EDELMANN / SCIENCE PHOTO LIBRARY. EDELMANN / SCIENCE PHOTO LIBRARY.
The human embryo after 28 days (4 weeks) 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.
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