SEXUAL VESTIGES. “In early development, the embryo develops along ‘indifferent’ lines as a sort of unisex model that later differentiates into a male or female phenotype. Often, leftovers of earlier stages can be seen in adults. Many of these embryonic ‘vestiges’ cannot be seen externally; others, like nipples on the chest, are readily visible.”
https://evolution-outreach.springeropen.com/articles/10.1186/s12052-014-0012-5