It appears as a midline swelling from the first pharyngeal arch late in the fourth week of embryogenesis.

[1] In the fifth week, a pair of lateral lingual swellings (or distal tongue buds) develop above and in line with the median tongue bud.

These swellings grow downwards towards each other, quickly overgrowing the median tongue bud.

[1] The line of the fusion of the distal tongue buds is marked by the median sulcus.

[1] This article incorporates text in the public domain from page 1102 of the 20th edition of Gray's Anatomy (1918)