Limb formation is currently explained by a long-standing model, which states that signals from tissue in the limb bud called the apical ectodermal ridge (AER) instruct cells of the upper arm to form ...

The similarity isn't restricted to the ends of the limbs -- the bones of the arm, forearm, and hand of different vertebrates form a recognizable pattern, even though they have been adapted to ...

Three construction teams, each with their own blueprint One type of precursor cell forms the skull and facial bones, another the spinal column and ribs, and a third type the skeleton of the limbs.