The material was taken from 18 and 20 day embryos and 2, 4 and 6 day old rats. It was fixed in 10% neutral formalin, embedded in paraffin, sectioned at 5u. and stained in toluidin blue. Chromotrope (metachromatic) substances were observed in the tooth germ from very early stages in the development of the incisor and 1st molar teeth to the commencement of production of both dentine and enamel. The metachromatic material also occurred at the junction between the down-going epithelium and the connective tissue, the amelo-dentinal junction, and basal part of ameloblasts. When enamel and dentine begin to form it is found in the pulp, the predentine and the stratum intermedium.