Prenatal ordering and postnatal sequence in dental development.

The chronology of human development involves complex systems of sequence or order that can scarcely be hinted at in the standard texts. Anatomy students learn one order of ossification for the carpal bones, beginning with the capitate and hamate, and proceeding through the triquetral, lunate, and the multangulars. Dental students learn the median order of tooth eruption as first molar (Ml), central incisor (Ii), lateral incisor (12), first premolar (P1), canine (C), second premolar (P2), second molar (M2), and third molar (M3). Students of embryology learn the sequence of development of the deciduous teeth in simple mesial to distal sequence, from the deciduous central incisor (ii) to the deciduous second molar (m2) (A through E), and this is as far as most texts currently go. Such orders as these are, of course, species specific. The carpal bones do not ossify in exactly the same order in Macaca, and the carpus is more complex in Macaca and Hylobates than in man. The order of permanent tooth eruption shows a progressive change through the primates, from a probable Ml-M2-M3-Il-12-Pl-P2-C order, to that in recent man. The order of epiphyseal union is notably different in the macaques and langurs from that in man today.' The changes in relative growth of component organs and tissues, which bring the embryo to adult proportions, are together grouped under the label morphogenesis, and together describe how the human embryo becomes, recognizably, a human adult. The uniquenesses of sequence or order of events in developmental timing do not as yet have a comparable name, although chronogenesis is one appropriate label. The order of events and relative timing in man are uniquely human. The remarkable relative delay in the second and third molars, and delayed carpal development relative to dental development, are both part of the sequential process whereby man is developmentally differentiated from monkeys.