Sequence of mouse ruga appearance as determined by Shh expression pattern. Embryos were ranked according to their weight (A: 80–90 mg; B-: 95 mg; C: 120–130 mg; D: 120–130 mg; E: 130–140 mg; F: 160–170 mg; G: 160–170 mg; H: 190–200 mg; I: 270–280 mg; J: 300–310 mg; K: 370–380 mg; L: 590–600 mg). Age of embryos (ED = embryonic day) is shown along the time scale. Dissected upper jaws hybridized with a mouse Shh probe are shown (left), together with a corresponding scheme showing ruga numbering (right). In this scheme, starting Shh expression is represented with dotted lines and the new forming (latest) ruga is underlined in yellow. Drawings of mouse heads are shown at ED 12.0, 14.0 and 15.0 to underline that the A/P elongation of the region delimited by ruga 2 (in green) and ruga 8 (in red) parallels the elongation of the snout. Note that in some cases ruga 9 has been removed during separation of the upper from the lower jaw.