The heart tube develops from angioblasts which form small blood vessels within the region anterolateral to the neural plate. This is the cardiogenic area.
With enlargement of the cephalic region of the embryo over the cardiogenic area in a cephalic direction, the region of the angioblasts within the splanchnic mesoderm is displaced ventrally and rostrally. In tandem, there is a ventral folding of the same mesoderm which acts to fuse one relatively large, angioblast-derived vessel from each side. The vessel resulting from fusion is the heart tube. It lies at the level of the primordial thorax. Also, folding and fusion of the mesoderm produces a space within which the heart tube is suspended from the dorsal wall by further mesoderm termed the dorsal mesocardium. The space is the pericardial cavity. Loss of cells within the dorsal mesocardium leads to the formation of the transverse pericardial sinus in the mature heart.
The primitive heart tube gains a connection with the dorsal aortae, also formed from angioblasts, via the primitive aortic arches at the cephalic end. At the caudal end, the tube is in continuity with the primitive venous system. The heart tube becomes contractile soon after its condensation; it sends pulsatile waves of blood into the dorsal aortae.
Subsequently but with a large amount of temporal overlap, the heart tube:
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