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Embryonic stage - Embryonicstage...

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Embryonic stage At the age of four weeks, the embryo is about the size of a pea. A primitive heart is beating, the head  is defined with rudimentary eyes and ears, and tiny bumps represent arms and legs. The embryo  also contains a primitive nervous system, and the head has begun to enlarge. A cartilage skeleton  has appeared, and muscles have taken shape. By the end of eight weeks, the embryo is somewhat human looking. Facial features are evident, and  most of the organs are well developed. From this point onward, development consists chiefly of  growth and maturation. The embryo is about 1.5 inches in length. Henceforth it is known as a  fetus.   Nourishment of the embryo, and then the fetus, is accomplished through the  placenta.  The maternal  and embryonic blood supplies meet at this organ, but the blood does not mix. Instead, diffusion  accounts for the passage of gases, nutrients, and waste products across the membranous barriers. 
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