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The transition from oogenesis to embryogenesis has received little attention in insects so far, owing largely to internal fertilization that prevents direct observation. Fertilization is characterized by extreme sperm economy, sometimes linked to gigantic sperm length. Egg activation as a rule is not initiated by sperm entrance but by changes connected with egg deposition (deformation, water uptake etc.). Both fertilization and activation can be achieved in vitro. The ooplasmic movements prominent in many species during both oogenesis and early embryogenesis seem largely independent of cleavage nuclei but may require extranuclear sperm contributions.

The aberrant pattern, “double abdomen,” previously induced in the egg of Smittia by uv irradiation of anterior pole regions was also produced by puncturing of the egg at the anterior pole. Double abdomens and embryos with anterior defects developed in eggs in which puncturing had locally prevented the regular arrangement of cleavage nuclei in the periplasm. The resulting gap in the blastoderm at the anterior pole was subsequently closed under exclusion of a small amount of egg material. Double abdomens did not develop in eggs where exclusion of anterior egg material was not observed. Thus a basic switch in the developmental program of the egg appears to depend upon the functional elimination of some crucial components in the anterior egg region.

We measured the intracellular electrical potential in oocyte and nurse cells of Drosophila follicles at different developmental stages (6–14) and determined the intrafollicular potential difference. During stages 8–10B, when intrafollicular transport is known to occur, no significant potential difference was found. During late vitellogenic stages the nurse cells assume a more positive potential than the oocyte. This result contrasts with the published data on Hyalophora follicles, in which intercellular electrophoresis of negatively charged proteins occurs from nurse cells to oocyte as a result of an intrafollicular potential difference (nurse cells more negative than the oocyte). Such a potential difference was not observed in Drosophila follicles at any stage, not even after application of juvenile hormone. The extrafollicular electrical field is described with a dipole model. The hypothetical dipole is located in the long axis of the follicle and changes its calculated length stage-specifically.

Summary Mature chorionated oocytes (eggs) dissected from ovaries of the dipterans Psychoda and Smittia can be activated by brief exposure to de-ionized water, tap water or low osmolality saline. Under optimal conditions larvae will hatch from nearly 100% of the experimentally activated eggs. In Psychoda, the yield of activation decreases with increasing osmolality of the activating medium; the threshold is near 350 mosmol/1, well below the osmolalities of Grace's and Robb's tissue culture media. The temporal threshold for activation is above 3 min and below 5 min. The effects of water treatments lasting up to 1 min are completely reversible; treatments of 2 or 3 min caused irreversible damage while failing to initiate embryogenesis. Storage of Psychoda eggs in Grace's medium for up to (at least) 6 h has no adverse effects on subsequent experimental activation while storage for 24 h reduces the fraction of activable eggs and also tends to impair embryogenesis. The data presented here indicate that low osmo...

Summary Eggs taken from the genital tracts of Venturia (Nemeritis) canescens (Hym., Ichneum.) and Psychoda cinerea (Dipt., Psychod.) can be activated by brief exposure to distilled water. However, embryogenesis fails soon unless some other conditions are met. These are mechanical deformation of the egg in Venturia, and mating in Psychoda. Development ceases at or soon after meiosis in Psychoda and in most Venturia eggs; the remaining Venturia eggs fail to form an orderly blastoderm although they contain hundreds of nuclei. In Venturia the anomalies are probably due to insufficient activation of some cytoplasmic component(s) while Psychoda eggs fail in the absence of some paternal contribution, possibly the sperm which in this case must enter the oocyte within the follicle.

Drosophila females homozygous for the mutation dicephalic occasionally produce ovarian follicles with a nurse-cell cluster on each oocyte pole (dic follicles). Most dic follicles contain 15 nurse cells as in the normal follicle, but the total nurse-cell volume is larger in dic follicles; this is in keeping with the increase in DNA content recently described. However, the relative increase in oocyte volume during nurse-cell regression (from stage 10B onward) is not significantly larger in dic than in normal follicles. Time-lapse recordings in vitro show that, as a rule, both nurse cell clusters in a dic follicle export cytoplasm to the oocyte but nurse-cell regression remains incomplete at both poles and the persisting remnants of the nurse cells cause anomalies in chorion shape. The kinematics of cytoplasmic transfer are less aberrant at that oocyte pole which harbours the germinal vesicle. Possible links are discussed between these anomalies of oogenesis and the double-anterior embryonic patterns observed in the majority of developing dic eggs.

Drosophila embryos were locally irradiated with a 257-nm laser microbeam during blastoderm and germ band stages. Depending on stage and beam diameter (10–30 μm), from 0 to 45 nuclei were exposed to the uv radiation. The doses used, 5 or 10 erg, did not eliminate nuclei or cells at once, but up to 50% of the adult survivors from irradiated eggs carried defects in the thorax. These were scored with reference to the imaginal discs from which the affected structures derive. For each thoracic disc a “target center” was calculated as the weighted mean value of all beam locations affecting the respective adult derivatives. The target centers for the germ band stage map within the respective germ band segments. The pattern of target centers for the blastoderm stage is comparable to the thoracic region of published fate maps, and the distances between adjacent leg centers (approximately three cell diameters) agree with recent evidence based on mosaic flies. We discuss the question whether the target centers mark the position of the respective disc progenitor cells at the stages of irradiation and conclude that these positions are rendered rather correctly at least with reference to the longitudinal egg axis.

The bilateral symmetry of the larval body may arise epigenetically even in animal forms with bilaterally symmetrical eggs. In insects, this can be shown by development of parallel twins from eggs or early embryos split along their midlines. The present contribution explores the possibility that epigenetic symmetrization is based on circular asymmetries pre-existing in the egg cell. This possibility is suggested by phologenetic considerations and by data based on epimorphic regeneration of larval appendages. The author proposes an experimental test discriminating between circular and bilaterally symmetrical prepatterns in primitive insects.

In the Drosophila ovarian follicle, 15 polyploid nurse cells (NC) are linked by cytoplasmic bridges to the anterior pole of the oocyte. The mutant dicephalic occasionally produces aberrant follicles whose 15 nurse cells are separated into two clusters, each attached to one oocyte pole [split nurse cell (SNC) follicles]. Feulgen cytophotometry of isolated nuclei from such clusters and from normal follicles of the same females revealed an inverse correlation between NC number per cluster and mean DNA content per nucleus. The maximum DNA content of nuclei from split NC clusters falls into the 4,096 C class as opposed to the 2,048 C class in 15 NC clusters; the mean DNA content per NC nucleus is 1,940 C in the former and 1,340 C in the latter. These correlations are confirmed by measurements on follicles containing fewer or more than 15 NC, and on SNC follicles from another mutant (spindel C). These findings indicate that replication in NC nuclei is controlled with reference to the transcriptional capacity of the individual NC cluster rather than the total capacity available per oocyte or follicle. We discuss this finding with respect to embryonic patterning as influenced by nurse cell location and list and discuss conflicting published reports on over- and underreplication in Drosophila follicles.

We determined the extracellular electrical current pattern around Drosophila follicles at different developmental stages (7–14) with a vibrating probe. At most stages a characteristic pattern can be recognized: current leaves near the oocyte end of the follicle and enters at the nurse cells. Only at late vitellogenic stages was an inward-directed current located at the posterior pole of many follicles. Most striking was the observed heterogeneity both in current pattern and in current density between follicles of the same stage. Different media (changed osmolarity or pH, addition of cytoskeletal inhibitors or juvenile hormone) were tested for their effects on extrafollicular currents. The current density was consistently influenced by the osmolarity of the medium but not by the other parameters tested. Denuded nurse cells (follicular epithelium locally stripped off) show current influx, while an accidentally denuded oocyte produced no current. Our results show that individual follicles may be electrophysiologically different, though their uniform differentiation during vitellogenesis does not reflect such heterogeneity.

The pattern of intercellular connections between germ line cells has been studied in follicles of the mutantdicephalic (dic), which possess nurse cell clusters at both poles. Staining of follicles with a fluorescent rhodamine conjugate of phalloidin reveals ring canals and cell membranes and thus allows us to reconstruct the spatial organization of the follicle. Each germ line cell can be identified by the pattern of cell-cell connections which reflect the mitotic history of individual cells in the 16-cell cluster. The results indicate that in both wild-type anddicephalic cystocyte clusters one of the two cells with four ring canals normally becomes the pro-oocyte. However, in some follicles (dicephalic and wild-type) oocytes were found with fewer or more than four ring canals. Indic follicles, one or several nurse cells may become disconnected from the other cells during oocyte growth at stage 9-10. Such disconnected cells cannot later on empty their cytoplasm into the oocyte. This, in turn, might be of consequence for the determination of axial polarity of the embryo.

The segmental pattern of insect embryos depends on influences from morphogenetic centres near each of the egg poles1. In Drosophila, maternal effect mutations are known that impair the normal function of each centre2. Injection of wild-type cytoplasm into mutant eggs has revealed that morphogenetic signals localized at the anterior and posterior pole of eggs can be transplanted3,4. We show here that these activities can also be detected during oogenesis. Posterior activity can be recovered at an early stage (stage 10, ref. 5) from the oocyte-nurse cell complex, but anterior activity can only be detected in the mature oocytes (stage 14). We conclude that the bicoid-dependent anterior signal, although produced by the nurse cells, does not become active before it is localized to the anterior egg pole, whereas posterior activity can be detected in the nurse cells before, and therefore independently of, its localization to the posterior egg pole.

We have isolated and characterized a homeoboxcontaining gene from the honeybee Apis mellifera. Its homeobox region shows a high degree of sequence similarity to the homeobox of the Drosophila gene Deformed (Dfd). At the DNA level 82% of the basepairs are the same, whereas the putative amino acid sequences are identical between the bee and the fruitfly genes. Similarity is also present 5′ and 3′ to the homeobox. Using this isolate as a probe we have performed in situ hybridization on sections from blastoderm-stage embryos of the honeybee Apis mellifera. In early blastoderm stages we found a rather irregular pattern of labelled nuclei. In middle stages we found silver grains over each nucleus and also over the cytoplasm in a belt of blastoderm cells in the prospective gnathal region. These results indicate that the Deformed genes from honeybee and fruitfly are homologous both with respect to their DNA sequence and their spatial and temporal pattern of expression during embryogenesis.

MORPHOGENETIC movements which occur during the early stages of animal embryogenesis are difficult to study unless some kind of marking technique is used. In vertebrates vital staining of small cell patches1 or application of charcoal powder2 has permitted the construction of maps showing extensive morphogenetic movements during early embryogenesis. Similar techniques have proved useful for the study of morphogenesis in other animal phyla. There are, however, no reports on successful intra vitam staining of insect eggs. This may be because the protective covers of most insect eggs do not permit dyes to enter locally, while eggs deprived of their covers cease to develop. Morphogenetic movements taking place during the plasmodial stages3 of insect embryogenesis therefore had to be reconstructed from a series of fixed material4–6 or were analysed by using local defects as markers7,8 and by time lapse studies7,9,10. Defect marking more than other techniques involves the danger of misinterpretation. Time lapse studies and investigations on fixed material share the disadvantage that only a limited range of structures located in the egg plasmodium can be traced satisfactorily, for example, nuclei or special inclusions of the cytoplasm; furthermore, work with fixed material is technically difficult and cinematography fails to reveal movements within bulky eggs sufficiently clearly. Renewed attempts to overcome these handicaps have produced a technique which, although far from being as versatile as vital staining in amphibians, serves to reveal the type and extent of movements within the whole egg plasmodium of the house cricket (Acheta domesticus L.) during germ band formation and anatrepsis.