The best place for students to start is the Heart Tutorial. In human embryos the heart begins to beat at about 22-23 days, with blood flow beginning in the 4th week. The heart is therefore one of the earliest differentiating and functioning organs. The heart begins very early in mesoderm within the trilaminar embryonic disc.
Heart development (also known as cardiogenesis) refers to the prenatal development of the heart. This begins with the formation of two endocardial tubes which merge to form the tubular heart, also called the primitive heart tube.
Endocardial tubes. This early development is critical for subsequent embryonic and prenatal development. The heart is the first functional organ to develop and starts to beat and pump blood at around day 21 or 22. The heart begins to develop near the head of the embryo in the cardiogenic area.