Closure of the Secondary Palate in Three Strains of Mice

Abstract

At the time when Peter reviewed the subject of secondary mammalian palate embryology (Peter, 1924), it had been established that the secondary palate is derived from two shelves of tissue, which originate in the dorsolateral wall of the oral cavity and grow downward toward the lower jaw, passing lateral to the tongue. The shelves subsequently come to lie in the transverse plane dorsal to the tongue, where they fuse with the nasal septum and with each other, thus forming the roof of the mouth. The problem in this sequence is how the shelves manage to reach a position dorsal to the tongue after having had the tongue tightly wedged between them. Various mechanisms postulated to explain this change of position were reviewed by Lazzaro in 1940. The theories fell into three categories: (1) external forces such as muscular pressure by the tongue, (2) growth changes involving regression of the ventral portion and an outgrowth in the horizontal plane; and (3) a rapid rotation of the shelves due to some intrinsic force.