A small piece of tissue in the neural tube of Xenopus laevis was cut out and either replaced in the same embryo after rotating it by 180 degrees, or transplanted into another embryo. We show that, within 24 h after fertilization, the neural tube consists of a mosaic of regions, each with a fixed tendency to develop into a specific part of the nervous system. If a piece of tissue is grafted into another embryo, a pre-determined structure is formed without reference to its surroundings. If a piece of tissue is rotated by 180 degrees, the neural structures resulting from it are correspondingly reversed. Despite such determination of the anatomical polarity, the polarity for selective nerve connections remains labile. When a part or the whole of the tectum is reversed, the trajectory of the optic nerve is sometimes altered, but the array of optic axons invading the tectum ignores the anatomical reversal of the target structure and forms a normal retino-tectal projection with respect to the whole animal. Whenever the position of the diencephalon relative to the tectum is altered, however, the axial polarity of the retino-tectal projection is altered correspondingly.