Page:Popular Science Monthly Volume 82.djvu/438

434 jaws may be seen after they have reached the hæmal surface and are assembled around the new mouth (Fig. 8). Ultimately the first two

 showing the three pairs of primitive jaws derived from their invertebrate ancestors, and their union to form the unpaired jaws typical of the vertebrates.

pairs fuse to form the fixed upper jaw, and the third pair forms the lower jaw.

The same three pairs of invertebrate jaws are present in the embryonic stages of man. They occupy the same relative positions as in the frog, and their subsequent history is the same (Fig. 9, A). Their presence and mode of growth largely control the architecture of the

 A, human embryo, a little more than a month old, showing the traces of invertebrate jaws in the mandibular, m. d., maxillary, m. x., and premaxillary, p. mx., arches. B, embryo a little less than two months old, with the preceding parts nearly united. C, mouth of adult, the contours showing its elemental structure.

human face. In rare cases they fail to unite in the normal way, giving rise to such defects in the adult as hare lip, cleft palate and open tear duct (Fig. 9, B). Even in otherwise normal faces, the presence of a pronounced "Cupid's bow" mouth, or prominent lateral lobes on the