The remaining lateral part of the otic vesicle forms the utriculosaccular space, which remains connected to the endolymphatic duct. The upper part of the utriculosaccular space is called the utriculovestibular part (saccus vestibularis). At S16, its surface area increases. At the same time, it flattens into three mutually perpendicular planes, each of which forms a discoid structure. At S18, the central parts of the discs fuse together and degenerate, while the peripheral parts remain as semicircular ducts. The semicircular ducts open into the lower region of the utriculovestibular part, which becomes the utricule.