Later milestones include the work by W. J. Kerr and W. L. Bender in 1922, who produced an electrocardiogram showing ventricular tachycardia evolving into ventricular fibrillation. The re-entry mechanism was also advocated by DeBoer, who showed that ventricular fibrillation could be induced in late systole with a single shock to a frog heart. The concept of "R on T ectopics" was further brought out by Katz in 1928. This was called the “vulnerable period” by Wiggers and Wegria in 1940, who brought to attention the concept of the danger of premature ventricular beats occurring on a T wave.