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Abstract

In the first visual ganglion of the fly (Musca domestica) there are many similar visual channels (ldquocartridgesrdquo), connected to each other by various systems of fibers, the most regular of which consists of the collateral branches of the L4 neuron, which is contained in each cartridge.
The three collaterals of L4 run to three different cartridges, one of which is the parent cartridge of the neuron, the other two being neighbours of that cartridge in two directions of an hexagonal array (Figs. 1a, 3).
Within each cartridge there are thus the endings of three collaterals (from three different L4 neurons). These make close physical contact, and serial sectioning shows that each collateral is presynaptic to the other two (Fig. 2). It follows that there are reciprocal synapses between any pair of these collaterals.
The network of relationships thus set up is the simplest scheme of coupling in a hexagonal array (Fig. 1a, b, c).