Oral and nasal cavity

The nasal sac is closed off at the bottom and at the back by the epithelial lamina (nasal fin). While it deepens, the nasal fin at the back of the nasal sac shrinks to become the oronasal membrane. By S18, it finally disappears. The primary nasal cavities now open through the primary choanae into the primary mouth cavity. At S17, the roof of the mouth cavity bulges upward. Near the former oropharyngeal membrane a slit-shaped sac (Rathke's pouch) evaginates towards the infundibulum of the hypophysis.