Intracellular translocation of symbiotic bacteroids during late oogenesis and early embryogenesis of Bradysia tritici (syn. Sciara ocellaris) (Diptera: Sciaridae)

Abstract. The distribution of symbiotic prokaryotes (bacteroids) in ovarian follicles and young embryos of Bradysia ( Sciara ) was studied using light and electron microscopy. In mid-vitellogenic follicles (prior to oosome formation) isolated from 8-h-old midges, most symbionts were scattered in the ooplasm. Later, during oogenesis and concomitant with the formation of the oosome, symbionts aggregated at the posterior pole of the follicle. During early embryogenesis, large symbiont clusters were seen between the oolemma and oosome and lateral to the oosome. When the presumptive pole-cell nuclei moved into the oosome, the bacteroids became scattered in and around the prospective germ plasm, and many of them became incorporated into the pole cells. At the anterior pole, a nearly symbiontfree area could be recognized at first (anterior cone), but later on symbionts aggregated there (usually best seen in 1-h embryos). Thereafter, smaller symbiont clusters spread into more lateral parts of the thickened anterior cortex. Finally, the aggregates became dispersed when cleavage nuclei moved into the anterior egg cortex. The distribution of symbionts may reflect cytoplasmic streaming or other transport phenomena during development which could participate in oosome formation or in the histological differentiation of the anterior egg cortex.     

Die Ooplasmabewegungen während der Furchung vonPimpla turionellae L. (Hymenoptera), eine Zeitrafferfilmanalyse

Summary In the eggs ofPimpla turionellae, which are characterized by a long germ anlage (long-germ egg type), the cleavage nuclei primarily populate the anterior part and only later appear in the posterior of the egg lumen during the intravitelline cleavage. Gastrulation and segmentation also start within this anterior region. Time-lapse motion pictures served to observe and to check quantitatively even slow movements during cleavage and blastogenesis. In motion diagrams made by means of microkymographic technics the flow within the ooplasm along the longer axis of the egg has been timed.Shortly before the first cleavage in thestrictly unfertilized male eggs a short-timeunipolar flow sets in from a primary initial region at 90% of their length. Thus a pillar of central plasm between both of the poles becomes shifted towards the posterior, while its outer coating layer of marginal-plasm is displaced forwards by the same distance. In eggs from fertilized females two successive flows of the same unipolar type have been observed.At the end of the third cleavage the energids, heretofore loosely grouped together, become distributed within the central plasm to form a nuclear column. At the same time a fluently pulsatory bipolar flow sets in, within asecondary initial region at 80% of the egg length. Comparable to two mirror-image fountains, parts of the central plasm are carried towards the front pole and to the rear pole of the egg, respectively, while the marginal plasm, together with the oolemma, flows in opposite directions at times. With each pulsation the moving areas of the bipolar flow are shifted more and more towards the egg poles. The occurrence of bipolar flow pulsations, amounting to five, is correlated with the nuclear divisions in a still unknown way. In the rhythm of the bipolar flow, the energids become dispersed within the central plasm with a certain spatial lagging.After the bipolar flow has come to a halt, four further cleavages are indicated by faint local pulsations of the ooplasm. The cleavage nuclei move to the egg surface and pole cells become separatedtied off During blastoderm formation another four faint pulsations are observed, especially within the central ooplasm, all of them clearly synchronized with superficial cleavages. Occurring in mitotic waves, these cleavages indicate a third initial region, with the individual position varying between 10 and 28% of the egg length.Furthermore the technics of time-lapse motion pictures permit a local and temporal determination of extravitelline pole space formation, of a ring-shaped contracted region of slightly thickening periplasm within the secondary initial region, and the dislocation of the oosome towards the egg surface, which results from the activity of the posterior fountain during the phase of bipolar flow. Invagination and segmentation of the embryo become distinct within the secondary initial region, thus identifying this region as a differentiation centre.The correlation of plasm flow and nuclear divisions is discussed as well as the correlation of the initial regions to the different patterns of egg architecture in the longgerm egg type. The correlation between bipolar pulsations and the development of the metameric pattern including the function of the oosomal region is also discussed. The ooplasmic movements as known from egg types other thanPimpla are compared to the above observations.

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Durchgeführt mit Hilfe der Leihgaben von der Deutschen Forschungsgemeinschaft und von Euratom (Verträge Nr. 041-65-10 BIOD, Nr. 077-69-1 BIOC mit Heiligenberg-Institut). Wir danken Frau Dr. C. Meng und Frl. Dipl. Biol. E. Bruhns für die freundliche Überlassung von Daten und Zeichnungen zur normalen Entwicklung desPimpla-Eies und damit für einen wesentlichen Beitrag zum Verständnis unserer Befunde.     

Experimental test fails to confirm gradient interpretation of embryonic patterning in leafhopper eggs

Posterior pole material was incorporated into middle egg fragments of standard length but different mean position along the longitudinal egg axis. Middle egg fragments of rather posterior location formed only posterior pattern elements, i.e., thoracic and abdominal segments. More anterior middle egg fragments, on the other hand, were able to form complete embryos. The results are at variance with basic suppositions of a recent model of pattern formation in the Euscelis egg.     

Subcortical microtubule network separates the periplasm from the endoplasm and is responsible for maintaining the position of accessory nuclei in hymenopteran oocytes

Oocytes of hymenopterans are equipped with peculiar organelles termed accessory nuclei. These organelles originate from the germinal vesicle (oocyte nucleus) and gather preferentially at the anterior pole. To gain insight into the mechanism of uneven (asymmetrical) distribution of accessory nuclei, the organization of the microtubule cytoskeleton in the oocytes of two hymenopterans Chrysis ignita and Cosmoconus meridionator has been studied. It is shown that during late previtellogenesis two networks of microtubules are present along the contact zone between the oocyte and enveloping follicular epithelium. The external one is associated with belt desmosomes connecting neighbouring follicular cells. The internal network is composed of randomly orientated microtubules and separates transparent, organelle-free periplasm from the endoplasm. All cellular organelles and the germinal vesicle are localized in the endoplasm. Accessory nuclei are accumulated in the anterior endoplasm; they always lie in direct contact with the subcortical network. Treatment with colchicine results in the disappearance of the periplasm as well as in the redistribution of cellular organelles including accessory nuclei. Presented findings suggest that subcortical microtubules play an important role in the positioning of accessory nuclei throughout the ooplasm.     

External features of the developing embryo of the stonefly,Kamimuria tibialis (pictet) (plecoptera,perlidae)

External features of the egg, developing embryo, and first instar nymph of Kamimuria tibialis are described. The embryonic development from the germ disc to the full-grown embryo is divided into 12 stages. The saclike embryonic rudiment is formed by the bending and folding of the germ disc. The embryo first elongates at the egg surface and then sinks into the yolk due to caudal flexure. In the head, four paired protocerebral lobes differentiate and the fourth lobes are thought to be the rudiments of preantennal ganglia. The columnar serosal cells appear at the posterior pole of the egg and they disappear before katatrepsis. The coniform chloride cells occur at the hind margins of the first nine abdominal segments in the full-grown embryo and first instar nymph. Amnion formation in K. tibialis is very similar to that of Allonarcys proteus and the Isoptera. It is proposed that the immersed type of growth pattern of embryos is divided into two subtypes in hemimetabolous insects; one is in the Palaeoptera and Paraneoptera, and the other is in the Plecoptera, Orthoptera, Notoptera, Isoptera, Embioptera, and the blattarian, Periplaneta americana.     

Germ cell development in the Honeybee (Apis mellifera); Vasa and Nanos expression

Background  

Studies of specification of germ-cells in insect embryos has indicated that in many taxa the germ cells form early in development, and their formation is associated with pole plasm, germ plasm or an organelle called the oosome. None of these morphological features associated with germ cell formation have been identified in the Honeybee Apis mellifera. In this study I report the cloning and expression analysis of Honeybee homologues of vasa and nanos, germ cell markers in insects and other animals.     

Pattern formation fails after blastoderm formation by rapid cell cycles in an artificially activated insect egg

Oocytes explanted from adult ovaries of the arrhenotokous Hymenopteron Pimpla turionellae remain in an inactive state, because development has not been initiated by mechanical deformation during natural oviposition. However, they could be induced to enter development by injecting cleavage energids into the posterior pole. After lag phases of up to 32 h, the implanted nuclei initiated a normal cleavage process, except that the polarity of its progress was reversed. In other oocytes, the injected energids congregated in a ring-shaped region at the egg surface to form a superficial nuclear front, which slowly advanced towards the anterior egg pole, thereby successively stimulating portions of the quiescent ooplasm to take part in development. Up to 41 rapid cell cycles started from that front, each of them with an anaphase wave running backwards into the region already peripherally occupied by nuclei. Thus, the blastoderm was formed extremely metachronously and by rapid obviously biphasic cell cycles, which never occur at the egg surface during normal cleavage. A germ band, however, was only formed under the following conditions: (1) that cleavage did not follow the nuclear front mode, and (2) that ooplasm from the donor's posterior pole was co-injected with the graft nuclei. We conclude that embryonic differentiation requires some of the events which had been omitted in eggs where development failed, especially the exponential increase of the cell cycle length, and the activity of some posterior factor(s) during egg activation.     

Pattern formation in fragmented eggs of the short germ insect Schistocerca gregaria

Summary The effect of transverse fragmentation on the segment pattern of the short germ embryo of the locust Schistocerca gregaria has been investigated at two stages subsequent to the formation of the germ anlage. Following fragmentation both anterior and posterior partial embryos were observed, although rarely in a single egg. Anterior partial patterns usually terminated with a segment visible at the time of fragmentation or with the next segment due to appear. Posterior partial patterns began with a wide range of segments depending on the level of fragmentation.Anterior and posterior partial patterns developing in a single egg were usually not complementary and the segments missing sometimes included some segments visible when the embryo was fragmented. Non-complementary patterns resulted following fragmentation in all regions, while complementary patterns only occurred after fragmentation in the visibly-segmented region.The results suggest that following fragmentation isolated posterior portions of the embryo continue to form segments, while isolated anterior regions usually do not. This effect could result from variable damage to an existing pattern of unequally-sized segment primordia, or from the disruption of a process of sequential segmentation in the elongating posterior region of the embryo. The results are broadly compatible with the progress zone model proposed by Summerbell et al. (1973).     

Development of the embryonic segment pattern in the pea-beetle Callosobruchus maculatus. Supracellular,cellular and molecular aspects

Summary

This review deals with the question of how cells in the early embryo of the pea-beetle differentiate into a sequential pattern of segments. Anterior and posterior fragments of an egg have different options for development depending on whether they are exposed, before cellularization, to decaying ooplasm in the complementary fragment. Without such exposure all fragments produce fewer segments than corresponding fragments obtained at cellularization. With exposure a fraction of anterior and posterior fragments produces considerably more segments than corresponding fragments obtained at cellularization. In addition, posterior fragments are uniquely different from anterior ones in that they also produce reversal of segment sequence which can be restricted to longitudinal strips of the larval cuticle.

The difference in reaction to decaying ooplasm between anterior and posterior fragments suggests an asymmetry in the control of metamerization. Lateral inhibition by an asymmetric gradient of a diffusible morphogen can describe these observations [18] except for the restriction of reversal to longitudinal strips. The latter requires either that morphogen transport be polarized, possibly by a voltage gradient in the egg, or that the interpretation of cell position is polarized. The induction of double abdomens with UV-light and RNase suggests that RNA is part of the control mechanism. This and strip-restricted reversal are features shared by eggs of Coleoptera and Diptera.     

Inhibition by ultraviolet light of pole cell formation in Smittia sp (Chironomidae,Diptera): Action spectrum and photoreversibility

The formation of pole cells (primordial germ cells) in Smittia sp can be inhibited by ultraviolet (uv) irradiation without causing significant mortality. Until 70 min after egg deposition, pole cells are suppressed by low uv doses applied to the posterior pole region. Microbeam irradiation of a target area including the oosome inhibits pole cell formation; this is not observed after irradiation of other target areas. The action spectrum for uv inhibition of pole cells shows a distinct peak at 260 nm; its shape suggests that a nucleic acid or nucleic acid-protein complex acts as an effective target. Independent evidence for the involvement of a nucleic acid moiety is derived from the fact that uv inhibition of pole cell formation is photoreversible. The results are discussed in the context of pole cell determination by localized cytoplasmic components.     

Oosome formation in two ichneumonid wasps

During oogenesis of two hymenopteran species. Cosmoconus meridionator and Lissonota catenator, the place of origin and the ways of transport of two oosome components have been shown. The first constituent is produced in the trophocytes cytoplasm as the nuagemitochondria complexes (NM complexes). The NM complexes subsequently are transported via intercellular bridges to the oocyte and along the oolemma to its posterior pole. At the same time, near the posterior pole, the other structures connected with the oosome formation appear. Most probably they are produced in the ooplasm and have the appearance of nuage of moderate density (NMD). The two structures are composed of proteins and do not contain RNA (toluidine blue test). The NM complexes as well as the NMD disappear as the definite oosome forms at the posterior pole of the oocyte. At the same time the oosome becomes RNA positive. It is suggested that the oocyte nucleus is not involved in the process of the oosome formation.     

Genetic analysis of pattern-formation in the embryo ofDrosophila melanogaster

Summary The mutationbicaudal (Bull, 1966) causes embryos to develop a longitudinal mirror image duplication of the posteriormost abdominal segments, while head and thorax are missing. These embryos occur with varying frequencies among eggs laid by mutant females, irrespective of the paternal genotype. Recombination and deletion mapping indicate thatbicaudal (bic) is a recessive, hypomorphic, maternal-effect mutation mapping at a single locus on the second chromosome ofDrosophila melanogaster close tovg (67.0±0.1). The frequency of bicaudal embryos depends on the age of the mother, her genetic constitution and the temperature at which she is raised. Best producers are very young females hemizygous forbic (bic/Df(2)vg ^B ) at 28° C. Under these conditions 80% to 90% of the eggs which differentiate can show the bicaudal embryo phenotype. Upon ageing of the mother the frequency of bicaudal embryos declines rapidly, and most of the eggs develop the normal body pattern. Temperature shift experiments suggest a temperature-sensitive period at the onset of vitellogenesis.The mutation causes several types of abnormalities in the segment pattern of theDrosophila embryo, which are interpreted as various degrees of expression of the mutant character. The most frequent abnormal phenotype is the symmetrical bicaudal embryo with one to five abdominal segments duplicated. Less frequent are asymmetrical types, in which the smaller number of segments is always in the anterior reversed part. Other phenotypes are embryos with missing or rudimentary heads, and embryos with irregular gaps in the segment pattern. In bicaudal embryos, the pole cells, formed at the posterior pole of the egg prior to blastoderm formation, are not duplicated at the anterior. The significance of thebicaudal phenotypes for embryonic pattern-formation inDrosophila is discussed.     

Maternal information and localized maternal mRNAs in eggs and early embryos of the ascidian Halocynthia roretzi

Summary

The mosaic behavior of blastomeres isolated from ascidian embryos has been taken as evidence that localized ooplasmic factors (cytoplasmic determinants) specify tissue precursor cells during embryogenesis. Experiments involving the transfer of egg cytoplasm have revealed the presence and localization of various kinds of cytoplasmic determinants in eggs of Halocynthia roretzi. Three cell fates, epidermis, muscle and endoderm, are fixed by cytoplasmic determinants. The three kinds of tissue determinants move in different directions during ooplasmic segregation. Prior to the onset of the first cleavage the three kinds of determinants reside in egg regions that correspond to the future fate map of the embryo and then they are differentially partitioned into specific blastomeres. In addition to tissue-specific determinants, there is evidence suggesting that ascidian eggs contain localized cytoplasmic factors that are responsible for controlling the cleavage pattern and morphogenetic movements. Transplantation of posterior-vegetal egg cytoplasm to an anterior-vegetal position causes a reversal of the anterior-posterior polarity of the cleavage pattern. Localized cytoplasmic factors in the posterior-vegetal region are involved in the generation of a unique cleavage pattern. When vegetal pole cytoplasm is transplanted to the animal pole or equatorial position of the egg, ectopic gastrulation occurs at the site of transplantation. This finding supports the idea that vegetal pole cytoplasm specifies the site of gastrulation. Recently, we started a cDNA project to analyze maternal mRNAs. An arrayed cDNA library of fertilized eggs of H. roretzi was constructed, and more than 2000 clones have been partially sequenced so far. To estimate the proportion of the maternal mRNAs that are localized in the egg and embryo, 150 randomly selected clones were examined by in situ hybridization. We found eight mRNAs that are localized in the eight-cell embryo, of which three were localized to the myoplasm (a specific region of the egg cytoplasm that is partitioned into muscle-lineage blastomeres) of the egg, and then to the postplasm of cleavage-stage embryos. These results indicate that the proportion of localized messages is much higher than we expected. These localized maternal messages may be involved in the regulation of various developmental processes.     

Changes in the apparent localization of anterior determinants during early embryogenesis (Smittia spec., Chironomidae,Diptera)

Summary InSmittia embryos, anterior UV irradiation causes the formation of double abdomens in which head and thorax are replaced by a mirror-image duplication of abdominal segments. The transformation is ascribed to the inactivation of cytoplasmic ribonucleoprotein particles acting as anterior determinants. During the first 5 h after egg deposition (h.a.d.), high yields of double abdomens are produced by microbeam irradiation of a small target area (diameter 20 m) behind the anterior pole. This localization coincides with a yolk-free cytoplasmic cone observed behind the anterior pole of newly depositedSmittia eggs (Zissler and Sander 1973). Irradiation of this area becomes less efficient between 5 and 7 h.a.d., while double abdomen yields increase after irradiation of adjacent target areas. A dramatic increase in the double abdomen yield is also observed after irradiation of the entire anterior surface. Under these conditions, the UV fluence required for a given yield decreases by a factor of 3.9 between 5.5. and 7 h.a.d. This can be explained quantitatively by the assumption that the effective targets move an average distance of 8 from the strongly absorbing endoplasm towards the egg surface. During the period of apparent target deshielding, the distance of nuclei from the egg surface actually decreases from 11 to 3 m. However, the timing and direction of the nuclear migration and of the apparent deshielding differ, indicating that the shift of effective targets may be triggered by, but does not exactly follow, the nuclear movement. The data suggest that a major fraction of anterior determinants shifts from a small volume behind the anterior pole to a spread-out distribution over the surface of the anterior half.     

Double abdomen induction by UV in Bradysia tritici (syn. Sciara ocellaris,Sciaridae): sensitive stages and conditions for photoreversal

Summary The pattern anomaly double abdomen was induced in embryos of Bradysia tritici (syn. Sciara ocellaris) by irradiation of the anterior egg pole with far UV (254 or 285 nm) using low UV fluences. The maximum yield of 18% of double abdomens was obtained when 2.5 h embryos were irradiated (late intravitelline cleavage stage); earlier irradiation failed to yield double abdomens, as did irradiations after the early syncytial blastoderm stage. Exposing irradiated embryos to photoreverting light (366 nm) reduced the yield of malformations. Most double abdomens were symmetrical and the number of segments ranged from 3 to 8 in each set, with the mean value at 6.4 segments.     

Determination in Drosophila Embryos

SYNOPSIS. In this review we describe data of experiments whichinterfere with the formation of the metameric pattern duringembryogenesis. Ligating embryos before blastoderm stage leadsto a gap in the segmentation pattern of the differentiated embryo.The gap can extend up to 6 segments but terminal segments arealways recognizable. In posterior but not in anterior fragmentswe find abnormally large but fewer segments. This increase insegment size results from a different determination of blastodermcells after ligation. During nuclear multiplication stages whena gap can be produced, the zygotic genome is not yet active.Information to develop the metameric pattern in ligated embryosmust therefore have been made during oogenesis. Recently Nüisslein-Volhard and Wieschaus (1980) have describedthree zygotic mutations which form embryos with a gap of segmentssimilar to our ligated embryos. We have discussed these mutantphenotypes in connection with our experimental data. Segmentation is controlled at several levels. During oogenesisthe anterior-posterior and dorsal-ventral axes become established(Nüsslein-Volhard, 1979). Also during oogenesis, but extendinginto early embryonic life, information is generated to subdividethe embryo into blocks of cells forming the metameric pattern.At blastoderm the identity of segments becomes established.     

The embryonic development of the primitive moth,Neomicropteryx nipponensis Issiki (lepidoptera,micropterygidae): Morphogenesis of the embryo by external observation

The micropterygid moth Neomicropteryx nipponensis belongs to the most primitive suborder Zeugloptera of the Lepidoptera. During embryogenesis the small circular germ disk formed on the ventral egg surface invaginates deeply into the yolk. It finally separates from the egg periphery or rudimentary serosa, and becomes a sac-shaped germ rudiment. Its anterior part later develops into the germ band, while its posterior part is the future amnion. Just before revolution of the embryo, the embryo assumes a completely superficial position beneath the yolk. Neither amnion nor serosa rupture during revolution; by completion of dorsal closure they have been incorporated into the yolk to form the secondary dorsal organ. The formation of the germ rudiment and embryonic membranes in N. nipponensis resembles those of swift moths, Endoclyta (suborder Monotrysia) and of the caddisflies, Stenopsyche (Trichoptera), but differs from those of ditrysian Lepidoptera. The secondary dorsal organ has never been found in any other lepidopteran embryos; however, it is formed in N. nipponensis and in the Trichoptera. The results of the present study strongly support the general phylogenetic views that the Zeugloptera have a close affinity to the Trichoptera.     

Embryonic development of a whirligig beetle,Dineutus mellyi,with special reference to external morphology (insecta: Coleoptera,Gyrinidae)

The egg morphology and successive changes of developing embryos of the whirligig beetle, Dineutus mellyi (Adephaga: Gyrinidae) are described from observations based on light and scanning electron microscopy. The egg surface is characterized by minute conical projections covering the entire egg surface, a stalk‐like micropylar projection at the anterior pole of the egg, and a longitudinal split line along which the chorion is cleaved during the middle embryonic stages. The germ band or embryo is formed on the ventral egg surface, and develops on the surface throughout the egg period; thus, the egg is a superficial type, as is the case in most coleopteran species. A pair of lateral tracheal gills (LTGs) of the first abdominal segment originates from appendage‐like projections arising at the lateral side of pleuropodia, and the LTGs of the second to ninth abdominal segments are arranged in a row with that of the first segment. Therefore, LTGs are structures with serial homology. The paired dorsal tracheal gills (DTGs) of the ninth abdominal segment are formed on the regions just latero‐dorsal to the LTGs of this segment. Regarding the pleuropodia as the structures being homologous with thoracic legs, neither the LTGs nor DTGs are homologous with thoracic legs, but originate in the more lateral region corresponding to the future pleura of the thoracic segments. The last (10th) abdominal segment in the larva is formed by the fusion of the embryonic 10th and 11th abdominal segments. Four terminal hooks at the end of the last abdominal segment originate from two pairs of swellings on the posterior end of the embryonic 11th abdominal segment. It is proposed that the terminal hooks possibly correspond to the claws of medially fused cerci of the embryonic 11th abdominal segment. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

尖唇散白蚁胚胎发育激光共聚焦扫描显微镜观察

【目的】本研究旨在探究尖唇散白蚁Reticulitermes aculabialis胚胎在不同发育阶段的变化特征。【方法】每日收集尖唇散白蚁的卵,并固定其胚胎发育状态,采用DAPI染剂对白蚁胚胎进行染色,通过激光共聚焦扫描显微镜观察记录尖唇散白蚁胚胎在不同发育阶段的形态特征。【结果】在25℃下尖唇散白蚁胚胎发育过程历经25~30 d,按照发育特征将其划分为12个阶段。胚胎发育早期,卵黄细胞均匀分布在卵内部,卵内细胞核向卵的中间浓缩,在细胞到达卵的后表面时形成浓缩的囊胚细胞作为胚盘;胚胎发育中期,胚胎开始进行“反转型”的囊胚运动,头部和前后轴从后极到前极反转,胚带出现明显的“双弯”结构。胚胎发育中后期,胚胎变宽,内部器官逐渐开始发育,出现明显的伸长与分节;胚胎发育后期,附肢发育明显,内部器官发育成熟。【结论】尖唇散白蚁胚胎发育过程历经12个阶段,属于短胚带型,胚带出现“双弯”结构,发育中期经历两次囊胚反转。本研究为真社会性昆虫白蚁的胚胎发育过程提供了形态学和生物学依据。     

A morphogenetic determinant in the anterior pole of an insect egg (Smittia spec., Chironomidae, Diptera): localization by combined centrifugation and ultraviolet irradiation

In chironomid midges, the development of the head and thorax in the embryo requires the function of cytoplasmic determinants localized near the anterior pole of the egg. Experimental inactivation of these determinants causes a dramatic switch in the developmental program of the embryo. Instead of the normal segment pattern, the aberrant pattern “double abdomen” is formed. Head, thorax, and anterior abdominal segments are then replaced by an additional set of posterior abdominal segments jointed with reversed polarity to the original abdomen. To determine the cellular fraction which contains the effective targets for uv induction of double abdomens, Smittia eggs were centrifuged prior to uv irradiation. Accumulation of proteid spheres or lipid droplets in the irradiated anterior pole region caused a considerable decrease in the double abdomen yield. Removal of these components from the target area enhanced double abdomen formation. The maximum yield of double abdomens was obtained after uv irradiation of a cytoplasmic layer in which organelles larger than ribosomes could not be detected. The results of these and other experiments, suggest that ribosomes, ribosomal subunits, or other ribonucleoprotein particles act as effective targets for the uv induction of double abdomens in Smittia eggs.