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A sex organ or primary sexual characteristic, as narrowly defined, is any anatomical part of the body involved in sexual reproduction and constituting the reproductive system in a complex organism, especially the external sex organs; the external sex organs are also commonly referred to as the genitalia or genitals.
Flowers are the reproductive organs of flowering plants, cones are the reproductive organs of coniferous plants, whereas mosses, ferns, and other similar plants have gametangia for reproductive organs.
The Latin term genitalia, sometimes anglicized as genital area, is used to describe the externally visible sex organs, known as primary genitalia or external genitalia: in males, the penis and scrotum; and in females, the clitoris and vulva.
The other, hidden sex organs are referred to as the secondary genitalia or internal genitalia. The most important of these are the gonads, a pair of sex organs, specifically the testes in the male or the ovaries in the female. Gonads are the true sex organs, generating 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.
In humans, women have two genital orifices, the vagina and urethra, while men have only one, the urethra. Male and female genitals have many nerve endings, resulting in pleasurable and highly sensitive touch. In most human societies, particularly in conservative ones, genitals are considered a public indecency and sometimes even illegal if left uncovered in public.
In mammals, sex organs include:
In typical prenatal development, sexual organs originate from a common anlage anatomy during early gestation and differentiate into male or female variations. The SRY gene, usually located on the Y chromosome and encoding the testis determining factor, determines the direction of this differentiation. The absence of it allows the gonads to continue to develop into ovaries.
Thereafter, the development of the internal reproductive organs and the external genitalia is determined by hormones produced by certain fetal gonads (ovaries or testes) and the cells' response to them. The initial appearance of the fetal genitalia (a few weeks after conception) looks basically feminine: a pair of "urogenital folds" with a small protuberance in the middle, and the urethra behind the protuberance. If the fetus has testes, and if the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce the scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings grow, wrap around the penis, and fuse in the midline to form the penile urethra.
Each sexual organ in one sex has a homologous counterpart in the other one. See a list of homologues of the human reproductive system. In a larger perspective, the whole process of sexual differentiation also includes development of secondary sexual characteristics such as patterns of pubic and facial hair and female breasts that emerge at puberty. Furthermore, differences in brain structure arise, affecting, but not absolutely determining, behavior.
Intersex is the development of genitalia somewhere between typical male and female genitalia. Once the child is born, the parents are faced with decisions that are often difficult to make, such as whether or not to modify the genitalia, assign the child as male or female, or leave the genitalia as is. Some parents allow their doctors to choose. If they do decide to modify the genitalia, they have approximately a 50% chance of getting genitalia that will match the child's gender identity. If they pick the wrong one, their child may begin to show symptoms of transsexualism, which can lead them to a life of discomfort until they are able to remedy the issue.
Sexual reproduction in flowering plants involves the union of the male and female germ cells. The sex organs, contained within the flower, may contain both male and female sex organs (these are known as perfect, bisexual, or hermaphrodite) or only one of the two (known as imperfect or unisexual). Also, those plants whose flowers are unisexual may contain both male and female flowers, or there may be purely male and female plants of the same species.
During a plant's sexual reproduction the stamen (male sex organ) produces pollen from an anther. These male germ cells are carried to the pistil (female sex organ), with the ovary at its base where fertilization can take place. The male germ cells can be carried by air, rain, water, insects or other symbiotic animals, or simply by gravity.
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