Studies on the mechanism of blastula formation in starfish embryos denuded of fertilization membrane

A starfish egg, denuded of the fertilization membrane and placed on a glass surface, becomes a cell monolayer after several cleavages. This sheet of cells folds and forms a hollow sphere resembling a normal blastula at the 2⁹-2¹⁰-cell stage ('closing movement'). A marked morphological change was observed in each cell, preceding the closing movement. The surface of each blastomere differentiated into two parts: one was smooth, whereas the other was rough with microvilli. The smooth surface was more adhesive and flexible than the rough surface, suggesting that the closing movement may be driven by a local increase in cell adhesiveness.