The embryo-forming potency of the posterior marginal zone in stages X through XII of the chick.

At stage X a small posterior marginal zone (PM) fragment, when transplanted into similar-size hole in the lateral marginal zone, can initiate the development of an ectopic axis. The laterally transplanted PM, inhibits the regeneration of an axis at the original posterior side from the lateral section of the marginal zone (LM) inserted to replace it. At stage XI both the axis-forming and inhibitory capacities of the PM fragment become weaker and an axis-forming capacity starts to build up anterior to the PM, resulting in the formation of two primitive streaks at 90 degrees to each other. At stage XII the change of potencies exhibited at stage XI is more pronounced, the ability of the transplanted PM to promote axis formation at the new site is lost, and an axis is formed from the original posterior side of the blastoderm.