<P> From splanchnopleuric mesenchyme, the cardiogenic region develops cranially and laterally to the neural plate . In this area, two separate angiogenic cell clusters form on either side and coalesce to form the endocardial tubes . As embryonic folding continues, the two endocardial tubes are pushed into the thoracic cavity, where they begin to fuse together, and this is completed at about 22 days . </P> <P> At around 18 to 19 days after fertilisation, the heart begins to form . This early development is critical for subsequent embryonic and prenatal development . The heart is the first functional organ to develop and starts to beat and pump blood at around day 21 or 22 . The heart begins to develop near the head of the embryo in the cardiogenic area . Following cell signalling, two strands or cords begin to form in the cardiogenic region As these form, a lumen develops within them, at which point, they are referred to as endocardial tubes . At the same time that the tubes are forming other major heart components are also being formed . The two tubes migrate together and fuse to form a single primitive heart tube, the tubular heart which quickly forms five distinct regions . From head to tail, these are the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus . Initially, all venous blood flows into the sinus venosus, and contractions propel the blood from tail to head, or from the sinus venosus to the truncus arteriosus . The truncus arteriosus will divide to form the aorta and pulmonary artery; the bulbus cordis will develop into the right ventricle; the primitive ventricle will form the left ventricle; the primitive atrium will become the front parts of the left and right atria and their appendages, and the sinus venosus will develop into the posterior part of the right atrium, the sinoatrial node and the coronary sinus . </P> <P> The central part of cardiogenic area is in front of the oropharyngeal membrane and the neural plate . The growth of the brain and the cephalic folds push the oropharyngeal membrane forward, while the heart and the pericardial cavity move first to the cervical region and then into the chest . The curved portion of the horseshoe - shaped area expands to form the future ventricular infundibulum and the ventricular regions, as the heart tube continues to expand . The tube starts receiving venous drainage in its caudal pole and will pump blood out of the first aortic arch and into the dorsal aorta through its polar head . Initially the tube remains attached to the dorsal part of the pericardial cavity by a mesodermal tissue fold called the dorsal mesoderm . This mesoderm disappears to form the two pericardial sinuses the transverse and the oblique pericardial sinuses, which connect both sides of the pericardial cavity . </P> <P> The myocardium thickens and secretes a thick layer of rich extracellular matrix containing hyaluronic acid which separates the endothelium . Then mesothelial cells form the pericardium and migrate to form most of the epicardium . Then the heart tube is formed by the endocardium, which is the inner endothelial lining of the heart, and the myocardial muscle wall which is the epicardium that covers the outside of the tube . </P>

What germ layer gives rise to blood islands