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The Umbilical Cord

     The umbilical cord is a life support system, containing two arteries and one vein that connects the baby to the placenta. Very small molecules oxygen, water, salt, food from the mother's blood. It is necessary to note that large molecules cannot pass through the placental wall, these include red blood cells and harmful substances, such as most bacteria, material wastes, and hormones. The mechanisms that govern the transfer of substances across the placental barrier are complex.
    The amnion, a bag or an envelope that contains a clear fluid, in which the developing embryo floats, is another important life support system. Like the placenta and umbilical cord, the amnion develops from the fertilized egg, not from the mother's own body. At approximately 15 weeks, the Kidneys of the foetus begin to produce urine. This fetal urine remains the source of the amniotic fluid until the third trimester, when some of the fluid is excreted from the lungs of growing foetus. Although the amniotic fluid increases in the volume tenfold from the 12th to the 40th week of pregnancy, it is also removed in various ways. Some are swallowed by the foetus, and some are absorbed through the umbilical cord and the membranes covering the placenta. The amniotic fluid provides an environment that is temperature and humidity controlled as well as shockproof.
   
   We should know that before some women are aware that they are pregnant, some important embryonic development take place. In the third week, the neural tube that eventually becomes the spinal cord forms. At about 21days, eyes begin to appear, and at 24 days the cells for the heart begin to differentiate. During the fourth week, the first appearance of the urogenital system is apparent and arm and leg buds emerge. Four chamber of the heart take shape, and blood vessels surface. From the fifth to the eighth week arms and legs differentiate further, at this time, the face starts to form but still is not very structures fuse. 
     Organogenesis is the process of organ formation that takes place during the first two months of the prenatal development. It is necessary to note that when organs are being formed, they are especially vulnerable to environmental changes. 

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