In biology, that's unusual. Tracking which embryonic cells become what body part is an important part of figuring out the basic rules for making life. For example, scientists know, in detail, what cells turn into limbs and eyes. Today, two teams of biologists are publishing the first reports of cell-fate-tracking for phalluses in snake, chick, and mouse embryos. Yay! Considering the similarities between mouse and human embryo development, the mouse results likely apply to you, too, says Patrick Tschopp, a geneticist at Harvard University who worked on one of the teams. "We can be fairly confident that in humans, it's pretty similar," he tells Popular Science.