Physiological context: The philtrum is the dip or groove in the middle of the upper lip below the nose (link). It is a feature of most mammals, thought to draw water from the mouth to the nose to keep the nose moist but is vestigial in humans. It is formed around the second month as the parts of the embryo's face grow together and meet there (link).
Anyway, I've heard that it is formed by an angel striking the lip just before a baby is born so it forgets all the Torah it learned in the womb. What is the source for this tradition? The best I found was Nidah 30B (PDF link) (point by point Daf Yomi link) which says, "Once he is born, an angel slaps him on the mouth, and he forgets everything." What is the link between that statement and it forming the philtrum? (I'm sure the chachamim would have seen the feature on stillborn fetuses, non-Jews, and maybe even noticed it on mammals, so it might not even make sense for them to say the angel forms the philtrum. I'm not sure if they do or don't; I'm asking for the source for the part of the tradition that says it changes the face.)