Fig. 4. Absence of semicircular duct formation and formation of a common macula in a fused utriculosaccular chamber in Hmx2^lacZ null mutants. (A) Section illustrating normal development of the semicircular ducts in the control embryo at E13.5. (B) Hmx2^lacZ–/– inner ear at E13.5 illustrating the formation of two vestibular diverticulae that are dorsal and lateral to a fused utriculosaccular chamber. (C) Control inner ear at E16.5 illustrating normally developed utricle and saccule chambers with associated maculae and horizontal ampulla. (D) Hmx2^lacZ–/– inner ear at E16.5 illustrating a common macula within a fused utriculosaccular chamber, lacking any distinction between the utricle and saccule other than the location of the maculae, as both appear to be combined ventrally. (E) Section illustrating normal development of the horizontal crista and ampulla coming off the utricular chamber in a control embryo at E18.5. (F) Hmx2^lacZ–/– inner ear at E18.5 demonstrating a more severe dysgenic vestibular system and increased fusion and enlargement of the common utriculosaccular chamber relative to earlier stages. D, dorsal; HC, horizontal crista; L, lateral; M, medial; MS, macula of saccule; MU, macula of utricle; S, footplate of the stapes; SD, semicircular duct; V, ventral; VD, vestibular diverticum.