The ear consists of three parts that have different origins, but function as one unit. The internal ear originates from the otic vesicle, which is detached from surface ectoderm during the fourth week of development. The vesicle is then divided into a ventral component, which gives rise to the saccule and the cochlear duct, and a dorsal component, which gives rise to the utricle, semicircular canals, and endolymphatic duct. The epithelial structures thus formed are known collectively as the membranous labyrinth. Except for the cochlear duct, which forms the organ of hearing, all other structures derived from the membranous labyrinth compose the organ of balance. Structures of the middle and external ear develop from the structures and derivates of the pharyngeal apparatus. The tympanic cavity and auditory tube derive from the endoderm of the first pharyngeal pouch. The ossicles develop from the mesenchyme of the first (malleus and incus) and second (stapes) pharyngeal arches. The muscles of the middle ear and their nerves derive from the pharyngeal apparatus derivates. The tympanic membrane arises from the mesenchyme covered by the ectodermal epithelium on the outer side, and with the endoderm of the first pharyngeal pouch on the inner side. The external auditory meatus derives from the first pharyngeal groove. The auricle develops from six mesenchymal hillocks along the first and second pharyngeal arch that surround the first pharyngeal groove. The genes responsible for the development of the internal ear are: Pax2, which is responsible for the formation of the cochlear duct, Nkx5, which is responsible for the formation of the semicircular ducts, and Notchl, Mathl, Jag2, Hesl and Hes2, which are responsible for the differentiation of sensory hair cells within the hearing and balance organs. The retinoblastoma gene has a distinct role in the maturation of cochlear and vestibular hair cells.