The term sex is variously employed. In the broad sense it includes everything from the sex cells to sexual behaviour. Primary sex, which is generally all that distinguishes one kind of individual from another in the case of many lower animals, denotes the capacity of the reproductive gland, or gonad, to produce either sperm cells or eggs or both. If only sperm cells are produced, the reproductive gland is a testis, and the primary sex of the tissue and the individual possessing it is male. If only eggs are produced, the reproductive gland is an ovary, and the primary sex is female. If the gland produces both sperm and eggs, either simultaneously or successively, the condition is known as hermaphroditic. An individual, therefore, is male or female or hermaphrodite primarily according to the nature of the gonad.
As a rule, male and female complement each other at all levels of organization: as sex cells; as individuals with either testes or ovaries; and as individuals with anatomical, physiological, and behavioral differences associated with the complemental roles they play during the whole reproductive process. The role of the male individual is to deliver sperm cells in enormous numbers in the right place and at the right time to fertilize eggs of female individuals of the same species. The role of the female individual is to deliver or otherwise offer eggs capable of being fertilized under precise circumstances. In the case of hermaphrodite organisms, animal or plant, various devices are employed to ensure cross-fertilization, or cross-pollination, so that full advantage of double parentage is obtained. The basic requirement of sexual reproduction is that reproductive cells of different parentage come together and fuse in pairs. Such cells will be genetically different to a significant degree, and it is this feature that is essential to the long-term well-being of the race. The other sexual distinctions, between the two types of sex cell and between two individuals of different sex, are secondary differences connected with ways and means of attaining the end.

LI 3.9.25