At week 10, a liquid-filled space called the scala tympani develops in the capsule around the cochlear duct, along the primordium of the spiral organ. Toward the end of month 4, a similar space forms on the opposite side of the cochlear duct, called the scala vestibuli. Both spaces are continuous at the apex of  the cochlea, which is known as the helicotrema. The wall between the cochlear duct and the scala vestibuli becomes the vestibular membrane; the wall towards the scala tympani becomes the basilar membrane. On the basilar membrane, in the cavity of the cochlear duct (scala media), the spiral organ differentiates; this is comprised of the inner and outer hair cells and the tectorial membrane.