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jaws may be seen after they have reached the hasmal surface and are 

 assembled around the new mouth (Fig. 8). Ultimately the first two 



Fig. 8. Head of an Embryo Frog, 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 em- 

 bryonic 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 



Fig. 9. Figures Illustrating the Mode of Growth of the Human Face. 



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 



