The differentiating epithelial otic vesicle lies in a loose mesenchyme. Cavities develop around the membranous labyrinth in the mesenchyme situated directly near to the otic vesicle, forming the perilymphatic space. The liquid filling this space is called perilymph. At S14-15, the mesenchymal wall of the perilymph condenses. At S17-18, it forms the cartilaginous otic capsule which later ossifies. The perilymphatic cavities around the utricule and saccule merge together to form the vestibule or cisterna vestibuli. This vestibule continues into the perilymphatic spaces of the scala vestibuli and tympani. The perilymphatic and the subarachnoid spaces can remain in contact with each other through the perilymphatic duct.