You and Your Heart Go Way Back
The heart of an embryo begins to form within 16 to 19 days of gestation, and goes through some very interesting changes before it winds up shaped like a miniature adult heart.
First, the heart of an embryo starts off as a little tube, similar to the heart of a goldfish. As it grows outward, it runs out of room and is forced to double back into a horseshoe shape. Soon, it develops into a two-chambered heart, like a frog's. In time, another chamber develops -- now it looks a bit like a snake's heart. Eventually, the fourth chamber develops, and it takes on the appearance of what it is: a human heart. The embryonic heart starts to beat around the 22nd day of pregnancy. At around four weeks, it's capable of circulating blood.