A fold forms as the semicircular ducts develop. This separates the utriculovestibular part from the sacculocochlear part (saccus cochlearis). This fold deepens towards the endolymphatic duct, thus dividing its end part into two branches: the utricular duct and saccular duct, known collectively as the utriculosaccular duct. The utricle and saccule remain in contact with each other through these two narrow ducts.
The sacculocochlear part becomes subdivided into an upper region called the saccule, and a lower region called the cochlear duct. At S19, the cochlear duct is initially L-shaped. It becomes further elongated and begins to roll up into a spiral. At S23, by the end of the embryonic period, it has already gone through 2½ rotations to establish its final shape. The connection between the saccule and cochlear duct later narrows to form the ductus reuniens.