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“Entwickelungsmechanik, like every new approach in science, will have to acquire its deserved status step by step. Nonetheless, posterity might well be amazed on learning that the now ruling descriptive school ignored the secure findings of Entwickelungsmechanik for a long time, until descriptive research had led to the same views, and especially that this school placed more confidence in the latter kind of findings than in the former. This will bear permanent testimony to the insufficient appreciation that those researchers had for the value of experiments.” This was Wilhelm Roux’s verdict at the end of a discussion of gastrulation and the fate map in frogs (Roux 1892, p. 441). In the preceding paragraph he had summarized his conclusions on these topics, and at the same time paved the way for the verbal showdown just quoted: “The conclusion, based on three different types of experiment, that gastrulation in the frog occurs by way of bilateral epiboly and concrescence on the lower side of the egg and that the outer face of the blastoporal lip is the site of the neural folds, is therefore so secure that its certainty could neither be increased by the consent of descriptive researchers, which is per se pleasing, nor could it be diminished by their opposition; the facts thus established must rather be viewed as the firm foundations of our knowledge of developmental processes, meaning at the same time that all views clearly incompatible with these facts can for sure be called mistaken” (Roux 1892, p. 440/41).

The title of this Essay embodies an anachronism, because the term aneuploidy was not coined until several years after Theodor Boveri’s death. But that term would be meaningless if all chromosomes were functionally equivalent, and consequently no such term existed at the time when Boveri demonstrated the so-called individuality of chromosomes for the first time, by analysing dispermy in sea urchins. It is his exploitation of this “experiment of Nature” and its developmental implications that we shall now discuss, returning thereby from the role of the egg cytoplasm in development (see the previous Essay) to the roles that the nucleus, and more specifically the chromosomes, play. The work reviewed in this Essay and the next was, according to E.B. Wilson (1918, p. 74/75), Boveri’s “crowning achievement, whether in respect to excellence of method or importance of result.”

There are two definitions for the classic paper: it is one that has opened new perspectives and thereby influenced research in its field, and it is something people quote without having read. Wilhelm Roux’s treatise “Uber die Bedeutung der Kerntheilungsfiguren — Eine hypothetische Erorterung” (1883) meets either definition. It was considered a landmark paper by his contemporaries (e.g. Wilson 1896, pp 183, 221) and also declared highly influential by later generations (e.g. Oppenheimer 1965, p. 212), but developmental biologists — in contrast to geneticists (e.g. Sturtevant 1966 pp. 19, 136) — have quoted it from the beginning in a way that makes one wonder whether they had indeed read and understood it. The causes of this intriguing situation are complex. In addition to problems hailing from Roux’s prose (described in Sander 1991), they involve a possibly inadvertent shift in terminology and a transposition of Weismannian ideas that can both be traced back to the extremely influential book of Edmund B. Wilson (1896); Roux himself may have contributed to the confusion by his urge to claim paternity for almost any idea that arose in developmental biology during his lifetime.

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.

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.

Medial clefts in the axis of the trunk region are malformations known from many chordates and are mostly referred to as rachischisis anterior. In teleosts, rachischisis was previously ascribed either to secondary rifting of a single uniform axial rudiment, or to the establishment of two (half) axes and body halves physically separate from the very beginning. In order to decide between these conflicting interpretations, we treated zebrafish embryos during blastodisc stages and epiboly with several chemical teratogens causing rachischisis anterior. Treatment with ethanol, Colcemid, hydroxyurea, or cycloheximide was found to delay the proliferation and movements of the deep cells more strongly than the timing of cell differentiation, so that the deep cells embark on organogenesis before having reached their destinations in the uniform germ shield. Treatment with alpha-amanitin, on the other hand, seems primarily to affect the periblast and enveloping layer; the incomplete epiboly observed in these layers appears to restrain deep cell epiboly physically and thus to cause rachischisis. In both instances, the split condition of the embryo's trunk region is clearly due to the ectopic formation of physically separate body halves right from the beginning, a mode we call bipartite axiation. We also describe secondary anomalies specific for individual teratogens, and briefly discuss the possible origins of rachischisis anterior among other chordates including man.

U.v. irradiation of the anterior pole of nuclear multiplication stage Drosophila eggs produces embryos with defective anterior structures. At a low frequency embryos resembling some phenotypes of the bicaudal syndrome of Drosophila were observed. These embryos had no head or thorax and the eight abdominal segments were spread to the anterior of the embryo. Sometimes spiracles, characteristic of the most posterior embryonic segment were observed at the anterior of the embryo. The development of these embryos was followed, and abnormalities occurred as early as blastoderm formation. The extent of the blastoderm defects correlated well with the final abnormality in the embryo.

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 prosobranch snail Assiminea grayana is living on muddy shores at a level flooded only during spring tides. Its spawn consists of 2-layered capsules containing one egg cell each. They are deposited in clusters protected by a cap formed of faecal pellets. The veliger larva ready to hatch may survive within the egg capsule for several months until suitable hatching conditions occur. These are provided by lengthy or

UV-irradiation (maximum intensity at 253,7 nm) of the anterior 1/3 of the egg during nuclear migration stage frequently is followed by the formation of "double abdomens". The type of malformation shows a germ band in which head, thorax and anterior abdominal segments are replaced by a set of 6-8 posterior abdominal segments joined in mirror-image symmetry to the corresponding parts of the original abdomen. The course of development leading to this malformation is visibly aberrant from the blastoderm stage to germ band segmentation only. The two abdomens develop in strict synchrony provided their positions are symmetrical to the equatorial plane of the egg. Asymmetric positions are less frequent and delay temporarily the development of one or the other of the abdomens due to the lack of space. The results are compared to similar malformations in other insect eggs and possible interpretations are reviewed.

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.

Investigations on pterygote insect oogenesis have yielded a multitude of data on the architecture and development of early and late oocytes (for reviews see King, R. C., 1970; Telfer and Smith, 1970; Mahowald, 1972; Anderson, 1974; Telfer, 1975; see also Miya et al., 1969; Truckenbrodt, 1970; Buning, 1972). Oogenesis has also been studied in some Apterygota (Cone and Scalzi, 1967; Cantacuzene and Martoja, 1972; Palevody, 1972, 1973; Matsuzaki, 1973; Bilinsky, 1976, 1977, 1979; Klag, 1977, 1978). Yet the end product of oogenesis was usually exempted from investigation; the newly laid insect egg thus remains largely unknown with respect to ultrastructure. This may mainly be due to technical reasons: the prevalence of yolk and the extremely resistant egg covers of insect eggs provide considerable handicaps for fixation and sectioning. Descriptions of the egg shell are therefore much more frequent than publications dealing with the egg cell.

Mutations altering the spatial pattern of embryonic primordia are valuable tools for studying the earliest steps of embryonic pattern formation. As the spatial organisation of the egg cell arises during oogenesis, such mutants are likely to affect the embryo via the maternal genome (maternal effect). In Drosophila, most maternal-effect mutants studied so far lead to early developmental arrest. A few others yield pattern defects apparently linked to local failure of blastodermal cellularisation rather than to abnormal arrangement of primordia or ooplasmic determinants1,2. A small number of mutants, however, are known in which the embryonic fate map is affected, for example, bicaudal3,4, which profoundly alters the antero-posterior pattern of embryonic organisation. The bicaudal phenotypes suggest that the mutation causes a perturbation of a morphogen gradient that defines the antero-posterior coordinate in early embryonic pattern formation5,6. A new maternal-effect mutant, dorsal, seems to exert a similar influence on the dorso-ventral coordinate for patterning6. Dorsal (dl) has both a recessive and a dominant maternal-effect phenotype. The dl recessive phenotype is marked by an apparently complete lack of dorso-ventral polarity from early embryogenesis onward6. The dl dominant (dlD) phenotype, which we report on here, is temperature sensitive: at 22 °C, dl/+females lay eggs which develop normally while eggs produced at high temperature (29 °C) yield abnormal larvae which fail to hatch. These mutant dlD larvae vary in phenotype but nearly always lack muscles and frequently part of the ventral hypoderm. They are defective in structures that arise from the ventral region of the blastoderm in normal embryogenesis6,7 (Fig. 1).