The material makes contact with the cell membrane, which then invaginates. The ectomesenchyme tissue also invaginates superficially to shape the primitive dental pulp. During clathrin-mediated endocytosis, the cell membrane invaginates to form a budding vesicle. During the earlier stages of embryogenesis, the otic placode invaginates to produce the otic cup. Lingual tonsils are covered externally by stratified squamous nonkeratinized epithelium that invaginates inward forming crypts. Next, the placode invaginates and forms a pouch referred to as the lens pit. An area of the ectoderm thickens, then invaginates to become a "shell gland".