In this way, the nasal sac is formed. At S16-18, by the fusion of the lateral and medial nasal processes an epithelial lamina (nasal fin or "Epithelmauer" of Hofstetter) appears. Later on, this shrinks to a small caudal remnant called the oronasal membrane. At S18, the fin is replaced by a mesenchymal bridge which separates the nasal sac from the mouth cavity. The mesenchyme from premaxilla and maxilla forms the primary palate (animation: facial development stage 15-17).
At S17, the ventral lower end of the medial nasal process becomes spherical (globular process, processus globularis).