Figure 15.30 A. Sagittal section through the nasal pit and lower rim of the medial nasal prominence of a 6-week embryo. The primitive nasal cavity is separated from the oral cavity by the oronasal membrane. B. Similar section as in A showing the oronasal membrane breaking down. C. A 7-week embryo with a primitive nasal cavity in open connection with the oral cavity. D. Sagittal section through the face of a 9-week embryo showing separation ofthe definitive nasal and oral cavities by the primary and secondary palate. Definitive choanae are at the junction of the oral cavity and the pharynx.
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