The Role of Competence in the Cranio‐Caudal Segregation of the Central Nervous System

Left to right thirds of Triturus presumptive prosencephalon show identical developmental potencies after implantation in a neutral Ambystoma environment. Such equipotential grafts were excised from stages between late‐gastrula and mid‐neurula and implanted into the neural plate of an Ambystoma host at different cranio‐caudal levels. Their regional differentiation was independent of the age of the host, but dependent upon the age of the donor material; the older the latter the smaller the portion of the graft which was transformed into more posterior neural structures. Full transformation occurred in stage 11/12 grafts, while pure prosencephalic differentiation took place in stage 16 grafts, demonstrating that the period of competence of the neurectoderm for transformation extends from stage 11/12 up to stage 16. Irrespective of the level of implantation all grafts older than stage 11/12 and younger than stage 16 showed an uninterrupted cranially‐oriented regional differentiation. The medio‐lateral extension of the transformation process is primarily determined by the temporal loss of competence of the implanted neurectoderm. A comparison of grafts implanted at different cranio‐caudal levels showed that transformation is more pronounced the more caudal the level of implantation, so that another factor(s) than competence must also play a role in the regional segregation of the CNS.