The embryonic heart starts developing from two distinct vascular tubes of the embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 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

Neural plate

In human embryology, formation of neural plate is the first step of neurulation. It is created by a flat thickening opposite to the primitive streak of the ectoderm.-Development:...

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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

Endocardium

The endocardium is the innermost layer of tissue that lines the chambers of the heart. Its cells are embryologically and biologically similar to the endothelial cells that line blood vessels....

. A primitive heart tube is formed.

Initial Beats

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.

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