THE ORGANIZATION OF THE NUCLEOLUS IN MERISTEMATIC PLANT CELLS : A Cytochemical Study

The architecture of the nucleolus in Allium porum and Triticum vulgare meristematic cells has been investigated by means of digestions with various enzymes. After staining with azure B at pH4, plant nucleoli exhibit lighter regions which, under electron microscopy, correspond to the fibrillar zones characterizing these organelles. Evidence is presented indicating that these latter zones contain coarse convoluted filaments quite similar to the loops first demonstrated by La Cour (24) and which are assumed to originate from the nucleolar-organizing chromosomes. These coarse, 0.2µ wide filaments are remarkably resistant to the action of deoxyribonuclease, ribonuclease, pepsin, trypsin, or of various combinations of these enzymes and, moreover, they show insignificant incorporation of labeled thymidine even after long exposure to this DNA precursor. The clearing action of pepsin on different regions of the nucleolus lends support to the hypothesis that an amorphous material or matrix pervades the mass of this organelle. This effect is particularly striking within the particulate nucleolar zones themselves. Both ribonuclease and trypsin disorganize the RNP (ribonucleoprotein) nucleolar particles. The effect of the latter enzyme on the RNP particles is taken to indicate that they contain proteins particularly susceptible to trypsin which are essential for maintenance of their morphological integrity. Trypsin also interferes with azure B-staining of the nucleolar mass as a whole and, according to radioautographic data, extracts RNA throughout this organelle. Accordingly, the hypothesis is considered that RNA is complexed with proteins not only within the particulate nucleolar portions, as is already well known, but also in the fibrillar zones.     

Isolation of spherosomes (oleosomes) from onion, cabbage, and cottonseed tissues

Subcellular particles, identical in appearance to spherosomes observed in situ, were isolated from onions (Allium cepa L.) and cabbages (Brassica capitata L.). They were minute spherules about 1 micron in diameter, filled with an evenly stained osmiophilic matrix and delimited by a single, fineline membrane 20 to 30 Å thick.     

EVIDENCE FROM ELECTRON MICROGRAPHS FOR THE PASSAGE OF MATERIAL THROUGH PORES OF THE NUCLEAR MEMBRANE

1. The nurse cells of Rhodnius possess nucleoli that stain with Heidenhain's hematoxylin but give a negative Feulgen reaction. In localized positions adjacent to the nuclear membrane are seen masses of material both within the nucleus and the adjoining cytoplasm that stain with Heidenhain's hematoxylin, but, like the nucleolus, give a negative Feulgen reaction. 2. Electron micrographs of the nurse cells of Rhodnius reveal the nuclear membrane to contain pores approximately 400 A in diameter. 3. In electron micrographs the nucleolus is seen to be composed of a reticulum containing tightly packed granules. Between the centrally located nucleolus and the nuclear membrane are observed relatively small bunches of granules of the same relative size as those occurring in the nucleolus. Aggregated at certain positions adjacent to the nuclear membrane both within the nucleus and in the adjoining cytoplasm are irregularly shaped masses of granules. Certain of these masses within the nucleus are seen to be continuous with those in the cytoplasm through narrow isthmuses of material extending through pores of the nuclear membrane. Other masses of granules show evidence of preparing to enter the pores by projecting tongues of material toward and into them. In the adjacent cytoplasm pear-shaped masses of granules are seen in front of and in contact with the pores which suggests that they were fixed in the process of or just after completing passage through the pores.     

Structural components of the nuclear body in nuclei of Allium cepa cells

INTRODUCTIONSmall spherical nucleax bodies have long beenobserved in both hamal and plain interphasenuclei. In the case of animal cells, these nuclear bodies are generally called coiled bodies[1].As for plant cells, they have been vaxiously described as coiled bodies, ~somes, micronucleolior nucloolus-associated bodies because they sometimes appeared in the vicinity of nucleolusl2-4].Eaxly cytologists noted that nuclear bodies in platcells appeared as a tangle of coiled threads forming a …     

A CORRELATED LIGHT AND ELECTRON MICROSCOPE STUDY OF THE NUCLEOLAR MATERIAL DURING MITOSIS IN VICIA FABA

Root meristematic cells of Vicia faba were examined, with both light and electron microscopes, in order to study the behaviour of the nucleolar material during the mitotic process. Under light microscopy, the preprophase nucleolus is seen to consist of a densely stained material in which are embedded several unstained vacuole-like structures of varying size. The electron microscope reveals that the dense nucleolar material is formed of two structurally distinct components, each segregated into irregularly shaped zones blending with one another. One of these components is represented by 150 A granules which, in places, are arranged into thread-like structures approximately 0.1 µ in diameter; the other component apparently consists of fibrils 60 to 100 A in diameter. The large and medium sized intranucleolar vacuoles contain loosely scattered granules and fibrils similar to those just described. The granular and fibrillar components of the denser portion of the nucleolus persist as such during prophase and disperse throughout the nuclear cavity at the time of nucleolar disintegration. After nuclear membrane breakdown, these granules and fibrils, as well as those of the nucleoplasm, mix freely with similar elements already present within the forming spindle. No evidence has been obtained that, during or after nucleolar disintegration, the structural components of the nucleolus become associated as such with the chromosomes to form an external or internal matrix. Our observations suggest the existence, of a matrix substance within late prophase, metaphase, and anaphase chromosomes, the fine structure of which bears strong resemblance to that of their constituent coiled chromonemata. Data are presented, moreover, that indicate that part of this matrix substance, presumably formed at some time during prophase, is released from the chromosomes during their anaphasic movement. A number of observations indicate that the main bulk of the next nucleolus is derived from a prenucleolar fibrillogranular material, arranged into thread-like structures some 0.1 µ in diameter, which collect in the interchromosomal spaces during early and midtelophase. Finally, our data would seem to favour the view that most of this prenucleolar material results from a resumption of the synthetic activity of the early and midtelophase chromosomes rather than from a mere shedding of a preexisting matrix substance.     

ULTRASTRUCTURE OF THE SHOOT APEX OF CHENOPODIUM ALBUM AND CERTAIN OTHER SEED PLANTS

The ultrastructure of cells of the vegetative shoot apices is described for Chenopodium album, Kalanchoë blossfeldiana and K. laxiflora, Bryophyllum daigremontianum, Nicotiana rustica, and N. tabacum (Maryland Mammoth), and Ginkgo biloba. A less intensive study was made of the last three listed. The structures and organelles usually associated with meristematic cells were observed: dictyosomes, plastids (in various stages of development), mitochondria, endoplasmic reticulum (ER), vacuoles, lipid droplets, and plasmalemma. In addition, spherosome-like structures were observed in all zones of the shoot apices. Also, multivesicular bodies were observed in C. album and B. daigremontianum. Ribosome density is greater in cells of the flank meristem. Proplastids, plastids with prolamellar bodies, or grana have a differential distribution in the apex, characteristic for a particular species. Confirmation could not be given to the concept that vacuoles arise as a series of local dilations in long extensions of the so called "smooth ER." The tonoplast and ER are distinguishable at the time of inception of a vacuole, although the tonoplast may arise from the ER. Rapid growth of a vacuole and/or fusion with other vacuoles may result in irregularly shaped prevacuoles. No vacuoles were observed to originate from cisternae of dictyosomes in the species studied.     

Ultrastructural,cytochemical and immunocytochemical investigation of the interphase nucleus in the unicellular green alga Chlamydomonas reinhardtii

Summary— The ultrastructural organization of the interphase nucleus of the green alga Chlamydomonas reinhardtii was investigated and found to be largely dependent on the fixation conditions. In specimens stained with bismuth, densely contrasted granules ranging from 25 to 45 nm in diameter were localized throughout the interchromatin space and often formed clusters. These granules were labeled by RNase A-gold complexes and may represent the counterparts of animal and higher plant cll interchromatin granules. Within the nucleolus the Ag-NOR and pyroantimonate stains and, to a lesser extent, the bismuth stain reacted with the nucleolar dense fibrillar component (DFC). When cells were subjected to a heat shock at 42°C, the nucleolar DFC was found to progressively separate from the nucleolus and, after 3 h, appeared as a continuous meandering thread about 0.1 μm in width. Within the nucleolus, labeling on conventional preparations occurred as small clusters with antibodies to H3 histones or to DNA whereas RNase A-gold complexes labeled most of it including fibrillar centers. Improved ultrastructural preservation in cryofixed, cryosubstituted specimens gently fixed in glutaraldehyde permitted to localize nucleolar DNA predominantly at the outer edge of fibrillar centers and to a lesser extent within the neighbouring DFC. Our results indicate that the structure and composition of Chlamydomonas interphase nuclei are comparable, despite particularities, to those of animal and higher plant nuclei.     

Phase contrast and electron microscopy of the mycetocytes and symbiotes of the pea aphid, Acyrthosiphon pisum

Phase contrast, scanning electron, and transmission electron microscopy of the symbiotes of Acyrthosiphon pisum was undertaken. Some staining properties of the symbiotes were also studied.The symbiotes of the pea aphid were found to be coccoid bodies 2 to 5 μ in diameter, gram negative, stained slightly by Fuelgen's, and stained blue by Machiavello's. The symbiotes appear to be surrounded by three membranes. Ribosomes may occur within the cytoplasm of the symbiotes. The cytoplasm of the mycetocytes contains large numbers of mitochondria, endoplasmic reticulum, ribosomes, and a large nucleus, and nucleolus.A discussion of the classification of the symbiotes is also presented.     

Structural modifications of the nuclear components during lizard oogenesis in relation to the differentiation of the follicular epithelium

This paper deals with an electron microscope study of nucleolar ultrastructural modifications that occur in the oocytes of the lizard Podarcis sicula during ovarian follicle differentiation. In small diplotene oocytes around which a monolayered follicular epithelium forms, the nucleolus appears as a fibrillo-granular structure. Afterwards, simultaneously with the beginning of pyriform cell differentiation inside the granulosa, the nucleolus progressively condenses and breaks into fragments, forming dense spherical bodies. In larger follicles, with well differentiated pyriform cells, a typical nucleolus is no longer detectable in the oocyte nucleus. These ultrastructural modifications suggest a possible impairment of the oocyte nucleolus in ribosome organization. A possible involvement of pyriform cells in supplying ribosomes to the growing oocyte is discussed.     

Glomerulosomes: morphologically distinct nuclear organelles of unknown nature

The nucleus of some representatives of the genus Pelomyxa (Amoebozoa, Archamoebae, Pelobiontida) contains specific bodies (membrane-less organelles). They may be either embedded in the nucleolar mass or detached from the nucleolus. We termed these nuclear bodies the glomerulosomes for their characteristic ultrastructural appearance. The glomerulosomes are distinct nuclear bodies, about 1 μm in diameter. The morphological and diagnostic unit of a glomerulosome is an electron-dense thread/string, about 30–40 nm in thickness. These threads are not direct continuation of the nucleolar material. The threads create the unique geometric appearance of the glomerulosome by being organized into precisely parallel rows/cords. Each cord of the threads can curve at different angles within the glomerulosome body, but the threads themselves are not coiled. Nowadays, the glomerulosomes have been discovered in P. palustris, P. stagnalis, P. paradoxa, and Pelomyxa sp. Despite the unique appearance of glomerulosomes, their existence may be a more common phenomenon in eukaryotic cells than just a specific feature of the nucleus of elected pelomyxes.

     

Differences in the fine structure and chemical constitution of nucleolar bodies and the true nucleolus in Xenopus laevis embryos

Numerous bodies resembling nucleoli, named “prenucleolar bodies”, were seen in the interphase nucleus of Xenopus laevis embryos between stages 7 and 11 of Nieuwkoop and Faber (1956) but not at stage 12. These bodies are composed of thick strands, 200 A in diameter, and apparently differ from the fibrillar component of the true nucleolus which consists of thin fibrils, 50 A in diameter. The granular component of the true nucleolus consists of fibers and granules which are both also 150–200 A in diameter, but which differ in chemical nature from the prenucleolar bodies. The granular component and fibrillar component are readily digested by RNase with or without pretreatment with trypsin, while the prenucleolar body is only digested with RNase after pretreatment with trypsin. This suggests that the prenucleolar body consists of strands of RNA coated with protein. At stage 9, another type of nucleolus-like body is formed, which is larger (2–2.6 μ in diameter) than the prenucleolar body (0.2–1 μ) and consists of thin fibrils of 50 A. This body resembles the fibrillar component of the true nucleolus in the size of the elemental fibrils as well as in its susceptibility to actinomycin D, RNase and trypsin. It seems to be a precursor of the true nucleolus and for this reason was named the “primary nucleolus.” From stage 9 to stage 10, each nucleus in the presumptive ectodermal and mesodermal areas contains 2 primary nucleoli together with multiple prenucleolar bodies. At stage 12, the prenucleolar body is not seen at all, but a new type of nucleolus-like body appears. There are usually 2 of these bodies in each nucleus, and they consist of 2 components: a network of 50 A fibrils, and a group of strands, 150–200 A in diameter, containing some granule-like elements. The former has the same susceptibility to actinomycin D, RNase and/or trypsin as the fibrillar component of the definitive nucleolus and the primary nucleolus, while the latter has the same susceptibility as the granular component of the definitive nucleolus. Thus, this body may     

Are unusual plasmodesmata in the embryo-suspensor restricted to species from the genus Sedum among Crassulaceae?

The Crassulaceae family comprises mainly herbaceous leaf succulents, some of which have an ornamental value. During embryogenesis, they produce a suspensor with a giant polyploid basal cell. It has recently been shown that in Sedum acre and S. hispanicum this cell has compound plasmodesmata with an unusual dome of electron-dense material associated on the cell's side. These compound plasmodesmata differ from the typical ones occurring in other angiosperms. In this study, the hypothesis was tested that the unusual plasmodesmata in the embryo-suspensor are a feature not only restricted to species from the genus Sedum, but are also found in other Crassulaceae genera. Suspensors of example species from the genera Sempervivum and Jovibarba, which have vegetative morphologies quite different from Sedum and which are placed in the traditional classification into another subfamily, were first examined using an electron microscope. It was found that the unusual compound plasmodesmata in the suspensor are not only restricted to species from the genus Sedum but are also found in species from other Crassulaceae genera (Sempervivum arachnoideum and Jovibarba sobolifera). It should be noted that some ultrastructural features of compound plasmodesmata in the analyzed genera (e.g. the character of the wall with plasmodesmata, plasmodesmata diameter or occurrence of the electron-dense material) are different from the suspensor plasmodesmata recorded in species from the Sedum genus. We found that in Sempervivum arachnoideum the size of the plasmodesmata diameter varies according to the micropylar-chalazal axis of the embryo. This is the first report of variation in the diameter of the plasmodesmata within the embryo of angiosperms. Further study will be needed to show whether compound plasmodesmata occur in other Crassulaceae clades, whether they are a stable feature at the genus level in this family, and also whether they have evolved several times or only once in Crassulaceae.     

Morphological Diversity of the Rod Spherule: A Study of Serially Reconstructed Electron Micrographs

Purpose

Rod spherules are the site of the first synaptic contact in the retina’s rod pathway, linking rods to horizontal and bipolar cells. Rod spherules have been described and characterized through electron micrograph (EM) and other studies, but their morphological diversity related to retinal circuitry and their intracellular structures have not been quantified. Most rod spherules are connected to their soma by an axon, but spherules of rods on the surface of the Mus musculus outer plexiform layer often lack an axon and have a spherule structure that is morphologically distinct from rod spherules connected to their soma by an axon. Retraction of the rod axon and spherule is often observed in disease processes and aging, and the retracted rod spherule superficially resembles rod spherules lacking an axon. We hypothesized that retracted spherules take on an axonless spherule morphology, which may be easier to maintain in a diseased state. To test our hypothesis, we quantified the spatial organization and subcellular structures of rod spherules with and without axons. We then compared them to the retracted spherules in a disease model, mice that overexpress Dscam (Down syndrome cell adhesion molecule), to gain a better understanding of the rod synapse in health and disease.

Methods

We reconstructed serial EM images of wild type and Dscam^GoF (gain of function) rod spherules at a resolution of 7 nm in the X-Y axis and 60 nm in the Z axis. Rod spherules with and without axons, and retracted spherules in the Dscam^GoF retina, were reconstructed. The rod spherule intracellular organelles, the invaginating dendrites of rod bipolar cells and horizontal cell axon tips were also reconstructed for statistical analysis.

Results

Stereotypical rod (R1) spherules occupy the outer two-thirds of the outer plexiform layer (OPL), where they present as spherical terminals with large mitochondria. This spherule group is highly uniform and composed more than 90% of the rod spherule population. Rod spherules lacking an axon (R2) were also described and characterized. This rod spherule group consists of a specific spatial organization that is strictly located at the apical OPL-facing layer of the Outer Nuclear Layer (ONL). The R2 spherule displays a large bowl-shaped synaptic terminal that hugs the rod soma. Retracted spherules in the Dscam^GoF retina were also reconstructed to test if they are structurally similar to R2 spherules. The misplaced rod spherules in Dscam^GoF have a gross morphology that is similar to R2 spherules but have significant disruption in internal synapse organization.

Conclusion

We described a morphological diversity within Mus musculus rod spherules. This diversity is correlated with rod location in the ONL and contributes to the intracellular differences within spherules. Analysis of the Dscam^GoF retina indicated that their R2 spherules are not significantly different than wild type R2 spherules, but that their retracted rod spherules have abnormal synaptic organization.     

ELECTRON MICROSCOPIC OBSERVATIONS ON THE SUBMICROSCOPIC MORPHOLOGY OF THE MEIOTIC NUCLEUS AND CHROMOSOMES

Thin sections of the testicular follicles of the grasshopper Laplatacris dispar were studied under the electron microscope. In the primary spermatocytes, during meiotic prophase, three main regions can be recognized within the nucleus: (1) the nucleolus and associated nucleolar material; (2) the interchromosomal regions with the dense particles; and (3) the chromosomes. The nucleolus is generally compact and is surrounded by nucleolar bodies that comprise aggregations of dense round particles 100 to 250 A in diameter. A continuous transition can be observed between these particles and those found isolated or in short chains in the interchromosomal spaces. Particles of similar size (mean diameter of 160 A) can be found associated with the nuclear membrane and in the cytoplasm. The chromosomes show different degrees of condensation in different stages of meiotic prophase. The bulk of the chromosome appears to be made of very fine and irregularly coiled filaments of macromolecular dimensions. Their length cannot be determined because of the thinness of the section but some of them can be followed without interruption for about 1000 to 2000 A. The thickness of the chromosome filaments seems to vary with different stages of prophase and in metaphase. In early prophase, filaments vary between 28 ± 7 A and 84 ± 7 A with a mean of 47 A, in late prophase the mean is about 70 A. In metaphase the filaments vary between 60 and 170 A with a mean of about 100 A. Neither the prophase nor the metaphase chromosomes have a membrane or other inhomogeneities. The finding of a macromolecular filamentous component of chromosomes is discussed in relation to the physicochemical literature on nucleoproteins and nucleic acids and as a result it is suggested that the thinnest chromosome filaments (28 ± 7 A) probably represent single deoxyribonucleoprotein molecules.     

CYTOMORPHOLOGICAL CHARACTERIZATION OF THE PLANKTONIC DIAZOTROPHIC CYANOBACTERIA TRICHODESMIUM SPP. FROM THE INDIAN OCEAN AND CARIBBEAN AND SARGASSO SEAS1

Trichodesmium Ehrenberg species were collected in the Caribbean Sea, Sargasso Sea, and coastal areas of Tanzania (Indian Ocean). The specimens were divided into five species on the basis of morphometric characters such as cell dimensions and colony formation: T. tenue Wille, T. erythraeum Ehrenberg, T. thiebautii Gomont, T. hildebrandtii Gomont, and T. contortum Wille. In addition, Trichodesmium sp., a spherical colony of uncertain taxonomic position was examined. The cell structure of each species was investigated by means of light, scanning electron, and transvnission electron microscopy. Particular attention was paid to the presence and ultrastructural arrangement of gas vacuoles and glycogen fiber clusters (GFCs). This resulted in identification of two major groups of species: 1) T. tenue, Trichodesmium sp. with spherical-shaped colonies, and T. erythraeum with GFCs and more or less localized gas vacuoles; and 2) T. thiebautii, T. hildebrandtii, and T. contortum lacking GFCs and with gas vacuoles spread at random. The species within each group were further characterized with respect to the dimension of the gas vesicles, cylindrical bodies, scroll bodies, and a new cellular inclusion body, Differences in colony formation and cell dimensions correlated with specific ultrastructural characters in five of the six forms. This is the first ultrastructural study comparing different forms of Trichodesmium sampled at geographically remote areas and shows that one species appears identical regardless of the sampling site. Some of the species had not been investigated earlier, and probably more species are to be identified and analyzed.     

Abnormal swellings in the tegument of the acanthocephalan parasite Metechinorhynchus salmonis

A yellow mass up to 1 mm in diameter was found in the tegument of acanthocephalans of the species Metechinorhynchus salmonis taken from the intestine of adult coho and chinook salmon, Oncorhynchus kisutch and O. tschawytscha, in a tributary to Lake Ontario. Histochemical and ultrastructural observations indicated that it consisted of colored phospholipid bodies in a matrix containing glycogen. No protozoan, bacterial, or viral pathogen was found, suggesting that the abnormality was due to a metabolic dysfunction. Five to ten percent of the worms found were affected.     

Nucleolar structure across evolution: the transition between bi- and tri-compartmentalized nucleoli lies within the class Reptilia

Two types of nucleolus can be distinguished among eukaryotic cells: a tri-compartmentalized nucleolus in amniotes and a bi-compartmentalized nucleolus in all the others. However, though the nucleolus' ultrastructure is well characterized in mammals and birds, it has been so far much less studied in reptiles. In this work, we examined the ultrastructural organization of the nucleolus in various tissues from different reptilian species (three turtles, three lizards, two crocodiles, and three snakes). Using cytochemical and immunocytological methods, we showed that in reptiles both types of nucleolus are present: a bi-compartmentalized nucleolus in turtles and a tri-compartmentalized nucleolus in the other species examined in this study. Furthermore, in a given species, the same type of nucleolus is present in all the tissues, however, the importance and the repartition of those nucleolar components could vary from one tissue to another. We also reveal that, contrary to the mammalian nucleolus, the reptilian fibrillar centers contain small clumps of condensed chromatin and that their surrounding dense fibrillar component is thicker. Finally, we also report that Cajal bodies are detected in reptiles. Altogether, we believe that these results have profound evolutionarily implications since they indicate that the point of transition between bipartite and tripartite nucleoli lies at the emergence of the amniotes within the class Reptilia.     

ORIGIN, DEVELOPMENT, AND NATURE OF INTRANUCLEAR RODLETS AND ASSOCIATED BODIES IN CHICKEN SYMPATHETIC NEURONS

Correlative data are presented here on the developmental history, dynamics, histochemistry, and fine structure of intranuclear rodlets in chicken sympathetic neurons from in vivo material and long-term organized tissue cultures. The rodlets consist of bundles of ~70 ± 10 A proteinaceous filaments closely associated with ~0.4–0.8 µ spheroidal, granulofibrillar (gf) bodies of a related nature. These bodies are already present in the developing embryo a week or more in advance of the rodlets. In early formative stages rodlets consist of small clusters of aligned filaments contiguous with the gf-bodies. As neuronal differentiation progresses these filaments increase in number and become organized into well-ordered polyhedral arrays. Time-lapse cinemicrography reveals transient changes in rodlet contour associated with intrinsic factors, changes in form and position of the nucleolus with respect to the rodlet, and activity of the gf-bodies. With the electron microscope filaments may be seen extending between the nucleolus, gf-bodies, and rodlets; nucleoli display circumscribed regions with fine structural features and staining reactions reminiscent of those of gf-bodies, We suggest that the latter may be derivatives of the nucleolus and that the two may act together in the assemblage and functional dynamics of the rodlet. The egress of rodlet filaments into the cytoplasm raises the possibility that these might represent a source of the cell's filamentous constituents.     

Cell morphology, ultrastructure, and calcification pattern of Oocardium stratum, a peculiar lotic desmid

Cell morphology and ultrastructure of the desmid Oocardium stratum and its habitat conditions in two limestone-precipitating spring habitats in the Alps were studied. In spite of specific cell geometry, we found ultrastructural features (nucleus with nucleolus, Golgi apparatus, chloroplast structure, lipid bodies, cell wall texture) closely related to other desmids. The type of the mucilage pore apparatus perforating in high densities extended areas of the cell wall of Oocardium is of the Cosmarium type. Oocardium contrasts to Cosmarium by a peculiar bilateral cell geometry (lateral sphenoid shape) which is combined with a dislocated nucleus. Although the cell features of Oocardium did not differ between the two habitats, different calcification types (rhombohedral calcite versus fascicular-fibrous calcite) and calcification intensities were recorded. The spatial positioning and extension of the Oocardium niches differed considerably between the two springs in spite of high CO₂ oversaturation at both sites.     

THE FINE STRUCTURE AND FUNCTION OF THE CONTRACTILE AXOSTYLES OF CERTAIN FLAGELLATES

The axostyles of the flagellates Oxymonas, Saccinobaculus, and Notila are large ribbon-shaped structures which undulate actively in the cytoplasm. The form of their movements is described and illustrated. Axostyles consist of regular arrays of longitudinal fibres, the number of which varies between 100 and 5000 in different species. The fibres are about 240 A in diameter, apparently hollow, regularly cross-banded with a periodicity of about 150 A, and connected by delicate cross-links, also at regular intervals of about 150 A. They resemble very closely the central fibres of cilia and flagella. No other structural components are present, except at the anterior end, where the fibres are attached to one or more basal bodies, and at the posterior tip, where they are anchored to the plasma membrane. The relevance of the findings to an understanding of the mechanism of ciliary and flagellar movements is discussed.