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Fig. 12B. —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. Medial-developing external auditory canal becomes filled with meatal plug (MP) consisting of proliferating ectodermal cells that subsequently recanalize. Tubotympanic recess (TR) has deepened to form tympanic cavity (TC). Otic vesicle forms endolymphatic duct and sac (ES), early semicircular canals (SC), and cochlear diverticulum (CD). Membranous labyrinth induces surrounding mesenchyme to condense and ultimately form bony labyrinth. S = stapes, I = incus, M = malleus.