Four-week-old embryo. Most of the human embryo's organ systems have been formed, especially the heart that is already beating. The embryo's curved appearance is due to the symmetrical growth of the brain, which now comprises 5 divisions. The spinal cord has thickened on the underside, forming clusters of nerve cells. These unite with the upper and lower sides of the neural canal to form spinal nerves, which connect with other organs as they are formed. Lungs bud, forming bronchi, bronchioles and finally air sacs. Glands such as the thyroid, pancreas, thymus and parathyroid arise from the primitive gut. After eight weeks of development the embryo is known as a foetus.