The status of the palatal shelves of shelves of human fetuses (before, during and after palatal shelf elevation) was correlated with mouth opening reflexes recorded cinematographically by Doctor Davenport Hooker for many of the same fetuses. Some deductions and conclusions follow.
Mouth opening reflexes begin before palatal shelf elevation and are a significant factor in tongue withdrawal from between the vertical shelves through traction resulting from mandibular depression. (2) The upward pull on the palatal shelves created by differences in pressure between the closed nasal area and the oral region when the tongue is withdrawn (epithelial plugs fill external nares completely) is an active force for palatal shelf elevation. (3) Palatal shelf movements are variable in character and closure sometimes results in the shelf borders enfolding each other. (4) On mouth closure, pressure on the shelves by the tongue and the amniotic fluid entering the mouth helps maintain shelf position before fusion. (5) Human palatal shelf elevation without shelf contact in the midline is not necessarily part of normal closure, but, especially if the shelves are far apart, a stage in cleft palate formation. (6) Increased mandibular growth follows rather than precedes tongue withdrawal. Other factors influencing closure are discussed.