Forces generated within microfilament rings are important during cellularisation of the early Drosophila embryo, according to Thomas and Wieschaus (see p. 863). During cellularisation, the embryonic membrane invaginates to surround the syncytial nuclei of the embryo. The cellularisation front - the leading edge of the invagination - is rich in cytoskeletal proteins, which suggests that contractile forces may be involved in invagination. Thomas and Wieschaus report that src64 and tec29, which encode tyrosine protein kinases involved in cytoskeletal regulation, are essential for the contraction of the microfilament rings present at the cellularisation front of the Drosophila embryo. They also show that Bottleneck, a protein previously shown to regulate cellularisation, counters the src64-dependent contraction of the microfilament rings. The researchers use these and other results to model how src64-dependent and src64-independent forces may be involved in cellularisation.