This report presents r e s u l t s of a study in Schistocerca gregaria of large dorsal unpaired median neurons found p r e v i o u s 5 in ganglia of several insects (Crossman e t a l . , '71a; Hoyle, unpublished). Most of these neurons give off a single median neurite t h a t makes a T-branch, providing symmetrical l e f t and r i g h t e f f e r e n t axons. These axons emerge i n ganglion nerve trunks and terminate in each of two nerves. They then travel t o major skeletal leg muscles. One of the neurons supplies only the extensor t i b i a e ( j u m p i n g muscle).The f i n a l branches of t h i s neuron parallel the f a s t axon and supply muscle units t h a t receive the f a s t axon only. All the muscle f i b e r s in a unit a r e innervated (multiterminally) by the f a s t axon, b u t only a few receive terminals of the dorsal neuron i n addition. The endings a r e of neurosecretory type, containing large numbers of dense core vesicles. Their function i s unknown.I n 1971, Crossman e t al. ( ' 7 1 a ) reported large nerve cell bodies in the mid-lines of the dorsal surfaces of cockroach and locust thoracic ganglia t h a t give large, overshooting action potentials. Each neuron makes a T-branch, the arms o f which are symmetrical, l e f t and right neurites.forms axons t h a t emerge from ganglion nerve trunks. appear t o be e f f e r e n t neurons supplying muscles, although t h e i r i d e n t i t y could not be ascertained. Each of these branchesThe large dorsal c e l l s thusIn both cockroach and locust, Crossman e t a l . ( ' 7 2 ) consistently found eight o f these neurons, apparently a l l identical and with similar branching patterns.In t h e i r f i r s t detailed account Crossman e t a l . ( ' 7 l a ) , showed the neuron as having b u t a single pair of o u t p u t s , in N5 (designation of Pringle, '39). In a 159

The large octopaminergic dorsal unpaired median neuron of the locust that innervates the extensor tibiae muscle, DUMETi, was examined electronmicroscopically. Its soma contains many Golgi complexes apparently making dense-core vesicles similar to those found in peripheral branches and terminals. There are also larger stores of the dense material in the soma, especially near the exit of the principal neurite, that are not in vesicular form. Since the neurons can be penetrated and stimulated by microelectrodes, they form favorable subjects for direct studies of the control of neurosecretion. Preterminal fine branches of the neuron were located in proximal outer bundles of muscle fibers into which they had been traced electrophysiologically. They contain numerous large dense-core vesicles arrayed in rows near microtubules. These fine branches have a thick layer of collagenous connective tissue between the axon and the muscle fiber. Final terminals have varicosities containing many vesicles, lying inside the outer layers of the sarcolemmal complex of muscle fibers. They do not form synaptic structures. Terminals of another DUM neuron, one that innervates the dorsal longitudinal flight muscles (DUMDL), were similar in detail to those of DUMETi. DUMETi swelled about 20-fold in cross-sectional area above a ligature, in a 12-hr period, indicating that there is an extensive centrifugal flow of material in it, and sprouted a branch.

Each of the paired cephalic eyes of the marine gastropod, Bulla, is about 0.5 mm in diameter and contains about 1000 large photoreceptors, small photoreceptors, numerous pigmented support cells and about 130 neurons. The photoreceptors are of three types: large (90 micron X 20-30 micron) dense ones (PRLD) with elaborate narrow microvilli and aggregates of 650 A clear vesicles in the cytoplasm; large clear ones (PRLC) with elaborate wide microvilli and aggregates of 650A clear vesicles; small slender receptors (PRS) with a tuft of microvilli and lacking vesicle aggregates. Neurons (15-25 micron) containing dense-core 1000 A vesicles are in the periphery of the retina or grouped in a collar around the neuropil below the photoreceptor layer. The 4-5 largest neurons are in the collar area. Correlation of neuron morphology with electrical activity was done with intracellular recording and Lucifer yellow injection of some of the larger neurons in the collar area whose action potentials contribute to the optic nerve impulses. Each one has an axon in the optic nerve and processes that go to the neuropil. They are the pacemaker neurons of the circadian rhythm in impulse frequency that is recorded from the optic nerve, since only their action potentials are correlated 1:1 with the optic nerve impulses. Gap junctions (with pentalaminar structure) are common between photoreceptors, glial cells, photoreceptors and glial cells, and neuronal processes in the neuropil, providing a basis for electrotonic coupling among retinal cells. There are about 2000 axons (diameter less than 3 micron) in the optic nerve, possibly one from each retinal photoreceptor and neuron plus efferent fibres from the brain. Accessory nerves, containing a few large axons, are seen in the optic nerve sheath.

We have found that ultrathin Formvar films are easily and reliably made at an air-water interface by the drop method. By varying the concentration of Formvar in the drop, films of different characteristics can be obtained. Concentrations of 0.25-0.4% in ethylene dichloride produce extremely flat, ultrathin, and stable films that are especially suited for shadowed and negatively stained preparations. Low concentrations ( < 0.1%) produce nets consisting of many tiny holes which, after carbon stabilization, are ideal for supporting high-resolution samples. Above 0.5%, films made by the drop method develop bubbles, and this bubble defect makes them unsuitable for section support.For section support, Formvar films made by the stripping method off mica are far superior to those made off glass. The films are more uniform in surface contour and thickness. They are less readily attacked by alcohols. Consequently, they are more resistant to staining procedures involving organic solvents and continue to be strong and uniform for section support.