Key stages in the development of the mammalian secondary palate. Panels depict
ventral views of the oral cavity with the tongue removed. Formation of the secondary
palate occurs between gestational day (GD) 12 and GD 14 in mice (shown) and
gestational weeks 6.5 to 10 in humans, in which the developmental steps depicted are
similar. During this process, the pair of secondary palatal shelves (P2) grow toward
each other, make contact along their respective medial edge epithelia (MEE; GD 12 and
GD 13) to form the medial edge seam (MES; GD 14), effectively separating the oral from
the nasal cavities. Mx, maxilla; P1, primary palate; P2, secondary palate; T, tongue;
U, upper lip.