Each spermatozoon can fuse with a larger female gamete, or ovum , in the process of fertilization. A male cannot reproduce sexually without access to at least one ovum from a female, but some organisms can reproduce both sexually and asexually. Most male mammals , including male humans, have a Y chromosome , which codes for the production of larger amounts of testosterone to develop male reproductive organs. Not all species share a common sex-determination system. In most animals , including humans , sex is determined genetically ; however, species such as Cymothoa exigua change sex depending on the number of females present in the vicinity.
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They can exist in any combination, and a person's placement on one spectrum does not necessarily determine their placement on any of the others. Intersex people are individuals born with physical sex markers genitals, hormones, gonads, or chromosomes that are neither clearly male nor female. The existence of intersex people shows that there are not just two sexes, and the lines between sexes can be blurry. The sex spectrum is the concept of a continuum of people with sexes ranging from people with typical male physiology to people with typical female physiology. The gender spectrum visualizes gender as a continuum stretching from men to women and masculine to feminine. Gender identities other than man or woman are considered to be non-binary. The middle range of this spectrum might include:. The spectrum model also creates room for a range of gender expressions that fall between masculine and feminine.
The one-sex and two-sex theories are two models of human anatomy or fetal development discussed in Thomas Laqueur 's book Making Sex: Body and Gender from the Greeks to Freud. He theorizes that a fundamental change in attitudes toward human sexual anatomy occurred in Europe in the 18th and 19th centuries. Prior to the eighteenth century, it was a common belief that women and men represented two different forms of one essential sex: that is, women were seen to possess the same fundamental reproductive structure as men, the only difference being that female genitalia was inside the body, not outside of it. Anatomists saw the vagina as an interior penis , the labia as foreskin , the uterus as scrotum , and the ovaries as testicles. There was an abundance of literature written in the 18th century supporting the two-sex model. Jacques-Louis Moreau wrote that "not only are the sexes different, but they are different in every conceivable aspect of body and soul, in every physical and moral aspect. To the physician or the naturalist, the relation of woman to man is a series of opposites and contrasts". According to Laqueur, prior to the eighteenth century it was acknowledged that there were physical differences between the sex organs of men and women, but these differences were never made to be of significance; "no one was much interested in looking for evidence of two distinct sexes, at the anatomical and concrete physiological differences between men and women, until such differences became politically important. Laqueur uses examples from ancient thinkers to help support his claim to the dominance of the one-sex model prior to the eighteenth century.