Examinations were made by means of the Golgi-Cox method of the developing neuronal elements of the cortex of the dog during the first ten weeks of life. The neuronal elements undergo gradual morphological differentiation during the first week. From 2–4 weeks of age the most dramatic changes in the development of dendritic arborizations occur. After this time the changes are slower with terminal maturation of cellular elements, cessation of neuronal growth and final organization of apical and basilar dendrites and their collaterals. Comparison with the development of the kitten shows close similarity in the development of canine and feline neocortex. In dogs, however, an essentially mature neocortex is not reached until after four weeks of age. The relation of these data to biochemical and electrophysiological development and behavioral patterns in the young dog is discussed.