Limb reduplicating effects of chorio-allantoic membrane and its components

The abilities of the chorion, the allantois, the chorio-allantoic membrane (CAM) and the yolk sac of the chick embryo to induce limb reduplications when grafted to the anterior margin of the embryonic chick limb were investigated. One chorion graft out of 27 (3.7%), two yolk sac grafts out of 30 (6.7%), five chorio-allantoic membrane grafts out of 35 (14.3%) and 26 allantois grafts out of 82 (31.7%) resulted in the formation of an extra Digit 2 or other digit of more posterior character. Allantois therefore possesses much greater ability to induce reduplications than the other membranes tested, and in view of its ease of isolation, offers a possible route for the further characterisation of this activity. The implications of these findings for theories of limb development are considered.