Mammary Glands, source of milk for the young and a characteristic of all mammals, including the egg-laying monotremes. Although the mammary glands evolved recently in the development of vertebrates, their origin is uncertain. In monotremes, they resemble sweat glands in structure, leading to speculation that they are sweat or sebaceous glands modified through evolution. In higher orders, the glands at maturity are developed only in females; in monotremes they evolve equally in both sexes. The organ in human males occasionally develops functional ability, giving support to the belief that lactation occurred in both sexes in prehistoric mammals.