This report is concerned primarily with the chronological relations among stages of palatal closure, changes in facial profile, and the sex of a select group of human embryos representing the critical period of palate development. Histological serial sections of 40 human embryos ranging in age from 6 to 9 weeks and free from gross defects were studied using light microscopy and graphic reconstructions. Maxillary prognathism usually occurred concomitantly with vertically oriented palatal shelves. As the shelves progressively assumed a horizontal position prior to palatal closure, a profile reversal occurred, i.e., the lower jaw became prognathic. Both profile reversal and reorientation of the palatal shelves occurred earlier in males (seventh week) than females (eighth week). These patterns suggest a sexual dimorphism in which the palatal shelves in females are apart and vertical for a relatively longer time than in males. Also, the concordance between palatal-shelf elevation and mandibular prognathism suggests that the forward repositioning of the lower jaw may contribute to the removal of the tongue from between the palatal shelves.
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http://deepblue.lib.umich.edu/bitstream/2027.42/38119/1/1420020404_ftp.pdf