Origin of the retina from both sides of the embryonic brain: a contribution to the problem of crossing at the optic chiasma.

Cells originating from one of the first two blastomeres of the frog embryo were labeled either by injecting them with horseradish peroxidase or by changing the ploidy of one blastomere. Both methods show the labeled cells confined to the same side of the brain as the labeled blastomere except for cells that have moved from the opposite side into the ventral diencephalon and ventral part of the retina. Reciprocal movement of cells from each side of the prospective forebrain into the prospective retina on the opposite side starts before the neural tube closes and results in the formation of an incipient optic chiasma which may provide the pathway for optic axons to grow from the retina to the opposite side of the brain.