Period in which the developing tooth takes on a bell shape in cross section, immediately preceding the "advanced bell stage", in which the hard tissues (dentin and enamel) form the crown of the tooth.
Source:
PAC, 2016, 88, 713. 'Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)' on page 729 (https://doi.org/10.1515/pac-2015-1202)