Frontonasal prominence

During the third week two areas of thickened ectoderm, the olfactory areas, appear immediately under the fore-brain in the anterior wall of the stomodeum, one on either side of a region termed the frontonasal prominence (or process).

By the upgrowth of the surrounding parts these areas are converted into pits, the olfactory pits, which indent the frontonasal prominence and divide it into a medial and two lateral nasal processes.