In mammals, a mammary duct is present in order to produce milk. The mammary duct is an organ known as an exocrine gland, which is a type of gland that secretes a substance. The mammary duct is an enlarged sweat gland. The mammary gland, or mammary duct, is composed of alveoli. Alveoli are a few millimeters in size and form cavities in the breast. These cavities fill with milk-creating cells called cuboidal cells, which are surrounded by the myoepithelial cells. When the alveoli combine they are called lobules. In the anatomy of the nipple, the lactiferous duct drains from each of the lobules. Humans have two complex mammary glands (one in each breast), which is made up of about 10-20 simple mammary glands. “Simple mammary gland” refers to the milk-secreting tissue that leads to the lactiferous duct in each breast. A complex mammary gland is comprised of simple mammary glands that serve one nipple. The mammary gland is stimulated by oxytocin, a hormone that is released when an infant suckles on the breast. This sends a message to the body that the milk that had been stored within the mammary glands is ready to be released and secreted through the nipple. The mammary duct will grow at various times over the lifecycle of a human, including the embryonic stages and puberty stages.