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Notes: Summary The peripheral retina and lamina ganglionaris of Musca domestica have been investigated electronmicroscopically. Peripheral Retina 1. The four Semper cells of each ommatidium form thread-like processes which extend proximally through the retinula and end at a level just distal to the basement membrane. 2. Rhabdomeres No. 1–6 have a conical form, being approximately 2 μ in diameter at the distal ends and 1 μ at the proximal ends. The consequences of this special shape are, a) that the mean number of absorbed quanta per volume unit remains approximately constant throughout the length of the rhabdomeres and, b) that about half the amount of photopigment per rhabdomere compared with a cylindrical rhabdomere of 2 μ diameter is adequate in order to allow the same total absorption. In contrast to the above, rhabdomeres No. 7 and 8 have a diameter of about 1 μ throughout their length. 3. The spatial arrangement of the rhabdomeres in Musca domestica is of a definite rotationally asymmetrical pattern. This pattern is not universal in Dipterans as is witnessed by the rotationally symmetrical pattern found in a more primitive species, Wilhelmia equina (Simuliidae). The microvilli in the distal ends of Musca rhabdomeres are oriented in a specific manner, whereby their long axes are parallel in rhabdomeres No. 1 and 4, 2 and 5, and 3 and 6 respectively. 4. Pigment granules within retinula cells No. 1–6 migrate to the bases of the microtubules upon bright adaptation. No pigment migration is observed in retinula cells No. 7 and 8. Migration of particles has been previously proposed as a protective mechanism for rhabdomeres No. 1–6 during bright illumination. Lamina Ganglionaris 1. Pour monopolar neuron somata have been found distal to each optical cartridge in the lamina. The four fibers of these neurons form a bundle and enter the crown of photoreceptor cell axon terminals. Two large diameter fibers which are derived from two type I somata form the central second order neurons, L1 and L2 in each cartridge. The two small diameter fibers, L3 and L4 withdraw between the photoreceptor axon terminals R5 and R6 to a point on the periphery of the cartridge where they remain throughout the lamina. L4 sends a short collateral back into the base of its parent cartridge. 2. Paired centrifugal fiber endings are located between receptor axon terminals on the periphery of each cartridge. In medial and proximal sections through the cartridges there is one pair of centrifugal endings per receptor axon terminal. The pair of centrifugal endings located between axon terminals No. 5 and 6 bifurcates distally so that L3 and L4 are surrounded by two pairs of centrifugal endings as they withdraw from the center of the cartridge. 3. The following cell contacts, which appear to be synaptic in nature, have been observed between various fibers within the cartridges and are characterized by the presence of a T-shaped synaptic ribbon and synaptic vesicles within the presynaptic fibers and occasionally by a membranous postsynaptic structure (see the Table and Pig. 33). Commonly, two postsynaptic fibers share one presynaptic site. The receptor axon terminals R1–6 provide synaptic inputs to L1 and L2 as well as to the centrifugal endings α, β. L3 receives an input from at least one of the receptor axons and from one centrifugal fiber. L4 receives an input from a centrifugal fiber and is possibly presynaptic to L1 and L2. The centrifugal fibers make synaptic contact with one another and are presynaptic to small unidentified fibers. Neurons L1 and L2 are also presynaptic to small fibers of unknown origin. Furthermore T-shaped ribbons have been observed within the centrifugal fibers as well as within the receptor axon terminals at points of contact with epithelial glial cells. 4. The epithelial glial cells are arranged in a strict and orderly fashion between the optical cartridges. Throughout most of the eye, each glial cell touches upon exactly three cartridges and each cartridge is surrounded by three cells. Exceptions to this rule occur within the equatorial and marginal regions of the eye. The distal surfaces of the epithelial cells are characterized by extensive outfoldings of the limiting membranes. The greater portion of the cytoplasm is filled with numerous mitochondria and infolded membranes. 5. The mirror image pattern inversion of the ommatidia at the equator in the peripheral retina is projected onto the lamina as witnessed by the location of fibers L3, L4 and R7, R8 in relation to the cartridges. This inversion can also be observed in the arrangement of the epithelial glial cells between the cartridges. 6. Whereas six retinula cell axon terminals are usually found in the cartridges a different situation exists in the equator region of the eye. In this region the cartridges contain seven to eight axon terminals of retinula cells No. 1–6. This agrees with the result that in the equator region seven to eight rhabdomeres No. 1–6 view one and the same point in the environment rather than the usual six rhabdomeres (Kirschfeld, 1967).