The ear, too, is a complex organ developmentally. Its initiation occurs at about the same time as that of the lens, with the appearance of an auditory placode on either side of the anterior part of the hindbrain. Development of the membranous labyrinths varies in different vertebrate groups; in mammals the basal papilla develops the coiled cochlea, and in most groups there is elaboration and ... [Show full abstract] subdivision of the sensory areas. The lappets on the utriculus flatten, obliterating their cavities except around the edges where they persist as tubes, forming the semicircular canals in their several planes. The first or hyoid branchial pouch lies immediately ventral to the developing labyrinth and, indeed, its dorsal wall is pressed against the ventral side of the sacculus. The area of contact between the pouch and the skin grows as the middle ear develops and the middle-ear cavity swells, increasing the area of the tympanum. The bones of the arch soon appear as a bulge on its posterior wall and, as the cavity swells still further, it pushes around them and meets behind, leaving the arch as a pillar, the columella auris, surrounded by the endodermal lining of the middle ear, and outside this by the cavity of the middle ear.