The author has studied 194 cases of polydactyly in the Japanese, and found 87% of the cases to be preaxial polydactyly. Preaxial polydactyly differed from postaxial polydactyly in many points. In preaxial polydactyly, the more proximal the level of bifurcation, the more frequent was the complication by a brachymesophalangy 5. Hypoplasia of the thenar muscles, while also varying with the level of bifurcation, was more closely connected with the presence of triphalangism. In almost all cases of preaxial polydactyly, the apical ectodermal ridge may be the site of pathogenesis.
