A comparison of eutherian and marsupial optic chiasms: a brief review

There are four fiber components in the mammalian optic chiasm. Two crossed and two uncrossed components, one of each from each eye. The two crossed components meet at the midline, forming coarse interdigitating bundles in some species, and extremely fine crossing bundles in others. The two components from one eye mingle in the optic nerve. Then in some species they separate, before reaching the chiasm, but in others they do not separate until they reach the midline of the chiasm. Once past the chiasm, the two components that form the tract reunite and one more, the crossed and the uncrossed fibers mingle. The review considers that difference between the marsupial and the eutherian mammals that have been studied, and looks at the developmental changes that are likely to underlie these different types of chiasmatic organization.