The pectoral region of the mammary line eventually pushes into the connective tissue and forms the mammary ridge. The ridge then invaginates and forms 15-20 buds. These buds will eventually become lobes and lobules. The original solid stalk becomes canalized to form the lactiferous ducts. The interlobar and connective tissue septa are formed to separate the adjacent lobes.
Fatty tissue is deposited within the breast as well.
At the time of birth, the pit from which the buds originated raises above the surface and becomes the nipple. At the same time, the lactiferous ducts open to the apex of the nipple. After birth, the ducts in males will remain in a rudimentary state. Females' ducts begin to elongate and branch, which accelerates upon reaching puberty. The adolescent period usually begins somewhere between the ages of 9 and 13 years old.