BRAIN GROWTH AND NEOCORTICAL DEVELOPMENT IN THE OPOSSUM

General brain growth and differentiation of the neocortex have been st udied in the marsupial, Didelphis virginiana from the 10.5 day embryo through adulthood. Didelphis is born after a short gestation period of about 12.5 days, at a time when the telencephalic wall consists only of two layers and is considered to be at an embryonic stage of develop ment. The cortical plate does not appear until late in the first postn atal week, thus neocortical development is totally a postnatal phenome non in Didelphis as has been shown in other marsupial species examined to date. The general pattern of development and the establishment of the six-layered adult neocortex in Didelphis is similar to that descri bed in eutherian mammals. Signs of cortical lamination can be seen as early as postnatal day 35 and the cytoarchitecture of a typical mammal ian neocortex is well defined by postnatal day 60 in Didelphis prior t o the onset of weaning.