Limbs

All limbs begin as limb buds, tiny bumps that are visible from about the fourth week. To grow a new limb, the cells in the limb bud have to know two things: the position of the limb in space, and where to grow bone, muscle and tendons. To grow an arm, for example, the cells must be able to tell apart the shoulder and hand end, the thumb and little finger side, and the palm and knuckle side.

Four limb buds in a chick embryo. Imelda McGonnell, King's College, University of London