The milk lines are two parallel lines, formed by thickenings of the epidermis (the mammary ridge) 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.
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Mammary glands begin developing during the embryonic stage. In many mammals, these glands first appear as elevated ridges along the milk lines, which then separate into individual buds located in regions lateral to the ventral midline. The location of these buds varies according to species; they are located in the thoracic region in primates, in the inguinal area in ungulates, and along the entire length of the trunk in rodents and pigs.[1]
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 humans have two nipples, but in some cases more than two will develop. In most cases, these additional nipples grow along the milk line.