Comparative studies on the statoblasts of higher phylactolaemate bryozoans

Statoblasts of five higher phylactolaemates were compared morphologically. As a result, they were divided into two groups: Group I comprising Lophopus crystallinus, Lophopodella carteri, and Pectinatella gelatinosa, and Group II comprising Pectinatella magnifica and Cristatella mucedo. These two groups are thought to represent independent evolutionary series. In Group I and in P. magnifica, the statoblasts are curved to varying degrees after the manner of a saddle. When the dorsal and ventral valves are flattened, therefore, the contour is different between the two. In Group I, the outermost layer of a mature statoblast is hard-gelatinous and basophilic; it remains intact after the statoblast is set free. The statoblast does not float until it is dry, and the float is similar in size on both valves. In Group II, a mature statoblast is covered by a softgelatinous basophilic layer, which decays after the statoblast is released. The statoblast floats without drying, and the float is better developed on the dorsal valve than on the ventral. Moreover, in the members of Group II, large yolk granules are first formed, followed by much smaller yolk granules. When their statoblasts are treated with KOH, the shell is separated completely into two valves. These characters are common to many lower phylactolaemates. By contrast, in L. carteri and P. gelatinosa, the yolk granules are uniformly small and the capsule proper resists KOH treatment. On these points, L. crystallinus is somewhat different from these two species, suggesting its primitive nature.     

Ultrastructural studies on the formation of yolk granules in the statoblast of a fresh-water bryozoan,Pectinatella gelatinosa

The formation of protein-carbohydrate yolk in the statoblast of a fresh-water bryozoan, Pectinatella gelatinosa, was studied by electron microscopy. Two types (I and II) of yolk cells were distinguished. The type I yolk cells are mononucleate and comprise a large majority of the yolk cells. The type II yolk cells are small in number; they become multinucleate by fusion of cells at an early stage of vitellogenesis. In both types of yolk cells, electron-dense granules (dense bodies) are formed in Golgi or condensing vacuoles, which are then called yolk granules. For the formation of yolk granules, the following processes are considered: 1. Yolk protein is synthesized in the rough-surfaced endoplasmic reticulum (RER) of the yolk cells. 2. The synthesized protein condenses in the cisternal space of the RER and is packaged into small oval swellings, which are then released from the RER as small vesicles (Golgi vesicles, 300-600 A in diameter). 3. The small vesicles fuse with one another to form condensing vacuoles, or with pre-existing growing yolk granules. 4. In the matrix of the condensing vacuoles or growing yolk granules, electron-dense fibers are fabricated and then arranged in a paracrystalline pattern to form the dense body. 5. After the dense body reaches its full size, excess membrane is removed and eventually the yolk granules come to mature. Toward the end of vitellogenesis of the yolk cells, the cytoplasmic organelles are ingested by autophagosomes derived from multivesicular bodies and disappear.     

Fine Structure of the Ovarian Development in the Orb-web Spider, Nephila clavata

ABSTRACT Fine structural changes of the ovary and cellular composition of oocyte with respect to ovarian development in the orb-web spider, Nephila clavata were examined by scanning and transmission electron microscopy. Unlike the other arthropods, the ovary of this spider has only two kinds of cells-follicle cells and oocytes. During the ovarian maturation, each oocyte bulges into the body cavity and attaches to surface of the elongated ovarian epithelium through its peculiar short stalk attachments. In the cytoplasm of the developing oocyte two main types of yolk granules, electron-dense proteid yolk and electron-lucent lipid yolk granules, are compactly aggregated with numerous glycogen particles. The cytoplasm of the developing oocyte contains a lot of ribosomes, poorly developed rough endoplasmic reticulum, mitochondria and lipid droplets. These cell organelles, however, gradually degenerate by the later stage of vitellogenesis. During the active vitellogenesis stage, the proteid yolk is very rapidly formed and the oocyte increases in size. However, the micropinocytosis invagination or pinocytotic vesicles can scarcely be recognized, although the microvilli can be found in some space between the oocyte and ovarian epithelium. During the vitellogenesis, the lipid droplets in the cytoplasm of oocytes increase in number, and become abundant in the peripheral cytoplasm close to the stalks. On completion of the yolk formation the vitelline membrane, which is composed of an inner homogeneous electron-lucent component and an outer layer of electron-dense component is formed around the oocyte.     

鹅掌楸油细胞发育过程中超微结构的变化与挥发油产生的关系

鹅掌楸[Liriodendron chinense(Hemsl.)Sargent.]油细胞的发育过程可依据细胞壁的结构变化依次划分为3个阶段,即仅具初生纤维素壁层阶段、木栓质化壁层形成阶段和内纤维素壁层形成阶段。在发育早期,仅具初生纤维素壁层时,油细胞因其体积大,核仁显著,含极少淀粉粒和质体几乎无类囊体而与周围的组织细胞不同。对其3个发育阶段中内部结构变化分析表明,挥发油合成于细胞质和质体中。细胞质中,挥发油就以小滴形式产生,然后逐渐与油囊融合直接贮入油囊,与此同时,在各种细胞器中,质体的变化最为明显,质体中合成的锇物质,随质体解体进入细胞质中,再经转化通过杯形构造积累入油囊。油囊中积累的油经OsO4染色后呈灰色,且分为2层,外层较内层深,推测与油的2种来源有关。     

Demonstration of a phagocytic cell system belonging to the hemopoietic lineage and originating from the yolk sac in the early avian embryo.

It is well established that hemopoietic cells arising from the yolk sac invade the avian embryo. To study the fate and role of these cells during the first 2.5-4.5 days of incubation, we constructed yolk sac chimeras (a chick embryo grafted on a quail yolk sac and vice versa) and immunostained them with antibodies specific to cells of quail hemangioblastic lineage (MB1 and QH1). This approach revealed that endothelial cells of the embryonic vessels are of intraembryonic origin. In contrast, numerous hemopoietic cells of yolk sac origin were seen in embryos ranging from 2.5 to 4.5 days of incubation. These cells were already present within the vessels and in the mesenchyme at the earliest developmental stages analyzed. Two hemopoietic cell types of yolk sac origin were distinguishable, undifferentiated cells and macrophage-like cells. The number of the latter cells increased progressively as development proceeded, and they showed marked acid phosphatase activity and phagocytic capacity, as revealed by the presence of numerous phagocytic inclusions in their cytoplasm. The macrophage-like cells were mostly distributed in the mesenchyme and also appeared within some organ primordia such as the neural tube, the liver anlage and the nephric rudiment. Comparison of the results in the two types of chimeras and the findings obtained with acid phosphatase/MB1 double labelling showed that some hemopoietic macrophage-like cells of intraembryonic origin were also present at the stages considered. These results support the existence in the early avian embryo of a phagocytic cell system of blood cell lineage, derived chiefly from the yolk sac. Cells belonging to this system perform phagocytosis in cell death and may also be involved in other morphogenetic processes.     

Coordinated cell-shape changes control epithelial movement in zebrafish and Drosophila

Epithelial morphogenesis depends on coordinated changes in cell shape, a process that is still poorly understood. During zebrafish epiboly and Drosophila dorsal closure, cell-shape changes at the epithelial margin are of critical importance. Here evidence is provided for a conserved mechanism of local actin and myosin 2 recruitment during theses events. It was found that during epiboly of the zebrafish embryo, the movement of the outer epithelium (enveloping layer) over the yolk cell surface involves the constriction of marginal cells. This process depends on the recruitment of actin and myosin 2 within the yolk cytoplasm along the margin of the enveloping layer. Actin and myosin 2 recruitment within the yolk cytoplasm requires the Ste20-like kinase Msn1, an orthologue of Drosophila Misshapen. Similarly, in Drosophila, actin and myosin 2 localization and cell constriction at the margin of the epidermis mediate dorsal closure and are controlled by Misshapen. Thus, this study has characterized a conserved mechanism underlying coordinated cell-shape changes during epithelial morphogenesis.     

Biochemical and morphological stratification inAcer saccharum root callus

Summary Root callus of sugar maple (Acer saccharum, Marsh.) exhibits differential metabolic activities and cellular morphologies at different depths. A thin layer of acid phosphataseactive cells encloses a second layer of cells that reduce nitroblue tetrazolium (NBT). Mitosis occurs within the cells of this NBT reducing layer (which is one-third of the total depth of the callus), and the cells beneath it show acid phosphatase activity. Morphologically the cells at the outer surface appear tubular and loosely attached, with intact nuclei and cytoplasm. The cells of the NBT region are isodiametric and possess a large vacuole. The cells below the NBT region show, progressive degeneration with depth by loss of nuclei, cytoplasm, and cell wall integrity. There is new cell growth only from the NBT region when aseptically cut callus sections are placed on new medium, regardless of whether or not this region is in direct contact with the agar. Several hypotheses to explain sugar maple callus organization are discussed. This paper is Vermont Agricultural Experiment, Station Journal No. 457.     

胭脂鱼眼早期形态发生研究

采用组织学方法观察了胭脂鱼(Myxocyprinus asiaticus) 眼的发生过程, 结果显示: 胭脂鱼眼的发育经历了眼原基形成期、眼囊形成期、视杯形成期、晶体板形成期、晶体囊形成期、角膜原基形成期、角膜上皮形成期、视网膜细胞增殖期、晶状体成熟期、眼色素形成期以及眼成型期等11个时期。视网膜发育最早, 起始于眼原基的形成, 直至眼成型期分化完成, 形成了厚度不一的8层细胞, 由内向外依次为神经纤维层、神经细胞层、内网层、内核层、外网层、外核层、视杆视锥层和色素上皮层, 且发育历时最长, 约264h。晶状体的发育在视网膜之后, 始于晶体板的形成, 于出膜前期成熟, 发育历时最短, 约74h。角膜发育最晚, 始于角膜原基的形成, 出膜1 d分化为透明的成熟角膜, 发育历时约96h。出膜4 d仔鱼眼色素沉积明显, 视网膜各层分化明显, 晶状体内部完全纤维化, 眼的形态结构基本发育完全。     

Acid phosphatase activity during spore differentiation of the red algaeGigartina teedii andChondria tenuissima

The acid phosphatase activity during carposporogenesis inGigartina and tetrasporogenesis inChondria was studied using the Gomori technique. During the first steps of gonimoblast maturation ofGigartina, portions of cytoplasm are ensheathed by ER cisternae with acid phosphatase activity, giving rise to autolysosomal concentric membrane bodies. In a similar way large mucilage sacs are severed. They extrude their contents in a kind of exocytosis. Multivesicular bodies, concentrically arranged cisternae and extracytoplasmic compartments, each with acid phosphatase activity, remain in young carpospores for some time, probably as remnants of the autophagocytotic and exocytotic events. The Golgi apparatus is poorly developed in gonimoblast cells and young carpospores. It becomes a prominent cell component in maturing carpospores and then participates in cell wall formation. Only some of the dictyosomal cisternae contain acid phosphatase; these are irregularly distributed in the dictyosome. — In pre- and postmeiotic tetraspore mother cells ofChondria massive lead deposits are found in the dictyosomes and in adjacent Golgi vesicles. Finer lead precipitates occur in ER cisternae, especially in those which are sequestering starch-grain-containing portions of the cytoplasm to give rise to autolysosomes. During cell cleavage, the dictyosomes aggregate. They become devoid of acid phosphatase activity with the exception of vesicles at the trans face. Later, Golgi stacks associate and have common, Gomori positively reacting, narrow cisternae at the cis face. The Golgi apparatus derived cored vesicles do not contain lead precipitates whereas the Golgi cisternae in the final stage of tetrasporogenesis show acid phosphatase activity. Variations in acid phosphatase distribution are explained in the light of current models of membrane flow.Dedicated to Univ.-Prof. DrO. Härtel on the occasion of his 80th birthday.     

ULTRASTRUCTURE OF TETRASPOROGENESIS IN THE PARASITIC RED ALGA LEVRINGIELLA GARDNERI (SETCHELL) KYLIN12

Ultrastructural studies on tetraspore formation in Levringiella gardneri revealed that 3 stages may be recognized during their formation. The youngest stage consists of a uninucleate tetraspore mother cell with synaptonemal complexes present during early prophase of meiosis I. Mitochondria are aggregated around the nucleus, dictyosome activity is low, and chloroplasts occur in the peripheral cytoplasm. A 4-nucleate tetraspore mother cell is formed prior to tetrahedral cell cleavage, and an increase in the number of chloroplasts and mitochondria occurs. Small straight-profiled dictyosomes secrete vesicles into larger fibrous vesicles or contribute material to the developing tetraspore wall. During the second stage of tetraspore formation, striated vesicles form within endoplasmic reticulum, semicircular profiled dictyosomes secrete vesicles for fibrous vesicles or wall material, and starch formation increases. The final stage is characterized by the disappearance of striated vesicles, presence of straight, large dictyosomes which secrete cored vesicles, and an abundance of starch grains. Cleavage is usually complete at this stage and the tetraspore wall consists of a narrow outer layer of fibrillar material and an inner, electron transparent layer. These spores are surrounded by a tetrasporangial wall which was the original wall surrounding the tetraspore mother cell.     

不同发育时期哲罗鱼卵黄的超微结构

应用透射电镜观察了不同发育时期哲罗鱼(Hucho taimen)卵黄的超微结构.根据哲罗鱼卵黄物质在卵母细胞中的加工合成、积累以及卵母细胞中参与卵黄颗粒形成的细胞器的变化,可将该鱼卵黄发生分为4个特征时期,即卵黄发生前期、卵黄泡期、卵黄积累期和卵黄积累完成期.卵黄发生前期是指卵母细胞发育过程中的卵黄物质开始积累前的时期,此时期核仁不断分裂,出现线粒体云和早期的滤泡细胞层、基层和鞘细胞层;卵黄泡期特点主要是细胞器不断变化产生卵黄泡和皮层泡;卵黄积累期的滤泡膜由内向外依次为放射带、颗粒细胞层、基层和鞘细胞层,此时外源性卵黄前体物质不断经过血液汇集于鞘细胞层,后经微胞饮作用穿过胶原纤维组成的基层,经过多泡体作用转运至颗粒细胞内,在细胞内经过加工和修饰形成小的卵黄蛋白颗粒,卵黄蛋白颗粒经微胞饮穿过放射带进入卵母细胞边缘形成的空泡中,不断积累形成卵黄球;进入卵黄积累完成期,卵黄球体积变大,向细胞中心聚集,填满大部分卵母细胞,卵黄积累完毕.     

Macrophages of hemangioblastic lineage invade the lens vesicle-ectoderm interspace during closure and detachment of the avian embryonic lens

Summary An area of cell death is apparent in the lens vesicle margin and the lens stalk during closure and detachment of the lens anlage from the cephalic ectoderm. Free phagocytic cells closely associated with this area of cell death have been interpreted as cells migrating from the lens epithelium. Scanning and transmission electron microscopy, light-microscopic histochemical staining for acid phosphatase and immunostaining using MB1 (a monoclonal antibody specific for quail endothelial and hemopoietic cells) of chimeras of chick embryo and quail yolk sac were used to analyze these lens vesicle-associated free phagocytic cells. The cells have morphological features identical to those of macrophages in other embryonic tissues. In contrast to epithelial cells phagocytosing cell debris, they exhibit strong acid phosphatase activity, a feature typical of macrophages. In addition, free phagocytic cells are MB1 positive in chick embryo-quail yolk sac chimeras, hence they proceed from cells of hemangioblastic lineage originating in the yolk sac. These results indicate that the lens vesicle-associated free phagocytic cells are macrophages. Observations of MB1 positive amoeboid cells in the juxta-retinal mesenchyme and on the borders of the optic cup suggest that these macrophages migrate through the mesenchyme surrounding the eye primordium. Macrophages are seen in both the interspace between lens vesicle and ectoderm and in the lumen of the lens as well as within both the ectoderm and the lens epithelium. In these locations they remove cell debris, and thereby contribute to the complete disappearance of the area of cell death. Macrophages remain in the lens vesicle-ectoderm interspace until developmental stages at which it is invaded by corneal endothelial cells.     

Oogenesis: From Oogonia to Ovulation in the Flagfish,Jordanella floridae Goode and Bean, 1879 (Teleostei: Cyprinodontidae)

We provide histological details of the development of oocytes in the cyprinodontid flagfish, Jordanella floridae. There are six stages of oogenesis: Oogonial proliferation, chromatin nucleolus, primary growth (previtellogenesis [PG]), secondary growth (vitellogenesis), oocyte maturation and ovulation. The ovarian lamellae are lined by a germinal epithelium composed of epithelial cells and scattered oogonia. During primary growth, the development of cortical alveoli and oil droplets, are initiated simultaneously. During secondary growth, yolk globules coalesce into a fluid mass. The full‐grown oocyte contains a large globule of fluid yolk. The germinal vesicle is at the animal pole, and the cortical alveoli and oil droplets are located at the periphery. The disposition of oil droplets at the vegetal pole of the germinal vesicle during late secondary growth stage is a unique characteristic. The follicular cell layer is composed initially of a single layer of squamous cells during early PG which become columnar during early vitellogenesis. During primary and secondary growth stages, filaments develop among the follicular cells and also around the micropyle. The filaments are seen extending from the zona pellucida after ovulation. During ovulation, a space is evident between the oocyte and the zona pellucida. Asynchronous spawning activity is confirmed by the observation that, after ovulation, the ovarian lamellae contain follicles in both primary and secondary growth stages; in contrast, when the seasonal activity of oogenesis and spawning ends, after ovulation, the ovarian lamellae contain only follicles in the primary growth stage. J. Morphol. 277:1339–1354, 2016. © 2016 Wiley Periodicals, Inc.     

Cellular Events of Wrinkled Blastula Formation and the Influence of the Fertilization Envelope on Wrinkling in the Seastar Patiriella exigua

Like many echinoderms, the seastar, Patiriella exigua has a wrinkled blastula rather than the smooth-walled blastula typical of most phyla. The cellular events of wrinkled blastula formation in P. exigua were documented using light, confocal and electron microscopy. Wrinkled blastulae have a highly infolded epithelium. Prior to wrinkling, the blastomeres are cuboidal with lipid droplets and yolk granules distributed throughout their cytoplasm. During wrinkling, the cells become columnar and the lipid and yolk reserves become redistributed to the basal and apical ends of the cells, respectively. Gastrulae have a tall columnar epithelium, with a basal accumulation of lipid. Interdigitation of numerous cell projections, including short lateral processes, basal lamellipodia and apical filopodia, assists in maintaining epithelial integrity during wrinkling. Apical filopodia have not been observed in other echinoderm embryos. Although 1 M urea caused elevation of the fertilization envelope, the embryos did not expand into the newly-created space. This is suggested to be due to the adhesive properties of the hyaline layer. Embryos removed from their envelope were enlarged with shallower and fewer wrinkles compared with controls. It appears that the integrity of the hyaline layer and fertilization envelope both influence the compact wrinkled profile of P. exigua blastulae.     

A fine structural study of cytodifferentiation during cleavage, blastula, and gastrula stages of Fundulus heteroclitus

The fine structure of cleavage, blastula, and gastrula stages of Fundulus heteroclitus was investigated. Cleavage blastomeres are relatively unspecialized, containing few or poorly developed organelles. Beginning in blastula stages, signs of differentiation were noted, including development of the endoplasmic reticulum and Golgi apparatus and appearance of a primary nucleolus and polyribosomes. More extensive structural specializations occur in gastrula stages, including further development of the endoplasmic reticulum and appearance of a granular component in the nucleolus. These changes are associated with cell differentiation and an increased capacity for protein synthesis, and may be preparatory to subsequent histogenesis. The periblast is a continuous syncytial cytoplasmic layer located between the blastodisc and yolk and is formed during late cleavage by incomplete division of the cytoplasm of the blastodisc. Cytoplasmic projections extend from the periblast (and from the basal region of cleavage blastomeres prior to formation of the periblast) into the yolk and function in uptake of yolk material in the absence of pinocytosis. Yolk material appears to be digested by the periblast and transferred into the segmentation cavity where it is available to the blastomeres. Protein granules, lipid droplets, glycogen, crystalline arrays, and multivesicular bodies are related to food storage and utilization by blastomeres. The yolk gel layer enclosing the yolk sphere was found to be a thin layer of cytoplasm continuous with the margin of the periblast and is renamed the yolk cytoplasmic layer.     

THE ULTRASTRUCTURE AND CYTOCHEMISTRY OF RESTING CELL FORMATION IN AMPHORA COFFAEFORMIS (BACILLARIOPHYCEAE)1

The ultrastructure of logarithmic-growing cells and of resting cells in laboratory cultures of Amphora coffaeformis (Ag.) Kütz. isolated from deep ocean water was examined using electron and light microscopy. The acid Phosphatase activity, chlorophyll a and lipid content were assessed at weekly intervals of resting cell formation during cold-dark treatment, simulating deep ocean water. Approximately 4 wk are required to complete resting cell formation. During the first week, the cytoplasm undergoes extensive transformation and lysosomal activity is observed. Large vacules decrease in size and many small ones develop, the mitochondria become fewer and one or more massive mitochondria appear possibly by fusion of smaller ones; the cytoplasm becomes densely granular. During the second and third week, the cytoplasm continues to contract, lipid bodies begin to develop and the plastid becomes densely stained. At the fourth week, the mature resting cell is formed containing one or more massive mitochondria, a well-formed plastid, and granular cytoplasm containing occasional lipid droplets. There is no change in frustule morphology and the cytoplasm does not produce a protective layer. The variation in chemical constituents correlates with microscopic structure of the cells. The fine structure of cells during growth resumption when exposed to light at 25 c is presented. Previous reports of viable, chlorophyll-containing cells at great depths in the ocean may be explained by the results reported in this paper.     

Ultrastructural and histochemical study of previtellogenic oogenesis in the desert lizard Scincus mitranus (Squamata,Sauropsida)

The structure of the granulosa in reptilian sauropsids varies between groups. We investigated the follicle development in the desert lizard Scincus mitranus. In the germinal bed, oogonia, and primary oocytes were identified and found to be interspersed between the epithelial cells. Previtellogenesis was divided into three stages: early, transitional, and late previtellogenic stages. During the early previtellogenic stage (diplotene), the oocyte is invested by small epithelia cells that formed a complete single layer, which may be considered as a young follicle. The transitional previtellogenic stage was marked by proliferation and differentiation of the granulosa layer from a homogenous layer consisting of only small cells to a heterogeneous layer containing three cell types: small, intermediate, and large cells. The late previtellogenic stage was marked by high-synthetic activity of large cells and the initiation of cytoplasmic bridges between large granulosa cells and the oocyte. Small cells were the only type of granulosa cells that underwent division. Thus, these cells may be stem cells for the granulosa cell population and may develop into intermediate and subsequently large cells. The intermediate cells may be precursors of large cells, as suggested by their ultrastructure. The ultrastructure of the large granulosa was indicative of their high synthetic activity. Histochemical analysis indicated the presence of cholesterol and phospholipids in the cytoplasm of large cells, the zona pellucida, among the microvilli, in the bridges region, and in the cortical region of the oocyte cytoplasm. These materials may be transferred from large cells into the oocyte through cytoplasmic bridges and provide nutritive function to large cells rather than functioning in steroidogenesis or vitellogenesis.     

The histology and histochemistry of oogenesis in the water strider, Gerris remigis Say

In each ovariole of Gerris remigis, nurse cells arise by mitotic divisions at the anterior end of the germarium. These cells enlarge as they move posteriorly. This size increase is possibly caused by fusion of cells, but probably by endopolyploidy as well. The nurse cells then establish connections with a central trophic core, which receives the products of subsequent nurse cell degradation. Two possible pathways of nuclear degradation are suggested: one involves the condensation of chromatin within the nucleus; the other, the release of DNA as fine granules into the cytoplasm. Cytoplasmic areas containing such DNA are also rich in proteinaceous granules, but have a meager content of RNA. The remainder of the cytoplasm of the mature nurse cells contains a high concentration of RNA, as do the nucleoli. Posteriorly the trophic core connects via nutritive cords with each developing oocyte in the prefollicular region and in the anterior vitellarium. RNA is apparently contributed to the ooplasm via the trophic stream. Patches of cytoplasmic DNA are present in the young oocytes; the origin and fate of this DNA is uncertain. During early oocyte maturation chromosomal stainability decreases, and the nucleolus enlarges. In previtellogenic stages, numerous proteinaceous bodies appear in association with the nucleolus-chromosome complex. These bodies, like the nucleolus, have only a low RNA content. They may pass to the cytoplasm, but cannot be traced with certainty. During the latter part of this period a complex population of small proteinaceous and lipid preyolk bodies accumulates peripherally in the oocyte. Definitive protein and lipid yolk are probably derived by the enlargement and inward migration of these bodies. The oocytes are each surrounded by a layer of follicle cells proliferated in the prefollicular region. These become binucleate and enlarge as the enclosed oocytes grow and elongate. RNA also increases in the nucleoli and cytoplasm of the follicle cells as they move posteriorly in the vitellarium. There is no evidence of transfer of nucleic acids or protein from the follicle cells to the oocyte. The nurse cells are therefore implicated as the major source of nucleic acids for the maturing oocyte.     

ULTRASTRUCTURE AND DEVELOPMENT OF THE MICROSPORANGIUM OF PSEUDOTSUGA MENZIESII (MIRB.) FRANCO. I. DORMANT STAGES

The dormant (mid-November to mid-February) microsporangia of Pseudotsuga menziesii (Douglas-fir) contain pollen mother cells (PMC's) in diffuse diplotene, surrounded by 1–2 layers of tapetal cells and 3–4 layers of microsporangial wall cells. At the beginning of dormancy, PMC's are large and their walls are lysed. The cell walls contain a thick layer of loosely-arranged fibrils which are produced in large vesicles in the PMC cytoplasm and are secreted across the plasma membrane. PMC's contain several layers of rough ER. The inner tangential and the radial walls of the tapetal cells are lysed. During dormancy the PMC's form many new autophagic vacuoles, the chromatin consists of a network of fine threads comprised of medium-sized granules of uniform size and the nucleoli split. The outer tapetal wall is thick and becomes encrusted by an irregular lipid layer. The tapetal cytoplasm is similar to the PMC cytoplasm but is devoid of amyloplasts. The tapetal cytoplasm shows secretory activity at the beginning of dormancy and again near the end of dormancy. The later secretory activity results in the deposition of a spongy material, especially along the radial and inner walls of the tapetal cells. Tapetal cells contain 1–2 large nuclei which show prominent and irregular clumps of chromatin. Subcellular developmental changes occur in the dormant microsporangia of Pseudotsuga in much the same manner as has been reported for Pinus.