The first manifestation of the mammary gland in many mammals is an elevated ridge or milk line, which then fragments into individual buds in specific regions lateral of the dorsal midline. After initial development of the milk lines they go into remission., 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.[MP]

consider splitting ridge (precursor) and milk line

uberon

milk line, mammary line

UBERON:0008425

These buds are either located in the thoracic region in primates, in the inguinal area in ungulates or along the entire length of the trunk in rodents and pigs. In humans, milk lines appear in the seventh week of embryonic development

• nipple
• mammary placode