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The milk lines are two parallel lines (thickenings of the epidermis) along the ventral surface of mammals of both sexes; they extend from the upper limbs (arms) to the lower limbs (legs) and are developed in the embryo. They give rise to the mammary glands and nipples but are otherwise usually not visible in the adult.
Milk lines in humans
In humans, milk lines appear in the seventh week of embryonic development. After initial development of the milk lines, they go into remission.
If the milk lines do not go fully into remission, polythelia (extra nipples) may result. Occasionally, the milk lines even create extra mammary glands, or polymastia.
"Milk line"