During mouse preimplantation development, the formation of the blastocoel, a fluid-filled lumen, breaks the radial symmetry of the blastocyst. What controls the formation and positioning of this basolateral lumen remains obscure. We find that accumulation of pressurized fluid fractures cell-cell contacts into hundreds of micron-size lumens. Microlumens eventually discharge their volumes into a single dominant lumen, which we model as a process akin to Ostwald ripening, underlying the coarsening

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... of foams. Using chimeric mutant embryos, we tune the fracking of cell-cell contacts and steer the coarsening of microlumens, allowing us to successfully manipulate the final position of the lumen. We conclude that hydraulic fracture of cell-cell contacts followed by directed coarsening of microlumens sets the first axis of symmetry of the mouse embryo.