Only two weeks after fertilization, the first sign of the formation of the three axes of the human body (head/tail, ventral/dorsal, and right/left) begins to appear. At this stage, known as gastrulation, a flat and featureless sheet of cells folds into a living blueprint for the body, a fleeting transformation into axes and layers that will determine how every tissue develops. This all-important moment has, however, long stood beyond the reach of science, occurring too early and deeply within the uterus to study directly.