Remarkably, the fetal brain generates far more neurons than are found in the adult. Why? During late fetal development, there is a dramatic competition in much of the brain, with winning neurons being the ones that migrate to the correct location and maximize synaptic connections to other neurons. And neurons that don't make the grade? They undergo "programmed cell death"-genes are activated that cause them to shrivel and die, their materials then recycled.
( Robert M. Sapolsky )
[ Behave: The Biology of Humans ]
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