The 5-week embryo is characterized by the presence of four pharyngeal clefts (Fig. 15.6), of which only one contributes to the definitive structure of the embryo. The dorsal part of the first cleft penetrates the underlying mesenchyme and gives rise to the external auditory meatus (Figs. 15.10 and 15.11). The epithelial lining at the bottom of the meatus participates in formation of the eardrum (see Chapter 16).
Active proliferation of mesenchymal tissue in the second arch causes it to overlap the third and fourth arches. Finally, it merges with the epicardial ridge in the lower part of the neck (Fig. 15.10), and the second, third, and fourth clefts lose contact with the outside (Fig. 15.10B). The clefts form a cavity lined with ectodermal epithelium, the cervical sinus, but with further development this sinus disappears.
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