(C) 3-5 weeks[?]The Beginning:The human heart begins to beat and pump blood through the embryo around day 22 of gestation.The electric stimulus that triggers contractions in the myocardium arise spontaneously within the myocardium itself, and propagate from cell to cell. Input from the central nervous system can modify the heart rate (the frequency of heart beats), but it does not initiate beats.The ability of cardiac myocytes to beat is an intrinsic property of these cells. It has been found that myocytes removed from the early heart and grown in culture will beat sporadically, and if they become connected to each other, will then begin to beat rhythmically, in unison.As a functional organ, the heart begins to beat very early, even before it has assumed its final form.Interestingly, the heart begins to beat even before structures such as valves and septa (singular: septum; the muscular walls that divide the chambers) have formed. The initial contractions are peristaltic - that is, they proceed in a wave like fashion along the length of the heart. IMPORTANT EVENTS FOLLOWING FERTILIZATION0' hourFertilization (day-15 from LMP)30 hours2 cell stage (blastomeres)40-50 hours4 cell stage72 hours12 cell stage96 hours16 cell stage. Morula enters the uterine cavity5th dayBlastocyst4-5th dayZona pellucida disappears5-6th dayBlastocyst attachment to endometrial surface6-7th dayDifferentiation of cyto and syncytiotrophoblast layers10th daySynthesis of hCG by syncytiotrophoblast9-10th dayLacunar network forms10-11th dayTrophoblasts invade endometrial sinusoids establishing uteroplacental circulationInterstitial implantation completed with entire decidual coverage13th dayPrimary villi16th daySecondary villi21st dayTertiary villi21st-22nd dayFetal heart, Fetoplacental circulation