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Summary The acini of the bovine mandibular gland consist of mucous and demilunar seromucous cells. The mucous cells contain electron-lucid mucigen droplets or mucoid masses, only rarely delimited by membranes. The cisternae of the RER communicate freely. Mitochondria are scarce. The seromucous demilunar cells contain considerably more granules, and perigranular membranes are far more common. Many granules have characteristic alternating electron opaque and less electron dense areas. The prominent RER appears to be associated with a peculiar labyrinth of interconnecting tubules. Mitochondria are numerous. Numerous myoepithelial cells are observed. The cuboidal cells of the intercalated ducts lack signs of secretory activity. The columnar cells of the striated ducts have extensive infoldings of the basal plasma membrane, associated with mitochondria. The cell apex contains many granulated vesicles. Endogenous lactoperoxidase activity could be demonstrated only in the demilunar cells, within the secretory granules, the RER, including the perinuclear cisternae, as well as the labyrinth mentioned above. A positive reaction could be observed over acinar lumina and in intercellular canaliculi. The possible role of peroxidase as a component of a bactericidal system of the bovine saliva is discussed. Zusammenfassung Feinstruktur und Peroxidase-Aktivitat der Drusenzellen der Gld. mandibularis des Rindes Die Acini der Gld. mandibularis des Rindes bestehen aus mukosen Zellen und seromukosen Halbmondzellen. Die mukosen Zellen enthalten elektronen-durchlassige Mucigentropfchen oder mukoide Massen, die nur selten membranbegrenzt sind. Die Zisternen des RER kommunizieren frei. Mitochondrien sind selten. Die Sekretionsprodukte der Halbmondzellen sind viel korniger, und gut erhaltene perigranulare Membranen sind viel haufiger. Das deutliche RER scheint mit einem besonderen Labyrinth miteinander verbundener Tubuli zusammenzuhangen. Mitochondrien sind zahlreich. Zahlreiche Myoepithelzellen sind zu beobachten. Die kubischen Zellen der interkalaren Gange sezernieren nicht. Die hohen Zylinderzellen der Streifenstucke besitzen ausgedehnte Einfaltungen der basalen Plasmamembran. Endogene Laktoperoxidase-Aktivitat zeigten nur die Halbmondzellen, und zwar in den Sekretgranula, im RER einschlieslich der perinuklearen Zisternen und im erwahnten Labyrinth. Eine positive Reaktion lag im Lumen der Acini und in den interzellularen Kanalchen vor. Die mogliche Bedeutung der Peroxidase als Komponente im bakteriziden System des Rinderspeichels wird diskutiert. Resume Structure fine et activite Peroxydasique des cellules de la glande mandibulaire du boeuf Les acinus de la glande mandibulaire bovine component des cellules a mucus et des croissants sero-muqueux. Les cellules a mucus contiennent des gouttelettes de mucigene transparentes aux electrons ou des masses mucoides, rarement delimitees par des membranes. Les citernes du R. E. R. communiquent librement. Les mitochondries sont rares. Les cellules des croissants seromuqueux contiennent beaucoup plus de granules et les membranes perigranulaires sont bien plus frequentes. Beaucoup de granules montrent une juxtaposition caracteristique d' aires opaques aux electrons et d' autres moins denses. L' important R. E. R. parait associea un labyrinthe particulier de tubules intercommunicants. Les mitochondries sont nombreuses. On a observe de nombreuses cellules myoepitheliales. Les cellules cubiques des canaux intercalates ne montrent aucun signe d' activite secretoire. Les cellules columnaires des conduits stries montrent d' importantes invaginations de la mebrane plasmatique basale, associee aux mitochondries. L' apex des cellules contient de nombreuses vesicules granuleuses. Une activite de lactoperoxydasique endogene n'a pu etre demontree que dans les cellules des croissants, a l' interieur des granules de secretion, du R. E. R. y compris les citernes perinucleaires, ainsi que dans le labyrinthe mentione ci-dessus. Une reaction positive a pu etre observee dans la lumiere des acinus et dans les canalicules intercellulaires. Le role possible de la peroxydase en tant que constituant d' un systeme bactericide de la salive desBovins est discute. Resumen Estructura fina y actividad de la peroxidasa en las celulas de la glandula mandibular del buey Los acinos de la glandula mandibular del buey se componen de celulas mucosas y de otras, semilunares seromucosas. Las celulas mucosas contienen gotas de mucigeno o masas mucoides visibles en el microscopio electronico, que solo raras veces estan delimitadas por membranas. Las cisterns del RER se comunican libremente. Las mitocondrias son raras. Las celulas semilunares seromucosas contienen un numero considerablemente mayor de granules y las membranas perigranulares son mucho mas frecuentes. Muchos de los granulos demuestran areasopacas en el microscopio electronico que alternan con otras menos densas. El RER prominente parece asociarse con un laberinto peculiar de tubulos conectados entre si. Mitocondrias existen en gran numero. Ademas se observan muchas celulas mioepiteliales. Las celulas cuboideas de los conductor intercalares no muestran senales de actividad secretora. Las celulas altas de los conductos estriados tienen una membrana basal muy plegada hacia adentro, asociada con las mitocondrias. El apice de las celulas con-tiene muchas vesiculas granuladas.

The epithelium of the Malpighian tubules in the housefly is comprised of four distinct cellular types. Type I cells are characterized by the presence of intimate associations between infoldings of basal plasma membrane and mitochondria. On the luminal surface, cytoplasm is extended into microvilli which contain mitochondria. Membrane-bound vacuoles in the cytoplasm seem to progressively accumulate granular material. Type II cells have dilated canaliculi. Microvilli lack mitochondria. The Type III cell has not been reported previously in Malpighian tubules. It has very well-developed granular endoplasmic reticulum which contains intracisternal bundles of tubules. Cytoplasm contains numerous electron dense bodies. Type IV cells occur in the common duct region of the Malpighian tubules. Mitochondria do not extend into the microvilli.

The gland cells of Lyonet's gland, which is accessory to the silk gland in the silkworm larva, is characterized by the presence of complicated canaliculi bearing microvilli on their inner surface, large numbers of mitochondria and remarkably convoluted basal plasma membrane. On the other hand, the cell lacks the well-developed cytoplasmic membrane system such as rough- and smooth-surfaced endoplasmic reticula and Golgi bodies, though free ribosomes are numerous. Secretory vesicles are absent, and the canaliculi contain no dense material. From such ultrastructural observations, it was suggested that a possible role of the gland may be the exchange of the small molecules such as water and ions, rather than the hitherto supposed secretory role of a cementing sunstance of silk proteins. The lumen of the proximal part of the glandular duct contains a kind of proteinaceous substance which can be demonstrated histochemically and is regarded as similar to one of the silk proteins in the silk gland, not to the real product of the Lyonet's gland.

The filter chamber is a complex junction of anterior and posterior extremities of the midgut and Malpighian tubules. The sac-like anterior extremity, or filter chamber proper, comprises two cell types. These are large cuboidal cells which secrete a mucoprotein, and extremely thin cells which have regular tubular invaginations of the basal plasma membrane. The posterior extremity of the midgut and the internal Malpighian tubules coil round the filter chamber proper. They consist of thin epithelial cells identical in ultrastructure. The basal plasma membrane in these cells is formed into leaflets. A thin cellular sheath and thick muscle layers surround the filter chamber. The filter chamber proper is lined by the mucoprotein secretion of the cuboidal cells. This secretion appears to bind potassium ions. ATPase and alkaline phosphatase cannot be detected in the filter chamber epithelia. The structure and cytochemistry of the filter chamber suggests that water flows from filter chamber proper to midgut and Malpighian tubules by passive osmosis. This may be facilitated by ion binding in the filter chamber proper and by hydrostatic pressure engendered by contraction of the muscular coat. The Malpighian tubules appear to be structurally and chemically adapted for ion secretion by active transport and possibly for reabsorption in the Malpighian duct segment.

The spatial arrangement of the five basic segments of the tubular nephron of the sea lamprey is reversed between ammocoetes and adults due to differences in the position of the glomus. The initial segment, the ciliated neck segment, resembles the pronephric nephrostome. The proximal segment is divided into two distinct regions, the pars convoluta followed by the pars recta, and has a brush border which reacts positively with the periodic acid-Schiff stain. Ultrastructurally the cells of the proximal segment resemble the corresponding cells of other vertebrates except that they lack the usual infoldings of the basal plasma membrane and they often show intercellular spaces. The mitochondria of these cells often contain filamentous and crystalline material and are usually associated with “flocculent bodies” resembling “non-nucleoid” renal microbodies. The structure of the cells of the proximal segment suggest that they are involved in endocytosis.

Apocrine sweat glands of human axillary skin of four cases removed at operations for osmidrosis axillae were observed with the electron microscope.1. The nucleus of the gland cell is shaped slightly flattened sphere and is enclosed with double layered porous nuclear envelope. The karyoplasm consists of compound helical strands, chromonemata. The nucleolus is made up of well developed nucleolonema which makes a tangled glomerulus. In the nucleolonema, beadlike chains of granules are arranged in a helical disposition.2. Spherical or ellipsoidal mitochondria possess outer and inner double membranes and matrix. Less developed inner double membranes (cristae mitochondriales) are situated at the periphery in a form of arch. As mitochondria enlarge, the inner double membranes are disappeared and the organelles may be converted into the secretory granules.3. Well developed GOLGI apparatus is seen at the supranuclear region. In this case, the GOLGI lamellae are only rarely observed, but grouped small vesicles and large vacuoles are recognized. Rough surfaced endoplasmic reticulum is generally less developed and small number of cisternae are closely applied to the mitochondria.4. Electron microscopically, two kinds of secretory granules are recognized. One of them is light granule probably derived from mitochondria and the other is dark, irregularly shaped granule. The latter granule may be formed within the GOLGI vacuole or smooth surfaced endoplasmic reticulum. The internal density of this granule is uneven and it may be assumed that this granule is of lipid nature containing iron and pigment. Some features suggesting the disintegration of dark granules prior to the extrusion were observed.5. Marked infoldings of basal plasma membrane are clearly observed. Moreover, beaded arrangement of small vesicles continues to the tip of infolding. It is considered that these structures may be intimately associated with the absorption and transport of water.6. On the free surface of the gland cell, many microvilli are observed. These are disappeared at the formation of apocrine secretory projecton, interior of which is quite clear and contains small number of vesicles.7. The myoepithelial cell is characterized by the presence of longitudinally oriented myofilaments. Grouped small vesicles were found immediately beneath the plasma memebrane facing the membrana propria.

Salivary gland cells of Pseudolynchia canariensis are ultrastructurally similar to those of many other diptera. Heavy infections with Haemoproteus columbae sporozoites cause disruption of the basement membrane, basal plasma membrane, and disorganization of the functional organelles of the cell. Cytoplasmic microtubules appear to increase in number.

The epithelial cells of the midgut and hindgut in the crayfish were studied by electron microscopy. The epithelial cells of the midgut are characterized by formation of cytoplasmic tubules or lamellae in the basal half of the cells. These structures appear to be specified infoldings of lateral plasma membranes, and are assumed to be similar in terms of structure and function to the cytoplasmic tubular structures in the chloride cell of fish gill filaments. In the hindgut, the epithelial cells are prominently provided with cytoplasmic microtubules. Thick bundles of them are attached to both luminal and basal plasma membranes. When attached with microtubules, the basal plasma membrane of the epithelial cells usually makes an epithelio-myonal junction with that of the processes of striated muscle fibers derived from the muscular coat, in a manner similar to that of the intercalated disc of the cardiac muscles.

In an attempt to elucidate some of the mechanisms utilized in the placental transfer of large molecules morphological changes in the visceral yolk sac have been studied in relation to the disposition of ferritin molecules at different stages of gestation. From the tenth through the eighteenth days of pregnancy, ferritin molecules were readily absorbed by the visceral endodermal cells when this material was injected either into the maternal blood stream or directly into the uterine lumen. These large molecules entered the endodermal cells through a progressively developed apical canalicular system and became localized within relatively large, heterogenous vacuoles. These vacuoles were seen to retain the ferritin for long periods, except during the last few days of gestation. At the end of pregnancy, when normally antibodies are being transferred in the largest quantities to the fetus, relatively smaller vesicles which contain ferritin appeared to have migrated away from the larger apical storage vacuoles and to have approached the endodermal basal plasma membrane. Once the large protein molecules left these cells they appeared to pass either directly into vitelline capillaries, or through mesothelial cells which line the exocoelom. Transport of large protein molecules through visceral endodermal cells involves an apical tubular system, large storage vacuoles, and smaller vesicles which appear to migrate basally near the end of pregnancy. Regulation of this process appears not to depend on the failure or success of entry of such a substance into these cells, but rather on its subsequent disposition once it is intracellular.

The tubular hindgut of the intertidal herbivorous isopod, Dynamene bidentata, consists of a long dorso-ventrally flattened anterior region, surrounded by a network of muscles, and a short muscular sphincter which grades into a pair of anal flaps. The monolayer of epithelial cells forming the wall of the hindgut appears to take no part in the production of digestive enzymes, food absorption, or glycogen and lipid storage. One function of the hindgut is to propel undigested food material, enclosed within a peritrophic membrane, to the sphincter and anal flaps where faecal pellets are formed and ejected. At the fine structural level lateral plasma membranes, often partially obliterated by microtubules, are visible. The basal plasma membrane of a typical epithelial cell is deeply infolded, associated with mitochondria, and pinocytotic. The apical plasma membrane is irregularly folded, engaged in pinocytosis, and often encloses subcuticular spaces containing an acid mucopolysaccharide substance. An intima, composed of a thin double-layered epicuticle, and a thick acid mucopolysaccharide-positive endocuticle, overlies the cells. The endocuticle may selectively bind substances to the apical plasma membrane before they are engulfed by pinocytosis. The cells resemble those of osmoregulatory organs and may help counterbalance changes in the haemolymph concentration resulting from the intertidal existence of this isopod.

SUMMARY.— The fine structure of periderm and germinative cells of the bilaminar epidermis of a 14 mm. human embryo is described. Desmosomes at different stages of differentiation are present at intervals along the apposed plasma membranes of cells of both layers, but hemi-desmosomes have not yet developed along the basal plasma membranes of the germinative cells. Cytoplasmic tono-filaments are not diffusely distributed within the germinative cells, but a continuous band of filamentous material lies just within, and parallel with the basal plasma membrane. Cells occasionally present between periderm and germinative layers exhibit features suggesting they may be immigrant mesenchymal elements.

The mature ultimobranchial follicle of the rat consists of two or more layers of cells (U cells) surrounding a lumen containing cell debris. The ultrastructure of the outer, or basal, U cell is characterized by the presence of half desmosomes on the basal plasma membrane and pinocytotic vesicles near it, by little granular reticulum but an abundance of free ribosomes, by clusters of fibrils connected to desmosomes and possibly free in the cytoplasm. The cell ages by accumulation of clusters of fibrils and it undergoes differentiation to form more apical U cells which contain fewer ribosomes and have fibrils dispersed in the cytoplasm. The apical U cell is desquamated into the lumen and ultimately becomes a carcass containing a dense matting of fibers and vacuoles containing a reticulated material resembling that in the lumen. U cells are observed associated with typical thyroid epithelium in the thyroid of the newborn rat in relatively large follicles containing colloid and desquamated cells. They also form rods of cells in the very young rat. Mixed follicles containing both U cells and typical thyroid epithelium occur at all ages.

The fine structure of the lacrimal gland and its changes of the secretory cycle have been studied by light and electron microscopy, on the rat. 1. The acinar cell contains only one kind of the secretory granule which is surrounded by a thin membrane originated from the Golgi membranes, and its contents are low in density, water-rich and not mucous. The secretory granule is extruded in a typical mode of merocrine secretion into the lumen. 2. Both in the small and larger ducts, dense secretory granules are found in their epithelial cells and seem to be formed within the Golgi complex. The mechanism of granule release is not clear but the apocrine-like processes are often found. No typical infolding of the basal plasma membrane is observable and intercellular boundaries are characterized by well-developed cellular interdigitation in complex arrangement. 3. The myoepithelial cells are present in the terminal portion and duct system which is lined with the epithelial cells containing secretory granules. 4. Unmyelinated nerve fibers are sometimes found in the interstitial connective tissue among acini, but there is no evidence of direct attachment of nerve fibers to the acinar cells or myoepithelial cells.

Rate of urine formation is very sensitive to potassium concentration. Potassium is concentrated in the urine by a mechanism which is independent of other monovalent cations. Rubidium, caesium and sodium are also capable of maintaining a flow of urine. At low external potassium concentrations, sodium stimulates potassium secretion. Rate of urine secretion is stimulated by low osmotic pressures; the osmotic pressure of urine was slightly hypertonic throughout the range of external osmotic pressure employed. Addition of sucrose depresses rate of urine secretion; the potassium concentration of the urine increased by 1 mM/l. for each 2 mM/l. of sucrose added to the bathing medium. Urine formation is insensitive to sulphanilamide, acetazolamide, ouabain and a wide variation of pH. These observations are discussed in relation to the hypothesis that potassium secretion takes place across both surfaces of the cell. The pump on the basal surface may be a coupled sodium-potassium pump, whereas that on the apical surface may be electrogenic. Microvilli at the apical surface or channels formed by a complex infolding of the basal plasma membrane may represent structural devices by which standing osmotic gradients can be established during solute-linked water transport across the cells of Malpighian tubules.

The rat submaxillary salivary gland has five distinct parenchymal zones. 1Acini consist of secretory and myoepithelial cells. An extensive network of canaliculi connect the many cells within an acinus to the main lumen. The fine structure of acinar secretory cells suggests that they are capable of great synthetic capacity; each cell having a large amount of ergastoplasm, many Golgi zones, and a great amount of secretory material. It is proposed that these cells are of the continually secreting type. 2Intercalated ducts consist of cuboidal cells and myoepithelium. This segment connects the acini to the main conduit system of the gland. The fine structure of the cuboidal cells indicates that they are essentially nonsecretory. 3The granular duct consists of three types of columnar cells; (a) dark narrow cells which contain many free ribosomes but no ergastoplasm or granules, (b) light granular cells which have varying amounts of ergastoplasm and granules, (c) dark granular cells which are full of granules while the other cell constituents including the nucleus, occupy a basal position. It is proposed that these three cells represent different secretory stages of the same cell type. This supports the interpretation that secretion in these cells is not continuous, but cyclic in nature. 4The striated duct forms a small portion of the total gland parenchyma and consists of tall columnar cells with extensive infolding of the basal plasma membrane, relatively little ergastoplasm and very few granules. It seems likely that ion and water metabolism is a specialized function of this segment. 5The excretory duct consists of three cell types: (a) tall columnar light cells, (b) dark columnar vesiculated cells and (c) small basal cells. The basal infoldings of these cells and the arrangement of many capillaries around these ducts suggests that this segment is primarily concerned with water transport.

Prostatic carcinoma, renal cell carcinoma and melanoma were studied by electron microscopic enzyme cytochemistry. ACPase of prostatic cancer cells was found in lysosomes, secretory vacuoles and vesicles as well as some Golgi apparatuses. The enzyme localizations remained unchanged in tumor cells two weeks after castration when cells were viable. Renal cell carcinomas revealed to have positive ALPase activity at microvilli resembling brush border and lateral plasma membranes facing adjacent tumor cells. The enzyme was negative at basal plasma membrane of the tumor cells. Proximal tubule cells of human kidney showed ALPase activity only at apical membranes. This indicated functional dedifferentiation of plasma membrane in the tumor cells. These observations were verified and substantiated by TPPase, IDPase and G6Pase localizations in tumor cells. The tumor cells also showed positive G6Pase in endoplasmic reticulum. DOPA oxidase was successfully detected in smooth endoplasmic reticulum close to Golgi apparatus, innermost Golgi cisternae and premelanosomes both in cultured melanoma cells and melanoma tissues, which may have diagnostic value. Thus, EM enzyme cytochemistry was considered to be applicable and useful for human cancer study concerning histogenesis, diagnosis and to assess the treatment.

Unfixed human foetal and post-natal skin has been examined by the technique of freeze-fracture replication without sublimation (i.e. they were not ‘etched’). General features of epidermis and dermis are described, but the bulk of the observations relate to the epidermis. Tonofilaments are prominent within the epidermal keratinocytes and their fractured ends project appreciably beyond the general cytoplasmic plane. Desmosomes are identifiable at intervals along cross-fractured cell appositions (i.e. where the plasma membranes establish contact with one another), but nothing corresponding to the intermediate dense line of thin sections is seen in replicas. Vesicular elements are present in the general intercellular space. At appositions two complementary fracture faces related to the plasma membrane are revealed. One of these faces, directed towards the cytoplasm, exhibits localized arrays of particles at the sites of desmosomes; these are referred to here as ‘desmosomal particles’, and are highly characteristic of this face in both foetal and adult epidermis. The other fracture face, directed towards the intercellular space, is devoid of such particles. It has not proven possible to locate with certainty the exact plane of fracture of the plasma membrane which reveals these faces, and the merits of different possibilities are discussed. At the epidermal-dermal junction, apparently corresponding complementary fracture faces of the basal plasma membrane are revealed. To date, nothing which can be equated either with a basal lamina, or hemi-desmosomes is evident in replicas.

The initial loss in the rabbit brain of non-electrolytes injected into the carotid artery was measured with ‘Indicator Diffusion’ technique, 3–0–methyl-D-glucose shared with D-glucose the ability to pass into the brain by a mediated transport mechanism. The rate of transport of the following substances could not be distinguished: D-mannose, D-galactose, D-arabinose and 1)-ribose. They all penetrated more slowly than D-glucose. D-mannitol was also lost from the blood during the passage through the brain. This observation, together with the known inability of D-mannitol to pass the blood-brain barrier, suggests that the loss takes place into the endothelial cytoplasma. Thus the basal plasma membrane of the endothrlial cells together with the basal membrane proper may define blood-brain barrier chararteristirs.

Two main cell types constitute the defensive osmeterium gland of Papilio larvae. Ellipsoid gland cells have an extensively infolded basal plasma membrane, abundant ribosomes and whorls of smooth endoplasmic reticulum. The apical plasma membrane bears long microvilli extending into a mass of granular material containing electron-lucid cavities. Tangential slits occur in the epicuticle. Tubular arm cells contain heterogeneous, electron-dense inclusions, extensively-branched nuclei and large mitochondria sometimes distended with electron-dense material. The apical plasma membrane bears short microvilli. The inner, dense epicuticle forms a complex ramifying system. The two-phase defensive fluid consists mainly of water, 2-methyl propionic acid, and 2-methyl butyric acid.

A new cell with relatively little cytoplasm has been identified in the second kind of follicle in the C3H mouse thyroid gland. It has as ultrastructural characteristics the presence of clusters of fiber in the cytoplasm, vesicles near the basal plasma membrane, and half desmosomes in the basal plasma membrane. It resembles the U cell found in the ultimobranchial follicle of the Fischer rat thyroid, but has a somewhat larger amount of granular reticulum. It is found in the follicle wall between other epithelial cells and the basement membrane, but occasionally is in contact with the lumen. It also occurs as a double layer in the follicle wall. Cell debris in the lumen of the follicle contains fibrils and may be the result of desquamation of this fibril‐containing cell. Copyright © 1972 Wiley‐Liss, Inc.

1. Electron micrographs of the ultimobranchial body in sexually mature Rana pipiens were prepared to describe the cytology of the parenchymal cells. 2. Cytoplasmic granules, enclosed within a limiting membrane, are approximately 1000 A in diameter and are found at the apical and basal poles of the nucleus as aggregations and also in the basal cytoplasm in contact with the basal plasma membrane. 3. Lamellar bodies (myelin figures) of various complexities are found in all cells but are more prevalent in degenerating cells. Those structures found in the central lumen are a part of the cellular debris. 4. Lysosomes with densities similar to those of cytoplasmic granules suggest that some granules are not utilized. 5. Ergastoplasm within the parenchyma is scarce. The cytoplasm contains smooth membrane vesicles and free ribonucleoprotein (RNP) particles. 6. The ultimobranchial body is considered to have a low rate of protein synthesis. The degree of endocrine activity, if any, could not be determined at this time.

Endocytosis in thyroid follicle cells from untreated, suppressed and TSH-stimulated glands were studied. In untreated glands, images indicating a micropinocytic process resulting in the production of bristle-coated vesicles were observed at the apical surface. In suppressed glands, signs of apical micropinocytosis were only rarely encountered. After TSH stimulation, the coated vesicles reappeared. In addition, pictures indicating endocytosis through engulfment of colloid by large cytoplasmic protrusions were observed. Similar pictures were rarely encountered in normal glands. In all the experimental groups, signs of micropinocytosis by coated vesicles were observed along the lateral cell surface as well as along narrow infoldings in the basal plasma membrane. Along parts of the basal cell surface bordering on basement membrane, smooth vesicles suggesting a different form of endocytosis were observed. Differences in structure and thickness of the plasma membrane of the different surfaces of the cells were described, as well as differences in the localization of ATPase activity. Small vesicular structures, displaying ATPase activity were observed both in the basal portions of follicle cells and in the thyroid capillary walls. Heavy deposits of ATPase reaction products were found on the plasma membrane of parafollicular cells and in the interspace between a parafollicular and a follicle cell. Only scattered deposits were found between the lateral surfaces of adjacent follicle cells. Desmosomes were found between adjacent follicle cells as well as between parafollicular cells, but were not observed along the contact between a parafollicular and a follicle cell.

The central canal and the urinary bladder of the leech Hirudo medicinalis (L.) were studied by electron microscopy. The central canal consists of several different parts: the initial, intermediate and terminal canal which border only on connective tissue, and the inner and the wide canal which on the other hand are closely connected to the canaliculus cells which produce the primary urine. Structures related to transcellular transport of solutes such as microvilli and basal plasma membrane infoldings in association with mitochondria have been found in all parts of the central canal. Evidence has been obtained that the surface enlargement by basal plasma membrane infoldings increases from the proximal to the distal parts of the central canal while the luminal surface enlargement remains nearly constant. The increasing basal surface enlargement is especially obvious in those parts of the central canal which border only on connective tissue. The mechanism of reabsorption is discussed with regard to physiological data so far available. A recycling process for solutes may take place between the epithelium of the inner and the wide canal on one hand and the bordering canaliculus cells on the other hand.

By repeated intraperitoneal injections of rabbit anti-guinea-pig thyroglobulin serum into guinea-pigs, a high level of precipitating antibody to thyroglobulin was obtained in their serum for periods as long as 40 days. Although rabbit γ-globulin, most probably anti-thyroglobulin antibodies, was found as large deposits intercellularly and between the basal plasma membrane and the basement membrane of the follicle cells, no intracellular changes could be observed. No accumulation of mononuclear cells was detected in the interstitium, and in contrast to the heavy infiltration of granulocytes observed 24 hours after a single injection, only scattered eosinophils were found after repeated injections.

A combined study using electron probe (Stereoscan) and transmission electron microscopy has revealed the existance of surface specialisation of the adhesive organ of adult Diplostomum phoxini Faust, 1918. The lateral walls of the adhesive organ bear stout spines, whereas the surface which comes into contact with the host tissues consists of numerous small chambers approximately 2 μ high and 0.5–1 μ in diameter. Within the chambers are slender microvilli. This surface is cytoplasmic and is in continuity with unicellular gland cells, lying within the body of the adhesive organ, via cytoplasmic processes. The chambered adhesive organ surface and the processes contain mitochondria, granular material and membrane bounded secretion bodies 0.18 μ in diameter containing densely staining granular contents. A fibrous basement layer is poorly developed and the basal plasma membrane of this surface is in close contact with adjacent parenchymal cells and the lacuna excretory system. Histochemistry has demonstrated the presence of non-specific acid phosphatase activity in the walls of the chambers, in the microvilli and in the cisternae of the gland cell endoplasmic reticulum. The gland cells contain extensive granular endoplasmic reticulum and numerous golgi complexes. The concept of surface specialisation in relation to function is discussed.

The rectal gland of the spiny dogfish, Squalus acanthias has been described by light and electron microscopy. The cell outline shows remarkable complexity in electron micrographs. The basal plasma membrane has numerous complex infoldings and interdigitations separated by groups of mitochondria. The lateral plasma membrane also shows extensive interdigitation. The apical region demonstrates profiles of interleafing cell processes with much membrane interdigitation occurring between them but containing few mitochondria. The apical surface has microvilli which extend into the tubular lumen and are covered with nap-like filaments. Glands of some fish were catheterized and attempts were made to correlate the fine structure of these glands with the known secretory rates. The cellular morphology of the secretory cells of the rectal gland correlates well with that seen in other ion-transporting tissues, although some unique specializations were noted.

Human chorionic gonadotropin (HCG) was localized in human placenta at the ultrastructural level by an immunocytochemical method which linked the antigen HCG to a peroxidase label through the antigen-antibody reactions of an immunoglobulin-peroxidase bridge. Material on the maternal surface of the apical plasma membrane and in the cisternae of rough endoplasmic reticulum of the syncytiotrophoblast displayed strong immunostaining for HCG and the outer surface of the basal plasma membrane appeared equivocally immunoreactive. Lamellae and vesicles of the Golgi complex of the syncytium lacked electron-opaque reaction product indicative of immunostaining for HCG. Deposits of peroxidase reaction product in the large variety of cytoplasmic dense granule varied from negligible to abundant in immunostained and in control specimens alike and, accordingly, the large granules appeared to contain intrinsic peroxidase but not HCG. The question of the immunoreactivity of the small variety of cytoplasmic dense granule was not conclusively settled, but the occurrence of reactive and unreactive small granules in both immunostained and control specimens weighed against the presence of HCG in these rather infrequent granules. The cytotrophoblast lacked evidence of immunoreactivity.

Ultrastructural observations on the intestine of Metastrongylus sp. have shown that it is composed of a single layer of epithelial cells each with a basally placed nucleus and resting on a basement layer. The basal plasma membrane of each cell is highly infolded and at the cell apex the lateral plasma membranes of adjacent cells are thickened to form a terminal bar.The brush border consists of numerous microvilli which possess dilated tips. The tips contain a granular material and the evidence obtained suggests that they are released into the lumen indicating a form of micro-apocrine secretion. Golgi complexes, glycogen, lipid droplets and granular endoplasmic reticulum are present in the intestinal cells. The Golgi complexes are associated with the production of an electron dense body. Other inclusions are cytolysome-like in their structure. The discharge of lamellar bodies into the intestinal lumen is described and the role of the intestine in secretion, absorption, and excretion is discussed.The authors wish to acknowledge the co-operation and facilities provided by Professor J. Brough in the Department of Zoology and Mr J. H. M. Hughes (Veterinary Officer) for allowing unrestricted access to infected material at the Roath abattoir, Cardiff. Grateful acknowledgement is also expressed for the financial support given to one of us (T. J.) by the Pig Industry Development Authority. Any views expressed in this paper are not necessarily those of the Authority.The authors also wish to express their appreciation of the excellent technical assistance provided by Mr T. W. Davies.

A unique specialization of the basal plasma membrane of the tegumental syncytium of Hymenolepus diminuta is described and called a multitubular complex. The complex consists of rows of tubules oriented either vertically or horizontally with respect to the basal plasma membrane. A complex is formed by either one set of vertical rows and one set of horizontal rows, or two sets of horizontal rows which interdigitate; mitochondria are frequently associated with the apical or lateral margins of the complexes. On the basis of the morphology of these complexes and other experimental evidence, it is suggested that they may function in water or ion transport, or both.

Anal papillae from fourth instar larvae of the salt-water mosquito Aedes campestris maintained in normal (hyperosmotic) lake water are made up of a single epithelial layer. The only type of cell present in this layer shows several ultrastructural features characteristic of transporting tissues. The apical plasma membrane (facing the external medium) is infolded into a series of lamellae which bear a particulate coat on the cytoplasmic surface. The basal plasma membrane (facing the haemolymph) is also highly infolded to form a system of interconnecting channels throughout the cell. These channels may be considerably dilated. Dense mitochondria are abundant in anal papillae cells and tracheoles frequently penetrate deeply into the cells. No qualitative or statistically significant quantitative differences are observed in the ultrastructure of anal papillae taken from A. campestris larvae maintained in diluted (hyposmotic) lake water. It is suggested that in both hyperosmotic and hyposmotic external media the anal papillae are actively engaged in ion transport and that adaptive changes in transport rates which have been demonstrated physiologically may require only minor structural modifications such as permeability changes or activation of enzyme systems already present in the cell membranes.

Active ion transport against a high electrochemical potential gradient occurs across the epithelium of the gill processes of crayfish. The transport occurs in a thin sheet of cytoplasm underlying cuticula. This cytoplasm is supplied with irregular microvilli on the apical side. The basal plasma membrane invaginates into the cytoplasm forming a dense membrane system which is in close contact with numerous elongated mitochondria. The cytoplasm of the adhering cells is divided by a septated desmosome. It is suggested that the infolded membranes are the main place of ion transport and the close contact of mitochondria with them supplies the optimal energetic conditions for transport.

Small pieces of skin from 8 cm long Rana clamitans larvae were fixed in OsO4, washed, dehydrated, and embedded in a methacrylate mixture. Ultrathin sections were cut on a Porter-Blum ultramicrotome and were examined in an RCA electron microscope, type EMU 2D. The sections showed that aggregates of fibrous material in the cells of the inner layer of epidermal cells are identical in disposition and size with the classical figures of Eberth. It is conclusively shown that these figures do not arise from an aggregation of mitochondrial filaments. The tendency of the fibrils to concentrate on attachment points, or thickenings of the basal plasma membrane, is noted. It is also observed that numerous mitochondria are located in the distal region of the cells of the outer layer of epidermis in association with the secretory vacuoles. Microvilli are seen occasionally on the free surface of the skin. Cisternae are found only in the cells of the outer epidermal layer, while vesicular endoplasmic reticulum is found in the cells of both epidermal layers.

The copper-accumulating region of the midgut is a mosaic of interstitial and cup-shaped, copper-accumulating cells. The cup of each cuprophilic cell is lined with a highly refractile border of long microvilli except in one strain where it is predominantly lamellar. The nucleus lies basally; the basal plasma membrane is fairly extensively infolded. Cytolysomes are abundant and increase in number with increasing copper content of the diet. The interstitial cells bear short, less regular microvilli and have a less electron-dense cytoplasm. The nucleus is apical, the mitochondria-associated basal membrane is very extensively infolded and cytolysomes are less abundant. Virus-like particles present in nuclei of both cell types increase in number with increasing copper concentration.

Sweat glands from different representative areas of the horse have been studied in 6 animals with light microscopy and the intermandibular and coccygeal regions from 2 animals with electron microscopy. The sweat glands were numerous and well developed in all areas examined. The columnar cells, dominated by secretory PAS-positive diastase resistant vesicles, were surrounded by myoepithelial cells resting on a well developed basal lamina. The cytoplasmic organelles characteristic for cells involved in secretion were present. The extensively folded basal plasma membrane, the numerous microvilli at the luminal border and the intraepithelial canaliculi lined with microvilli were morphological structures typical of cells involved in water and electrolyte transport. The observation of cytoplasmic protrusions were suggestive of apocrine secretion.

The monolayered, intestinal epithelium of the ascaroid, Phocanema decipiens is composed of an innumerable number of uninuclear cuboidal to columnar cells. In the strongyloid, Ancylostoma caninum only the anterior and posterior parts of the intestine are cellular, and are composed of only two longitudinal rows of polynuclear cells, whereas cell boundaries are not found in the middle part of the intestine, i. e. the epithelium is a syncytium. This latter fact is here demonstrated for the first time by electron microscopy in the intestine of nematodes. The basal plasma membrane covering the intestine of both nematodes has invaginations, and towards the perienteric cavity a basal lamella is present. Towards the intestinal lumen the epithelium is covered with microvilli. The central cores of the microvilli as well as the terminal web, with which the cores are connected, appear homogeneous in the strongyloid but are composed of filaments in the ascaroid. In A. caninum, the epithelium is characterized by the presence of mitochondria and rough-surfaced endoplasmic reticulum, the latter often in characteristic whorl-like arrangements. A Golgi complex, various lysosome-like bodies and pigment granules are also present. In P. decipiens the intestinal cells are filled with deposits of glycogen, lipid droplets and lipo-protein granules. The mitochondria are characteristically concentrated in the apical part of the cells and along the cell boundaries. It is pointed out that the structures of the intestinal cells are in accordance with the feeding biology of the nematodes.

Infoldings of basal plasma membrane which were found in kidney tubules and other various epithelial cells (1–8) have been postulated as a special device concerning the active transport of water (6, 7). It is expected that this special structure may occur in cells of the sweat gland, which is comparable to the kidney in function that a great deal of water is excreted. We previously found basal infoldings in eccrine sweat glands of the pig ( carpal organ) (9) . The eccrine sweat glands in human axillary skin were observed at this work and more highly developed infoldings were found. These results may be a strong evidence to support the theory that the basal infoldings play a role in the water transport as claimed by Pease (6) and Ruska et al. (7).

The cytoplasm of maize coleoptile cells was displaced to either the apical or basal ends of the cells by centrifuging (1750xg for 10 min) segments in which protoplasmic streaming had been stopped by pretreatment with cytochalasin B. Centrifugation toward the base of the segment promotes the subsequent basipetal transport of indole-3-acetic acid, whereas apical centrifugation dramatically inhibits this transport. Apical centrifugation neither promotes acropetal transport nor reverses the polarity of auxin transport. Experiments in which the amyloplasts were separated from the bulk of the cytoplasm indicate that the basipetal transport is independent of both the position and pressure exerted by the amyloplasts but is strongly dependent on the amount of cytoplasm at the basal end of the cells. These effects of centrifugation on auxin transport lead to the conclusion that the metabolic component of the transport is a polar secretion of auxin localized in the basal plasma membrane of each cell.

The bacillary band is composed of tall columnar cells. The basal plasma membrane is folded and interdigitates with adjacent cells. The cytoplasm is filled with RNP particles and a fine endoplasmic reticulum. The apical area contains numerous mitochondria and a complex system of lamellae formed by invaginations of the plasma membrane. A homogeneous secretory mass fills the cuticular pore. The stichosome consists of a row of cells each of which partially or completely encloses the esophagus. The entire stichosome and esophagus are enclosed by a membranous sheath consisting of many fine layers. The central nucleus has many fine blebs arising from its surface. Numerous electron-dense granules are scattered in the cytoplasm. Large vesicles with connecting canaliculi occur throughout the cell. The esophageal lumen is enclosed by tubular cells. The fine structure suggests that the stichocytes may be secretory in function but no evidence of a secretion leaving the cell has been observed. The bacillary band also appears to secrete a substance through the cuticular pores but the nature of this product is undetermined. Comparison of the fine structure of the Malpighian tubules of insects and mammalian kidney tubule cells with that of the bacillary cells suggests an excretory function. The stichosome and bacillary band of the anterior portion of the members of the superfamily Trichuroidea have been studied by many investigators. Eberth (1859) reported many rodlike shafts in the bacillary band and described the esophagus as being embedded in a row of large cells termed the "Zellenkorper." Bastian (1866) interpreted the bacillary band as an area of cuticular pores. He considered the granular cells of the stichosome to have replaced the usual esophageal muscle fibers. Rauther (1918), in a detailed study, described the bacillary band and noted the presence of a secretory product. He also reported intracellular canals opening into the esophagus in the cells of the stichosome. Jagerskiold (1901) studied the bacillary Received for publication 10 May 1963. * From a dissertation submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Louisiana State University. This work was supported in part by U.S.P.H.S. Grant E-2347. t Present address: Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland. bands of several nematodes and concluded that the bacillary band consisted of many hypodermal glands, each having a cuticular pore. Since these nematodes lack the usual excretory system Jagerskiold thought that the hypodermal glands were excretory in function. Miller (1929) reported that the bacillary band was important in the feeding of the worm. He believed the esophagus to be nonfunctional since muscle fibers were not observed. The bacillary band and the stichosome were related in nutrition while the intestine served as an excretory organ. Chitwood (1930, 1935) rejected Miiller's theory and reported the presence of muscle fibers in the esophageal wall. He also noted that each stichocyte had an opening into the esophageal lumen and thus confirmed the opinion that the stichocytes were esophageal glands. A comparison of electron microscopic studies with previous light microscopy investigations should clarify and expand present knowledge of the structure of the bacillary cells and stichocytes of the genus Trichuris, and give insight into the relationship of form to function in these nematodes.

Ruthenium red staining demonstrated the intercellular cement substance more distinctly and specifically than staining with uranyl acetate and lead citrate. The intercellular cement substance was seen in direct contact with the outer leaflet of the plasma membrane. In the acantholysis of pemphigus vulgaris the cement substance underwent dissolution first in nondesmosomal areas and later in desmosomal areas and at intermediate junctions where the cement substance was present as a thicker layer and appeared condensed. Occluding zonules persisted even in degenerated acantholytic cells, indicating a high degree of resistance against separation. In separated desmosomal areas and separated intermediate junctions, the trilaminar structure of the plasma membrane remained intact. In early acantholyzed cells, the attachment plaques and tonofilaments appeared normal. Thus, acantholysis in pemphigus is initially an extracellular phenomenon. Fine fibrillar structures were found crisscrossing the space between the basal plasma membrane of the basal cell and the basal lamina. The presence of these fibrils partially explained why acantholysis does not occur in this space.

The reserve bladder system of Cyathocotyle bushiensis (Trematoda: Strigeoidea) consists of a series of lacunae extending over the dorsal and lateral regions of the body. The lacunar wall is composed of a layer of cytoplasm containing nuclei, mitochondria, and ribosomes. The wall possesses an outer plasma membrane elevated to form stacks of lamellae and a basal plasma membrane demarcating the layer from other structures. In the adult (5-day) parasite the cytoplasm contains lipid droplets only, which become associated with the lamellae and are then released into the lumen of the lacuna. In the 18-hr parasite the cytoplasm and lamellae are associated with excretory corpuscles consisting of layers arranged concentrically around a central core. At 3 days, the excretory cytoplasm possesses both fat and excretory corpuscles. Histochemical tests suggest that the corpuscles contain calcium salts and the lipid droplets neutral fat. The lamellae exhibit acid and alkaline phosphatase activity. The excretory system of the strigeoid trematodes is distinctive in that the excretory bladder becomes enlarged to form an extensive system described as the reserve bladder (Hughes, 1927). These trematodes also possess the basic protonephridial system and this is in continuity with the reserve bladder at specific points. In some genera (e.g., Diplostomum) the reserve bladder comprises narrow tubules terminating in vesicles containing excretory corpuscles. In other genera (e.g., Cyathocotyle, Holostephanus, and Apatemon) the reserve bladder consists of a series of large, fluid-filled lacunae. It has been suggested that the reserve bladder lacuna may function as a hydrostatic skeleton, as a vehicle for the circulation of dissolved nutrients through the body, as well as being involved in excretion (Van Haitsma, 1931; La Rue, 1932; Erasmus and Ohman, 1963; Ohman, 1966a, b). The lappets, adhesive organ, and reserve bladder characteristic of the adult strigeoid make their appearance during the development of the metacercaria in the second intermediate host. The development of the reserve bladder at this period has been followed by Komiya (1938), Erasmus (1958), and Pearson (1961) and these authors have demonstrated that the system is fully formed and functional in the completely developed metacercaria. In this way the system exhibits morphological and physiological continuity between metacercaria and adult. Received for publication 12 August 1966. The appearance of lipids in the excretory system of adult Digenea has been conclusively demonstrated in only a few genera: Fasciola hepatica (by von Brand and Weinland, 1924; Vogel and von Brand, 1933; Stephenson, 1947); Diplostomum sp., Apatemon gracilis minor, and Holostephanus luiihei (by Ohman, 1965, 1966a, b). The present study was undertaken to demonstrate the presence of lipid in the reserve bladder system of C. bushiensis and to determine as far as possible the place of origin of the free droplets and the nature of the wall enclosing the excretory lacuna. In addition, age variation in relation to the structure of the reserve bladder lining and the associated excretory products was

The intestinal epithelium of Dirofilaria immitis from an untreated dog and dogs treated with Caparsolate Sodium was studied by electron microscopy. The intestine of D. immitis is composed of a single layer of thin, columnar epithelial cells, joined laterally by terminal complexes. Each cell is divisible into an apical, central, and basal region. The latter contains a labyrinth of cisternae formed by deep invaginations of the basal plasma membrane. The apical cell surface is modified into irregularly arranged microvilli, which lack a dense central core. No terminal web is present. The central region of the cell contains a large nucleus and a limited number of organelles. These include a few rudimentary mitochondria, poorly defined endoplasmic reticulum, scanty ribosomes, microbodylike structures, and well-developed Golgi complexes. Glycogen was not observed. Lipid and pigment granules constitute the major cell inclusions and are closely associated with each other. Pigment granules are polymorphic and probably iron-positive as determined histochemically. They possess myelin figures in a dense matrix. These granules occur abundantly in the anterior portion and are almost absent from the posterior portion of the intestine. Their probable polyphasic role during absorption of host hemoglobin is discussed. A reduction in the number of these granules was observed 24 hr following administration of Caparsolate Sodium. They also seemed to aggregate, lose most of their myelin figures, and assume varying densities. Dense deposits covering the surface of the microvilli and the apical cell membrane are assumed to be a protein-arsenical complex which alters the morphology and function of the striated border. This is considered the probable mechanism of the filaricidal action of the therapeutic agent. Carbonell and Apitz (1959) described pigment granules from the intestinal cells of Ascaris lumbricoides which they regarded as intermediary products of metabolism. Similar granules were reported from the intestine of Ancylostoma caninum and their relationship to host hemoglobin has been indicated (Lee, 1968a, b). Another blood-feeding nematode, Dirofilaria immitis, also contains intestinal pigment granules. Infections with this nematode respond well to treatment with organic arsenical compounds (Otto and Maren, 1950; Drudge, 1952; Kune and Ohishi, 1957; Bailey, 1958). Little is known, however, about the mechanism of their vermicidal action. The intestinal fine structure of filariae from an untreated dog and those treated with Caparsolate Sodium [Sodium (p-carboamyl-phenylarsylene-dithio)diacetate] was studied to ascertain the possible mechanism of action of the therapeutic agent and to elucidate further the role of intestinal pigment granules. Received for publication 29 November 1968. * This investigation was supported by PHS Research Grant AI-02347 from NIAID of the NIH. MATERIALS AND METHODS Two dogs, naturally infected with D. immitis, were treated with Caparsolate Sodium at doses of 1 mg/lb of body weight. One naturally infected dog served as a control. One treated dog received 2 consecutive doses of the drug intravenously within 24 hr. The other was given 4 consecutive injections in 2 days. Both were killed with an overdose of nembutal 24 hr after the last injection of arsenical compound. The control dog was killed at the same time. Adult D. immitis were collected from the right ventricles and washed with warm saline. Segments of about 1 inch were taken from the anterior, middle, and posterior portions of worms and fixed in cold (5 C) glutaraldehyde (Pease, 1964) for 24 hr. Each segment was then subdivided into pieces of about 1 mm and postosmicated (Millonig, 1961) overnight at 5 C. They were embedded in Araldite 502 (Luft, 1961) after dehydration with ethanol. Sections were obtained with diamond knives on a PorterBlum MT-1 microtome, stained with lead hydroxide (Watson, 1958), and viewed with a Hitachi HU-11A. Histochemical tests for iron were performed on paraffin sections using a combination of microincineration and Prussian blue technique as described by Fenton et al. (1964).

The gastrodermis of the frog rectal fluke, Megalodiscus temperatus, was examined with the electron microscope for acid phosphatase activity following starvation periods of 12 and 24 hr. After 12 hr starvation, enzyme activity was observed associated primarily with a saccular membrane system that appears to be associated with the basal plasma membrane of the gastrodermis. Reaction product is also observed in the cisternae of the nuclear envelope, the rough endoplasmic reticulum, and, to a lesser extent, associated with the microvilli of the brush border. A number of vacuoles are also observed in the gastrodermis. These often contain recognizable organelles such as mitochondria. After 24 hr starvation, enzyme activity is observed only in these vacuoles, and no enzyme activity is seen at other sites. The difference in localization of enzyme activity is believed to reflect an increase in autophagy following prolonged starvation. In a report on the cytochemistry of the gastrodermis of the trematode, Megalodiscus temperatus, Bogitsh (1972) observed nonspecific acid phosphatase (AcPase) activity associated with the brush border and with infoldings of the basal plasma membrane. A change in localization of the enzyme activity occurred when no food was present in the lumen of the gut. The purpose of the present investigation was to determine the localization of nonspecific AcPase activity in the gastrodermis of M. temperatus following starvation. MATERIALS AND METHODS Adult Megalodiscus temperatus were removed from the rectal areas of naturally infected Rana pipiens and placed in 0.69% saline at room temperature. The digestive tracts of worms at the time of removal from their hosts were completely filled with blood. One group of these well-fed worms was removed from the saline after 12 hr, at which time the deep red color was virtually absent from the lumen of their guts, although subsequent Received for publication 18 July 1972. * Supported, in part, by a research grant from NIH (AI-08058). microscopic examination did reveal small quantities of blood. The other group of worms was kept in the saline for 24 hr. Subsequent examination of the digestive tracts of these worms revealed no blood in the gut lumen. At this time, the worms were considered completely starved. Both groups of worms were fixed under coverslip pressure either in 3% distilled glutaraldehyde-cacodylate (0.1 M, pH 7.2) for 3 hr at 4 C for morphological studies or in 2% distilled glutaraldehyde-cacodylate (0.1 Wi, pH 7.2) for 20 min at 4 C for cytochemical studies. Each group of fixed worms was sectioned at 40 Aum on a Sorvall TC-2 Tissue Sectioner and rinsed in cacodylate buffer (0.1 M, pH 7.2) overnight at 4 C. The latter group of sections was incubated for 30 min at 37 C either in the medium of Novikoff (1963) for acid phosphatase localization using cytosine monophosphate (CMP) as a substrate at pH 5.0 or in the medium of Novikoff and Goldfischer (1961) for nucleoside diphosphatase (NDPase) or thiamine pyrophosphatase (TPPase) localization using inosine diphosphate and thiamine pyrophosphate, respectively, as substrates at pH 7.2. Control sections were incubated either in a medium from which the substrate was omitted and, in the case of acid phosphatase, in a medium to which sodium fluoride (NaF) (0.01 M) was added. Following incubation, the sections were rinsed for 10 min in the buffer. These sections and those to be used for morphological studies were postFIGURES 1, 2. Gastrodermis of Megalodiscus temperatus starved for 24 hr. 1. Morphology of gastrodermis showing distribution of autophagic vacuoles. Arrows are pointing to several. Section stained with lead citrate and uranyl acetate. X 7,800. 2. Gastrodermis reacted for acid phosphatase. Reaction product is observed in autophagic vacuoles (arrows), and, to a lesser extent, associated with microvilli of brush border (b). X 13,200. Inset. Higher magnification view of autophagic vacuole with reaction product. X 30,000.