A thickened and indented region in the midline in the floor of the rostral part of the pharyngeal region of the foregut, located between the second and third branchial arches that is the first indication of the thyroid primordium. [Kaufman_MH_and_Bard_JBL, The_anatomical_basis_of_mouse_development_(1999)_San_Diego:_Academic_Press, p.132] [ https://www.worldcat.org/search?q=bn%3A0124020607 ]