The New York Times, April 6, 2009
For its first four weeks, a human embryo looks like a crumpled tube. But around its twenty-seventh day of development, four buds bulge from its sides. Over the next few days, the buds grow like tulips, stretching out into flattened stalks and blooming into crowns of fingers and toes. Inside these developing limbs, bones condense. Muscle cells, tendons, blood vessels and nerves all find their respective places. The embryo now has hands with thumbs to suck, legs ready to deliver a kick.
For developmental biologists, the development of limbs captures all that is marvelous about embryos: how a few cells can give rise to complicated anatomy. In fact, biologists understand the development of the limb much better than any other part of the body.
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