Auditory vesicle
When the mouth of the auditory pit
Auditory pit
The auditory pit, also known as the otic pit, is the first rudiment of the internal ear. It appears shortly after that of the eye, in the form of a patch of thickened ectoderm, the auditory plate, over the region of the hind-brain. The auditory plate becomes depressed and converted into the...

 is closed, and thus a shut sac, the auditory vesicle (or otic vesicle), is formed; from it the epithelial lining of the membranous labyrinth
Membranous labyrinth
The receptors for the senses of equilibrium and hearing are housed within a collection of fluid filled tubes and chambers known as the membranous labyrinth...

 is derived.

The vesicle becomes pear-shaped, and the neck of the flask is obliterated.

From the vesicle certain diverticula are given off which form the various parts of the membranous labyrinth.

One from the middle part forms the ductus and saccus endolymphaticus, another from the anterior end gradually elongates, and, forming a tube coiled on itself, becomes the cochlear duct, the vestibular extremity of which is subsequently constricted to form the canalis reuniens.

Three others appear as disk-like evaginations on the surface of the vesicle; the central parts of the walls of the disks coalesce and disappear, while the peripheral portions persist to form the semicircular ducts; of these the superior is the first and the lateral the last to be completed.

The central part of the vesicle represents the membranous vestibule, and is subdivided by a constriction into a smaller ventral part, the saccule
The saccule is a bed of sensory cells situated in the inner ear. The saccule translates head movements into neural impulses which the brain can interpret. The saccule is sensitive to linear translations of the head, specifically movements up and down...

, and a larger dorsal and posterior part, the utricle.

This subdivision is effected by a fold which extends deeply into the proximal part of the ductus endolymphaticus, with the result that the utricle and saccule ultimately communicate with each other by means of a Y-shaped canal.

The saccule opens into the cochlear duct, through the canalis reuniens, and the semicircular ducts communicate with the utricle.
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