[1] 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.

[2] A mammary ridge, or crest, usually stops growing at eight weeks and its length is regressed starting at the caudal end and extending cranially,[1] so that what remains is a round, ectodermic placode where the axilla develops.

[3] A basement membrane separating the expanding ectodermal crest structure and the underlying mesoderm usually remains.

In humans, milk lines form as thickenings of the epidermis of the mammary ridge, along the front surface of both sexes.

A recently published study demonstrates that focal fat pads on the front of human torsos are of mammary ridge origin.