In the heart of Passer the specialized conducting tissue develops earlier than the nervous elements. The sinuatrial and the atrioventricular nodes appear simultaneously at the 15 mm stage and the atrioventricular bundle at the 23 mm stage. During development the sinuatrial node shifts its position due to the gradual absorption of the sinus venosus into the right atrium. The caudal end of the interatrial septum is enlarged to accomodate the compound atrioventricular node. In the adult bird the bundle branches extend through the atrioventricular valves of their sides and reach the other wall of the ventricular cavity. Among the specialized tissues, the sinuatrial region is the first to receive the nerve supply. The atrioventricular nodal and bundle regions are innervated by the ganglia present at the atrioventricular sulcus. The ganglionic cells are bipolar or multipolar. The nerve fibers terminate in “end-bulbs.” Only the atrioventricular node shows a few fine nerve fibers within its substance. No direct nerve connection is present between the nodes and the bundle. It seems that the nervous elements present in the regions of the specialized tissues are concerned only with the control of the activity of these structures.