Scanning electron microscopy of post-ejaculatory spermiogenesis in the tick Ornithodoros moubata

The final stages of spermiogenesis in ticks occur in the female genital tract. Scanning electron microscopy was used to follow the morphologic changes that occur in the sperm during this post-ejaculatory spermiogenesis in the African soft tick, Ornithodoros moubata, and to determine a time sequence for its occurrence in vivo. Characteristic features of the maturing and mature cell described include (1) differentiation and detachment of the operculum, (2) changes in cell shape corresponding to different developmental stages, (3) passive migration of the nucleus and acrosome from an anterior to a posterior position, and (4) eversion of that portion of the acrosomal canal containing the nucleus and acrosome. A possible fate for the remainder of the acrosomal canal is suggested by extrusion and detachment of spherical structures, the ‘posterior bubbles’, from the posterior end of the mature supermatozoon. A mechanism for cellular elongation resulting from contractions of the outer sheath is proposed.     

Fine structure and development of the sperm of the tick Ornithodoros (Pavlovskyella) erraticus (Ixodoidea, Argasidae)

Sperm development in Ornithodoros (Pavlovskyella) erraticus includes the formation of subsurface cisternae in the primary spermatocytes, which divide meiotically to secondary spermatocytes and ultimately to spermatids. During spermiogenesis the spermatid undergo morphological transformation including polarization of the nucleus and subsurface cisternae, formation of a cisternal tube, and modification of the subsurface cisternae to cellular processes surrounded by cisternal vesicles. Further transformation occurs after spermatids are introduced into the female. The spermatid cisternal tube now invaginates to form an inner cord surrounded by an outer sheath. The invaginated inner cord elongates anteriorly as the outer sheath continues to invaginate posteriorly during spermiogenesis. With further elongation, the spermatid membrane ruptures anteriorly, leaving the inner cord exposed as the outer surface of the maturing sperm. Posteriorly, the original plasma membrane invaginates to form an acrosomal canal which becomes surrounded by an acrosome. The hemispherical anterior end of the mature sperm is covered with rows of projections separated from the remainder of the sperm by a row of fringed processes. Except for the posterior end, the rest of the sperm is covered by longitudinally distributed electron-dense cellular processes and an outer mat of more electron-lucent tubular elements. Mitochondria and bundles of microfibrils are found beneath the cellular processes. Microfibrils are suggested to be the principal contractile organelles responsible for sperm motility. Cellular processes appear to be the main external motile structures, while movements of tubular elements and fringed processes may also contribute to sperm motility.     

Developmental expression of spermatid-specific thioredoxin-1 protein: transient association to the longitudinal columns of the fibrous sheath during sperm tail formation

The mammalian sperm tail presents a complex organization in which a number of additional structures, namely outer dense fibers and fibrous sheath, surround the central axoneme and are thought to regulate flagellar motility. We have previously described a novel member of the thioredoxin family of proteins with a spermatid specific expression pattern, spermatid-specific thioredoxin-1 (Sptrx-1). We report here the developmental analysis of Sptrx-1 expression during murine spermiogenesis. Immunocytochemical analysis of Sptrx-1 through the different steps of spermiogenesis in rat seminiferous tubule sections showed that its expression begins at step 9, gets progressively stronger until steps 14-16 (where a peak is reached), and then diminishes in steps 17 and 18 until practically no immunolabeling is detected in step 19 spermatid. During its transient expression in spermiogenesis, Sptrx-1 is most concentrated in the periaxonemal compartment of the tail of the elongating spermatid, except in the very last steps (steps 17-19), when periaxonemal labeling disappears and a residual buildup of Sptrx-1 occurs in the shrinking cytoplasmic lobe. Electron microscopic analysis by immunogold labeling pinpointed the localization of Sptrx-1 to the assembling longitudinal columns of the fibrous sheath, whereas the forming ribs of the fibrous sheath were unlabeled. Immunoblotting of isolated fibrous sheath and tails obtained from epididymal or ejaculated sperm of rat and human confirmed our immunocytochemical observation: Sptrx-1 is no longer a component of the mature fibrous sheath. To our knowledge, this is the first report of a protein that specifically associates to the fibrous sheath during development but does not become a permanent structural component. The expression pattern of Sptrx-1 during rat spermiogenesis suggests that it could be part of a nucleation center for the formation of the longitudinal columns and transverse ribs that bridge the latter.     

Systematic and morphological studies of the genus Chaetopleurophora Schmitz (Diptera: Phoridae) occurring in Japan

Japanese species of the genus Chaetopleurophora are reviewed. All belong to the C. erythronota group. The following three species from Japan are described: C. rhomboidea sp. nov., C. pygidialis Schmitz and C. dividua sp. nov. The male and female genitalia are studied and further examples of unique characters of the genus including asymmetric features are added. The male aedeagus of the genus is illustrated for the first time. The aedeagus of the species treated in this study consists of only two components, the inner core plate and the outer jacket plate. The jacket plate wraps sinistrally around the core plate. The combination of the core plate and the jacket plate forms complex, asymmetric features of the aedeagus in the Phoridae. The structure around the genital opening in the female genitalia protrudes posteriorly under the segment IX + X, and shows asymmetric features in C. rhomboidea sp. nov. with a bilaterally different degree of sclerotization, shifted genital opening to the right side and a membranous ribbon just on the left side. In addition, C. dividua sp. nov. is different from most of the species in the C. erythronota group, and very closely related to C. multiseriata (known in North America) in the male and female genitalia, wing venation and bristle formation on the scutellum. It is suggested that C. dividua sp. nov. forms a monophyletic group with C. multiseriata and the related species.     

DEVELOPMENT OF MUCILAGINOUS SURFACES IN EUGLENOIDS. I. STALK MORPHOLOGY OF COLACIUM MUCRONATUM1

The envelope and stalk of Colacium mucronatum Bourr. & Chad, were examined in living cells with light microscopy and in fixed preparations with scanning electron microscopy using critically point dried (CPD) and freeze dried (FD) preparations. The envelope of palmelloid cells is formed over the entire cell surface by many individual strands attached at right angles to areas of articulation of the pellicular strips. Strands were observed to anastomose on the posterior tip of otherwise naked cells. Stalks of living cells in India ink preparations had an optically dark inner core with a lighter outer sheath. In FD stalks a definite inner core was not evident, whereas CPD stalks had an outer surface composed of thick strands which may be the collapsed and aggregated strands of the FD stalks. In both there was also an amorphous matrix. The stalk forms from the aggregation of many strands from the anterior cell tip back to a point encompassing the cell surface anterior to a cross section of the tip 9 μm diam. The outer surface of the stalk comes from the pellicular surface joining that area and the core from the cell tip in the area of the canal opening. Any possible participation of the inner canal surface in stalk formation could not be determined because of the great density of the mucilage at the cell-tip/stalk junction.     

ELECTRON MICROSCOPY OF THE PACINIAN CORPUSCLE

The Pacinian corpuscle has a framework of cytoplasmic lamellae arranged concentrically in the outer zone, and bilaterally in the core. Between these is an intermediate growth zone. The inner core shows an unexpected complexity in that its component lamellae are arranged in two symmetrical groups of nested cytoplasmic sheets. Longitudinal tissue spaces form clefts separating the two groups. The perikarya of the core lamellae lie in or near the intermediate growth zone, and send arms into the clefts. The arms then branch and terminate as lamellae which interdigitate with those of neighboring cells. The single nerve fiber loses its myelin sheath just before it reaches the inner core but retains its Schwann cell cytoplasmic covering for a short additional distance. The Schwann sheath is not continuous with the lamellae of the inner core. Inside the core the fiber contains a striking circumferential palisade of radially disposed mitochondria. The fiber does not arborize. Vascular capillaries penetrate the hilar region of the corpuscle only as far as the myelinated sheath of the nerve, and they have not been seen elsewhere in the corpuscle. There is direct continuity between the clefts of the core and tissue spaces in the vicinity of the capillaries. It is likely that this provides a route whereby metabolites reach the active nerve ending, as well as the cells of the growth zone. The outer zone consists of at least 30 flattened concentric cytoplasmic lamellae separated from one another by relatively wide fluid-filled spaces. Collagenous fibrils are present, particularly on the outer surface of lamellae, and tend to be oriented circularly. The girdle of proliferating cells constituting the growth zone, which is prominent in corpuscles from young animals, is the layer from which the outer lamellae are derived. Osmotic forces probably elevate the lamellae, and maintain turgor pressure.     

The ultrastructure of the peculiar synspermia of some Dysderidae (Araneae, Arachnida)

The present study reports on the ultrastructure features of spermatozoa and spermatogenesis of several species of Dysderidae (Dysdera crocata, Dysdera erythrina, Dysdera ninnii, Harpactea arguta, Harpactea piligera, Dasumia taeniifera). Dysderid spiders are known to possess a peculiar sperm transfer form known as synspermia, characterized by fused spermatozoa surrounded by a secreted sheath. Until now the exact mode of formation of the synspermia is unknown. The present study demonstrates that the spermatids are connected via narrow cell bridges during the entire spermiogenesis as is usual, although in Dysderidae they do not separate at end of the spermiogenesis. Instead, they fuse completely within the testes shortly after the spermatid has coiled to get a spherical shape. The number of fusing sperm cells is different in the different observed species. The species of the genus Harpactea thus have synspermia consisting of two fused spermatozoa; whereas in the species of the genus Dysdera four sperm cells are fused and in D. taeniifera at least three spermatozoa are fused. In contrast with other known families with this peculiar form transfer of sperm, the synspermia in Dysderidae are mainly characterized by a conspicuous vesicular area which extends through the entire synspermium surrounding the cell organelles. Thus, all main cell components (e.g., nucleus, acrosomal vacuole, and axoneme) are covered by the vesicular membrane. The vesicular area seems to be functional and probably it is important during sperm activation in female genital system. Simultaneously to the extension of the vesicular area, the synspermium accumulates large amounts of glycogen. The glycogen is mainly located around the centriolar adjunct and along the axoneme accompanying the postcentriolar elongation of the nucleus. A further peculiar feature is the extremely elongated acrosomal vacuole, which seems to be synapomorphic trait for sperm cells of dysderids. Interestingly, spermatogenesis, including the fusion, exclusively occurs within the testes (in contrast to the formation of coenospermia). In the vas deferens only synspermia were found. The secreted sheath surrounding the spermatozoa is finally synthesized in the parts of the vasa deferentia, which are close to the genital opening where numerous vacuoles and microvilli are seen in the epithelial cells.     

Ultrastructure of receptaculum seminis in Haemaphysalis longicornis (Acari: Ixodidae) in relation to inserted endospermatophore

The receptaculum seminis, opening into the female genital tract, is found only in the metastriate ixodid ticks. An endospermatophore that has been inserted into the female genital aperture at copulation is first stored in the receptaculum seminis, where spermiogenesis is completed before the sperm ascend the oviducts. The receptaculum seminis consists of a simple cuticularized epithelium. Epithelial cells in sexually matured females develop during feeding and become active in secretion. Secretions discharged from epithelial cells are released into the lumen of this organ through the cuticle and may act on the wall of the inserted endospermatophore. The fact that resumption of spermiogenesis (spermateleosis) has already occurred before destruction of the endospermatophore just after copulation suggests that secretions from epithelial cells of the receptaculum seminis are not the trigger of spermateleosis, but a destructive agent of the endospermatophore wall. J Morphol 231:143–147, 1997. © 1997 Wiley-Liss, Inc.     

Development of the calyx and lateral oviduct during oogenesis in Aedes aegypti

Summary The lateral oviduct and calyx of nulliparous Aedes aegypti on a sucrose diet are both flattened sacs, lacking a well defined lumen. Both are formed of an inner epithelial and an outer muscular layer, each one cell thick. The lateral oviduct is surrounded by a circular muscle sheath which is continuous with the ovarian sheath. Each ovariolar sheath is continuous with the outer layer of the calyx. The structure of both the lateral oviduct and the calyx is greatly modified after the initial blood meal. A distinct lumen develops; there is an extensive development of the outer muscular layers, and the inner epithelial layers become invaginated forming deep crypts lined with extensive microvilli. The follicular stem, which joins the primary follicle to the calyx in each ovariole, is not hollow and does not mark the opening into the calyx through which the mature egg can pass. The eggs gain access to the oviductal system after the calyx extends around the follicular epithelium of the primary follicle, when breaks appear in the calyx wall opposed to the follicular epithelium, until the mature eggs, eventually lie in a highly distended thin-walled sac of calyx from which they have direct and easy access to the lateral oviduct. After oviposition, this sac contracts to occupy once more a compact axial position in the ovary. Remnants of the follicular epithelium, containing many lysosomes are attached to the calyx at this time.     

Fine Structure of Spirochaeta stenostrepta, a Free-living, Anaerobic Spirochete

The fine structure of Spirochaeta stenostrepta strain Z1, a free-living anaerobic spirochete, was studied by electron microscopy. The organism possessed a coiled protoplasmic cylinder, an axial filament inserted subterminally, and a loosely fitting sheath which enclosed both the protoplasmic cylinder and the axial filament. The axial filament consisted of two fibrils partially overlapping in a 1-2-1 arrangement. The axial fibrils appeared to possess a sheath surrounding an inner core. Both inner core and sheath were apparently enclosed in a cross-striated tubular structure, which was itself surrounded by an outer sheath. The axial filament exhibited a basal hook. A disc- or mushroom-shaped structure, possibly consisting in part of cytoplasmic membrane, was observed at the insertion end of isolated filaments. The protoplasmic cylinder had a distinctive surface structure consisting of an array of tightly packed, longitudinally arranged helices measuring 2.0 to 2.5 nm in diameter. This layer of helices lay below the outer cell sheath and the axial filament. Ballistic disintegration loosened the helical array, causing individual helices or segments of helices to become separated from the cell. The function of this layer of helices is still obscure.     

The fine structure of Gnathostomulid reproductive organs

Summary The male copulatory organs of five species of Gnathostomulida Scleroperalia have been studied by TEM techniques. These observations provide a more solid basis for classification in the light microscope: inLabidognathia longicollis (fam. Mesognathariidae) the stylet is composed of eight, and inSemaeognathia sterreri, Gnathostomula jenneri, Gnathostomula mediterranea andGnathostomula microstyla (Gnathostomulidae) of ten stylet rods. Each rod consists of a microtubule-filled inner rod, and of an outer rod, filled with crystallized inclusions. The inner rods are continuous with eight — or ten — rod formation cells which are located in the proximal stylet sack. Bipartition of rods occurs by a longitudinal invagination of the basement lamina, underlying the rod cells and the gland cells and continuous with that of the body wall epithelium. InLabidognathia, the outer rods are interlocked, in Gnathostomulidae, the stylet rods are surrounded by an extracellular (cuticular) tube-like stylet sheath of variable fine structure, which is believed to provide extra rigidity. In the species investigated, one single stylet gland, consisting of a monolayered epithelium showing different gland cell types, surrounds the stylet. In the apical gland cell portions, medially and distally membrane-bound secretory granules lie adjacent to the stylet sheath. In Gnathostomulidae, two anterior gland cells are seen in connection with the formation of the stylet sheath. In the muscular sheath the cross-striated fibers, basically derived from the longitudinal body wall musculature, show a tendency towards helical and circumferential arrangement. Musculature is especially prominent in the proximal stylet sack, which is rather a propulsive element than a sperm-storing vesicle, and lacks glands. InGnathostomula species, atrial cells underlie the distal tip of the stylet. The entrance into the male opening is lined with ciliary receptor cells and specialized gland cells.Stylet evolution in Scleroperalia is characterized by progressive differentiation of the muscular sheath, in particular of the proximal stylet sack, and of the stylet — the occurrence of a stylet sheath is seen in connection with increasing diversity of stylet shape.Abbreviations ac atrial cell(s) - ag anterior gland cell(s) - b bursa - bl basal lamina - c rod-crystal in outer rod - cj cuticle of jaw - d desmosome - di dictyosome - e body wall epithelium - ej pharyngeal epithelium - g stylet gland (cell) - gm median gland cell - i gut (cell) - ir inner rod - jc junctional complex - m muscular layer - mo male opening - mv microvillar protrusions - nu nucleus - o ovary - or outer rod - po proximal opening of the proximal stylet sack - ps proximal stylet sack - r stylet rod - rc rod cell - sg secretory granule - sj septate junction - sp sperm - ss stylet sheath - st stylet - te testes - v ventral - z centriole     

Fine-structural and chemical analyses on inner and outer sheath of the cyanobacteriumGloeothece sp. PCC 6909

Cells of the unicellular cyanobacteriumGloeothece sp. PCC 6909 are surrounded by an inner (enclosing 1–2 cells) and an outer (enclosing cell groups) sheath. Using conventional Epon-embedding in combination with ruthenium-red staining, the inner and outer sheaths appeared similar and displayed multiple bands of electron-dense subunits. However, embedding in Nanoplast resin to avoid shrinkage led to the detection of two distinct zones (inner and outer zone) each with several distinct layers. The zone delimited by the electron-dense thick inner sheath layer, and the zone enclosed by the thin electron-dense outer sheath layer, are composed of a homogeneous material of little electron-contrast. Whereas the outer zone appears to be of even contrast, the inner zone is characterized by a distinct electron-transparent layer. Element distribution analysis revealed that the electron-transparent layer contained relatively large amounts of sulfur, carbon, and oxygen but only little nitrogen.Inner and outer sheath fractions were isolated by differential mechanical cell breakage and centrifugation. The outer sheath fraction was less hydrated than the inner one. The two fractions differed little in their contents of uronic acids, carbohydrate and protein, although the outer sheath fraction contained less sulfate. A soluble polysaccharide with a chemical composition similar to that of inner and outer sheath fractions was also obtained from the culture supernatant.     

Electron microscopy and histochemistry of oncospheral hook formation by the cestode Catenotaenia pusilla

Swiderski Z. 1973. Electron microscopy and histochemistry of oncospheral hook formation by the cestode Catenotaenia pusilla. International Journal for Parasitology3: 27–33. Ultrastructure and histochemistry of oncospheral hook development in the cestode C. pusilla are described. The six anlagen of embryonal hooks appear in six specialized hook forming cells (oncoblasts) in the advanced phase of the preoncosphere.Electron microscopy shows a close connection of hook primordium with an abundance of free ribosomes, extended Golgi regions, and mitochondrial aggregations, evidently engaged in the hook morphogenesis. The shank completion occurs simultaneously with the progressive degeneration of hook forming cells, which completely disappears in the last phase of hook development. The mature oncospheral hook is a heterogeneous, bipartite structure composed of a dense outer sheath or cortex and a less dense inner core.Histochemistry shows evident changes in the reactivity for —SH, and —S—S— groups through the consecutive stages of hook development. The early hook anlage shows strongly positive reaction for sulfhydryl (—SH) groups (unconsolidated prekeratin), which remains in the subsequent stages mainly in the zone of keratinization, undergoing continuous displacement toward the base during hook maturation.The sulfhydryl groups of prokeratin through the oxidation process form bisulfite (—S—S—) links of mature hook keratin; reactivity for —SH groups completely disappears. The difference in electron density between the outer and inner part of the hook corresponds to a different reactivity for —S—S— links; the outer sheath shows evidently stronger reaction than the inner core.     

Some structural and cytochemical observations on the axial filament complex of lung-fluke spermatozoa

As seen in transverse section, doublet elements of the axial unit of spermatozoa of Haematolocchus medioplexus, a frog lung-fluke, possess walls made up of protofibrillar subunits 50–60 Å in diameter. The partition between A and B members of a doublet element often show extra protofibrils which may partially occlude the “lumen” of the A tubule. Each A tubule possesses outer and inner lateral arms which repeat at longitudinal intervals of about 215 Å and which appear to be structurally dissimilar; the outer arm is expanded at its free end and the inner arm often connects to the B tubule of the adjacent doublet element. Regularly-spaced radial links connect the central sheath of an inner core complex to the A tubules of the peripheral doublet elements. Tests for magnesium-activated ATPase activity provide evidence that the enzyme is associated with the surfaces of doublet elements and the surface of the central sheath. Finally, study of an axial unit which developed in an abnormal manner suggests that normal differentiation of an axial unit may depend on the elaboration of a core complex and radial links.     

The functional morphology of mating in the Silurian eurypterid, Baltoeurypterus tetragonophthalmus (Fischer, 1839)

The functional morphology of the genital appendages, horn organs and modified prosomal limb III of the Silurian eurypterid, Baltoeurypterus tetragonophthalmus (Fischer, 1839) (Chelieerata: Eurypterida) is investigated. The longer type A genital appendage, interpreted as a female structure, is associated with a pair of horn organs, regarded as spermathecae. The shorter type B appendage, interpreted as male, is associated with scimitar lobes on prosomal limb III, regarded as clasping structures. A mechanism is proposed by which the type A appendage was lowered through flexure of the genital operculum acting along sutures in the cuticle. A model of mating is suggested, involving the male depositing an unstalked spermatophorc on the substrate and the female retrieving it for storage in the spermathecae. This model is more arachnid-like than mating in Limutus (an extant aquatic chclicerate), and suggests that sperm transfer using spcrmatophores and storage of sperm in spermathecae, allowed the timing of egg production in curyptcrids to be controlled.     

Ultrastructure of Licea kleistobolus (Myxomycetes) and its bearing on the taxonomic affinity of the species

The peridium of Licea kleistobolus Martin consists of two distinct layers. Abundant refuse matter is evenly distributed on the surface of the fructification. The upper surface of the operculum is smooth and punctated by pores of variable size. The internal structure of the operculum is essentially the same as that of the peridium. Processes along the periphery of the lower surface of the operculum and along the upper edge of the sporangium function as an opening mechanism. The operculum bears hollow finger-like outgrowths and rounded warts, both being continuous with the inner wall of the operculum. The results of a SEM- and TEM-study indicate that the two different types of processes are homologous structures. Similarity between aberrant cap-illitial outgrowths along the margin of lobes in Listerella paradoxa Jahn and the processes bordering the sporangial edge and periphery of the operculum in L. kleistobolus , indicates affinity of the two species.     

Fine structure of unusual spermatozoa and spermiogenesis of the mite Megisthanus floridanus Banks, 1904 (Acari: Gamasida: Antennophorina)

The aflagellate spermatozoa of the gamasid mite Megisthanus floridanus are characterized by a large vacuole which contains a cytoplasmic column protruding into the vacuole from the region defined as the posterior part of the cell. The membrane of the column and the inner membrane of the posterior part of the cytoplasmic mantle (outer sheath) surrounding the vacuole bear numerous so-called cellular processes. However, most of the outer sheath is reduced and represented solely by a very thin membrane-like envelope. The posterior part of the cell bears extensive folds. The cell, or, more precisely, the column, shows a deep posterior invagination. This invagination contains extracellular material composed of thin filaments or strands. Peripheral folds emerging from the posterior rim of the cell form a thin-walled tube that contains the same material as the invagination. The elongated nucleus is attached to a peculiar acrosomal complex consisting of a flat acrosomal cisterna that parallels most of the cell membrane, an attachment cone, and a short acrosomal filament which is embedded in a narrow canal within the nucleus. The spermatozoa of M. floridanus represent a peculiar version of the vacuolated type of sperm known to be plesiomorphic within the anactinotrichid Acari. Some details of spermiogenesis are described and consequences for phylogenetic and systematic considerations are discussed.     

The structure of the cuticle and sheath of the infective juvenile of Trichostrongylus colubriformis

The infective third-stage juvenile of Trichostrongylus colubriformis is surrounded by its own cuticle as well as the incompletely moulted cuticle of the second-stage juvenile, which is referred to as the sheath. The sheath comprises an outer epicuticle, an amorphous cortical zone, a fibrous basal zone and an inner electron-dense layer. The basal zone of the sheath consists of three layers of fibres; the fibres are parallel within each layer, but the fibre direction of the middle layer is at an angle to that of the inner and outer layers. The cuticle comprises a complex outer epicuticle, an amorphous cortical zone and a striated basal zone. The lateral alae of the cuticle and the sheath are aligned and overlie the lateral hypodermal cords. The lateral alae of the sheath consist of two wing-like expansions of the cortical zone with associated specializations of the inner electron-dense layer which form a groove. The cuticular lateral alae consist of two tube-like expansions of the cortical zone. The lateral alar complex of the cuticle and the sheath may maximise locomotory efficiency and prevent rotation of the juvenile within the sheath.     

SPERMIOGENESIS OF THE TELEOST,ORYZIAS LATIPES,WITH SPECIAL REFERENCE TO THE FORMATION OF FLAGELLAR MEMBRANE *

In the early stage of Oryzias spermiogenesis, an axonemal bud appears at the distal end of a centriole characterized by its electron dense accessories. When the axoneme begins to grow in the cytoplasm, small vesicles come to surround it. These vesicles are similar to those produced by the Golgi apparatus which lies close to the growing axoneme. At this stage, the spermatid cell membranes disappear, causing transformation of the mononuclear spermatids into a multinucleated syncytium. As each axoneme elongates in the syncytium, it is enveloped by a cylindrical array of vesicles which are most likely derived from the Golgi apparatus. Shortly after this stage, the syncytium is again partitioned by cell membranes, restoring the existence of mononuclear spermatids. The arrayed vesicles fuse with each other to form two concentric membranes surrounding the axoneme. The inner membrane becomes the flagellar membrane and the outer one, the membrane of a flagellar sheath. These observations lead to the conclusion that the formation of the flagellar membrane is due to the fusion of vesicles surrounding the axoneme which are derived from the Golgi apparatus. In the course of spermiogenesis, no indication of an acrosomal structure is observed.