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Freeze-fracture replicas have been prepared from two fast-acting vertebrate muscles (frog sartorius and rabbit sacrospinalis) and are described with particular reference to the distribution of intercalated particles in the plasma membrane, T-tubule and SR cisternae. Orthogonal assemblies of small particles are present on the A face plasma membrane in each instance, and their distribution (in sartorius) is found to be random with respect to the underlying myofibrillar sarcomere repeat. Such assemblies are not present on A or B faces of T-tubule or SR cisternae. Asymmetric particle distribution is described for fracture faces of the T-tubules and SR: the profuse particle packing of the SR A face is uniform from terminal cisternae to medial fenestrated collar. Intercalated particles are present on A and B faces of T-tubule fractures: more commonly on the latter. These results are compared with studies on insect muscles, and a comparative approach to further studies on the correlation between membrane structure and function is discussed.

Gross ultrastructural and histochemical details of the integumental milk glands of the tsetse fly Glossina morsitans have been examined during the pregnancy cycle. Structural evidence for protein secretion is found between Days 3–8 of the nine-day cycle: termination of activity is completed on the day of parturition. Onset of lactation is synchronized with the eclosion of the first instar larva. The changes in cell volume (notably in the extracellular reservoir) occurring throughout the pregnancy cycle are illustrated in electron micrographs, and a one hundred-fold volume increase in the reservoir volume between the inactive phase and the active period is illustrated and discussed in terms of membrane modulation of the limiting membrane of the reservoir. Intracellular membrane changes during the cycle, particularly the development of extensive ER arrays in the actively secreting cell, are illustrated and discussed. It is suggested that cytoplasmic microtubules play a part in maintaining the form of the distended secretory cell, at the height of secretory release and storage. Histochemical observations on the milk secretion, and the contents of the larval gut are presented.

The disposition of the surface plasma membrane and its inwardly directed derivatives, corresponding to the T-system of other fibers, and of the corresponding sarcoplasmic reticulum (SR) elements has been examined in a dipteran asynchronous flight muscle ( Photmia regina ). The presence of uniaxonal neuromuscular junctions within clefts approaching the center of the fiber is described. The most conspicuous SR component is present in the dyads adjoining plasma membrane derivatives, but it is also sparsely represented elsewhere. The accessibility of the plasma membrane-limited compartments within the fiber to the ambient haemolymph, in the living insect, has been investigated by tracing the distribution of ferritin by the circulatory system. The proportion of fiber volume occupied by the T-system and SR components in asynchronous and synchronous muscle is compared and the functional implications of these proportions is discussed.

Lamellar body inclusions of the rat lung great alveolar cell (=Type II alveolar cell) have been studied by conventional thin sectioning and freeze-etching. The former confirmed previous reports that lamellar bodies are released into the alveolar space, and release inter alia tubular myelin, associated with lung surfactant. Replicas of fixed or unfixed freeze-etched material reveal the intracellular lamellar bodies as highly structured, in which lamellae may be concentric or disposed in parallel planar series. Fractured lamellae bear ca. 100 A particles, and linear sculpturing similar in spacing to extracellular tubular myelin. Transverse fractures show that lamellae are ca. 60 A in width, comprising two ca. 20 A components. The appearance of freeze-fractured and etched lipoprotein and phospholipid membranes is discussed, and it is proposed that the surfaces revealed within the lamellar body are associated with the elaboration of surfactant components.

The structure of peripheral nerves, and the organization of the myoneural junctions in flight muscle fibers of a beetle is described. The uniaxonal presynaptic nerve branches display the "tunicated" structure reported in the case of other insect nerves and the relationship between the axon and the lemnoblast folds is discussed. The synapsing nerve terminal shows many similarities with that of central and peripheral junctions of other insects and of vertebrates (e.g., the intra-axonal synaptic vesicles) but certain important differences have been noted between this region in Tenebrio flight muscle and in other insect muscles. Firstly, the axon discards the lemnoblast before the junction is established and the axon effects a circumferential synapse with the plasma membrane of the fiber, which alone shows the increased thickness often observed in both pre- and postsynaptic elements. Secondly, in addition to the synaptic vesicles within the axon are present, in the immediately adjacent sarcoplasm, great numbers of larger postsynaptic vesicles which, it is tentatively suggested, may represent the sites of storage of the enzymatic destroyer of the activating substance similarly quantized within the intra-axonal vesicles. The spatial relationship between the peripherally located junctions and the portion of the fiber plasma membrane internalized as circumtracheolar sheaths is considered, and the possible significance of this with respect to impulse conduction is discussed briefly.

Replicas of fractured chromaffin cells are indicative of a range of activities thought to characterize exocytosis, including attachment of secretory vesicles to the plasma membrane, fusion, extrusion of contents, and membrane retrieval. Exocytosis sites are abundant on stimulated cells but are infrequent when calcium is omitted from the system.

The fine structure of myoepithelial cells of the proventriculus of a marine annelid ( Syllis spongiphila ) is described. The contractile system of these muscle fibers includes a single medial Z band. The thick filaments possess a ~140 A paramyosin-like periodicity, but the filament disposition in these cells corresponds to that of other striated muscles, and actin orbitals in the A band number up to 20. The distance between the centers of the two H bands in the largest cells is ca . 40 µ Dyads involving T-system invaginations and isolated vesicles of the sarcoplasmic reticulum are situated at all sarcomere levels. Insertions of the contractile material onto invaginations of the inner and outer cell surfaces are described. Presumed polyaxonal neuromuscular junctions are established across wide synaptic clefts and include terminals with spherical and non-spherical synaptic vesicles. Inclusions occupying the core of each muscle cell appear to be rich in magnesium.

The fine structure of fibrillar flight muscle of the mature adult beetle Tenebrio molitor is described. Although the very high frequency of contraction of fibrillar muscle has previously been in part accounted for as the result of mechanical specialization of the wing-bearing segment rather than of a correspondingly high rate of motor impulse supply, the problem of the nature of the pathway by which excitation is conducted into these large fibers remained. Therefore, particular attention has been given to the disposition and relationships of the plasma membrane and sarcoplasmic reticulum in this tissue. The invading tracheoles draw with them a sheath of plasma membrane from the surface to all depths in the fiber, and it is suggested that these sheaths, together with the extensive tubular arborisations arising from them, reduce the maximum plasma membrane-to-fibril distance from the radius of the fiber to a value of less than 2 µ. The evidence presented here confirms Veratti's contention that in fibrillar muscle the "reticulum" is associated with, though entirely distinct from the fibrils. Unlike other muscles so far examined, these flight muscle fibers contain a plasma membrane reticulum only, but it is possible that elsewhere the general "sarcoplasmic reticulum" includes a component derived from the plasma membrane, likewise acting as the pathway for inward conduction of excitation. Profiles of the internalised plasma membrane in Tenebrio showing the usual triple-layered 25-25-25 A organization are frequently seen, in sections, in close association with isolated vesicles (defined by "simple" 50 A membranes) which are here considered to represent, in vestigial form, the portion of the sarcoplasmic reticulum which in other types of muscle is complex and highly developed. Such associations, in Tenebrio, between these two dissimilar elements are here termed "dyads" and the possible morphological and functional homology between these and the "triads" of other types of fiber is considered.

The organization of the luminescent organ of an adult firefly has been studied with the electron microscope, and particular attention has been given to the disposition of nerve terminals within the organ. The cytological structure of the cells of the tracheal system, the peripheral and terminal axons, the photocytes and the cells of the dorsal ("reflecting") layer is described. Previous observations on the peripheral course of nerve branches alongside the tracheal trunks at the level of the dorsal layer and photocyte epithelium have been confirmed, and specialised nerve endings containing axoplasmic components structurally identical with "synaptic vesicles" and "neurosecretory droplets" have been identified, not in association with the surface of the photocytes, but lying between the apposed surfaces of two components of the tracheal epithelium: the tracheal end-cell and the tracheolar cell. These cytological findings are discussed in terms of available biochemical and physiological evidence concerning the mechanism of light emission in the firefly, especially with respect to the possible role of chemical "transmitter" action in triggering a response in a luminescent effector system.

Pure preparations of Trypanosoma brucei , free from plasma and cellular components were isolated from rat blood, and concentrated into loose pellets by low-speed centrifugation. Pellets were either processed for thin sectioning as a control for general morphology, or glycerol-treated after glutaraldehyde fixation for preparation of freeze-fracture replicas. Concentration of cells of 50,000–100,000/mm 2 of sectioned or fractured surface facilitated identification of fracture faces of the cell body, invaginated flagellar pocket and flagellum. Particle distribution and A and B faces of these regions of the cell are described. A collar of B face particles occurs around the neck of the flagellar pocket, possibly associated with a junction controlling ingress of ingested materials to coated vesicles formed along the membrane defining the pocket. A and B faces of the flagellum and adjoining surface of the cell body have shown that the only intra-membrane specialization corresponding to the miniature ‘maculae adherentes’ described previously in thin sections is probably an uninterrupted series of small clusters (3–6) of 80 A particles on the A face of the flagellar membrane. It is proposed that these arrays represent attachment points for strands linking the axoneme and paraxial rod to the flagellar surface, and are not directly concerned with the physical adhesion of the flagellum to the cell body surface—a linkage that appears to be established within the extracellular gap between these apposed surfaces of the cell. The potential use of freeze-etching in further study of the external antigens of the infective cell is discussed.

The organization of the indirect flight muscle of an aphid (Hemiptera-Homoptera) is described. The fibers of this muscle contain an extensive though irregularly disposed complement of T system tubules, derived as open invaginations from the cell surface and from the plasma membrane sheaths accompanying the tracheoles within the fiber. The sarcoplasmic reticulum is reduced to small vesicles applied to the T system surfaces, the intermembrane gap being traversed by blocks of electron-opaque material resembling that of septate desmosomes. The form and distribution of the T system and sarcoplasmic reticulum membranes in flight muscles of representatives of the major insect orders is described, and the extreme reduction of the reticulum cisternae in all asynchronous fibers (to which group the aphid flight muscle probably belongs), and the high degree of their development in synchronous fibers is documented and discussed in terms of the contraction physiology of these muscle cells.

An electron microscopic investigation of the Malpighian tubules of a leaf hopper, Macrosteles fascifrons, shows that these organs comprise three quite distinct cell types, and the structure of these and of the mid- and hindgut epithelial cells is described. In particular, a comparison is made between the organization of the basal and apical surfaces of cells in the Malpighian tubule and in the vertebrate kidney, and it is suggested that similarities between these excretory epithelia reflect functional parallels between them. While the midgut and one region of the Malpighian tubule bear a typical microvillar brush border, elsewhere in the tubule and in the hindgut the apical surface bears cytoplasmic leaflets or lamellae. The sole solid excretory material of these insects consists of the brochosomes, secreted by cells of one region of the Malpighian tubule. The structure, geometry, and development of these unusual bodies, apparently formed within specialized Golgi regions, has been investigated, and histochemical tests indicate that they contain lipid and protein components.

The distribution of ATPase activity in the asynchronous flight muscles of Calliphora erythrocephala (Diptera) was studied at a fine structural level, using preparations of teased fibers, both unfixed and after brief fixation in hydroxyadipaldehyde, incubated in a medium for the histochemical demonstration of myosin or actomyosin ATPase. In relaxed fibrils, activity was found confined to the A bands and was absent from the H zones as well as from the Z and I band regions. At high magnification, deposits of final product, lead phosphate, appeared primarily related to the thick filaments, or to short lateral extensions from them. Evidence was gathered which indicated that this enzyme activity was that of a triphosphatase which did not act on dinucleoside or non-nucleoside substrates.

1. 1. The time dependence of the fluorescence enhancement of 1-anilinonaphthalene-8-sulphonate after addition to haemoglobin-free human erythrocyte ghosts was studied in 150 mM NaCl, 10 mM sodium phosphate (pH 7.0) at 25 °C. 2. 2. Enhancements were analysed in terms of “fast”, “medium” and “slow” phases. 3. 3. Ghost samples shown to be at least partly resealed towards Na + and/or osmotically responsive display a slow phase (half-time about 80 or 120 s depending on conditions at haemolysis). The medium phase (half-time about 14 s) is characteristic of physically intact membranes in a non-resealed state. The fast phase (half-time less than instrument response time) always comprises at least 50% of the total enhancement. 4. 4. 1-Anilinonaphthalene-8-sulphonate fluorescence parameters suggest that most probe binding sites are similar, and that they are not involved in membrane changes leading to resealing. 5. 5. It is proposed that 1-anilinonaphthalene-8-sulphonate sites are only distinguished by location on the extracellular or intracellular side of a membrane 1-anilinonaphthalene-8-sulphonate premeability barrier. Diffusion of 1-anilinonaphthalene-8-sulphonate across the barrier produces the observed medium or slow phases, depending on the state of the membrane.

An electron-microscopic examination of bone marrow mitochondria derived from 10 patients before and after chloramphenicol (CAP) therapy has shown that CAP induces an ultrastructural modification resulting in an increase in the density of the mitochondrial matrix. These changes resemble the condensed mitochondrial configurations which have previously been interpreted in terms of electron transfer and oxidative phosphorylation. However, in the concentrations employed in this study, CAP has no effect on mitochondrial respiration or on oxidative phosphorylation, but is a potent inhibitor of mitochondrial protein synthesis. Evidence relating this effect of the drug to its myelotoxicity has been supported by a comparative study demonstrating lack of effect of a drug such as tetracycline which is not known to be myelotoxic, and the excel lent correlation between the extensiveness of mitochondrial changes in the marrow and the serum level of free CAP. The results were confirmed in a parallel study on insect flight muscle mitochondria, in which the metabolic state under which the drug is administered can be controlled and where the regular disposition of the mitochondrial cristae provides an ideal model for the study of configurational changes.

The detailed structure of the cockroach spermatheca is described and discussed firstly as an example of an insect integumentary gland, and secondly, from the standpoint of its role in reproduction. The gland comprises a cortical rank of separate secretory units, each associated with an epithelial duct cell responsible for receiving secreted materials and transporting them through the cuticular intima lining the reproductive tract. Secretory activity is cyclic, and the probable mode of elaboration and release of secretory material is described, together with the fine structure of the markedly differing intimas associated respectively with the glandular and conducting units.

The distribution of esterase activity in the last abdominal ganglion, the connectives and the cereal nerves of the cockroach Periplaneta americana has been investigated cytochemically. Activity of an unspecific eserine-insensitive esterase (or esterases) has been found in glial elements in these regions of the nerve cord. In addition, sites of cholinesterase (eserine-sensitive) activity have been found in association with (a) the glial sheaths of the axons in the cereal nerves and connectives, (b) the glial folds encapsulating the neuron perikarya in the ganglion, and (c) in localized areas along the membranes of axon branches within the neuropile, often flanked by focal clusters of synaptic vesicles. These results are discussed with particular reference to the previously reported insensitivity of the insect nerve cord to applied acetylcholine, and to the probable existence of a cholinergic synaptic mechanism in the central nervous system of this insect.

The organization of the corpora cardiaca and corpora allata of the female alfalfa weevil, Hypera postica, generally parallels that reported for other insects with extrinsic neurosecretory axons, intrinsic neurosecretory cells, and interstitial or glial cells appearing in both glands. Connecting the brain with the cardiacum are the extrinsic neurosecretory axons containing granules between 600 and 2200 A. Granules with a diameter of up to 4000 A are found in the intrinsic secretory cells of the corpus cardiacum. From the lobed corpus cardiacum, axons extend along a short nerve trunk into the ovoid corpus allatum. A characteristic of this gland is the presence of a layer of axons, ensheathing most of the corpus allatum and situated beneath an external basement membrane. These axons are believed to have their origin in the protocerebral neurosecretory cells. Although the c. allatum is known to be engaged in the release of hormonally active material, the cytoplasmic organization does not strongly reflect such activity. The glands of the diapausing weevil appear similar to those of the active weevil with a reduction in cytoplasm and a consequent folding of the plasma membrane.

The fine structure of a spider myo-apodeme junction is described, and discussed in terms of other arthropod muscle attachments. This is contrasted with the situation in the venom gland, equipped with muscle fibers that control expulsion of the secreted material. The latter involves a cell-free collagenous matrix, lying between the muscle cells and the sheath of the gland. As in other arthropods, skeletal fibers are attached to the apodeme cuticle via specialized epidermal cells, containing oriented microtubules. Interdigitations between these cells and muscle, basally, and cuticle, apically, are described. Extracellular tonofibrillae described elsewhere are inconspicuous in the apodeme cuticle.

The electrophysiological properties and structure of heart muscle fibres of the lobster myocardium have been investigated. Discontinuities were observed in the expected decrement of electrotonic potentials in a study of the passive membrane properties of the muscle. Injection of Procion yellow revealed transverse barriers to dye movement, identified in electron micrographs as intercellular junctions. Unlike the intercalated discs of the vertebrate myocardium, which these regions superficially resemble, nexus regions of `close' or `gap' junctions are absent. The intercellular cleft is generally 400 A in width, locally reduced to 75-100 A The functional implications of these findings are discussed. Cardiac myoneural junctions, which include regularly repeating structures extending into the synaptic cleft from the postsynaptic membrane, are described.

The structure of the flight muscle of a dragonfly (Aeshna sp.) has been studied with the light and electron microscopes, and the organization of this specialized tubular muscle is described. This tissue is characterized by the great development of the sarcosomes, which are slab-like and are arranged within the fiber opposite each sarcomere of the radially oriented lamellar myofibrils. A well developed and highly ordered sarcoplasmic reticulum is present, consisting of perforated curtain-like cisternae extending across the face of each fibril, together with tubular invaginations of the fiber plasma membrane situated within indentations in the sarcosomes and traversing the fibril surface midway between the Z and M levels. The structure of these fibers, and notably the organization of the reticulum, is compared with that of other types of muscle, and the possible role of the two components of the sarcoplasmic reticulum in the contraction physiology of the dragonfly muscle fiber is discussed.

The membrane systems of cardiac muscle of the lobster Homarus americanus are described. Wide radial invaginations of the plasma membrane occur at the Z level. From these arise longitudinally disposed narrower tubules leading to a collar encircling the fibril at the H level, which is associated with cisternae of the sarcoplasmic reticulum in dyadic and triadic junctions. This situation is compared with the pattern of T-system invagination previously reported in muscles of crustacea and other animals. Its physiological significance is also discussed.

The cytological organization of flight muscle fibers of Odonata has been investigated. These fibers, in representatives of the Zygoptera and Anisoptera, have been compared and found to be similar, except that, in the former, pairs of lamellar fibrils, rather than single fibrils, alternate with the mitochondria. In each instance, in these synchronous muscles, the actin filaments of the myofibrils are found to lie opposite to and midway between pairs of myosin filaments—a configuration previously reported in asynchronous flight muscle fibers. The disposition of the T system and sarcoplasmic reticulum membranes in glutaraldehyde-fixed anisopteran muscle is described in detail: the T system tubules are shown to be radially continuous across the fiber, and are derived as openmouthed invaginations from the surface cell-membrane. The detailed organization of the dyad junctions between these tubules and the adjoining cisternae of the sarcoplasmic reticulum is described. The accessibility of the T system interior to diffusion exchange with the general extracellular milieu has been investigated by studies on the penetration of ferritin into the fiber: molecules of this marker have been found to diffuse solely along the T system tubules, and their presence in the tubule extremities adjoining the centrally placed nuclei confirms the morphological evidence suggesting that these tubules provide open diffusion channels extending across the radius of the fiber. The possible physiological role of these membrane components and their distribution in synchronous muscles of insects and vertebrates and in asynchronous insect flight muscle are discussed.

Scanning electron micrographs of the endothelium of the pulmonary artery reveal that the entire surface is covered by a meshwork of irregular projections which vastly increase the surface area. The size and density of the projections suggest that they may function to direct an eddying flow of plasma along the endothelial surface.

A child with severe accidental methadone intoxication is presented. The symptoms are discussed with particular reference to respiratory depression. The phenomenon of transient pulmonary edema secondary to this intoxication is described, we believe, for the first time in a child.

Experimental use of primary cultures of endocrine pancreas is constrained by early, vigorous proliferation of fibroblastoid cells. The addition of heavy metals, sodium ethylmercurithiosalicylate, phenyl mercuric acetate, phenyl mercuric nitrate and sodium aurothiomalate to the culture media selectively destroys these fibroblastoid cells yielding highly enriched, morphologically intact, functionally competent endocrine cells that are capable of cell replication. This action of heavy metals appears to be due to reversible inhibition of sulfhydryl enzymes since glutathione and thioglycolate were demonstrated to completely inhibit the cytotoxic effects of the mercury and gold containing agents, respectively. Certain variables in the application of the mercurial agents to pancreatic endocrine cell cultures were defined, most notably the enhanced sensitivity of fetalvs. neonatal tissue, and an inverse relationship of cell density to effective toxicity. After removal of the heavy metal agent from the culture media, many pancreatic islets send out cytoplasmic projections, containing large numbers of oriented microtubules which serve as bridging units to adjacent endocrine cells. The sustained availability of virtually pure pancreatic endocrine cell cultures, which results from the application of mercury to the culture media will undoubtedly permit many aspects of the cell biology of the endocrine pancreas to be directly and sequentially assailed.

There is increasing evidence that type II alveolar cells are capable of synthesizing surface active material like that obtained from the airways. However a number of problems remain to be solved before it can be stated conclusively that type II cells synthesize the surface active material of the terminal airspace. Among these problems is that of secretion. A number of previous studies have given evidence of the release of lamellar bodies by merocrine secretion. In this study morphologic evidence is presented which supports the view that secretion of lamellar bodies is accomplished by exocytosis. At the apical surface of type II cells, sites can be found where the limiting membrane of the lamellar body is clearly fused with the type II cell plasma membrane and an open channel exists between the contents of the lamellar body and the alveolar space. At these sites the lamellar contents extrude into the airspace with consequent loss of the highly compact organization of intracellular lamellar bodies. The intactness and continuity of the membranes can be traced for the full extent of the exocytosis site. Freeze-etch replicas of the membranes of type II cells show depressions which may represent the sites of discharged lamellae. In addition, tongue-like folds are seen which could be explained as the extensions of cytoplasm which surround the releasing lamellar body and which may flap over the exocytosis pit after discharge. Micrographs of the alveolar space show disorganized lamellar whorls which appear to be unravelling to produce tubular myelin. In view of the unusually large size and lipid composition of lamellar bodies, a mechanism involving hydration of mucopolysaccharide contents as an aid to expulsion of lamellar contents is suggested.

Little is known of how adrenal hormones pass from the interstitial to the vascular space. We have begun to examine the adrenal endothelium as a barrier to hormone passage, by the freeze-fracturing technique. The endothelium of both cortex and medulla is fenestrated. Fractures from both regions show endothelial cells to be extremely thin in regions where fenestrations are abundant. En face fractures show fenestrae disposed in tracts; the fenestrae reaching a distribution of 35/ μ 2 . In both cortex and medulla there are areas of continuous endothelium which contain caveolae. Structures believed to represent fenestra diaphragms contain randomly disposed particles and occasional pits. We have not identified in replicas the central ring and pore described in thin-sectioned material (Elfvin, 1965). The main differences between freeze-fractured aspects of cortical and medullary endothelium are the greater abundance of caveolae in the medulla and the size of the fenestrae (fenestra rims in the medulla are 525–780 A in diameter; in the cortex 570–1660 A). These differences may reflect the different embryological origins of the medulla and cortex. While caveolae may participate in hormone transport, there is no evidence for this. In the medulla the caveolae are more numerous and may have a function not necessarily related to transport. Possibly, caveolae play a role in processing hormones and related substances. For example, ATP and specific proteins are released as well as epinephrine during exocytosis from chromaffin cells. Epinephrine enters the vascular space but ATP does not. ATPase enzymes are a common feature of caveolae of other endothelia and may occur as well in adrenal endothelium.

The organization of flight muscle of a dragonfly, with special reference to the fibre membrane systems, has been investigated by means of platinum-carbon replicas of freeze-etched material. The fractured surface plasma membrane and origin of transverse tubules are described together with the radial course of the tubules into the cell, along grooves in the mitochondrial surface. The form of the sarcoplasmic reti, culum cisternae is also described. Replicas are interpreted as revealing up to three unfractured membrane surfaces and several fractured profiles. The illustrations include stereo-micrographs of replicas.

The fine structure of synapses in the central nervous system of lamprey (Petromyzon marinus) ammocoetes has been investigated. Both synapses within the neuropil and synaptic links between giant fibers (including Müller cells) and small postsynaptic units are described. The distribution of neurofilaments and microtubules in nerve profiles over a wide diameter range is described, and the possible role of these structures in intracellular transport is discussed. Electron micrographs indicate that small lucent "synaptic vesicles" occur sparsely throughout the axoplasm and in regular arrays in association with microtubules in the vicinity of synapses. Within a synaptic focus, immediately adjoining the presynaptic membrane, vesicles are randomly arranged and are not associated with microtubules. Neurofilaments are present, generally in large numbers, but these are not associated with vesicles or other particulates. The structural findings are considered in terms of current concepts of fast and slow transport in neurons and the mechanochemical control of intracellular movement of materials.

The fine structure of Methylococcus capsulatus is described. Particular emphasis is focused on the intracytoplasmic membrane system which is organized as a stacked array of flattened saccules. Each saccule is limited by a 75 A unit membrane and lies in close apposition to adjacent saccules. Methylococcus capsulatus is an obligate methylotroph whose sole source of carbon and energy is methane (or methanol). In this study methane oxidation is demonstrated for the first time in a cell-free system. Work is in progress to determine the cellular organelles which constitute the particulate fraction responsible for methane oxidation. The possible role of the intracytoplasmic membranes in energy transfer is considered in relation to the functions of stacked membrane arrays in other animal, plant and bacterial systems.

The dipteran haltere incorporates large numbers of regularly disposed mechanoreceptors providing the sensory input enabling the vibrating haltere to function as a gyroscopic organ of equilibrium. Campaniform sensilla of the basal and scapal regions have been investigated by light and transmission electron microscopy, and these observations are augmented by scanning electron studies of the cuticle overlying the groups of sensilla. Each sensillum possesses a specialized fan-shaped terminal containing a complex and ordered association of microtubules and filaments. The transmission of stress to this region via the cuticle, and its possible role in transduction is considered. The fine structure of apical and basal sections of the distal sensory process and associated sheath cells is described; the functional significance of the distribution of mitochondria and other components is discussed. The organization of haltere chordotonal sensilla is described briefly, and compared with other mechanoreceptors with particular reference to microtubules and scolopale structures.

We have undertaken a study to coordinate examination of thin sections with that of freeze-etched material. Results suggest that (1) lamellar bodies have a much more regular structure than hitherto believed. (2) The leaflets of some lamellar bodies reveal a unique substructure of ribs and intramembranous particles displayed in form and spacing (400-500 A) like those of tubular myelin as seen in the airspace. (3) As in other membranes of high metabolic activity the fracture face is covered with particles 80-100 A in diameter. Our freeze-etch findings of the previously undetected organization and suborganization of lamellar bodies suggest an entirely new set of morphological criteria for evaluating the maturation of lamellar bodies and their role in the intracellular assembly of the reticular components of the airspace lining.

IF the authors of the recent report1 had chanced to include an insect physiologist among those consulted about the unusual objects detected by electron microscopy in an atmospheric aerosol, their quest would have been at an end ! These structures are brochosomes, bizarre excretory products of the Malpighian tubules of leaf hoppers (Homoptera; Jassidae). They were discovered on wing membranes and elsewhere on the surface of these insects by Tulloch, Shapiro and Cochrane2 and Wilde and Cochrane3, who named these structures to accord with their net-like form. Subsequently it was found4, by examination of thin sections in the electron microscope, that brochosomes are produced intracellularly in the urine-secreting tubules; their eventual distribution over the surface cuticle seems to be achieved by the insect manipulating the excreted pellets with its posterior pair of legs5.

The chorion of the moth Hyalophora cecropia has been found to consist primarily of a complex of proteins rendered insoluble by disulfide and hydrogen bonds. The bulk of the chorion is lamellate, and as in many cuticles each lamella contains a regular helicoidal series of microfibrils. The lamellar organization is disrupted in a central zone containing narrow irregular channels, and elsewhere the mature chorion is traversed by wide cylindrical channels. These open to the outer chorion surface indirectly via a porous trabeculate zone, and are distinct from the channels leading to the aeropyles, described elsewhere, in lacking cytoplasmic processes during their formation.

1. 1. The haemolymph of Callinectes sapidus acclimated to fresh water is always negative with respect to the medium. 2. 2. This negative potential is the result of differential permeability and active transport processes. 3. 3. The whole gill is preferentially permeable to sodium with respect to chloride. The preferential permeability is primarily a function of the cuticle. 4. 4. Both sodium and chloride are actively transported inward. Sodium is moved against a concentration gradient but down an electrical gradient. Chloride moves against both an electrical and concentration gradient.