EXUBERANT DEVELOPMENT OF CONNECTIONS, AND ITS POSSIBLE PERMISSIVE ROLE IN CORTICAL EVOLUTION

The callosal visual connections of the cat provide a model for studyin g the phenotypes of cortical axons and their differentiation. The term inal arbor of a callosal axon develops in several successive stages. A t each stage, the arbor approximates the adult phenotype move closely. This is achieved through two mechanisms: (1) exuberant, but increasin gly constrained, growth and (2) partial deletion of previously generat ed parts of the arbor, This differentiation is controlled by interacti ons of the axon with its cellular environment, and by visual experienc e, It might have played a permissive role in the evolution of the cere bral cortex by enabling adjustments of cortical connectivity to change s in the number, size, internal organization and cellular composition of cortical areas.