Extensive cell migration and elaborate axonal growth are characteristic features of the developing nervous system. By the time many of the late generated neurons, such as cerebellar granule neurons, leave their site of origin in proliferating germinal zones and migrate to their permanent positions in the stratified nervous system, these migrating neurons are confronted by a morass of nerve fibers and extracellular matrix components. Although radial glial fibers1 and Purkinje cell dendrites2 may provide guidance toward the internal granule cell layer for migrating granule cells, the leading growth cone has been proposed to “push” its way through the tissue .3 Ramon Cajal3 likened these growth cones to battering rams which possessed an enormous power of penetration. More recently the possibility that these neuronal movements may be facilitated by the cell’s ability to “cut” its way through the tissue has been explored 4–10 A likely candidate for the cutting activity is cell secreted plasminogen activator which is capable of generating the broad acting protease plasmin from the extracellular proenzyme plasminogen.