Trajectories of axons in ectopic VIIIth nerves

Transplantation of the acousticolateral placode to the evacuated eye position in embryos of the frog Rana pipiens has been used to force axons of the VIIIth cranial nerve to penetrate the diencephalon. These ectopic axons establish a growth trajectory that is strikingly similar to their normal growth trajectory within the medulla oblongata despite the fact that no other axons within the diencephalon normally follow this route. The result is discussed in terms of the "blueprint" and substrates pathway hypotheses which have been advanced to explain the initial development of axon tracts within the central nervous system.