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Cardiac development begins in the third week with the migration of cells to a region called the cardiogenic region. Here the cells begin differentiation into a pair of endocardial tubes which will eventually fuse and differentiate into a primitive heart tube. Initially the center of the cardiogenic area is rostral to the buccopharyngeal membrane, but later is pushed caudally by folding of the embryo. As the embryo folds in laterally, the two endocardial tubes are pushed toward each other in the midline where they fuse.