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A sex organ, or primary sexual characteristic, as narrowly defined, is any of those anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; in mammals, these are:

• Female: Bartholin's glands, cervix, clitoris, Fallopian tubes, labia, ovaries, Skene's gland, uterus, vagina, vulva
• Male: Cowper's glands, epididymis, penis, prepuce, prostate, scrotum, seminal vesicles, testicles

The Latin term genitalia, sometimes anglicized as genitals and genital area, is used to describe the externally visible sex organs, known as primary genitalia or external genitalia: in males the penis and scrotum, 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.

A more ambiguously defined term is erogenous zone, subjectively, any portion of the body that when stimulated produces erotic sensation, but always prominently including the genitalia.

Main article: Development of the reproductive system

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 arises, affecting, but not absolutely determining, behavior.

The following is a list of anatomical terms related to sex and sexuality:

• areola
• Bartholin's gland — breast — bulbospongiosus muscle — bulbourethral gland
• cervix — Cleft of Venus — clitoris — common penile artery — corona glandis — corpora cavernosa — corpus cavernosum — corpus spongiosum — Cowper's glands — cremaster muscle
• dartos muscle — ductus deferens
• ejaculatory duct — endometrium — epididymis --- eggs
• Fallopian tube — foreskin — frenulum — frenulum preputii penis — frenulum labiorum pudendi — frenulum clitoridis — frenum — fundiform ligament
• G-Spot — Gartner's duct — genital tubercle — genitofemoral nerve — glans — glans penis — Gräfenberg spot
• hymen
• internal pudendal artery — intromittent organ — ischiocavernosus muscle
• labium — labia majora — labia minora
• mammae — mammary gland — meatus — mons pubis — mons veneris — Mullerian duct
• nipple
• ovary — ovum — oviducts
• penis — perineum — prepuce — prostate — pubic hair — pubic symphysis — pubococcygeus muscle — pudendal nerve
• raphe — ridged band
• scrotum — seminal vesicles — semeniferous tubules — Skene's glands — spermatozoon — spermatic cord — sphincter urethrae membranaceae — splanchnic nerves
• testes — testicle — tunica albuginea
• urethra — urethral sponge — urogenital diaphragm — uterus
• vas deferens — vagina

v • d • e Human anatomical features
Head	Skull · Forehead · Eye · Ear · Nose · Mouth · Tongue · Teeth · Jaw · Face · Cheek · Chin
Neck	Throat · Adam's apple
Torso	Shoulders · Spine · Breast (Tail of Spence) · Chest · Ribcage · Abdomen · Navel Sex organs (Clitoris · Vagina · Penis · Scrotum · Testicle) – Hip · Anus · Buttocks
Limbs	Arm · Elbow · Forearm · Wrist · Hand · Finger (Thumb · Index · Middle · Ring · Little) · Leg · Lap · Thigh · Knee · Calf · Heel · Ankle · Foot · Toe (Hallux)
Skin	Hair