That raised ridge of skin that runs up the center line of a man's scrotum --the one that looks like a seam when his balls are drawn up tight--is called the Scrotal Raphe (pronounced ray-fee. Of Greek origin for 'seam').
It is an extension of the Perineal Raphe, which runs along the perineum (taint) from the anus to the genitals in both men and women.
The Perineal and Scrotal Raphe exist because, in the embryonic stage, we all start off with a vagina-like structure. As the embryo develops and the male or female genitals form, the folds of that structure fuse together and leave behind a 'seam' where they meet.