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 ...

At about four weeks, the heart begins to pump blood. Tiny limb buds appear, ankles and wrists are formed, and fingers and toes develop. During the first trimester, which lasts up to 10 weeks ...