Periodic appearance and disappearance of microvilli associated with cleavage cycles in the egg of the ascidian, Halocynthia roretzi

The surface of eggs of the ascidian Halocynthia roretzi, observed with SEM, is essentially smooth until immediately before cell division when numerous microvilli appear and remain during cytokinesis. As the dividing blastomeres become closely adherent, however, the microvilli disappear and the eggs recover their smooth surface. This periodic appearance-disappearance of microvilli is repeated at each cleavage cycle up to at least the 32-cell stage. During blastomere adhesion, microvilli that have appeared near the plane of the first cleavage or of the bilateral symmetry seem to fuse together across the plane to form a zipper-like complex of cytoplasmic processes, which might be responsible for attachment of the two halves of these bilaterally symmetrical embryos via the blastomeres bordering the plane of symmetry.