Subsequent studies defined the inhibitory field ( Barlow 1969 ) , spatial aspects of the inhibitory interaction (

s (Fahrenbach 1983a, b; 98I4a). Familiarity with the general anatomical setting can be obtained by reference to a review of the Limulus visual system (Fahrenbach 1975). MATERIALS AND METHODS Animals were obtained from the Marine Biological Laboratory, Woods Hole, Massachusetts, and from the Gulf Specimen Company, Panacea, Florida. Very small specimens with a prosomal width of about 13 mm were used, which corresponds to the fourth posthatched juvenile with an age of about 8 months (Brown & Clapper 1981). The eyes were excised with a broad margin of cuticle and fixed for 18-24 h in a mixture containing 0.1 M phosphate buffer, 3 g 100 ml-1 NaCl, 6 g 100 ml-l sucrose, 8 ml 100 ml-1 glutaraldehyde (50% grade), 1.8 g 100 ml-~ paraformaldehyde, and 1 ml 100 mlacrolein at pH 7.4. This fixative is considerably different from any that are effective in larger animals. After a brief buffer wash, the eyes were postfixed in phosphate buffered osmium tetroxide (1 g 100 ml-'), passed through acetone and propylene oxide, and embedded in Araldite. The need for very long and flawless serial sections required renewed attention to, or modification of, every step in the process, which has been described in a separate publication (Fahrenbach 1984b). The method involves precise control of the dimensions and geometry of the trimmed block, coating of the block with contact cement for reliable ribboning, pick-up by a one-step method with a modified diamond knife trough, Formvar casting on plate glass for coated slot grids, staining in LKB grid holders under microscopic control, and a modification to the Philips 300 electron microscope for safe grid withdrawal. For scanning electron microscopy, tissue was fixed, cryofractured in absolute alcohol (Humphreys et al. 1974), critical point dried, and viewed in a AMR 1000 scanning electron microscope. The results reported here were derived from a series of 1073 sections (one section lost), cut at 500 nm and mounted on 86 slot grids. The trapezoidal block face (150 gm x 450 gm) covered an area of about 22 complete ommatidia, and the sectioned volume extended from the level of the eccentric cell bodies to the base of the plexus (about 50 gm). Photographic coverage was confined to one neuropil knot deep in the plexus and its directly overlying ommatidia. Tracing of eccentric cell axons, collaterals, and their ramifications was accomplished by marking of all the branches of one unit with coloured symbols and reconstruction of the arborization as a stick diagram. For publication, these markings were converted to black-and-white symbols. During a subsequent visual passage through the marked series, synapses were indexed, their origins and destinations were progressively catalogued, and their locations were indicated in correct scale position on the corresponding stick diagrams. Identification of synapses relies exclusively on presynaptic specializations and will be discussed in a subsequent section. Linear measurements of arborizations were compiled from scale drawings with a Zeiss MOP image analyser. These measurements are delimited by the last synapse on a neurite that passes out of the neuropil or traced volume. 221