CNRI / SCIENCE PHOTO LIBRARY CNRI / SCIENCE PHOTO LIBRARY
Five-week-old embryo. Most of the human embryo's organ systems have been formed, especially the heart whose output is 20% that of an adult. The embryo has all but lost its curved appearance, which was due to the symmetrical growth of the brain. Eye development has begun and there are approximately 40 pairs of muscles now present. The embryo's limbs are now sufficiently developed to be differentiated, and there is the appearance of fingers and toes. After eight weeks of development the embryo is known as a foetus.
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