The distribution of the large molecular weight tracer horseradish peroxidase is studied after intravenous injection in the enamel organ of the rat incisor with the electron microscope. The different zones of the enamel organ (differentiation, secretion, transition, maturation) are studied separately, and the functional implications of the results are considered for each zone. The cytology of tetracycline-induced abnormalities in the rat incisor enamel organ is studied with light and electron microscopes. The induced structural and functional abnormalities make it possible to improve present hypothesis of enamel formation and properties of enamel. The development of the ameloblast-odontoblast relationship is studied in the differentiation zones of the teeth of nurse shark and 3 species of anurans. The presence or absence of basal lamina at the time of enamel deposition is noted, and tracer studies are performed to find whether secretory material (of ameloblasts or odontoblasts origin) crosses the intact basal lamina.