Realization of P-D Polarity of Limb

Realization of P-D polarity of limb expressed in a specified proximo-distal sequence of differentiation of limb pans is controlled through the ( AER. Up to quite a late stage of '' limb bud development in chick embryo the AER can change the fate of mesodermal cells with respect to their ultimate position along the P-D axis as displayed by the following experiments:

1) While an additional AER is grafted on wing bud a duplicated set of distal wing structures is formed. It displays that under the influence of the additional AER a second P-D axis was created, regulation take place in the distal mesoderm so that what was normally destined to form one set of structures together one P-D axis now formed two sets, one along each P-D axis.

2) Distal half of the leg bud of stage 19 chick embryo was cut off and after that an AER was grafted on the remaining basal half of exposed mesoderm. A whole normal leg was formed. The basal half of mesoderm would have usually formed only the stylopodium parts, but under the inductive influence of AER regulation take place in its cells that gave rise to the various parts of all the three segments.