We have revealed the fundamental embryonic structure of deformation twins using in situ mechanical testing of magnesium single crystals in a transmission electron microscope. This structure consists of an array of twin-related laths on the scale of several nanometers. A computational model demonstrates that this structure should be a generic feature at the incipient stage of deformation twinning when there are correlated nucleation events. Our results shed light on the origin of twinning-induced plasticity and transformation toughening, critical to the development of advanced structural alloys with high strength, ductility, and toughness.