The auricle. The first pharyngeal cleft lies between the mandibular and the hyoid arch. The surface of both arches is at first smooth. At S15, each of the two pharyngeal arches gives rise to three hillocks. By S17, these hillocks can be seen distinctly. From S18 onward, they begin to fuse together. It is assumed that the tragus and the crus of the helix of the auricle arise from the hillocks of the mandibular arch; the three hillocks of the hyoid arch give rise to the helix and the antitragus. The external acoustic meatus has developed in the depth of the cleft. It ends at the tympanic membrane.