The synapses between the lateral giant axon and the giant motor axon found in the abdominal ganglia of the ventral nerve cord of the crayfish Procambarus clarkii are electronic. The junctional membrane rectifies, favoring impulse transmission from lateral giant fiber to giant motor fiber. This rectifying electronic junction consists of closely apposed membranes indistinguishable from ordinary arthropod gap junctions. The apposed membranes contain intramembrane particles that are -12.5 nm in width. These particles have a central depression and are arranged in a loosely ordered array with a center-to-center spacing of about 20 nm.The only obvious morphological evidence of asymmetry is the presence of vesicles (about 80 nm in diameter) in the cytoplasm adjacent to the junctional region of the presynaptic lateral giant fiber. Vesicles are not present in the adjacent cytoplasm of the postsynaptic giant motor fiber; however, mitochondria and smooth tubular endoplasmic reticulum are more frequent in the cytoplasm of the giant motor fiber.KEY WORDS cell-to-cell communication rectifying electrotonic junction gap junction freeze-fracture -electron microscopy Although most electrotonic junctions behave as fixed resistances connecting coupled cells, a few instances are known in which the junctions rectify. Furshpan and Potter (9) established that transmission at the giant motor synapse of the crayfish is electrical with rectification. Conduction is essentially a one-way transmission from lateral giant fiber to giant motor fiber. When the potential in the lateral giant axon (presynaptic) is positive relative to the potential in the giant motor fiber t Dr. Keeter died 17 February 1976.(postsynaptic), junctional resistance is low. At rest, and when the motor fiber is more positive, junctional resistance is high. Thus orthodromic transmission is favored over antidromic.Robertson (28) and Stifling (30) have suggested that the junctional membranes in this synapse form intimate contact. Our preliminary findings (12,15) indicate that these rectifying junctions may be similar morphologically to the more common linear electrotonic synapses. To positively identify topologically the location of these junctions, nerve cords were initially prepared for scanning electron microscopy. Further studies of this synapse on both sectioned and freeze-fracture preparations were carried out. We were unable to distinguish any structural differences between the 764 J. CELL BIOLOGY 9 The Rockefeller University Press

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SUMMARY Cells of Anacystis nidnlans were grown in synchronous culture using a light‐dark alternation to obtain synchronization. Two synchronous cycles were obtained with, decay of synchrony beginning with the third cycle. Cells of various ages in the growth cycle were treated with lysozyme to form spheroplasts. The percentage of spheroplast formation varied with age of the cells. After extended periods of lysozyme treatment, up to 90% of the cells of all ages showed spheroplast formation. Some cells were resistant to the action of lysozyme regardless of age or length of treatment. An ultrastructure study of the spheroplast was made. The electron‐dense inner layer of the cell wall was removed by the action of lysozyme on the glucosamine residues of the cell wall, indicating true spheroplast formation. The photosynthetic apparatus became more pronounced with extended treatment with lysozyme.