Fig. 23-11. The partitioning of the truncus arteriosus, and the development of the semilunar valves. A, Truncus arteriosus. B, C, and D, An intermediate position during the rotation that the heart undergoes during development; the terminology used by some authors for naming the cusps is shown. E, The usual position of the valves of the adult heart in situ; the terminology used in this book is shown. Click for high resolution image.