any structural anomaly of either of the presumptive bilateral epidermal ridges (milk lines) formed at the onset of mammogenesis and running in an anteroposterior direction ventrally between fore- and hindlimbs, one line along each flank of the embryo; in mouse, mammary gland development begins shortly after mid-gestation (about E10.5); epidermal cells within the milk line become columnar and multilayered, defining a ridge that protrudes above and below the plane of the single-layered primitive epidermis or periderm; by E11.5, five pairs of lens-shaped placodes form along the mammary line at the site of each future nipple