The organization of the insect visual system (Light microscopy) - II. The projection of fibres across the first optic chiasma

Each optic cartridge in the lamina of Diptera¹ gives rise to a bundle of fibres composed of the prolongations of at least 11 neurons (two first order receptors, R7 and R8, five monopolar cells, L1-L5 and four "centrifugal" cells, T1, T1a, C2 and C3). The bundles project to the outer surface of the second synaptic region, the medulla. The projection patterns of the bundles means that a point for point map of the cartridge arrangement in the lamina is conferred on the medulla. The cross-over of bundles, along the horizontal axis of the eye, merely reverses the lamina map on the medulla. All eleven fibres that enter a bundle at the lamina are contained within it as far as the medulla.