The philtrum is the vertical groove that runs from the underside of the nose to the top lip. Formed at between two and three months' gestation, it marks the place where different sections of the developing face are united. When something goes wrong with the process, the baby is born with a cleft lip. In many mammals, the philtrum assists with odour-detection but, in modern-day humans and other higher primates, who rely more on vision, it serves no particular role.