During the third week of embryonic development, 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.
^Abzhanov A, Cordero DR, Sen J, Tabin CJ, Helms JA (December 2007). "Cross-regulatory interactions between Fgf8 and Shh in the avian frontonasal prominence". Congenit Anom (Kyoto)47 (4): 136–48. doi:10.1111/j.1741-4520.2007.00162.x. PMID17988255.