Light- and electron-microscopic studies of growth and reproduction inCutleria (phaeophyta)

Summary Development of the plurilocular male gametangium inCutleria hancockii Dawson is fundamentally similar to that of the female gametangium. However, the sequence of mitoses is less regular and the number of divisions is more variable in the male structure. During mitosis the nucleolus disappears and the nuclear envelope breaks down into vesicles and cisternae. No well-defined chromosomal kinetochores were observed. The spindle does not persist during telophase. At least two types of vesicles, but no microtubules, are associated with cytokinesis. After cleavages are completed, each of the cells develops an eyespot and two flagella. The flagellar rootlet system consists of 4–5 bands of 5–10 microtubules radiating posteriorly from the basal bodies. Flocculent material surrounding the gamete at maturity may be involved with liberation. Prior to release, a pore is formed in each locule when the outermost layers of the surficial wall break, and the innermost layers expand out through this weakened region. The inner wall eventually bursts, releasing the gamete and flocculent material through the pore. The liberated gamete has a long, pleuronematic anterior flagellum, and a short, acronematic posterior flagellum which has a swollen base appressed to the plasmalemma.     

ULTRASTRUCTURE OF ZOIDOGENESIS IN UNILOCULAR ZOIDOCYSTS OF SEVERAL BROWN ALGAE1

Several different stages of the development of unilocular zoidocysts of small brown algae—Elachista fucicola, Hecatonema streblonematoides, Pylaiella littoralis—are observed by electron microscopy. 1. A slow growing phase is seen, during which nuclei and pheoplasts become associated by pairs and divide together, vacuoles and physodes are excreted through the plasmalemma, and Golgi bodies liberate vesicles with fibrillar material identical to the growing cell wall fibers. Mitochondria and Golgi bodies are concentrated under the very sinuous plasmalemma. 2. A very short spatial reorganization phase follows, during which organelles disperse between the nuclei-pheoplast pairs, cleavage vesicles appear, and flagella start developing. New pyrenoids form de novo. 3. The latter phase is followed by a longer maturation phase. Cleavage vesicles fuse and separate zoids grow as flagella. Mastigonemes formed in the endoplasmic reticulum are finally found in vesicles of a special Golgi body at the base of the anterior flagellum. They are liberated in parallel rows at the base of the already developed flagella by these Golgi's vesicles, and attach, on the flagella by an unknown process. Excretion of a mucilaginous substance takes place as the stigmas form de novo. 4. The ripe, swollen zoidocysts burst open, liberating the whole gelatinous mass. Naked zoids swim and settle on a substrate, retracting their flagella before excreting a new cell wall.     

Eyespot membranes in newly released zoospores of the green algaChlorosarcinopsis gelatinosa (Chlorosarcinales) and their fate during zoospore settlement

Summary Eyespot membranes in zoospores of the green algaChlorosarcinopsis gelatinosa were studied with the freeze-fracture technique. The PF of the plasmalemma overlying the eyespot lipid globules contains significantly greater numbers of intramembraneous particles (IMP; 8,200 IMP/m²) compared to other areas of the plasmalemma (2,100 IMP/m²). In the eyespot area the EF of the plasmalemma reveals no IMP, but regularly arranged depressions corresponding to the PF particles. Sizes of PF particles are not significantly different between the eyespot area and other areas of the plasmalemma. Zoospore settlement starts approximately two hours after release and involves in sequence, rounding up of the cells, retraction of the flagella and secretion of a cell wall. Eyespot membrane specializations on the PF of the plasmalemma disappear during flagellar retraction and before cell wall secretion.The functional significance of eyespot membrane specializations is discussed in accordance with the view that these membranes are engaged in photoreception and primary sensory transduction relating to green algal phototaxis.     

FINE STRUCTURE OF THE ZOOSPORE OF OEDOCLADIUM CAROLINIANUM (CHLOROPHYTA) WITH SPECIAL REFERENCE TO THE FLAGELLAR APPARATUS1, 2

Ultrastructure of the motile zoospore has been investigated in Oedocladium catolinianum & Hoffman. An unwalled zoospore is usually produced from the contents of a terminal vegetative cell and consists of two principal regions: a small anterior dome and a larger body region; a ring of flagella marks the juncture of these two areas. Chloroplast inclusions consist of thylakoids, mature and incipient pyrenoids, starch and striated microtubules; no eyespot has been observed. Zoospores appear to possess permanent contractile vacuoles with numerous accessory vacuoles, coated vesicles and occasionally coated tubules. The cytoplasm of the dome contains numerous mitochondria ER and golgi bodies, as well as two distinct types of vesicles. The first contains an electron-dense; granular core and is surrounded by a loose, sinuate membrane. The second vesicle is electron-opaque and is found at the apex of the dome: it contains mucopolysaccharides employed during zoospore adhesion. A complex flagellar apparatus encircles the lower region of the dome. It consists of ca. 30–65 flagella, a ring-shaped fibrous band, flagella roots and additional supporting material. The flagella and roots alternate with one another beneath the fibrous band. The compound flagellar roots consist of two superimposed components: an outer ribbon-like unit composed of three microtubular elements and a single striated inner component. A band of support material lies beneath the proximal end of the basal bodies. It is a continuous fibrous band, although it often appears as three distinct, repetitive units.     

Ultrastructure and ontogeny of a new type of eyespot in dinoflagellates

Summary Ultrastructure and ontogeny of a new type of eyespot in dinoflagellates is described. A marine tidal poolGymnodinium natalense is found to possess a highly organized eyespot whose structure is unique among dinoflagellates. The eyespot is rectangular in ventral view, C-shaped in apical view, and is located posterior to the sulcus. The eyespot is independent of the chloroplast and consists of several (typically six) layers of hemi-cylindrical walls which are concentrically arranged with narrow spacing between them. Each hemicylindrical wall is enclosed by a single unit membrane and is composed of many regularly arranged rectangular crystalline bricks. These crystalline bricks are produced in small vesicles which are formed in the invaginations of the chloroplast. The vesicles containing newly formed crystalline bricks are then transported to the sulcal area to assemble the eyespot. The crystalline bricks are arranged in a neat row within the vesicle termed “eyespot forming vesicle” (EFV), which is located near the sulcus. The hemi-cylindrical wall is constructed within the EFV. Based on the structure of the eyespot, viz. consisting of concentric multi-layered walls, the eyespot is thought to act as a quarter-wave stack antenna.     

Ultrastructural changes during sporangium formation and zoospore differentiation in Blastocladiella Emersonii

Samples from synchronized cultures of Blastocladiella emersonii were examined by electron microscopy from the late log phase to the completion of zoospore differentiation. Log-phase plants contain the usual cytoplasmic organelles but also have an unusual system of large tubules ca. 45 mμ diam that ramify in organized bundles throughout the protoplast. After induction, zoosporangium differentiation requires a 2-hr period in which the nuclei divide, a cross wall forms to separate the basal rhizoid region, and an apical papilla is produced. Nuclear division in B. emersonii is intranuclear with a typical microtubular spindle apparatus and paired, unequal, extranuclear centrioles at each pole. The papilla is formed by a process of localized cell wall breakdown and deposition of the papilla material by secretory granules. Differentiation of zoospores begins when one of the two centrioles associated with each nucleus elongates to form a basal body. The flagella fibers arise from the basal body and elongate into an expanding vesicle formed by the fusion of small secondary vesicles. The cleavage planes are formed by fusion of vesicles similar to those associated with flagellum initiation. When cleavage is complete, each sporangium contains ca. 250–260 uninucleate spore units with their flagella lying in the cleavage planes. Probable fusion of mitochondria to produce the single mitochondrion of the zoospore occurs after cleavage; the mitochondrion does not take its position around the basal body and rootlets until just before zoospore release. The ribosomal nuclear cap is organized and enclosed by a membrane formed through fusion of many small vesicles during a short period near the end of differentiation.     

Development of the pellicle and thecal plates following ecdysis in the dinoflagellateGlenodinium foliaceum

Summary The ultrastructure and development of the amphiesma of the dinoflagellateGlenodinium foliaceum was studied using conventional electron microscopy and immunocytochemistry. Ecdysis (shedding of the flagella, the outer two membranes of the cell, and the thecal plates) was induced by centrifugation. The cells were resuspended and the thickening of the pellicle and the development of the new thecal vesicles and plates was studied over a 9 h period. After ecdysis, the thin pellicle which underlay the thecal plates in the motile cells thickens to form a complex structure of four distinct layers: an outer layer of randomly oriented fibrils, a 50 nm layer of fibrils oriented perpendicular to the dense layer, the dense layer which has a trilaminate structure, and a wide inner homogeneous layer. The new thecal vesicles form in these pelliculate cells by the migration of electron translucent amphisomal vesicles over the layer of peripheral microtubules to a position directly under the plasmalemma. The thecal vesicles then flatten and elongate. A discontinuous pellicular layer appears within them. Subsequently, the thecal vesicles widen and are filled with a fibrillogranular substance overlying the pelliculate layer. The thecal plates form on top of this fibrillogranular material. By this time, most cells have escaped from the pellicle and are motile. At first, the outer thecal vesicle membrane is continuous with the inner thecal vesicle membrane at the sutures, but when this connection is broken, the dense pelliculate layers become continuous across the suture as does the inner thecal vesicle membrane. At ecdysis, this membrane becomes the new plasmalemma of the cell. Cells at each stage of pellicle thickening and thecal development were labelled with a polydonal antiserum raised against the 70 kDa epiplasmic protein ofEuglena acus. This antiserum labelled both the thecal plates of the motile cells and the inner homogeneous layer of the pellicle of ecdysed non-motile cells. No other amphiesmal structure was labelled, nor was any intracellular compartment.Abbreviations PBS phosphate-buffered saline - PIPES piperazine-N,N-bis[2-ethane sulfonic acid]     

Eyespot fine structure in Eudorina illinoiensis

Electron microscopy of the eyespot of the coenobic yolvocacean Eudorina illinoiensis (Kofoid) Pascher shows that besides the well-documented anterior-posterior decrease in size, congruity of structure is involved in eyespot differentiation.

In the anterior cells the eyespot lies adjacent to the plasmalemma with the thylakoid underlayers parallel to it, while in the posterior cells eyespot placement and orientation may vary considerably. Deep-sited eyespots of congruent organisation probably represent the mother-cell eyespot; discongruent ones are typical of posterior cells, but the smallest ones may be random associations of osmiophilic globules.

Eyespot structure is discussed in relation to photoreception and phototaxis.     

Two New Species of Nosema (Microsporida: Nosematidae) from the Mexican Bean Beetle Epilachna varivestis (Coleoptera: Coccinellidae)1,2

Two new species of the genus Nosema (Microsporida: Nosematidae) are described from the Mexican bean beetle, Epilachna varivestis (Coleoptera: Coccinellidae) and their life cycle stages studied by light and electron microscopy. Both species are monomorphic and disporous: they develop in direct contact with the cytoplasm of host cells and the nuclei of all stages are diplokaryotic. The more virulent species produces systemic infections most extensively in the adipose tissue, muscles, and Malpighian tubules of larvae and also invades the reproductive tissues of adult beetles. During merogonic development, it forms chains of diplokaryotic meronts. The fine structure of the sporoblast nuclei shows clumped material in the pole of each nucleus opposite their common plane of apposition. Spores are straight to slightly curved and ovocylindrical in shape and they measure 5.3 ± 0.13 × 2.1 ± 0.03 μm. The less virulent species also invades most host tissues but does not develop in the midgut epithelium; the Malpighian tubules are the principal site of its development and it also invades the ovaries and testes of adult beetles. Merogony occurs exclusively as the result of binary fission of diplokaryotic meronts. The plasmalemma of the meronts is covered with a thin deposit of exospore material upon which are located closely packed tubules that encircle the body transversely. A thickened deposit of exospore material on the surface of the diplokaryotic sporonts later obscures these tubules. Other tubules occur free in the host cell cytoplasm or attached to the plasmalemma of meronts and sporonts. Secretory granules also occur free or in chains in the host cytoplasm and are probably produced from the surface of the sporoblasts. Sporoblasts also contain an unusual cup-shaped organelle associated with a dense body, which is apparently involved in the formation of the polar tube and its associated organelles in the anterior part of the spore. Spores are ellipsoidal to slightly pyriform and measure 4.7 ± 0.06 × 2.6 ± 0.03 μm.     

Paramural bodies of Albugo candida

The paramural bodies of Albugo candida were formed solely by elaboration of the plasmalemma. Two major forms were recognized: one consisting of plasmalemmal invaginations projecting into the cytoplasm; the other appearing like a pocket containing a number of vesicles and tubules. It is suggested that the first is the basic form of paramural body. In sporangia the paramural bodies break away from the plasmalemma and undergo autodigestion while in vegetative hyphae their tubules and lamellae break up into vesicles that are finally sequestered into the wall.     

Ultrastructural Studies of Microgametogenesis and Macrogametogenesis of Eimeria truncata of the Lesser Snow Goose1

ABSTRACT. Microgamonts and macrogamonts of Eimeria truncata were observed in renal epithelial cells of collecting tubules and ducts and occasionally in macrophages of experimentally infected lesser snow geese (Anser c. caerulescens) beginning 8.5 days post inoculation. Intraparasitophorous vesicles in parasitophorous vacuoles of both types of gamonts appeared to originate in host cell cytoplasm and enter gamonts through micropores by budding of plasmalemma or by pinocytosis. Within the parasite's cytoplasm, the vesicles were broken down in Golgi-associated vacuoles. The surfaces of microgamonts were highly invaginated to facilitate extrusion of numerous microgametes. Formation and maturation of microgametes were similar to those of other eimerian species. Each microgamete had two flagella, a mitochondrion, and a peculiarly shaped electron-dense nucleus that was oval anteriorly in cross section and somewhat dumbbell-shaped posteriorly. A longitudinally arranged inner membrane complex lay between a portion of the mitochondrion and the plasmalemma. About five subpellicular microtubules extended the length of the microgamete body. Macrogametogony differed little from that described in other eimerian species. Type 1 wall-forming bodies (WFB) formed in Golgi complexes early in macrogametogony, and type 2 WFB formed in cisternae of endoplasmic reticulum in intermediate stages of macrogamont development.     

AN ELECTRON MICROSCOPE STUDY OF GAMETE RELEASE AND SETTLING IN THE COMPLANATE FORM OF SCYTOSIPHON (SCYTOSIPHONACEAE,PHAEOPHYTA)1

This paper describes the ultrastructural characteristics of gametes and their liberation from the gametangia in Scytosiphon sp., a brown alga showing only slight sexual differentiation. Both male and female gametes are released initially into a central cavity which forms in the gametangial sorus by the extensive dissolution of the internal cell walls. Scytosiphon sp. gametes possess structural features in common with the zoids of other species of brown algae. Gamete fine structure is discussed in relation to cell function. After release from the gametangial sorus, female gametes can be distinguished from males by the presence of a large number of Golgi-derived vesicles with electron dense cores. It is possible that these vesicles contain the sex attractant compound. When gametes settle they become spherical, the flagella and eyespot are withdrawn into the cell and adhesive material, apparently originating from the activity of the Golgi body, appears on the surface of the cell.     

Reproduction by zoospores inOedogonium

Summary Emergence of zoospores ofOedogonium and their subsequent developmental changes have been studied using live material and sections prepared for light and electron microscopy. Release commences with rupture of the cell wall at its pre-weakened site near the apical caps. The pliable protoplast of the zoospore becomes completely spherical once free of the wall; it is enclosed within the hyaline vesicle which expands continuously and then disappears. Meanwhile, as the flagella become active, the zoospore begins to elongate and its dome starts to protrude from a circular constriction where the flagella are inserted. Once free of the hyaline vesicle, it is actively motile for a variable period, during which elongation continues. The motile phase ceases when the zoospore begins to vibrate, whereupon the flagella are all violently shed. Soon after this, the constriction disappears from around the dome which becomes more pointed; the immobile cell now elongates further, increasing in volume. The cell periphery contains numerous contractile vacuoles. Zoospore elongation may be associated with a proliferation of longitudinal microtubules, and once the flagella are shed, the flagellar rootlet system disintegrates, probably releasing the rootlet microtubules. Mechanisms involved in the release of the zoospore are also discussed.     

Studies on marine algae of the British Isles. 8. Cystoseira tamariscifolia (hudson) papenfuss

The fine structure of the zoospores of Urospora penicilliformis (Roth) Aresch. (Chlorophyceae) is described. Of special interest is the flagellar apparatus. The proximal part of each of the 4 flagella is ribbon-shaped and contains nine wings attached to the peripheral double tubules. The flagellar root system originates from the flagellar bases and includes striated fibrous roots, passing close to the nucleus, and cruciate nine-stranded microtubular roots along the four corners of the cell. The Golgi bodies produce numerous vesicles, concentrating apically in the cell; they are presumed to be of importance for the attachment of the zoospore.     

THE CYTOSKELETON OF THE NAKED GREEN FLAGELLATE SPERMATOZOPSIS SIMILIS (CHLOROPHYTA):FLAGELLAR AND BASAL BODY DEVELOPMENTAL CYCLE1

Flagellar and basal body development during cell division was studied in the biflagellate green alga Spermatozopsis similis Preisig et Melkonian by light microscopy of immobilized living cells, statistical analysis of flagellar lengths during the cell cycle, and electron microscopy of cells and isolated cytoskeletons. Interphase cells display two flagella of unequal/subequal length. An eyespot located in an anterior lobe of the chloroplast is connected to the basal body bearing the shorter flagellum by means of a five-stranded microtubular root. Until cell division, the two parental flagella attain the same length. During cell division, each cell forms two new flagella that grow to a length of 1.5 μm before they are distributed in a semiconservative fashion together with the parental flagella to the two progeny cells at cytokinesis. During the following interphase, the flagella newly formed during the preceding cell division grow to attain the same length as the parental flagella until the subsequent cell division. The shorter of the two flagella of a cell thus represents the developmentally younger flagellum, which transforms to the mature state during two consecutive cell cycles. Interphase cells display only two flagella-bearing basal bodies; two nascent basal bodies are formed during cell division and are connected to the microtubular d-roots of respective parental basal bodies with which the newly formed basal bodies are later distributed to the progeny cells. During segregation, basal body pairs shaft into the 11/5 o'clock direction, thus conserving the 1/7 o'clock configuration of basal body pairs of interphase cells. Prior to chloroplast and cell division, an eyespot is newly formed near the cell posterior in close association with a 1s microtubular root, while the parental eyespot is retained. During basal body segregation, eyespot-root connections for both the old and newly formed eyespots are presumably lost, and new associations of the eyespots with the 2s roots of the newly formed basal bodies are established during cytokinesis. The significance of this “eyespot-flagellar root developmental cycle” for the absolute orientation of the progeny cells is discussed.     

The flagellar apparatus of the golden algaSynura uvella: Four absolute orientations

Summary Flagellated vegetative cells of the colonial golden algaSynura uvella Ehr, were examined using serial sections. The two flagella are nearly parallel as they emerge from a flagellar pit near the apex of the cell. The photoreceptor is restricted to swellings on the flagella in the region where they pass through the apical pore in the scale case and the swellings are not associated with the cell membrane or an eyespot. A unique ring-like structure surrounds the axonemes of both flagella at a level just above the transitional helix. The basal bodies are interconnected by three striated, fibrous bands. Four short (<100 nm) microtubules lie between the basal bodies at their proximal ends. Two rhizoplasts extend down from the basal bodies and separate into numerous fine striated bands which lie over the nucleus. Three- and four-membered microtubular roots arise from the rhizoplasts and extend apically together. As the roots reach the cell anterior, the three-membered root bends and curves clockwise to form a large loop around the flagella; the four-membered root bends anticlockwise and terminates under the distal end of the three-membered root as it completes the loop. There are four absolute orientations, termed Types 1–4, in which the flagellar apparatus can occur. With each orientation type the positions of the Golgi body, nucleus, rhizoplasts, chloroplasts and microtubular roots change with respect to the flagella, basal bodies and photoreceptor. Two new basal bodies appear in pre-division cells, and three short microtubules appear in a dense substance adjacent to each new basal body. Based upon the positions of new pre-division basal bodies, a hypothesis is proposed to explain why there are four orientations and how they are maintained through successive cell divisions.     

Ultrastructural study of the relationship between generative and vegetative cells inMagnolia ×soulangeana Soul.-Bod. pollen grains

Summary InMagnolia ×soulangeana pollen grains the generative cell (GC) does not become totally free within the vegetative cell (VC), at least until the pollen tube emergence. Due to a deviation in its detachment process from the sporoderm, the opposing ends of the VC plasmalemma do not fuse themselves when the GC moves away from the intine. Consequently, the interplasmalemmic space surrounding the GC does not become isolated but rather maintains continuity with the sporoderm through a complex formation that we have called plasmalemmic cord. The real existence of this formation was confirmed through serial sectioning showing the plasmalemmic cord to consist of the VC plasmalemma. In its initial portion it is occupied by a reasonably accentuated wall ingrowth of the inner layer of the intine (intine 3). In the remainder portion, neither of the cytochemical tests used in this work have revealed the presence of a significant amount of wall material. However, ultrathin sections of samples processed either chemically or by cryofixation showed the existence of an intricate system of tubules and vesicles, some of which are evaginations of the VC plasmalemma. The hypothesis that the plasmalemmic cord may have a role in the complex interactions between the two pollen cells is discussed.     

Lomasomes and plasmalemmasomes in fungi

Summary Electron microscope observations of fungal hyphae and yeast-like cells, using conventional fixation methods and freeze-etching, demonstrate that plasmalemmasomes are not fixation artifacts. The small invaginations of the plasmalemma observed in aldehyde-fixed preparations are also present in frozen-etched samples. The morphology of plasmalemmasomes in the species examined is variable and ranges from vesicles or tubules within the plasmalemma invagination to parallel arrays of membrane lamellae. Plasmalemmasomes thus appear to be primarily excess plasma membrane that has accumulated, perforce, endocellularly. Lomasomes, in contrast, appear to be accumulations of ejected material between the plasmalemma and cell wall that have become sequestered by the deposition of wall material.     

ULTRASTRUCTURAL ASPECTS OF CELL ELONGATION,CELLULOSE SYNTHESIS,AND SPORE DIFFERENTIATION IN ACYTOSTELIUM LEPTOSOMUM,A CELLULAR SLIME MOLD

Vegetative myxamoebae of Acytostelium leptosomum, a cellular slime mold, have the appearance of typical eucaryotic cells. The presence of dictyosomes has been established. Elongation of the cells during aggregation and culmination appears to be mediated by dense bundles of microfibrils traversing the cells longitudinally. Microtubules are present; however, they are randomly oriented and no correlation can be made with cell elongation or with the direction of the cellulose microfibrils within the stalk. A variety of vesicles, multivesicular bodies, and lysosome-like vacuoles seems to be involved in producing and transporting stalk material to the vicinity of the stalk. However, only rarely do the vesicles empty their contents directly to the outside of the cells. It seems rather that the fibrillar material of the stalk is assembled near or directly at the plasmalemma, and can then be seen to stream away and become an integral part of the stalk. An unusual structure, the H-body, is formed in great abundance during culmination indicating its possible involvement in stalk synthesis. The H-bodies are removed from the cells prior to spore formation together with other portions of the cytoplasm at least partly by a process involving autophagic vacuoles. These vacuoles, which are also present in the spores, appear to be part of a rather complex and extensive vacuolar apparatus including the food vacuoles, contractile vacuoles, lysosome-like structures, and possibly the H-bodies. The spore coat consists of a heavy outer wall with a fibrillar substructure and two thin, dense bands lining the inside of the plasmalemma. The fibrillar nature of both the outer spore wall and the stalk was accentuated by using barium permanganate to stain sectioned material.     

Behavior of flagella and flagellar root systems in the planozygotes and settled zygotes of the green alga Bryopsis maxima Okamura (Ulvophyceae,Chlorophyta) with reference to spatial arrangement of eyespot and cell fusion site

Behaviors of male and female gametes, planozygotes and their microtubular cytoskeletons of a marine green alga Bryopsis maxima Okamura were studied using field emission scanning electron microscopy, high‐speed video microscopy, and anti‐tubulin immunofluorescence microscopy. After fusion of the biflagellate male and female gametes, two sets of basal bodies lay side by side in the planozygote. Four long female microtubular roots extended from the basal bodies to the cell posterior. Four short male roots extended to nearly half the distance to the posterior end. Two flagella, one each from the male and female gametes, become a pair. Specifically, the no. 2 flagellum of the female gamete and one male flagellum point to the right side of the eyespot of the female gamete, which is located at the cell posterior and which is associated with 2s and 2d roots of the female gamete. This spatial relationship of the flagella, microtubular roots, and the eyespot in the planozygote is retained until settlement. During forward swimming, the planozygote swings the flagella backward and moves by flagellar beating. The male and female flagella in the pair usually beat synchronously. The cell withdraws the flagella and becomes round when the planozygote settles to the substratum 20 min after mixing. The axoneme and microtubular roots depolymerize, except for the proximal part and the basal bodies. Subsequently, distinct arrays of cortical microtubules develop in zygotes until 30 min after mixing. These results are discussed with respect to the functional significance of the spatial relationships of flagellar apparatus‐eyespot‐cell fusion sites in the mating gametes and planozygote of green algae.