This is an X-shaped crossing. The optic chiasma is where the nerve ?bres from the nasal half of each retina cross over the mid line to join the optic tract from the other side.
n. (pl. chiasmata) (in genetics) the point at which homologous chromosomes remain in contact after they have started to separate in the first division of *meiosis. Chiasmata occur from the end of prophase to anaphase and represent the point at which mutual exchange of genetic material takes place (see crossing over).
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