Patient with right sided axillary accessory breast tissue (polymastia). In the embryo, the ectoderm is the name given to the cells on the outside. They are responsible for the formation of several parts of the body, including the integumentary system (skin and the glands contained within it, such as sweat and mammary glands). A specific line of cells called the mammary streak are responsible for the formation of breast tissue. Originally, they span from the axilla to the groin on the ventrolateral sides of the embryo, but regress to only line the thorax as the embryo matures. Sometimes, the cells in the axillary region fail to degenerate, giving the potential for breast growth here during optimum hormonal conditions. Accessory breasts are known to grow during pregnancy, mimicking the response of normal breast tissue. They are treated by surgical removal.