BURSTThere’s a moment, just before the tight mass of cells that is a developing mouse embryo implants itself in the womb, that it all comes apart. Hundreds of tiny fluid-filled bubbles expand between each of the orb’s few dozen cells. The bubbles grow and press outward on cell membranes — and then, in a moment of fracture, pry them apart. Thin protein stands tether the cells together as the…