These placodes invaginate to form otic pits, and then otic vesicles. Two vascular fringes from the lower fold invaginate the roof and form the choroid plexus. The cementum and the enamel invaginate into the thick layer of dentin. These actomyosin rings invaginate to separate all nuclei for one another in the syncytial blastoderm. As development progresses, the lens placode begins to deepen and invaginate.