SECONDARY PALATE  Figure 7a. 6 1/2 week old embryo b. 7 1/2 week old embryo c. 10 week old embryo The secondary palate forms by 2 palatal shelves which grow out from the medial side of the maxillary swellings. This happens behind the primary palate. The shelves first appear in week 6 and initially grow vertically down the side of the tongue which is dominating most of the oral cavity at this time. They elevate at a precise time during week 7 to a horizontal position above the dorsum of the tongue. What causes the shelves to elevate is still unclear. It has been ascribed to changes in the extracellular matrix composition of the shelves, variations in blood flow, muscular movements and cell contractions. The withdrawal of the embryo's face from against the heart prominence by uprighting of the head facilitates jaw opening. Mouth opening reflexes have been implicated in withdrawal of the tongue from between the vertical palatal shelves. Pressure differences between the nasal and oral regions due to tongue muscle contraction may contribute to shelf elevation. The shelves approach each other and make first contact with each other in the midline around the end of week 8. They  fuse with the primary palate anteriorly and the incisive foramen forms at this junction. It also fuses with the nasal septum which has grown down from above as a derivative of the frontonasal process. This does not occur posteriorly in the region of the future soft palate. Fusion proceeds from anterior to posterior and is usually complete by week 11. In the weeks after fusion, mesoderm in the anterior half of the palate develops centres of ossification and gives rise to the hard palate. Posteriorly, mesenchymal tissue of the first and fourth branchial arches migrates to supply the musculature of the soft palate. Tensor palati is from the first arch and the others are derived from the fourth. This explains the differential nerve supply of these muscles.