotic division a polar body and a secondary oocyte are produced. At the second meiotic division the secondary oocyte produces an ovum and a second polar body. Oocytes may be present in the ovaries at birth and may represent the total number of eggs to be produced.

oogenesis The formation of eggs or ova including, in angiosperms, the formation of the embryo sac.

oogenesis (oh-ŏ-jen-i-sis) n. the process by which mature ova are produced in the ovary (see illustration). Primordial germ cells multiply to form oogonia, which start their first meiotic division to become oocytes in the fetus and complete it at ovulation. Fertilization stimulates the completion of the second meiotic division, which produces a mature ovum.

oogenesis The formation of eggs or ova, including the meiotic division of the oocytes and the formation of the yolk and the egg membranes.