Tooth development begins in the human embryo 28 to 40 days after ovulation. Epithelial cells grow into the ectomesenchymal parts of the jaw. An epithelial protrusion develops. Further penetration of the epithelial cells into the ectomesenchyma results in the formation of the dental papilla (Figs. 1, 2 and 3). At this point, the cells for the formation of the tooth hard substance are differentiated.