The clitoris (/ˈklɪtərᵻs/ or /klᵻˈtɔərᵻs/) is a female sex organ present in mammals, ostriches and a limited number of . In humans, the visible button-like portion is near the front junction of the labia minora (inner lips), above the opening of the urethra. Unlike the penis, the male homologue (equivalent) to the clitoris, it usually does not contain the distal portion (or opening) of the urethra and is therefore not used for urination. While few animals urinate through the clitoris, the spotted hyena, which has an especially well-developed clitoris, urinates, mates and gives birth via the organ. Some other mammals, such as lemurs and spider monkeys, also have a well-developed clitoris. The clitoris is the human female's most sensitive erogenous zone and generally the primary anatomical source of human female sexual pleasure. In humans and other mammals, it develops from an outgrowth in the embryo called the genital tubercle. Initially undifferentiated, the tubercle develops into either a penis or a clitoris, depending on the presence or absence of the protein tdf, which is codified by a single gene on the Y chromosome. The clitoris is a complex structure, and its size and sensitivity can vary. The glans (head) of the human clitoris is roughly the size and shape of a pea, and is estimated to have more than 8,000 sensory nerve endings. Extensive sociological, sexological and medical debate have focused on the clitoris, primarily concerning anatomical accuracy, orgasmic factors and their physiological explanation for the G-spot, and whether the clitoris is vestigial, an adaptation, or serves a reproductive function. Social perceptions of the clitoris range from the significance of its role in female sexual pleasure, assumptions about its true size and depth, and varying beliefs regarding genital modification such as clitoris enlargement, clitoris piercing and clitoridectomy; genital modification may be for aesthetic, medical or cultural reasons. Knowledge of the clitoris is significantly impacted by cultural perceptions of the organ. Studies suggest that knowledge of its existence and anatomy is scant in comparison with that of other sexual organs, and that more education about it could help alleviate social stigmas associated with the female body and female sexual pleasure; for example, that the clitoris and vulva in general are visually unappealing, that female masturbation is taboo, or that men should be expected to master and control women's orgasms.