Descent of the heart
The heart moves gradually downwards (descensus). At S9, it lies before the prechordal plate; at S12, it is at the level of C3-C4; by the end of the embryonic period, it is in the middle of the thorax; in the adult, it is in the region of the lower thoracic vertebrae.
The conducting system
Pacemaker cells are present from the tubular heart stage onwards. Each portion of the tubular heart has its own frequency, which is highest at the venous end and lowest at the arterial end. The sinus region has the highest frequency and, at this stage, assumes the pacemaker function.
The first recognizable structure in the conducting system is the sinu-atrial node (seen at S14). It develops in the wall of the right sinual horn, where the sulcus terminalis is later situated. At S15-16, the atrioventricular node is detectable for the first time. It lies at the dorsal aspect of the AV canal in the internal myocardial layer. The His' bundle, which derives from the atrioventricular node, soon becomes defined.