Script: As the first blood cells appear in the yolk sac, blood vessels form throughout the embryo, and the tubular heart emerges.
Almost immediately, the rapidly growing heart folds in upon itself as separate chambers begin to develop.
The heart begins beating 3 weeks and 1 day following fertilization.
The circulatory system is the first body system, or group of related organs, to achieve a functional state.
|All ages referenced to fertilization, not last menstrual period.|