The heart develops from a primitive structure called the heart tube (top left). As the heart tube increases in length, the top of the tube (known as the arterial pole; AP) and the bottom of the tube (venous pole; VP) remain in position, so the tube buckles and twists to form a helical loop (top right), which undergoes further development to form a mature heart (not shown). The loop is always in a rightward direction (which is on the left in this ventral view). Le Garrec et al. show that opposing rotations at the two poles, together with the asymmetric ingression of cells into the heart, ensures that the loop is to the right. Simultaneously, the dorsal mesocardium, which attaches the heart tube to the body wall of the embryo (orange; bottom left), breaks down as the heart grows. This releases the heart tube so that it is only attached at the arterial and venous poles, allowing the helical loop to form (bottom right).