Central projection of optic tract from translocated eyes in the leopard frog (Rana pipiens).

In Rana pipiens embryos, eye anlagen were moved to the evacuated ear position, where they continued to differentiate and sent their optic nerve fibers into the hindbrain. Upon entering the medulla, the optic fibers turned caudally, penetrated the spinal cord, and traversed the dorsolateral white matter to the caudal end. We found this pattern of growth in every animal; the optic fibers did not enter the tecta. These results suggest the existence within the neural tube of a three-dimensional gradient system to which embryonic optic fibers are responsive and which may guide the normal development of the visual pathway.