The embryonic heart starts developing from two distinct vascular tubes of the embryo around day 19 of the embryo's existence, under the signals of genetic expression arrived from the surrounding cardiogenic region — the embryonic "cranium", lateral to the neural plate.
Migration with Differentiation
For the next 3 days the embryonic folding will push the endocardial tubes from the cardiogenic region to a midline position, inside the future thoracic region. Once the endocardial tubes have reached this midline position they fuse, initiating the development of the future endocardium. A primitive heart tube is formed.
This heart attains functionality and starts beating at 22 days after the conception  (about 5 weeks after the last menstrual period). It can sometimes be seen as a flickering in the embryonic chest by an ultrasound performed during the fourth week after conception.