Box 2 | Description of chick limb development.

Studies in chick embryos have shown that cells receive signals to form limbs long before there are any visible signs of limb formation. Transplantation experiments indicated the regions that will give rise to wing or leg tissue when transplanted (see figure, part a). Cells in these regions give rise to swellings, which then become discrete buds. The buds elongate and grow out from the body and gradually form a limb-like shape. Development of the skeleton can be readily monitored by staining the developing limbs and making whole mounts. The skeleton is laid down in sequence, as the buds grow out, starting with structures nearest the body wall — for example, the humerus — and digits are the last structures to form (see figure, part b). It takes about 2 days to develop from the appearance of an early limb bud to a stage at which digit primordia have formed. The skeleton of the chick wing (see figure, part c) conforms to the general vertebrate plan, except that there are only three digits — 2, 3 and 4 — and each has a reduced number of phalanges. The chick leg (see figure, part d) has four toes — 1,2,3 and 4 — and each toe has a different number of phalanges.