Differentiation of avian retinotectal projection.

The topographic relationship between the retina and tectum is strict. Retinal fibers, however, overpass the target and distribute wider mediolaterally when they first enter the tectum. Retinal fibers find their target by sending collateral branches. Tetrodotoxin (TTX) treatment shows that neuronal activity plays a role in refinement of the retinal fibers. The rostrocaudal axis of the tectum anlage was rotated 180 degrees at about the 10-somite stage. The results showed that the polarity of the transplants was regulated and adjusted to that of the host embryo. It is suggested that the rostrocaudal polarity of the tectum anlage is not determined at about the 10-somite stage.