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Human Development

             Human development is how humans develop and live. Human development goes from fertilization to death. This essay will start at fertilization and end with one year of age. If it were not for Human Development humans would not be here.
             Fertilization is when a woman is ovulating and a man releases sperm during sexual intercourse. Only one third of the eggs that are fertilized actually survive the entire nine months of pregnancy. The rest are spontaneously aborted, frequently because that fetus was genetically or developmentally abnormal. After fertilization, the placenta develops from tissue contributed by both the embryo and the mother. .
             These are the stages of the development of the embryo and the fetus. The first two weeks of embryonic development are devoted to cleavage and the onset of gastrulation, the latter continuing during the third week. The circulatory system also starts to form in the third week. Neurulation begins as well, with the neural plate folding into a neural tube, which in turn will give a start to the brain and the spinal cord. The heart is beating by the end of the third week.
             The Neurulation continues posteriorly, the eyes and ears begin to develop, and the arm and leg buds form. By the end of the fourth week, the embryo is about 0.16 inches (4mm) long when measured from head to butt. The fifth week the eye lens and nasal pits are forming, a mouth is recognizable, and hands and feet become flattened and plate like, making the arms and leg buds like paddles.
             The embryo has by now doubled in size. The sixth week the face starts to take shape and the arms have become bent at the elbows.
             The eyelids start to for the seventh week. The eighth week the testes and ovaries are distinguishable, but the external genitalia are still not identifiable as being male or female. By the end of eight weeks all the internal and external organs are present and is the end of the embryonic period and the beginning of the fetal period.

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