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Fig. 12C. —Drawings reveal development of external, middle, and inner ear. Gray area = cartilage condensations of neural crest origin, areas with horizontal lines = ectoderm, areas with diagonal lines = endoderm. External auditory canal (EAC) is separated from tympanic cavity (TC) by tympanic membrane (TM), which has inner layer of endoderm, middle layer of mesoderm, and outer layer of ectoderm. Tympanic cavity has expanded around ossicles, leaving endodermal surface layer. Membranous labyrinth and surrounding bony labyrinth are in apposition with tympanic cavity via oval window (OW) and round window (RW). ES = endolymphatic sac, ED = endolymphatic duct, S = saccule, CD = cochlear duct, U = utricle, AT = auditory tube, SSC = superior semicircular canal, LSC = lateral semicircular canal, PSC = posterior semicircular canal, ET = epitympanum.