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Mammary glands are present in all mammals, although they are vestigial in the male of the species.

Mammary glands are most obvious in humans, as the female human body stores large amounts of fatty tissue near the nipples, resulting in prominent breasts. The mammary glands are modified sweat glands that produce milk, which is used to feed the young for some time after birth. Mammalian femaleĪ distinguishing characteristic of the class Mammalia is the presence of mammary glands. It is not etymologically related to the word male, but in the late 14th century the spelling was altered in English to parallel the spelling of male. The word female comes from the Latin femella, the diminutive form of femina, meaning " woman". In land plants, female and male designate not only the egg- and sperm-producing organisms and structures, but also the structures of the sporophytes that give rise to male and female plants.

The concept is not limited to animals egg cells are produced by chytrids, diatoms, water moulds and land plants, among others. Other than the defining difference in the type of gamete produced, differences between males and females in one lineage cannot always be predicted by differences in another.

There is an argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction. Oogamous species, which include humans in which the female gamete is very much larger than the male and has no ability to move.Anisogamous species with gametes of male and female types,.Isogamous species with two or more mating types with gametes of identical form and behavior (but different at the molecular level),.There is no single genetic mechanism behind sex differences in different species and the existence of two sexes seems to have evolved multiple times independently in different evolutionary lineages. Some organisms can reproduce both sexually and asexually. A female individual cannot reproduce sexually without access to the gametes of a male, or vice versa (an exception is parthenogenesis).

The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male.