Abstract
The developing jaws of embryonic mice, 9-12 days postconception, were dissected out, and the presumptive tooth-bearing areas were separated into the mesodermal and ectodermal portions. Recombinations were grown on the chick chorioallantois. It appeared that the morphogenic fields are determined by day 10 for molars and by day 10.5 for incisors. The ectodermal component determined tooth type from day 10.