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Human development

 

Down the back of the neural tube, cells begin to form what will become the spinal column. By about week 4, the embryo is 1/8 of an inch long. The embryo is growing quickly and the arm and leg buds will soon sprout. This is also a critical period for healthy development of brain tissue and internal organs because if they do not develop during this time, then they cannot develop later on as this is the only period where development of these things can take place. The embryo is now surrounded by the amniotic sac, which is surrounded by the placenta. The embryo is attached to the placenta by the umbilical cord. The mother nourishes the new embryo, as her blood system exchanges nutrients for waste products through the developing placenta. The placenta keeps the babies blood separate from the mother's blood, but allows the exchange of smaller particles. What this means is that small particles, such as oxygen and alcohol can get through the placenta, but larger particles such as blood cells can not. Unfortunately, this is also how harmful material, such as teratogens, can cause development problems to the embryo. At five weeks the arm buds and leg buds have sprouted. The embryo will gradually develop from the head area to the tail area. This is referred to as cephalo-caudal development. At about six weeks the sex organs of the embryo develop. A gene that is known as the SRY gene causes high level of testosterone to be released in to the embryo. The testosterone causes the development of male sex organs. Embryos without the SRY gene will develop female sex organs. By about the eighth week, the embryo's cartilage tissue turns in to bone tissue. It is now referred to as a foetus.
             The next stage is the foetal stage. The foetus is about 2 inches in length and growing rapidly. It has eyes, eyelids, nose, lips, fingers and other distinctive human features. At 12 weeks the foetus is about three inches long, and the system of the body are already well developed, with many organs more or less complete.


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