Summary—The synchronous cellularization of theDrosophilaembryo at the blastoderm stage provides a unique system for studying the molecular mechanisms involved in cytokinesis, using genetical and biochemical approaches. The cellularization process requires the major components of the embryonic cytoskeleton that are deposited into the egg during oogenesis. Genetical analysis indicates that it requires also the products of additional maternally‐acting genes, as well as that of a limited set of zygotically‐acting genes. The cellularization defective phenotypes associated with small deficiencies uncovering these latter loci reveal specific steps within this complex process. The molecular analysis of these genes will ultimately provide meaningful insights into the normal process of cellularization. Among them, theserendipityα gene encodes a membrane‐associated protein, which is exclusively accumulated during cellularization, and is required for the reorganization of the microfilaments as the onset of cellula