An EM autoradiographic study on ovarian follicle cells of Drosophila melanogaster with special reference to the formation of egg coverings.

Ovarian follicle cells of Drosophila melanogaster have been studied by ultrastructural and autoradiographic analyses. During their migration through the germarium, follicle cells undergo several structural changes and, of these, the most conspicuous one occurs at the level of the nucleolus. By the time the first ovarian chamber is formed, follicle cells have formed a layer of uniform thickness all around a cluster or nurse cells and the oocyte. Following the initiation of vitellogenesis, the follicle cells overlaying the oocyte become columnar while those over the nurse cells become very thin. During stages 9-10, the columnar follicle cells are involved in the formation of the vitelline membrane, while from stages 11 to 13 these cells produce the endochorion. An EM autoradiographic analysis has shown that the rate of 3H-uridine incorporation in follicle cells nuclei is low in previtellogenic chambers, while it becomes very high in nuclei of stage 9-10 chambers. After short exposure to uridine, silver grains are located predominantly over nucleoli. Evidence from incorporation studies with 3H-lysine indicates that the columnar follicle cells and the region of the various egg coverings are highly labelled within an hour of incubation in the tracer. The observations confirm that columnar follicle cells are the only cells in the chamber involved in the formation of materials which make up the egg coverings.     

A stereological analysis of developing egg chambers in Ephestia kuhniella

A polytrophic ovariole of the flour moth, Ephestia kuhniella, is composed of a linear series of increasingly mature egg chambers, each consisting of an oocyte, an interconnected cluster of seven nurse cells, and a covering layer of follicle cells. This study describes changes in the volume of each component as a function of the position of the egg chamber in the ovariole. Analysis of the growth curve of the Ephestia oocyte yields two possible correlations between accelerated oocyte growth and ultrastructural events enhancing the supply of yolk materials to the oocyte: the first is the initiation of yolk synthesis by the follicle cell layer and its transfer to the oocyte, and the second is the formation of channels between the follicle cells allowing hemolymph to gain access to the oocyte. An Ephestia oocyte increases in volume from approximately 2.5 × 10³ μm³ to approximately 2.0 × 10⁷ μm³ over an average series of 58 egg chambers.     

Oogenesis in the leech Glossiphonia heteroclita (Annelida, Hirudinea, Glossiphoniidae) II. Vitellogenesis, follicle cell structure and egg shell formation

By the end of previtellogenesis, the oocytes of Glossiphonia heteroclita gradually protrude into the ovary cavity. As a result they lose contact with the ovary cord (which begins to degenerate) and float freely within the hemocoelomic fluid. The oocyte's ooplasm is rich in numerous well-developed Golgi complexes showing high secretory activity, normal and transforming mitochondria, cisternae of rER and vast amounts of ribosomes. The transforming mitochondria become small lipid droplets as vitellogenesis progresses. The oolemma forms microvilli, numerous coated pits and vesicles occur at the base of the microvilli, and the first yolk spheres appear in the peripheral ooplasm. A mixed mechanism of vitellogenesis is suggested. The eggs are covered by a thin vitelline envelope with microvilli projecting through it. The envelope is formed by the oocyte. The vitelline envelope is produced by exocytosis of vesicles containing two kinds of material, one of which is electron-dense and seems not to participate in envelope formation. The cortical ooplasm of fully grown oocytes contains many cytoskeletal elements (F-actin) and numerous membrane-bound vesicles filled with stratified content. Those vesicles probably are cortical granules. The follicle cells surrounding growing oocytes have the following features: (1) they do not lie on a basal lamina; (2) their plasma membrane folds deeply, forming invaginations which eventually seem to form channels throughout their cytoplasm; (3) the plasma membrane facing the ovary lumen is lined with a layer of dense material; and (4) the plasma membrane facing the oocyte forms thin projections which intermingle with the oocyte microvilli. In late oogenesis, the follicle cells detach from the oocytes and degenerate in the ovary lumen.     

An EM autoradiographic study on ovarian follicle cells of Drosophila melanogaster with special reference to the formation of egg coverings

Summary Ovarian follicle cells of Drosophila melanogaster have been studied by ultrastructural and autoradiographic analyses.During their migration through the germarium, follicle cells undergo several structural changes and, of these, the most conspicuous one occurs at the level of the nucleolus. By the time the first ovarian chamber is formed, follicle cells have formed a layer of uniform thickness all around a cluster or nurse cells and the oocyte. Following the initiation of vitellogenesis, the follicle cells overlying the oocyte become columnar while those over the nurse cells become very thin. During stages 9–10, the columnar follicle cells are involved in the formation of the vitelline membrane, while from stages 11 to 13 these cells produce the endochorion.An EM autoradiographic analysis has shown that the rate of ³H-uridine incroporation in follicle cell nuclei is low in previtellogenic chambers, while it becomes very high in nuclei of stage 9–10 chambers. After short exposure to uridine, silver grains are located predominantly over nucleoli.Evidence from incorporation studies with ³H-lysine indicates that the columnar follicle cells and the region of the various egg coverings are highly labelled within an hour of incubation in the tracer.The observations confirm that columnar follicle cells are the only cells in the chamber involved in the formation of materials which make up the egg coverings.This work was partly supported by C.N.R. (Italy)I am indebted to Dr. J. Jacob from the Institute of Animal Genetics (Edinburgh) for introducing me to the use of EM autoradiography     

hold up is required for establishment of oocyte positioning,follicle cell fate and egg polarity and cooperates with Egfr during Drosophila oogenesis.

In Drosophila the posterior positioning of the oocyte within the germline cluster defines the initial asymmetry during oogenesis. From this early event, specification of both body axes is controlled through reciprocal signaling between germline and soma. Here it is shown that the mutation hold up (hup) affects oocyte positioning in the egg chamber, follicle cell fate and localization of different markers in the growing oocytes. This occurs not only in dicephalic egg chambers, but also in oocytes normally located at the posterior. Generation of mosaic egg chambers indicates that hup has to be at least somatically required. Possible interactions of hup with Egfr, the Drosophila epidermal growth factor receptor homolog, have been investigated in homozygous double mutants constructed by recombination. Stronger new ovarian phenotypes have been obtained, the most striking being accumulation of follicle cells in multiple layers posteriorly to the oocyte. It is proposed that the hup gene product is a component of the molecular machinery that leads to the establishment of polarity both in follicle cell layer and oocyte, acting in the same or in a parallel pathway of Egfr.     

Structure of the ovotestis and evidence for heterosynthetic incorporation of yolk precursors in the oocytes of the nudibranch mollusc,Spurilla neapolitana

The ovotestis of Spurilla neapolitana consists of a series of spherical lobes, each of which is composed of radially arranged, sac-like acini or follicles. The male and female portions of each acinus are separated by ovarian follicle cells and testicular accessory cells. A thick basal lamina serves as a barrier between adjacent acini. The surface of each ovotestis lobe is covered by several layers of myoepithelial cells resting on a connective tissue layer. Developing oocytes are intimately associated with follicle cells except in the last stages of vitellogenesis. Follicle cells are characterized by the presence of extensive arrays of rough endoplasmic reticulum (RER) and Golgi complexes and may play a role in vitellogenesis. An ultrastructural analysis of vitellogenesis suggests that oocytes utilize both auto- and heterosynthetic mechanisms of yolk formation. Autosynthetsis is suggested by the activity of the Golgi complex and RER, while heterosynthesis is indicated by high levels of endocytotic activity by the oocyte. Follicle cell development and high endocytotic activity in the oocytes may be a reproductive adaptation to accelerate yolk synthesis, resulting in more rapid egg production.     

Unusual modes of oogenesis and brooding in bivalves: the case of Gaimardia trapesina (Mollusca: Gaimardiidae)

Abstract. I describe an unusual case of follicular oogenesis in a bivalve, Gaimardia trapesina , a common marine bivalve from the Magellan Region and adjacent Sub-Antarctic waters, whose members brood their developing larvae. The gonad in G. trapesina is an acinus organ that infiltrates the perivisceral connective tissue; the walls of the acini are formed by tall, slender cells with distal nuclei, supported by a thin conjunctive tissue layer. At the onset of vitellogenesis, each developing oocyte becomes surrounded by a one-cell-thick layer of follicle cells, which may originate from the wall of the acinus. The cells form a follicle that completely encompasses single oocytes, except at the basal zone, where oocytes are in contact with the acinus wall. The follicle persists beyond the end of vitellogenesis and spawning. After gamete release, the persistent follicle participates in the attachment of ova and developing embryos to the interfilamental junctions of the inner and outer demibranchs of the gill, where embryos are incubated until hatching as late-stage pediveliger larvae. Ripe eggs are large (∼250 μm diameter), suggesting that development is entirely lecithotrophic. The follicle cells that mediate connections between developing embryos and the maternal individual probably have a mechanical role only, providing support and possibly facilitating the accommodation of a large number of embryos to maximize the branchial space available for brooding.     

Cytochemical characterization of the endomembranous system during oocyte secondary growth in Serrasalmus spilopleura (Teleostei, Characiformes, Characidae)

The endomembranous system of Serrasalmus spilopleura oocyte secondary growth was analysed using structural and ultrastructural cytochemical techniques. In vitellogenic oocytes, the endoplasmic reticulum components, the nuclear envelope intermembranous space, some Golgi dictiossomes, lysosomes, yolk granules, regions of the egg envelope and sites of the follicle cells react to acid phosphatase detection (AcPase). The cortical alveoli, some heterogeneous cytoplasmic structures, regions of the egg envelope, and sites of the follicle cells are strongly contrasted by osmium tetroxide and zinc iodide impregnation (ZIO). The endoplasmic reticulum components, some vesicles, and sites of the follicle cells also react to osmium tetroxide and potassium iodide impregnation (KI). The biosynthetic pathway of lysosomal proteins, such as acid phosphatase, required for vitellogenesis, involves the endoplasmic reticulum, Golgi complex, vesicles with inactive hydrolytic enzymes, and, finally, lysosomes. In S. spilopleura oocytes at secondary growth, the endomembranous system takes part in the production of the enzymes needed for vitellogenesis, and in the metabolism of yolk exogenous components (AcPase detection). The endomembranous system compartments also show reduction capacity (KI reaction) and are involved in the metabolism of proteins rich in SH‐groups (ZIO reaction).     

Cytochemical and fine structural analysis of oogenesis in the gastropod, Ilyanassa obsoleta

An analysis of differentiating oocytes of the gastropod, Ilyanassa obsoleta, has been made by techniques of light and electron microscopy. Early previtellogenic oocytes are limited by a smooth surfaced oolemma and are associated with each other by maculae adhaerentes. Previtellogenic oocytes are also distinguished by a large nucleus containing randomly dispersed aggregates of chromatin. Within the ooplasm are Golgi complexes, mitochondria and a few cisternae of the rough endoplasmic reticulum. When vitellogenesis begins, the oolemma becomes morphologically specialized by the formation of microvilli. One also notices an increase in the number of organelles and inclusions such as lipid droplets. During vitellogenesis there is a dilation of the saccules of the Golgi complexes and cisternae of the endoplasmic reticulum. Associated with the Golgi complexes are small protein-carbohydrate yolk precursors encompassed by a membrane. These increase in size by fusing with each other. The “mature” yolk body is a membrane-bounded structure with a central striated core and a granular periphery. At maturity a major portion of the ooplasmic constituents such as as mitochondria and lipid droplets occupy the animal region while the bulk of the population of yolk bodies are situated in the vegetal hemisphere. The follicle cells incompletely encompass the developing oocyte. In addition to the regularly occurring organelles, follicle cells are characterized by the presence of large quantities of rough endoplasmic reticulum and Golgi complexes whose saccules are filled with a dense substance. Associated with the Golgi saccules are secretory droplets of varied size. Amongst the differentiating oocytes and follicle cells are Leydig cells. These cells are characterized by a large vacuole containing glycogen. A possible function for the follicle and Leydig cells is discussed.     

Ultrastructural studies of the formation of the egg capsule in the hermaphroditic species, Isohypsibius granulifer granulifer Thulin, 1928 (Eutardigrada: Hypsibiidae)

The egg capsule of Isohypsibius granulifer granulifer Thulin 1928 (Eutardigrada: Hypsibiidae) is composed of two shells: the thin vitelline envelope and the multilayered chorion. The process of the formation of the egg shell begins in middle vitellogenesis. The I. g. granulifer vitelline envelope is of the primary type (secreted by the oocyte), but the chorion should be regarded as a mixed type: primary (secreted by the oocyte), and secondary (produced by the cells of gonad wall). During early choriogenesis, the parts of the chorion are produced and then connected into a permanent layer. The completely developed chorion consists of three layers: (1) the inner, medium electron dense layer; (2) the middle labyrinthine layer; (3) the outer, medium electron dense layer. After the formation of the chorion, a vitelline envelope is secreted by the oocyte.     

Ultrastructure of oogenesis in the bluefin tuna, Thunnus thynnus

Ovarian ultrastructure of the Atlantic bluefin tuna (Thunnus thynnus) was investigated during the reproductive season with the aim of improving our understanding of the reproductive biology in this species. The bluefin, like the other tunas, has an asynchronous mode of ovarian development; therefore, all developmental stages of the oocyte can be found in mature ovaries. The process of oocyte development can be divided into five distinct stages (formation of oocytes from oogonia, primary growth, lipid stage, vitellogenesis, and maturation). Although histological and ultrastructural features of most these stages are similar among all studied teleosts, the transitional period between primary growth and vitellogenesis exhibits interspecific morphological differences that depend on the egg physiology. Although the most remarkable feature of this stage in many teleosts is the occurrence of cortical alveoli, in the bluefin tuna, as is common in marine fishes, the predominant cytoplasmic inclusions are lipid droplets. Nests of early meiotic oocytes derive from the germinal epithelium that borders the ovarian lumen. Each oocyte in the nest becomes surrounded by extensions of prefollicle cells derived from somatic epithelial cells and these form the follicle that is located in the stromal tissue. The primary growth stage is characterized by intense RNA synthesis and the differentiation of the vitelline envelope. Secondary growth commences with the accumulation of lipid droplets in the oocyte cytoplasm (lipid stage), which is then followed by massive uptake and processing of proteins into yolk platelets (vitellogenic stage). During the maturation stage the lipid inclusions coalesce into a single oil droplet, and hydrolysis of the yolk platelets leads to the formation of a homogeneous mass of fluid yolk in mature eggs.     

A stereological analysis of developing egg chambers in the honeybee queen,Apis mellifera

Summary A polytrophic ovariole of the queen honeybee, Apis mellifera, is composed of a linear series of increasingly mature egg chambers, each consisting of an oocyte, an interconnected cluster of nurse cells, and a covering layer of follicle cells. This study describes changes in the volume of each of these components, as a function of the position of the egg chamber in the ovariole. An oocyte increases in volume from approximately 8.9 × 10³ m³ to approximately 9.6 × 10⁶ m³ over an average series of 20 egg chambers.     

Regulation of growth of nurse nuclei in the development of egg follicles inCecidomyiidae (Diptera)

InCecidomyiidae the number of trophocytes derived from the somatic tissue of the ovary and forming nutritive chambers of egg follicles is variable. The regulation of growth of the whole nutritive chambers and of the nurse nuclei was investigated in two species of the gall midges,Mikiola fagi andBoucheella artemisiae, at two different stages of the egg follicle development during the second period of the oocyte growth. The volume of a nutritive chamber is correlated with the size of the egg follicle as a whole and is not dependent on the number of nurse nuclei it contains. The total volume of nurse nuclei at each stage under investigation was found to have a constant value which is independent of their number. It was established that the growth of the nurse nuclei takes place through endomitosis, and that at a given stage of the egg follicle development the constant value of the total volume of the nurse nuclei reflects the constancy of degree of their total polyploidy. The results obtained indicate that at the early stages of the egg follicle development the rates of growth of the nurse nuclei and of the whole nutritive chambers in the egg follicles differing with respect to the number of their nurse nuclei must be different; the greater the number of nurse nuclei in a given nutritive chamber the slower the rate of growth of the chamber and their nuclei. As a result of this differential rate of growth the volumes of the nutritive chambers and total volumes of nurse nuclei reach at a certain stage of the egg follicle development certain values common for all egg follicles, irrespective of the number of the nurse nuclei they contain. Beginning with this stage the dependence between the endomitotic activity of the nurse nuclei and the rate of growth of the whole nutritive chamber on the one hand, and the number of the nurse nuclei in the chamber on the other, evidently disappears. The available evidence supports the hypothesis that in the egg follicle ofCecidomyiidae the growth regulation of nurse nuclei and, indirectly, also of whole nutritive chambers results from developmental interrelationships between the oocyte and the nutritive chamber, and that the oocyte plays a leading role in this process. In view of a syncytial character of the nutritive chambers inCecidomyiidae and distinctly expressed asynchrony of the growth-duplication cycles of nurse nuclei belonging to a given chamber it is concluded that the control mechanism for DNA synthesis and endomitosis in nurse nuclei must possess the property of a rapid switch. Processes of the growth regulation of the nurse nuclei are discussed in connection with the role of the nutritive chamber in production of RNA and its supply to the growing oocyte. It is suggested that in the egg follicles ofCecidomyiidae there exists a complex interrelationship between the control mechanism for DNA synthesis and endomitosis in the nurse nuclei and the synthetic processes regulated by the supply of the growing oocyte with RNA produced by the nuclei of the nutritive chamber.     

Electrotonic junctions in cecropia moth ovaries.

The steady-state potential of the oocyte, resistance between the ooplasm and the medium, and electronic coupling between oocytes in adjacent follicles were examined in vitellogenic ovarioles of Hyalophora cecropia. The steady-state potential had a constant value of ?40 mV throughout the 100-fold volume increase accompanying yolk deposition, while membrane resistance decreased gradually with increasing size. Resistance rose steeply with the onset of chorion deposition, but did not detectably change with either nurse cell collapse or termination of vitellogenesis. Nonrectified electrical coupling was found between oocytes in adjacent follicles, and fluorescein ions injected into the ooplasm moved readily from follicle to follicle. Large surface area and low membrane resistance made coupling difficult to detect electrically between more mature oocytes, but interfollicular fluorescein migration was found to persist until the end of vitellogenesis. Migration of fluorescein from the oocyte to the follicular epithelium could also be visualized and fingers of ooplasm that cross the vitelline envelope and terminate in dome-shaped attachments to the epithelial cells were implicated in this transfer. The termination of interfollicular coupling coincided with the termination of epithelial-oocyte coupling, and is proposed to result from thickening of the vitelline envelope and withdrawal of the ooplasmic processes.     

东方扁虾卵子发生的超微结构

根据卵细胞的形态、内部结构特征及卵母细胞与滤泡细胞之间的关系,东方扁虾的卵子发生可划分为卵原细胞、卵黄发生前卵母细胞、卵黄发生卵母细胞和成熟卵母细胞等四个时期。卵原细胞胞质稀少,胞器以滑面内质网为主。卵黄发生前卵母细胞核明显膨大,特称为生发泡;在靠近核外膜的胞质中可观察到核仁外排物。卵黄发生卵母细胞逐渐为滤泡细胞所包围;卵黄合成旺盛,胞质中因而形成并积累了越来越多的卵黄粒。东方扁虾卵母细胞的卵黄发生是二源的。游离型核糖体率先参与内源性卵黄合成形成无膜卵黄粒。粗面内质网是内源性卵黄形成的主要胞器。滑面内质网、线粒体和溶酶体以多种方式活跃地参与卵黄粒形成。卵周隙内的外源性物质有两个来源:滤泡细胞的合成产物和血淋巴携带、转运的卵黄蛋白前体物。这些外源性物质主要通过质膜的微吞饮作用和微绒毛的吸收作用这两种方式进入卵母细胞,进而形成外源性卵黄。内源性和外源性的卵黄物质共同参与成熟卵母细胞中富含髓样小体的卵黄粒的形成。卵壳的形成和微绒毛的回缩被认为是东方扁虾卵母细胞成熟的形态学标志。       

Stages of oocyte development in the zebrafish,Brachydanio rerio

Oocyte development has been divided into five stages in the zebrafish Brachydanio rerio, based on morphological criteria and on physiological and biochemical events. In stage I (primary growth stage), oocytes reside in nests with other oocytes (Stage IA) and then within a definitive follicle (Stage IB), where they greatly increase in size. In stage II (cortical alveolus stage), oocytes are distinguished by the appearance of variably sized cortical alveoli and the vitelline envelope becomes prominent. In stage III (vitellogenesis), yolk proteins appear in oocytes and yolk bodies with crystalline yolk accrue during this major growth stage. Ooctes develop the capacity to respond in vitro to the steroid 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) by undergoing oocyte maturation. In stage IV (oocyte maturation), oocytes increase slightly in size, become translucent, and their yolk becomes non-crystalline as they undergo final meiotic maturation in vivo (and in response to DHP in vitro). In stage V (mature egg), eggs (approx. 0.75 mm) are ovulated into the ovarian lumen and are capable of fertilization. This staging series lays the foundation for future studies on the cellular processes occurring during oocyte development in zebrafish and should be useful for experimentation that requires an understanding of stage-specific events. © 1993 Wiley-Liss, Inc.     

ORGANIZATION AND ACTIVITY IN THE PRE- AND POSTOVULATORY FOLLICLE OF NECTURUS MACULOSUS

The established follicle envelope of Necturus maculosus consists of a layer of follicle cells (granulosa) surrounding the developing oocyte, a layer of theca comprised of connective tissue cells, fibers, and matrix, and a layer of serosal cells. The changes in shape and fine structure of these layers during differentiation accompanying oogenesis are described. The cells and capillaries of the follicle envelope are engaged in an extensive pinocytotic activity, the details of which are described. We used cytochemical techniques to analyze the activity of the follicle envelope with respect to lipid accumulation and alkaline phosphatase activity. Radioautographic results indicate that cells of the follicle envelope are capable of incorporating tritium-labeled uridine and amino acids at certain times during oocyte growth. A comparative analysis was made of the soluble proteins in follicle envelopes isolated from immature oocytes and of those in follicle envelopes isolated from nearly mature oocytes and in postovulatory follicles. After the oocyte is ovulated, the cells of the follicle envelope are converted into a postovulatory follicle. The cells of the postovulatory follicle undergo further differentiation resulting in their becoming actively engaged in the formation of a secretion, the details of which are described at the electron microscope level. Analysis of the postovulatory follicle by thin-layer chromatography and cytochemistry demonstrated the presence of a wide variety of lipid substances and the possible presence of steroid. That the postovulatory follicle may be engaged in steroid biosynthesis is also suggested by studies involving the demonstration of 3 β-hydroxysteroid dehydrogenase activity with cytochemical techniques applied to frozen sections and to soluble proteins separated by gel electrophoresis.     

Egg Envelope Cytodifferentiation in the Colonial Ascidian Botryllus schlosseri (Tunicata)

Abstract The formation and cytodifferentiation of egg envelopes were studied at the ultrastructural level in blastozooids of Botryllus schlosseri. The process was divided into five recognized stages of oogenesis. First, the small young oocytes (stage 1) are contacted by scattered cells (primary follicle cells—PFC) which adhere to the oolemma at several junctional spots. PFC extend all around the growing oocyte, acquire polarity, and form a layer covered externally by a thin basal membrane (stage 2). At stage 3 isolated cells are recognizable between the PFC layer and oocyte. They never form junctions with the oocyte and represent prospective inner follicle cells (IFC) and test cells (TC), the latter being progressively received in superficial depressions in the oocyte. The layer of PFC, which maintains junctions with the oolemma, represents prospective outer follicle cells (OFC). PFC are considered to be the source of the three cellular envelopes because a contribution from mesenchymatous elements was not observed. At the beginning of vitellogenesis (stage 4), the vitelline coat (VC) becomes recognizable as a loose net covering the oocyte and TC. It is crossed by the oocyte microvilli and OFC projections which meet and form numerous small junctional plaques, some of them resembling gap junctions. IFC, VC and TC show marked signs of differentiation with approaching ovulation. OFC differentiate completely before ovulation (stage 5) and are engaged in intense synthesis of proteins which may be transferred and taken by endocytosis into the oocyte for yolk formation. Experiments with injected horseradish peroxidase also revealed that proteins present in the blood may reach the oocyte via the intercellular pathway, overcoming OFC and IFC. The possible roles of all the egg envelopes are discussed.     

Differentiation and distribution of annulate lamellae in growing oocytes in the newt, Cynops pyrrhogaster

Stages of oocyte development in Cynops pyrrogaster are defined, and changes of annulate lamellae in their fine structure, number, sizes and locations during oogenesis are described. The results show that two different types of annulate lamellae occur during oogenesis. One type differentiates in or at the periphery of vesicle-rich cytoplasm at the early stages of vitellogenesis and increases in number and size. The maximum number of about 40 stacks per median section of oocyte is reached at the stage of complete differentiation of the animal and the vegetal hemispheres. In these growing oocytes, all the stacks show elongate appearances and tetragonal arrangements of annuli as common characteristics. A second type of stacks of annulate lamellae is added anew in full-grown oocytes, increasing the number of stacks per median section of the oocyte to about 90. The new stacks occur in close contact with electron-dense bodies in the cytoplasm and have a massive appearance and hexagonal array of annuli. It is suggested that they appear coincidentally with the onset of oocyte maturation. The possible significance of the observed results is discussed.     

Formation and structure of egg capsules in scale insects (Hemiptera, Coccinea) I. Ortheziidae

The paired ovaries of the investigated species are composed of 20-30 ovarioles of a telotrophic-meroistic type. Each ovariole is subdivided into an apical tropharium (=trophic chamber) and a vitellarium that contains a single developing oocyte. This oocyte is surrounded by a mono-layered follicular epithelium that is responsible for synthesis of precursors of egg envelopes. In Orthezia, synthesis and secretion of precursors of egg envelopes (=choriogenesis) and accumulation of reserve substances in the oocyte cytoplasm (=vitellogenesis) start at the same time. The egg capsule is composed of two envelopes: an internal, thick vitelline envelope and an external, very thin chorion. The egg surface is covered with numerous, irregularly arranged waxy filaments of spiral shape. Eggs are devoid of the micropylar, aeropylar and hydropylar openings.