CD-1 mice (8 to 30 days old) were studied usinglight and electron microscopy. Scanning electron microscopy revealed that root formation precedes closure of the epithelial diaphragm. This results in the appearance of seams with gaps along the fusion lines of the leaflets. Leaflets are broad and irregular in outline. Seams appear in the interradicular areas as well as on the root surfaces. Cementum deposits form a ridge along the seams. Third molars often develop one leaflet less than required for normal development. Incomplete root separation results, with the roots connected internally. Apical ends stay wide open during development. Final constriction appears to be due to cementum deposition. Each root displays two or three apical foramina when fully formed.