The milk lines are two parallel lines, formed by 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.
Nipples develop on the milk lines of mammals. Most of us have two nipples, but in some cases humans can develop more than two nipples. We have heard of 'third nipples', but there can be more than three nipples in very rare cases, all which would grow along the milk lines.