The paired mammary glands develop in the mammary crest, which forms at S14 in the ectodermal ring between the limb buds (Development of the limbs). The mammary crest usually degenerates by the end of the embryonic period with the exception of the the middle part of the thoracic region. Incomplete degeneration can cause accessory nipples. At the level of the thoracic part of the mammary crest, epithelial buds grow inwards and ramify into 15 to 25 solid epithelial cords. These become canalized in the later stage of pregnancy to form lactiferous ducts. The ducts open into the areola which remains depressed up to birth (inverted nipple). After birth, it turns outwards through proliferation of the mesenchyme (eversion). Stroma and smooth muscle cells of the mammary glands come from the mesenchyme. In males, the mammary glands remain undeveloped. In females, they develop further under the influence of hormones at puberty, during late pregnancy and during lactation.