and vagina. In placental mammals, females have two genital orifices, the vagina and urethra, while males 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, exposure of the genitals is considered a public indecency. In humans, sex organs include: Development In typical prenatal development, sex organs originate from a common primordium during early gestation and differentiate into male or female sexes. The SRY gene, usually located on the Y chromosome and encoding the testis determining factor, determines the direction of the differentiation. The absence of it allows the gonads to continue to develop into ovaries. The development of the internal, and external reproductive organs 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 looks female-like: a pair of urogenital folds with a small protuberance in the middle, and the urethra behind the protuberance. If the fetus has testes and 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 sex organ in one sex has a homologous counterpart. The process of sexual differentiation includes the development of secondary sexual characteristics, such as patterns of pubic and facial hair and female breasts that emerge at puberty. Because of the strong sexual selection affecting the structure and function of genitalia, they form an organ system that evolves rapidly. A great variety of genital form and function may therefore be found among animals. Cloacae In many other animals a single posterior orifice, called the cloaca, serves as the only opening for the reproductive, digestive, and urinary tracts (if present). All amphibians, birds, reptiles, some fish, and a few mammals (monotremes, tenrecs, golden moles, and marsupial moles) have this orifice, from which they excrete both urine and feces in addition to serving reproductive functions. Excretory systems with analogous purpose in certain invertebrates are also sometimes referred to as cloacae. Insects The organs concerned with insect mating and the deposition of eggs are known collectively as the external genitalia, although they