soluble signaling morphogen known to play a critical role in controlling the proliferation, positioning and patterning of numerous mammalian cell populations during early development, including the node, notochord, floor plate and midline structures in the brain and spinal cord; regulates the zone of polarizing activity, anterior-posterior patterning, and distal outgrowth of the limb which results in the arrangement of fingers and toes in their proper sequences; controls the differentiation of regions in the gut and important in determining the bilateral nature of the human brain.