Blackburn, D.G. (1991). Evolutionary origins of the mammary gland. Mammal Review 21: 81-96.

Abstract: Because the mammary gland has no known homologue among the extant reptiles, attempts to reconstruct its evolution must focus on evidence from living mammals. Of the numerous structures that have been hypothesized to have given rise to the mammary gland, only three remain as plausible progenitors: sebaceous glands, eccrine glands, and apocrine glands. Ancestral mammary glands usually are assumed to have produced a copious watery secretion like that of human eccrine sweat glands. However, in terms of anatomy, physiology, development, and topographic distribution, mammary glands are more similar to apocrine and sebaceous glands than to typical eccrine glands. Nevertheless, each of the three populations of cutaneous glands exhibit specializations unlikely to be primitive for the mammary gland. The mammary gland probably either predated full differentiation of mammalian cutaneous glands, or more likely, evolved as a neomorphic mosaic that combined the properties of apocrine and sebaceous glands. Consequently, ancestral, prototypic lacteal glands may have had the capacity to synthesize and secrete small amounts of organic substances, as do sebaceous and apocrine glands of living mammals.