Before birth, the human heart begins to form as soon as the embryo starts to develop. At first, the heart is a simple tube that soon begins to beat regularly. Then it grows it grows so fast that there is no room for it to become longer inside the embryo. So it doubles back on itself, twists around, and begins to look like the heart as we know it.

A layer of tissue begins to grow down the middle of the tube, dividing it into the right and left sides. A little later, the partitions between the auricles and ventricles start to form. The amazing thing about this division is that it happens by about the sixth week of embryonic life, when the whole body is only a half inch long. At this early period, the heart is nine times as large in proportion to the size of the whole body as it is in an adult. It lies high up in the chest. Later it moves into its final position in the middle of the chest.

During this early period, the human heart goes through stages in which it resembles the hearts of various animals, before it is finally divided into four chambers. First, when it is a simple tube, it is like the heart of most fishes. Later, when the partitions begin to form, there is a time when the auricles are partly seperated but the ventricles have no wall between them, like the heart of a frog. When the auricles are divided but the division between the ventricles is incomplete, the human heart resembles that of the snake or turtle.