Summary of Development of the limbs
Limb buds appear at the end of the fourth week of development. They develop in a proximal-to-distal fashion, lead by the apical ectodermal ridge, a region of ectoderm on the distal border of the limb. Fingers and toes form when the cells of the apical ectodermal ridge start to die. Limb musculature and bones derive mainly from primitive streak mesoderm.