Primary sex organs are gonads; the rest are delivery gear
Sex organs, or reproductive organs, are body parts involved in sexual reproduction and form primary sex characteristics. They typically differentiate into male and female types, produce and move gametes, and together make a reproductive system—in animals and, with different parts, in flowering plants.
Reproductive organs make and move gametes, bring about fertilisation, and in many species nurture offspring until birth. Biologists split them into two tiers. The primary organs are the paired gonads, which become testes or ovaries depending on development; they produce the gametes carrying inheritable DNA and most of the hormones that steer sexual development, other sexual organs and sex-differentiated behaviour. Everything else in the reproductive tract, inside or out, counts as secondary, and the externally visible parts go by the Latin name genitalia.
In an embryo, both sexes start from one shared primordium. A gene called SRY, normally carried on the Y chromosome, encodes the testis-determining factor and tips the gonads toward testes; without it they go on to form ovaries. Hormones from the fetal gonads, and how tissues respond to them, then shape the rest of the anatomy, and early external genitals look female-like before any change. Further differences, such as body hair patterns and breasts, appear at puberty.
Genitals change fast over evolutionary time. Most researchers now agree that sexual selection is the main driver, producing strikingly divergent male structures and a huge variety across the animal kingdom. Gonads themselves probably arose independently several times. Many vertebrates, including all amphibians, birds and reptiles, some fish and a few mammals such as monotremes and golden moles, use a single rear opening, the cloaca, for reproduction and waste alike, while placental females have separate vaginal and urethral openings. Teleost fish are sexed by the shape of a fleshy genital papilla, and sexual planarian flatworms are hermaphrodites that swap sperm.
Plants and fungi follow different rules. Most plant species are hermaphrodite, carrying both kinds of organ. Land plants alternate between a spore-making sporophyte and a haploid gametophyte, and sex organs form only on the gametophyte. In flowers, stamens produce pollen, each grain holding a male gametophyte of just three cells, which travels to the pistil with its ovule at the base. Mosses and ferns use structures called gametangia, as do fungi, where male and female forms are usually distinct.
Source: Sex organ
