Abstract The cellular basis of the morphogenesis of cell sheets has been studied in sea urchin larvae ( Psammechinus miliaris) by means of time-lapse cinematography. Particular attention has been focussed on the role of localized increases in adhesion between cells in the ectoderm sheet, without loss of contact with the hyaline membrane, bringing about thickening and a resulting increase in curvature in such sheets. It seems as if the major changes in external form of the larva from the blastula stage up to the time of extension of the arms can be largely explained in such terms. The time-sp⊙ace pattern of such changes appear to be simple — they start in a field or ring-like zone around the vegetal pole, which results in the pear-like form of the early gastrula, and later appear in another ring-like zone around the ventral side, including the ventral surface of the animal pole, which produces the ventral flattening of the gastrula. The resulting thickenings in the latter case form the ventral ciliated band and the animal plate. The thickenings are sometimes associated with the formation of spaces between the cells in the adjacent ectoderm. The ectoderm cells bordering the spaces become active, and close the spaces by means of a pseudopodal mechanism. The relationship between these changes in the ectoderm and the other early morphogenetic processes is discussed.