Early in the fourth week, a thickening of the surface ectoderm on each side of the hindbrain (rhombencephalon) forms the otic placode. It invaginates to form the otic (auditory) vesicle, the primordium of the membranous labyrinth.
Each vesicle divides into the ventral component, which gives rise to the saccule and the cochlear duct (scala media), and the dorsal portion which forms the utricle, semicircular canals and endolymphatic duct.
The spiral organ of Corti differentiates from the cells in the wall of the cochlear duct.
The mesenchyme around the auditory vesicle differentiates into a cartilaginous otic capsula, which later ossifies to form the bony labyrinth. Part of the cartilaginous shell undergoes vacuolization, and two perilymphatic spaces (scala vestibuli and scala tympani) are formed.

1. Otic pit
2. Wall of rhombencephalon
3. Pharynx
4. Mesenchyme
5. Dorsal aorta
6. Endoderm

1. Statoacoustic ganglion
2. Otic vesicle
3. Surface ectoderm
4. Wall of rhombencephalon
5. Tubotympanic recess
6. First pharyngeal cleft
7. Mesenchyme
8. Dorsal aorta

Semicircular canals subsequently differentiate as evaginations of the utricular part of the vesicle.
Sensory areas developing from the walls of auditory vesicle form the crista ampullaris (in the semicircular canals) and maculae acusticae (in the utricle and saccule). Small groups of cells break away from the wall of the auditory vesicle to form the statoacoustic ganglion, which suplies the sensory cells of the organ of Corti, the crista ampullaris and the maculae acusticae.

1. Scala vestibuli
2. Scala media
3. Scala tympany
4. Mesenchyme
5. Vestibular membrane
6. Organ of Corti
7. Spiral ligament
8. Basilar membrane
9. Tectorial membrane
10. Auditory nerve fibers
11. Spiral ganglion