The first indication of the developing ear can be found in embryos of approximately 22 days as a thickening of the surface ectoderm on each side of the rhombencephalon (Fig. 16.1). These thickenings, the otic placodes, invaginate rapidly and form the otic or auditory vesicles (otocysts) (Fig. 16.2). During later development, each vesicle divides into (a) a ventral component that gives rise to the saccule and cochlear duct and (b) a dorsal component that forms the utricle, semicircular canals, and endolymphatic duct (Figs. 16.3— 16.6). Together these epithelial structures form the membranous labyrinth.
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