The mesenchyme between the two nasal sacs originating from the frontal process represents the primary nasal septum. The septum does not grow from the roof of the oronasal cavity but extends backwards and downwards. It fuses together with the secondary palatal processes. At S18, a groove appears on each side of the nasal septum, just above the primary palate. This enlarges to a pouch oriented towards dorsal and cranial directions, called the vomeronasal organ. This organ is generally present in adults and is considered to be a rudimentary olfactory organ.
With the fusion of the secondary palatal processes and the premaxilla, the initial oronasal cavity becomes subdivided into the mouth and the nasal cavities. The nasal septum leads to the formation of the right and left nasal cavities, which open through the definitive choanae into the pharynx. Lower, medial and upper nasal conchae form in the lateral wall of the nasal cavity. By S17, the first protrusions can already be seen. The paranasal sinuses only arise late in the fetal period and during the early postnatal period. The caudal extensions of the palatal processes grow further caudally beyond the limits of the definitive choanae and form the soft palate (palatum molle). The palatal processes and the primary palate fuse together with the exception of a small opening. In the adult, this is called the incisive foramen. The mouth cavity opens into the pharynx through the isthmus faucium.