Commitment to the cardiogenic cell lineage occurs early in development soon after gastrulation (the embryonic state following the blastula), approximately 48 h follow¬ing fertilization. The molecular basis for the formation of cardiac myocytes from the presumptive mesoderm requires further elucidation. The mesoderm consists of three germ layers that are incorporated in the building of the tissues and organs of the embryo. Cells committed to the cardiac
lineage are first seen to possess characteristics of cardiac myocytes prior to the formation of the tubular heart.
Within 10–14 days of gestation a pair of mainstem vessels are first differentiated; they are the primitive aortae. This pair of tubes comes to lie parallel and close to each other in the cephalic region of developing body cavity (see Fig. 1). These vessels appear in the splanchnic mesoderm of the pericardial region of the embryonic area and extend to the caudal end of the embryo. At the cephalic end the primitive aortae are continuous with another pair of stem vessels called the vitelline veins. Within a few days when the embryo is less than 2.5 mm long, the heart is formed in the septum transversum and the dorsal wall of the pericardium by fusion of the caudal parts of the ventral aortae. A section of main vascular tube specializes and possesses contractile elements within its walls. These walls of the heart tubes consist of a two-cell layer of myocardial cells and an internal single layer of endothelial cells separated from each other by a third layer called cardiac jelly.

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