The Origin and Evolution of Mammary Glands

Published works on the mammary glands are reviewed. Secretions in an incubation patch probably were important for the regimen of eggs and hatchlings of mammalian ancestors and subsequently evolved to milk. Viviparity succeeded oviparity. The nipple evolved. Hormones limited the development of mammae in male mammals. Mammae became aligned in two mammary lines, and subsequently localized along them.