During the fourth week of embryonic development, an area of thickened ectoderm develops, on each side of the frontonasal process called the nasal placodes or olfactory placodes, and appear immediately under the forebrain.

[2] The medial nasal process (nasomedial) on the inner side of each nasal pit merge into the intermaxillary segment and form the upper lip, crest, and tip of the nose.

The lateral nasal process from each side merge to form the alae of the nose.

[1] There is some evidence that development involves Sonic hedgehog and Fibroblast growth factor 8.

[3] This article incorporates text in the public domain from page 67 of the 20th edition of Gray's Anatomy (1918)