Axial twist theory

The axial twist theory is a proposed explanation for the optic chiasm and the contralateral organization of the vertebrate brain, holding that the front part of the head, including the face and parts of the brain such as the cerebrum and thalamus, is effectively turned around relative to the rest of the body. Under this model, the embryo rotates onto its left side during development and then undergoes two ninety degree twists to restore external bilateral symmetry, leaving the central nervous system with an overall 180 degree twist precisely at the optic chiasm and giving the vertebrate body a left handed chirality. The theory competes with several other proposed explanations for the same phenomenon, and none of them, axial twist theory included, has gained general acceptance; no direct evidence of the proposed developmental twist has yet been found.