the intraembryonic coelom during the third week splits this mesoderm into a splanchnic and a somatic layer. During the splitting process, the cardiogenic mesoderm becomes segregated in the splanchnic layer which is now called cardiogenic field. The intraembryonic coelom created in the cardiogenic area is called primitive pericardia! cavity.
The cardiogenic field is a horseshoe-shaped area which is located in the floor of the primitive pericardial cavity and overlies the endoderm of the embryonic disc. An inductive influence from the underlying embryonic endoderm appears to stimulate the formation of the cardiac primordium. This primordium appears in the form of a pair of endothelial tubes formed by the process of vasculogenesis in the mesoderm of the cardiogenic area. These tubes are called endocardia I heart tubes.
During the early 4th week\ the cephalic and lateral folding of the embryo brings the endocardial heart tubes in the thoracic region, where they meet along the midline and, by the day 22, bogin to fuse which each other to form a single heart tube (also called primary heart tube). The cranial end of the heart tube is the arterial end which is
connected to the two dorsal aorta by means of the first pair of aortic arches. The caudal end of the heart tube is the venous end which becomes connected to the vitelline, umbilical and common cardinal veins,:
As the heart tube forms, a series of constrictions (sulci) and dilatations appear along the heart tube. In the beginning, three divisions of the heart tube can be recognized. From the cranial to caudal direction, these divisions are: bulbus cordis, ventricle (also called primitive ventricle) and atrium (also called primitive atrium). The primitive atrium will give rise to parts of both (ie, right and left) atria, and the primitive ventricle will form most of the definitive left ventricle. The bulbus cordis and ventricle are separated from each other by the bulboventricular sulcus, whereas between the ventricle and atrium lies the atrioventicluar sulcus. Within the next 24 to 36 hours a fourth division becomes recognizable caudal to the atrium. This division, called sinus venosus, is formed by the confluence of the six veins (right and left vitelline, right and left umbilical and right and left common cardinal veins) which drain into the caudal end of the heart. The sinus venosus is a partially bifid chamber having two horns (called right and left sinus horns). Each sinus horn receives three veins (ie, vitelline, umbilical and common cardinal).
initially the primary heart tube consists only of endothelium. However, by day 22, a thick mass of splanchnic mesoderm invests the heart tube.