The Origin and Evolution of Mammary Glands

Abstract

Although there have been diverse investigations into thc origin and evo-lution of tbc mammary glands, several misconceptions have a wide currency, and many hypothetical explanations are often considcred fact. Recent textbooks on mammalogy and mammalian anat~ omy hardly consider the mammary glands except for brief and scattered descriptive comments. Thc aim of this paper is to review thc published evi-dence on thc origin and evolution of tbe marnmary glands. Origin The primary function of mammary glands i5 their secretion in aH the mam-mals of nutritious milk which is lapped or suckled by thc yaung. I nasmuch as the primitive monotremes lactate and are oviparous, lactation apparently pre-ccdcd viviparity. In thc. Marsupialia the mammae have attained their char~ aetcristie form (with nippIes). Mivart (perhaps) 1 and Darwin (1859) suggested that cutaneous ghinds evolved to a mammary funetion. Rein (1882) hypothcsized that mammae de~ veloped directly from ectoderm. Gegen-baur (1886) discussed the resemblance and probably homology of mono-treme lactiferous glands to tubular sweat glands. He believed, howcvcr, that other mammals derived their main-mae. from sebaceous.glands. Bendä (1894) and Brouha (1905) developed the now widely aeeepted theory that milk glands in all mammals are derived from sweat glands. Myers (1917) dis~ cussed the similarity of thc anlagen of mammary glands and hairs. No fossil evidenee on the origin of mammae is available. Inasmuch as Reeent mammals are endothcrmic and show a general asso-ciation of sweat glands, sebaceous Tbe author is assoeia te professor in the De