The human embryo after 28 days development, with the head at left. Most of the primitive organ systems are formed: the embryonic heart 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 becomes thickened on the underside, forming clusters of nerve cells: these unite with the upper & lower sides of the neural canal to form spinal nerves which connect with other organs as they are formed. Lungs bud in a branching manner, forming bronchi, bronchioles & finally air sacs. Glands such as the thyroid, pancreas, thymus & parathyroid arise from the primitive gut.