Basic structural pattern of limbs
The common structural pattern of tetrapod limbs is extremely clear expressed in their skeletal components and in the manner they are organized. The limb is composed of a series of bones that develop in a proximodistal sequence and are arranged in a definite common manner. Generally, a forelimb consist of a numerous, followed distally by two parallel bones, a radius and ulna and then various carpals, metacarpals and digits made of phalangeal elements in this order.
Figure: Scheme of serial homology of the bones of forelimbs and hind limbs of vertebrates
The corresponding skeletal elements of hind limbs are femur that is followed via parallel tibia and fibula, various tarsals, metatarsals and digits. Since there is a bone-for-bone correspondence in the forelimbs and hind limbs the several bones of these limbs are said to be serially homologous.