Developing rapidly and early, the heart is the first organ to function in the embryo, and it takes up most of the room in the fetus's midsection in the first few weeks of its life. During its initial stages of development, the fetal heart actually resembles those of other animals. In its tubelike, two-chambered phase, the fetal heart resembles that of a fish. In its three-chambered phase, the heart looks like that of a frog. As the atria and then the ventricles start to separate, the human heart resembles that of a turtle, which has a partial septum in its ventricle. The final, four-chambered design is common to mammals and birds. The four chambers allow low-pressure circulation to the lungs and high pressure circulation to the rest of the body.