A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, are called the primary sex organs. All others are called secondary sex organs, divided between the external sex organs—the genitals or external genitalia, visible at birth in both sexes—and the internal sex organs. Mosses, ferns, and some similar plants have gametangia for reproductive organs, which are part of the gametophyte. The flowers of flowering plants produce pollen and egg cells, but the sex organs themselves are inside the gametophytes within the pollen and the ovule. Coniferous plants likewise produce their sexually reproductive structures within the gametophytes contained within the cones and pollen. The cones and pollen are not themselves sexual organs. Terminology The primary sex organs are the gonads, a pair of sex organs, which diverge into testes following male development or into ovaries following female development. As primary sex organs, gonads generate reproductive gametes containing inheritable DNA. They also produce most of the primary hormones that affect sexual development, and regulate other sexual organs and sexually differentiated behaviors. Secondary sex organs are the rest of the reproductive system, whether internal or external. The Latin term genitalia, sometimes anglicized as genitals, is used to describe the externally visible sex organs: in male mammals, the penis and scrotum; and in female mammals, the vulva and its organs. In general zoology, given the great variety in organs, physiologies, and behaviors involved in copulation, male genitalia are more strictly defined as "all male structures that are inserted in the female or that hold her near her gonopore during sperm transfer"; female genitalia are defined as "those parts of the female reproductive tract that make direct contact with male genitalia or male products (sperm, spermatophores) during or immediately after copulation". Evolution It is hard to find a common origin for gonads but, gonads most likely evolved independently several times. At first testes and ovaries evolved due to natural selection. A consensus has emerged that sexual selection represents a primary factor for genital evolution. Male genitalia show traits of divergent evolution that are driven by sexual selection. Animals Mammals The visible portion of the mammalian genitals for males consists of the scrotum and penis; for females, it consists of the vulva (labia, clitoris, etc.)