Final answer:
The heart begins to beat during the embryonic period of development, usually by the end of the third week after fertilization.
Step-by-step explanation:
The heart begins to beat during the embryonic period of prenatal development. Specifically, the heart starts beating and pumping blood by the end of the third week after fertilization, which falls within the embryonic stage of development, occurring in weeks 3-8. This stage is critical for the development of the organ systems, where all the organ systems are structured in rudimentary form.