During the sixth week of development, semicircular canals appear as flattened outpocketings of the utricular part of the otic vesicle (Fig. 16.6, A and B). Central portions of the walls of these outpocketings eventually appose each other (Fig. 16.6, C and D) and disappear, giving rise to three semicircular canals (Fig. 16.6; see also Fig. 16.8). Whereas one end of each canal dilates to form the crus ampullare, the other, the crus nonampullare, does not widen (Fig. 16.6). Since two of the latter type fuse, however, only five crura enter the utricle, three with an ampulla and two without.
Cells in the ampullae form a crest, the crista ampullaris, containing sensory cells for maintenance of equilibrium. Similar sensory areas, the maculae
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