MORPHOLOGY AND ECOLOGY OF THE MARINE BENTHIC DINOFLAGELLATE SCRIPPSIELLA SUBSALSA (DINOPHYCEAE)1

The thecal surface morphology of Scrippsiella subsalsa (Ostenfeld) Steidinger et Balech was examined using the scanning electron microscope. This species is distinguished by a number of morphological characteristics. Apical plate 1′ is wide, asymmetric, and pentagonal, and it ends at the anterior margin of the cingulum. Intercalary plates 2a and 3a are separated by apical plate 3′. The apical pore complex includes a large P_o plate with a raised dome at the center and a deep canal plate with thickened margins at plates 2′, 3′, and 4′. The intercalary bands are wide and deeply striated. The cingulum is deep, formed by six cingular plates; its surface is transversely striated and aligned with a row of minute pores. The cingular list continues around postcingular plate 1′” to form a sulcal list. The sulcal list is a flexible ribbon with a rounded tip that protrudes posteriorly, partially covering the sulcal plates. The hypotheca is lobed, and the antapical plates are irregularly shaped and wide in antapical view. The thecal surface is vermiculate to reticulate. A comparison in morphology and ecology is presented between S. subsalsa and other known Scrippsiella species.     

PHYSIOLOGICAL AND ENVIRONMENTAL CONTROL OF GERMINATION IN SCRIPPSIELLA TROCHOIDEA (DINOPHYCEAE) RESTING CYSTS1

The effects of aging, temperature, and growth medium on germination in culture-produced resting cysts of the marine dinoflagellate Scrippsiella trochoidea (Stein) Loeblich ore examined. Cysts undergo a mandatory period of dormancy lasting approximately 25 days, during which germination does not occur. The duration of this period is not affected by temperature. Once the dormancy period is completed, germination is regulated by external factors. Cysts germinate optimally in nutrient replete medium at temperatures greater than approximately 14° C. At lower temperatures or in nutrient-depleted media germination rate is dramatically slowed, although the final germination frequency appears unchanged. The large Q₁₀ of this temperature effect (ca. 11) suggests that the reduction in germination rate at lower temperatures is not merely the reflection of generally reduced metabolic rates, but rather the result of a temperature response specific to germination. At the highest temperatures tested (22–25° C), germination rate remains maximal although vegetative growth is greatly reduced. A shift in temperature or nutrient conditions, per se, is not necessary for germination. The relatively short dormancy period combined with the absence of a requirement for a dramatic shift in environmental conditions could facilitate rapid cycling between resting and vegetative stages in natural S. trochoidea populations. At the same time, the dramatic reduction in germination rate at low temperatures would permit cysts of this species to serve as overwintering cells as well.     

BIOCHEMICAL COMPOSITION AND METABOLIC ACTIVITY OF SCRIPPSIELLA TROCHOIDEA (DINOPHYCEAE) RESTING CYSTS1

The composition and metabolic activity of cysts of the marine dinoflagellate Scrippsiella trochoidea (Stein) Loeblich were examined during dormancy, quiescence, and germination. On a per cell basis, newly formed cysts contained an order of magnitude more carbohydrate but significantly less protein and chlorophyll a than did exponentially growing vegetative cells. Loss of lipid and carbohydrate from cysts during the initial dormancy period reflected a respiration rate estimated to be 10% of the respiratory activity in vegetative cells. Among older, quiescent cysts the calculated respiration rate decreased further to approximately 1.5% of the vegetative rate and appeared to proceed largely at the expense of carbohydrate reserves. These estimated rates of respiration were in good agreement with direct measurements of cyst oxygen consumption. The transfer of quiescent cysts to conditions permissive for germination resulted in a rapid increase in respiration rate, as evidenced by carbohydrate loss and O₂ consumption. The increased respiratory activity was followed by an increase in protein content and, later, by an increase in chlorophyll a content and photosynthetic capacity. Just prior to germination the P/R ratio became greater than 1, and the estimated chlorophyll-specific photosynthetic activity reached 75% of the rate in vegetative cells. Complete restoration of photosynthetic and respiratory capacity apparently was not achieved until after excystment. These data confirm the common assumption that dinoflagellate cysts represent true “resting” cells, containing extensive energy reserves and displaying greatly reduced metabolic activity.     

CYST FORMATION IN THE RED TIDE DINOFLAGELLATE ALEXANDRIUM TAMARENSE (DINOPHYCEAE): EFFECTS OF IRON STRESS1

The toxic red tide dinoflagellate Alexandrium tamarense (Lebour) Balech (synonymous with Protogonyaulax tamarensis (Lebour) Taylor) was subjected to iron stress in batch culture over a 24-day time course. Monitoring of life history stages indicated that iron stress induced formation of both temporary (= pellicular) and resting (= hypnozygotic) cysts. Our experimental induction of sexuality appeared to be associated with iron limitation rather than the total depletion of biologically available iron. Degenerative changes in organelle (i.e. chloroplast, mitochondrion and chromosome) ultrastructure were largely restricted to pellicular cysts, suggesting that these temporary cysts were more susceptible to short-term iron stress effects than were hypnozygotes. These results are consistent with the hypothesized ecological roles of cysts in maintaining viability over brief (pellicular cysts) and extended (hypnozygotes) exposure to adverse environmental conditions.     

SPONTANEOUS AND STIMULATED BIOLUMINESCENCE OF THE DINOFLAGELLATE CERATOCORYS HORRZDA (PERIDINIALES)1

This is the first report of spontaneous bioluminescence in the autotrophic dinoflagellate Ceratocorys horrida von Stein. Bioluminescence was measured, using an automated data acquisition system, in a strain of cultured cells isolated from the Sargasso Sea. Ceratocorys horrida is only the second dinoflagellate species to exhibit rhythmicity in the rate of spontaneous flashing, flash quantum flux (intensity), and level of spontaneous glowing. The rate of spontaneous flashing was maximal during hours 2–4 of the dark phase [i.e. circadian time (CT)16–18 for a 14:10 h LD cycle (LD14:10)], with approximately 2% of the population flashing-min^?1, a rate approximately one order of magnitude greater than that of the dinoflagellate Gonyaulax polyedra. Flash quantum flux was also maximal during this period. Spontaneous flashes were 134 ms in duration with a maximum flux (intensity) of 3.1×10⁹ quanta-s^?1. Light emission presumably originated from blue fluorescent microsources distributed in the cell periphery and not from the spines. Values of both spontaneous flash rate and maximum flux were independent of cell concentration. Isolated cells also produced spontaneous flashes. Spontaneous glowing was dim except for a peak of 6.4× 10⁴quanta-s^?1 cell^?1, which occurred at CT22.9 for LD14:10 and at CT22.8 for LD12:12. The total integrated emission of spontaneous flashing and glowing during the dark phase was 4×10⁹ quantacell^?1, equivalent to the total stimulable luminescence. The rhythms for C. horrida flash and glow behavior were similar to those of Gonyaulax polyedra, although flash rate and quantum flux were greater. Spontaneous bioluminescence in C. horrida may be a circadian rhythm because it persisted for at least three cycles in constant dark conditions. This is also the first detailed study of the stimulated bioluminescence of C. horrida, which also displayed a diurnal rhythm. Cultures exhibited >200 times more mechanically stimulated bioluminescence during the dark phase than during the light phase. Mechanical stimulation during the dark phase resulted in 6.7 flashes. cell^?1; flashes were brighter and longer in duration than spontaneous flashes. Cruise-collected cells exhibited variability in quantum flux with few differences in flash kinetics. The role of dinoflagellate spontaneous bioluminescence in the dynamics of near-surface oceanic communities is unknown, but it may be an important source of natural in situ bioluminescence.     

LIGHT AND ELECTRON MICROSCOPICAL OBSERVATIONS OF THE DINOFLAGELLATE ACTIMSCUS PESTASTERIAS (DINOPHYCEAE)1

I examined the heterotrophic non-armored dinoflaget-late Actiniscus pentasterias (Ehrenberg) Ehrenberg by light and electron microscopy. Actiniscus pentasterias contains an internal skeleton consisting of two star-like siliceous elements. Special emphasis is given to the flagellar apparatus, the nucleus, and a new type of extrusome, named a docidosome. A three dimensional model of the flagerllar apparatus includes a fibrous nuclear connnective, a posterior striated root, and a dorsal striated component of the longitudinal microtabular root. The nucleus is surrounded by a conspicuous fibrous lamina, also visible in the light microscope. The nuclear pores are situated in annulated invaginations of the nuclear envelope, increasing the nuclear surface area by 15–25%. The docidosomes are rod-shaped membrane-bound structures that terminate in a distinct proximal head. They show very complex substructure, consisting of an inner medulla with highly ordered paired ribbons and an outer cortex.     

EFFECT OF LIGHT HISTORY ON THE ULTRASTRUCTURE AND PHYSIOLOGY OF PROROCENTRUM MARIAE-LEBOURIAE (DINOPHYCEAE)1

Ultrastructural and physiological responses of Prorocentrum mariae-lebouriae (Parke & Ballantine) Faust are reported for cultures maintained at growth irradiances (I_g) ranging from 20.6 to 0.3 E m^?2.d^?1 and following downward shifts in light intensity. We tested the hypothesis that Prorocentrum grown under light regimes that elicit different responses in photosynthesis and pigmentation exhibit distinctive cell ultrastructures. Prorocentrum from high-light conditions had high saturation intensities for photosynthesis (I_k) and low levels of Chl a, Chl c and peridinin-cell^?1 These cultures were morphologically distinguished by a large starch volume fraction (V_v), small chloroplast V_v and fewer thylakoids lamella^?1. I_k values were lower and pigment concentrations higher in low-light treatments, and cells showed reduced starch V_v, large chloroplast V_v, and higher numbers of thylakoids · lamella ^?1. Cells grown under extremely low-light conditions appeared stressed as indicated by the absence of starch reserves and the presence of large vacuoles within the cytoplasm. Results for presence of large vacuoles within the cytoplasm. Results for quantiative electron microscopy, photosynthesis-irradiance (P-I) relations and cell pigmentation indicate that photoadaptation in P. mariae-lebouriae involves a strategy that encompasses changes in both the “size” and “number” of photosynthetic units.     

HYPERMETHYLATION AT CPG-MOTIFS IN THE DINOFLAGELLATES AMPHIDINIUM CARTERAE (DINOPHYCEAE) AND SYMBIODINIUM MICROADRIATICUM (DINOPHYCEAE): EVIDENCE FROM RESTRICTION ANALYSES, 5-AZACYTIDINE AND ETHIONINE TREATMENT

Because of their peculiar chromatin features and phylogenetic position, dinoflagellates are potentially uniquely informative with respect to possible roles and evolution of DNA methylation systems. Here we report that DNA from Amphidinium carterae Hulburt and Symbiodinium microadriaticum Freudenthal was not significantly digested by a range of CpG methylation-sensitive endonucleases. Corresponding methylation-insensitive isoschizomers cleaved all DNA preparations. Treatment with the methylation inhibitors 5-azacytidine and ethionine resulted in major increases in digestibility of dinoflagellate DNA by CpG methylation-sensitive enzymes. The 5-azacytidine effect was not confined to heterochromatin. Our studies indicate that at least in some dinoflagellates, a high proportion of CpG motifs and a significant number of CpNpG motifs are normally methylated, implying methyltransferase substrate specificities similar to those of higher plants.     

PHOTOINHIBITION OF MECHANICALLY STIMULABLE BIOLUMINESCENCE IN THE HETEROTROPHIC DINOFLAGELLATE PROTOPERIDINIUM DEPRESSUM (PYRROPHYTA)1

Photoinhibition of mechanically stimulable bioluminescence (MSL) in the heterotrophic dinoflagellate Protoperidinium depressum Bailey was investigated using samples collected from the Massachusetts and southern Texas coasts. The times for both photoinhibition of MSL (ca. 10 min) and dark recovery from photoinhibition of MSL (ca. 45 min) in this species were similar to those reported for autotrophic dinoflagellates. The degree of photoinhibition of MSL was a linear function of the logarithm of photon flux density (PFD). The threshold PFDs for the photoinhibition of MSL were 0.02, 0.6, and 21 μmol photons · m^?2· s^?1 for broad-band blue, green, and red light, respectively. These PFDs are lower than those required for photoinhibition of MSL by the autotrophic dinoflagellates Pyrocystis lunula and Ceratium fusus. We speculate that photosynthetic pigments in autotrophic dinoflagellates shield the photoreceptor that causes photoinhibition of MSL, thus lowering the sensitivity of these dinoflagellates to light. When field-collected P. depressum were kept in the laboratory without growth for a week, photoinhibition of MSL's sensitivity to light increased progressively along with 1) a decrease in its bioluminescence capacity (BCAP), 2) a decrease in the ratio of MSL to BCAP (MSL/BCAP), and 3) a decrease in the orange pigmentation (probably carotenoid) of the dinoflagellate. The action spectrum for photoinhibition of MSL in P. depressum was characterized primarily with a broad peak in the blue extending into the green. We suggest that carotenoid was not a photoreceptor for the photoinhibition of MSL in P. depressum because the peak of the action spectrum was too broad and extended too far into the green part of the spectrum, and because the orange pigment present decreased as photoinhibition of MSL became more sensitive to light.     

CACHONINA ILLDEFINA SP. NOV. (DINOPHYCEAE): CHLOROPLAST TUBULES AND DEGENERATION OF THE PYRENOID1

A new species of the dinoflagellate genus Cachonina, C. illdefina sp. nov., was isolated from a red tide off El Capitan State Park, Santa Barbara County, California, in October 1973. The organism is light yellowgreen in color with deeply incised girdle and sulcal grooves. Electron microscopy of the organism, revealed a typical dinokaryotic nucleus. The chloroplasts of the organism are connected, and often contain microtubule-like elements, 25 nm diam. The pyrenoids are characterized as excluding chloroplast thylakoids and ribosomes, although containing an amorphous matrix and numerous tubular invaginations from the cytoplasm. The pyrenoids become detached from the chloroplasts and degenerate into small vesicles. C. illdefina is not bioluminescent.     

THE PAS/ACCUMULATION BODIES IN PROROCENTRUM LIMA AND PROROCENTRUM MACULOSUM (DINOPHYCEAE) ARE DINOFLAGELLATE LYSOSOMES1

Numerous spherical bodies containing electron-dense material, fibrous material, and membranous material are present in the cytoplasm of two dinoflagellate species, Prorocentrum lima (Ehr.) Dodge and Prorocentrum maculosum Faust. Similar bodies have been observed in other dinoflagellates and have been termed accumulation bodies or PAS bodies. In both Prorocentrum species, these bodies autofluoresce under blue light excitation and increase in size with cell culture age. They possess acid phosphatase activity, react positively with the periodic acid/Schiff reagent, and stain with acridine orange. All these properties are characteristic of eukaryotic lysosomes; thus, we propose that dinoflagellate accumulation bodies and PAS bodies are identical organelles and are, in fact, dinoflagellate lysosomes.     

PHYLOGENETIC ANALYSIS OF BRACHIDINIUM CAPITATUM (DINOPHYCEAE) FROM THE GULF OF MEXICO INDICATES MEMBERSHIP IN THE KARENIACEAE1

Brachidinium capitatum F. J. R. Taylor, typically considered a rare oceanic dinoflagellate, and one which has not been cultured, was observed at elevated abundances (up to 65 cells · mL^?1) at a coastal station in the western Gulf of Mexico in the fall of 2007. Continuous data from the Imaging FlowCytobot (IFCB) provided cell images that documented the bloom during 3 weeks in early November. Guided by IFCB observations, field collection permitted phylogenetic analysis and evaluation of the relationship between Brachidinium and Karenia. Sequences from SSU, LSU, internal transcribed spacer (ITS), and cox1 regions for B. capitatum were compared with five other species of Karenia; all B. capitatum sequences were unique but supported its placement within the Kareniaceae. From a total of 71,487 images, data on the timing and frequency of dividing cells was also obtained for B. capitatum, allowing the rate of division for B. capitatum to be estimated. The maximum daily growth rate estimate was 0.22 d^?1. Images showed a range in morphological variability, with the position of the four major processes highly variable. The combination of morphological features similar to the genus Karenia and a phylogenetic analysis placing B. capitatum in the Karenia clade leads us to propose moving the genus Brachidinium into the Kareniaceae. However, the lack of agreement among individual gene phylogenies suggests that the inclusion of different genes and more members of the genus Karenia are necessary before a final determination regarding the validity of the genus Brachidinium can be made.     

GROWTH AND PHOSPHORUS UPTAKE BY THE TOXIC DINOFLAGELLATE ALEXANDRIUM CATENELLA (DINOPHYCEAE) IN RESPONSE TO PHOSPHATE LIMITATION1

Alexandrium catenella (Whedon et Kof.) Balech has exhibited seasonal recurrent blooms in the Thau lagoon (South of France) since first reported in 1995. Its appearance followed a strong decrease (90%) in phosphate (PO₄^3?) concentrations in this environment over the 1970–1995 period. To determine if this dinoflagellate species has a competitive advantage in PO₄^3?‐limited conditions in terms of nutrient acquisition, semicontinuous cultures were carried out to characterize phosphorus (P) uptake by A. catenella cells along a P‐limitation gradient using different dilution rates (DRs). Use of both inorganic and organic P was investigated from measurements of ³³PO₄^3? uptake and alkaline phosphatase activity (APA), respectively. P status was estimated from cellular P and carbon contents (Q_P and Q_C). Shifts in trends of Q_P/Q_C and Q_P per cell (Q_P·cell?1) along the DR gradient allowed the definition of successive P‐stress thresholds for A. catenella cells. The maximal uptake rate of ³³PO₄^3? increased strongly with the decrease in DR and the decrease in Q_P/Q_C, displaying physiological acclimations to PO₄^3? limitation. Concerning maximal APA per cell, the observation of an all‐or‐nothing pattern along the dilution gradient suggests that synthesis of AP was induced and maximized at the cellular scale as soon as PO₄^3? limitation set in. APA variations revealed that the synthesis of AP was repressed over a PO₄^3? threshold between 0.4 and 1 μM. As lower PO₄^3? concentrations are regularly observed during A. catenella blooms in Thau lagoon, a significant portion of P uptake by A. catenella cells in the field may come from organic compounds.     

DIEL CHANGES IN PHASED-DIVIDING CULTURES OF CERATIUM FURCA (DINOPHYCEAE): NUCLEOTIDE TRIPHOSPHATES,ADENYLATE ENERGY CHARGE,CELL CARBON,AND PATTERNS OF VERTICAL MIGRATION1

The diel pattern of cell division, cell carbon, adenine nucleotides and vertical migration was determined for laboratory cultures of the photosynthetic marine dinoflagellate, Ceratium furca (Ehr.) Clap. & Lachm., entrained on an alternating 12:12 LD schedule at 20 C. Cell division was initiated during the latter portion of the dark period with ca. 30% of the population undergoing division. Cell C increased during the light period and exhibited a linear decrease with a loss of 33% during the dark period. ATP · cell^?1 increased during the light period and decreased by ca. 40–50% during the dark period. The diel patterns of cell C and ATP tended to “buffer” the magnitude of the change in C:ATP ratios around an overall mean value of 89. There was no obvious trend in the concentration of [GTP + UTP] · cell^?1 over the cell cycle. The cellular adenylate energy charge was maintained at values between 0.8 to 0.9 throughout the 24 h LD cycle, despite a ca. 40% decrease in total adenylates (A_T= ATP + ADP + AMP) during the dark period on 12:12 LD, and over a 68% decrease in ATP during 42 h of continuous darkness. These data lend experimental support to the theory of cellular metabolic control by the adenine nucleotides. With lateral illumination on 12:12 LD cycles, the cells began to concentrate at the surface of the experimental tubes shortly before the lights were turned on, and at the bottom of the tubes shortly before the lights were extinguished. This pattern continued for 6 days in continuous darkness, suggesting that the vertical migration pattern is independent of a phototactic response and may be under the control of an endogenous rhythm.     

THE EFFECTS OF SELECTED INHIBITORS ON CELLULOSE MICROFIBRIL ASSEMBLY IN BOERGESENIA FORBESII (CHLOROPHYTA) PROTOPLASTS1

Protoplasts of Boergesenia forbesii (Harvey) were treated with inhibitors of protein synthesis in order to investigate their effects on cellulose synthesis. Cellulose synthesis was reversibly inhibited by 10 μM cycloheximide as assayed by fluorescence microscopy of Tinopal binding to cellulose. Freeze fracture and image analysis of cycloheximide- treated cells indicated a reduction in the number of intramembrane particles; however, the terminal synthesizing complexes remained at all times. Treatment with 10 μM actinomycin D, when applied during the first hour of protoplast formation, irreversibly inhibits cellulose synthesis and terminal complex formation. De novo protein synthesis is required for cell wall regeneration by protoplasts. The data suggest that the structural subunits visualized in the terminal complex do not undergo significant turnover, but that there may exist an essential proteinaceous component of cellulose synthesis which must be continually renewed.     

THE DISCOVERY OF THE CRIBRIFORM PLATE IN SINOCONODON YOUNCI (TRICONODONTA,MAMMALIA)

ThepresenceofacribriformplateinlivingplacentalsandthemarsupialsandabsenceinmonotremesmayimplythatmammalsarediphyleticandtheoriginofcribriformplatetookplacewithinthcMammalia.Buttheconc1usioncamefromthe1ivingmammalsonly,andhasnotbeensupportedbypalaeontologicalevidence.Itsappearanceinoneoftheoldestandthemostprimitivemammals,Sinoconodon,possiblysuggeststhatthecribriformplatemayoccurearlier,i.e.inthetimeofthetransitionfromtheadvancedmammal-likereptiles,Cynodontia,tothetruemammals,andthatthedistrib…     

THE EFFECTS OF TEMPERATURE,IRRADIANCE, AND NITROGEN ON THE ENCYSTMENT AND GROWTH OF THE FRESHWATER DINOFLAGELLATES PERIDINIUM CINCTUM AND P. WILLEI IN CULTURE (DINOPHYCEAE)1

Effects of temperature, irradiance, and nitrogen availability on the encystment and growth of the freshwater dinoflagellates Peridinium cinctum Ehrenberg and Peridinium willei Huitfeld-Kaas were studied in culture. Lack of nitrogen was the main trigger of encystment in both species. Irradiance had a secondary effect on the percentage of the population of each species that encysted. Temperature did not significantly affect encystment in either species. In both species, only a small percentage of the population underwent encystment. Low light had an inhibitory effect on the growth of P. willei growing in nitrogen-sufficient medium.     

长期大强度运动对大鼠脑组织神经肽Y、亮氨酸脑啡肽、强啡肽A_(1-13)动态变化的影响

目的和方法：采用ＡＢＣ免疫组织化学法结合图象分析,观察大鼠脑组织神经肽Ｙ、亮氨酸脑啡肽、强啡肽Ａ１１３ 在长期（ 共７ 周）大强度（速度由１５ ｍ／ｍｉｎ 递增至３５ ｍ／ｍｉｎ、运动时间为２０ ～２５ ｍｉｎ／ｄ） 的运动下的变化。结果：安静状态下在丘脑室旁核（ＰＶ） 、下丘脑背内侧核（ＤＭ） 、下丘脑腹内侧核（ＶＭＨ）等核团ＮＰＹ 无显著性变化;在此基础上的末次急性运动结束后３ ｈ ＮＰＹ 变化尤为明显。安静状态下大鼠尾壳核ＬＥＮＫ 下降;而末次急性运动后大鼠下丘脑ＬＥＮＫ 被迅速激活而升高。该强度运动能激活下丘脑ＤＹＮＡ１１３ ,尤以运动结束后３０ ｍｉｎ 最为明显。结论： ＮＰＹ、ＬＥＮＫ、ＤＹＮＡ１１３ 在该强度运动下大鼠不同脑区呈现不同变化趋势     

MECHANISM OF GAMETE FUSION IN DINOPHYSIS FORTII (DINOPHYCEAE,DINOPHYTA): LIGHT MICROSCOPIC AND ULTRASTRUCTURAL OBSERVATIONS1

A variety of studies have examined the sexual life cycle of species belonging to the genus Dinophysis Ehrenberg. Here, we used TEM to investigate the mechanism of cellular fusion during the sexual life cycle in Dinophysis fortii Pavillard. We observed that fusion always occurred between a normal‐sized cell and a small cell following attachment of their ventral margins. After cell attachment, the small cell moved toward the epitheca of the normal‐sized cell, and the cingular and sulcal lists of the small cell shrunk or were almost lost. The epitheca of the normal‐sized cell then opened between the cingulum plates and the upper cingular list, after which the small cell was gradually engulfed. This is the first ultrastructural observation in a dinoflagellate of a larger cell opening its epitheca to engulf the smaller gamete. In another case, the normal‐sized cell did not open the epitheca, the cell wall of the attached small cell underwent extensive extracellular digestion, and the cytoplasm appeared to flow into the normal‐sized cell via the periflagellar area. Inflow of the nucleus was not observed in this case, suggesting that it represented a failure of sexual fusion. In both cases, membranous separations between the cytoplasm of the two cells were not observed. At the beginning of the fusion process, the nucleus of the small cell was substantially deformed. The planozygote, formed upon completion of sexual fusion, sometimes had two longitudinal flagella, but was identical to a normal vegetative cell in its cellular shape, as already mentioned by previous authors.