Publisher Summary Normal morphological and physiological developmental patterns in feline cerebral and cerebellar cortex are summarized in this chapter in order to provide a basis for the analysis of the pathophysiological effects of two types of injuries to the immature brain. Surgical interruption of axons of pyramidal neurons markedly accelerates the development of intracortical axon-collaterals that establish synaptic relations with adjacent arciform pyramidal neurons. One consequence of this re-organization of synaptic pathways is reflected in the development of powerful excitatory synaptic activities in traumatized regions of immature neocortex. Effects of x-irradiation of cerebellar cortex in newborn kittens are defined primarily in terms of loss of the external granular layer and modification of Purkinje cell morphogenesis. Major alterations in electrophysiological properties of irradiated cerebellar cortex are seen in evoked responses to folial surface and motor cortex stimulation. These alterations are considered expressions of radiation-induced maturation acceleration and re-organization of synaptic pathways and possible changes in membrane properties of Purkinje cell dendrites. The data obtained in these ontogenetic studies of pathophysiological processes indicate that different types of insults to the immature brain may produce similar overt effects, that is, changes in excitability by entirely different mechanisms.