Final answer:
The heart tube loops within the pericardium and forms an S shape between days 23 and 28 of embryonic development. The internal septa, which divide the heart into the atria and ventricles, begin to form around day 28.
Step-by-step explanation:
The heart tube loops within the pericardium and forms an S shape between days 23 and 28 of embryonic development. This looping process helps align the chambers and major blood vessels of the heart. The internal septa, which divide the heart into the atria and ventricles, begin to form around day 28. The development of the heart continues with the formation of valves that regulate blood flow.