The apical ectodermal ridge and the subjacent specialized somatopleuric mesenchyme constitute the growth zone or progress zone. A vein called the marginal sinus appears in the mesenchyme of this zone. The progress zone remains at the distal end of the limb until the fingers develop. It controls growth of the limbs and defines the proximo-distal axis (with respect to the limbs). With respect to this axis, the cranial border of the limbs is termed pre-axial and the caudal border is postaxial. A line perpendicular to the proximo-distal axis in a pre-axial to postaxial direction represents the cranio-caudal axis. The dorso-ventral axis is perpendicular to the two other axes. A group of cells at the postaxial border is thought to control development of the digits (zone of polarizing activity, ZPA). The 5th finger develops from cells near to the ZPA.