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A remarkable preparation for studying development and repair is the CNS of the newborn opossum which, removed in its entirety, survives in culture for more than 1 week. In suitable medium, cells continue to divide, mature and reflex activity is maintained. Moreover, nerve fibers grow rapidly, reliably and extensively across lesions made in the spinal cord. Restoration of conduction has been demonstrated by recording electrically; labeled fibres have been observed directly by light and electron microscopy as they traverse the lesion. Similar experiments have also been made in embryonic (E15) rat CNS in culture. Open questions concern the identity of the fibers that traverse the lesion and the specificity of connections that they make with targets. We are now also analysing mechanisms that favor repair in younger opossums and that prevent it in their older siblings. Of particular interest are oligodendrocytes and myelin that start to appear at about 8-9 days after birth.