- Linear thickening of surface ectoderm (milk line) appears on ventral body
- Extends from axilla to medial inguinal region
- Line disappears except in pectoral region

- 15 to 20 solid ectodermal buds arise in remaining milk line
- Buds form solid cords
- Which grow in underlying mesenchyme
- Cord grows and branches in fetal life
- Canalized before birth to form: Lactiferous sinus, Lactiferous duct, Secretory alveoli
- Open in depressed area, which elevates after birth – to form the nipple

At puberty:
- Males: have ducts without alveoli
- Females: Ovarian hormone increases branching of lactiferous duct, and increases fat between lobules. Therefore the mammary gland enlarges.
Congenital anomalies:
- Amastia – absence of one or two mammary glands
- Athelia – absence of one or two nipples
- Macromastia – large breasts
- Micromastia – small breasts
- Polythelia – accessory nipples
- Polymastia – accessory breast
- Gynaecomastia – male developing female breasts
- Klinefelter syndrome – XXY in males, therefore have enlarged breasts, wide hips and penis does not grow