The cell division process in specialised tissues of female ovaries and male testes which results in the production of sex cells, or gametes. Meiosis involves two divisions and produces four sperm cells in males and one ovum in females from a starting cell. Each sperm and ovum contains only half the original number of chromosomes--23 in the case of humans. Subsequently, meiosis is also called "reduction division. " Spermatogenesis is the term used for meiosis in males and oögenesis refers to the same process in females. See mitosis.