The Successive Steps in the Pattern Formation of the Amphibian Central Nervous System

Removal of the presumptive midbrain region of the central nervous system (C.N.S.) from stage 12/13 or 14 axolotl embryos, with or without removal of the underlying section of the archenteron roof, leads to a reformation of the midbrain when the presumptive fore‐ and hindbrain regions of the neural plate are brought into direct contact with each other. The younger the stage when the operation is performed, the more complete the reformed midbrain. These experiments suggest that the midbrain is formed by a spatial interaction between two preexisting formed domains, namely, the presumptive forebrain (generated by activation) and the hindbrain/spinal cord (generated by subsequent transformation), so that the main pattern of the C.N.S. is brought about by successive interactions between its segregating domains.