1. Temporal and spatial aspects of postembryonic optic lobe development in a Lepidopteran,Danaus plexippus plexippus L., were analyzed using serial section reconstructions and H3-thymidine radioautography to display loci of cell production and progressive movements of populations of cells. 2. Optic lobe development begins early in larval life and is continuous without perceptible fluctuations corresponding to molting. The production of new cells begins during the first larval stages and is completed within a few days after pupation. 3. Development of adult optic centers appears to be independent of the larval optic center and also of adult eye development which does not get underway until pupation. At pupation the larval stemmata migrate toward the brain along the stemmatal nerve which persists and later serves as the framework by which ommatidial neurones reach the brain. 4. Ganglion cells of the adult optic lobe are produced by two coiled rod-like aggregates of neuroblasts, the inner and outer optic lobe anlagen, which lie lateral to the protocerebrum and are already present in the brain of the newly hatched larva. Neuroblasts of the anlagen divide both symetrically to produce more neuroblasts and asymmetrically to yield one neuroblast and one smaller cell, the ganglion-mother cell. Subsequent ganglion-mother cell divisions produce the new ganglion cells which are continuously displaced from the anlage by additional cells. Following pupation mitotic activity in the anlagen diminishes and neuroblasts degenerate. By the fourth day after pupation the anlagen have disappeared. 5. Fiber differentiation begins within a few days of cell formation. Fibers travel in bundles usually toward the center of the coiled anlagen where they form the neuropile masses. With contributions from a growing population of ganglion cells, fibermasses grow rapidly in size and complexity. 6. The geometric arrangement of anlagen, cortices, and neuropile is dynamic and interdependent. Progressive changes in anlagen configuration result from the combined effects of an increasing neuroblast population, growing optic cortices, and expanding fibermasses between the arms of the anlagen. In turn, the cortices and fibermasses which follow anlagen contours also change form. The complex of these parts, initially small and coiled, gradually enlarges and uncoils until at the time of anlagen degeneration the three optic fibermasses and their cortices are in approximately their final arrangement. 7. The outer anlage forms cells of the lamina cortex at its lateral rim and cells of the medulla at its medial rim. Cells of the lobula cortex are produced by strands of inner anlage neuroblasts extending laterally between the arms of the coiled outer anlage. 8. Cells of the medulla cortex are first seen during the second larval instar and several days later the medulla fibermass is discernible. Cortex and fibermass lie medial to the outer anlage which is moved progressively more laterally as more cells are produced. Cells labelled with H3-Td R at the beginning of the third instar become the tangential cells of the adult optic lobe. Those labelled at the fourth and fith stages occupy positions near the tangential cells, and those labelled at pupation ultimately lie at the lateral edge of the cortex. 9. Production of the lamina cortex begins later and procedes more slowly. Cells here are first apparent during the fourth instar and form a cellular cap covering the lateral part of the optic lobe. Labelling studies show that the earliest formed cells finally occupy the most posterior region of the lamina cortex. The lamina fibermass is first seen in the mid-fifth instar brain. 10. For most of larval life the lobula cortex forms a plug of cells just inside the lamina. While the anlage remains coiled, the first-formed cells are at the center of the plug, but ultimately they lie at the most medial part of the cortex. Production of lobula cells begins during the third instar and by the mid-fourth instar the lobula neuropile can be seen medial to them. 11. As a result of these studies with H3-Td R injection and fixation after varying intervals it has been possible to estimate the age of cells at a particular developmental stage. Because this material offers an organized arrangement of cells of a wide range of identifiable ages and levels of maturation within a single individual, it provides an excellent model for the study of progressive neurone differentiation.