During the sixth week, the nasal pits deepen considerably, partly because of growth of the surrounding nasal prominences and partly because of their penetration into the underlying mesenchyme (Fig. 15.30A). At first the oronasal membrane separates the pits from the primitive oral cavity by way of the newly formed foramina, the primitive choanae (Fig. 15.30C). These choanae lie on each side of the midline and immediately behind the primary palate. Later, with formation of the secondary palate and further development of the primitive nasal chambers (Fig. 15.30D), the definitive choanae lie at the junction of the nasal cavity and the pharynx.
Paranasal air sinuses develop as diverticula of the lateral nasal wall and extend into the maxilla, ethmoid, frontal, and sphenoid bones. They reach their maximum size during puberty and contribute to the definitive shape ofthe face.
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