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This last essay in the series commemorating Wilhelm Roux (1850–1924) is devoted to speculative developmental theories (or models as they would be called today) proposed in the decade following Roux’s manifesto on Entwickelungsmechanik (see Sander 1991a). That decade, 1885–1895, not only saw Roux’s pioneering experimental papers discussed in previous essays and the influential books of August Weismann (1834–1914) on his Keimplasmatheorie, but also the competing theories on animal development proposed by Hans Driesch (1867–1941) and Oscar Hertwig (1849–1922). Towards the end of this period, Roux founded his Archiv fur Entwickelungsmechanik, demonstrating the new discipline’s consolidation.

The previous Essay traced some epistemological stages along Driesch’s road to neo-vitalism, a transgression that is singular in view of his previous merits (and recognition) as an experimental embryologist. Here we shall review his experimental work between the teleo-mechanistic Analytische Theorie der organischen Entwicklung of 1894 (see Essay 11) and the end of his career as an active embryologist, which came shortly after his neo-vitalistic Gifford Lectures of 1908.

The preceding Essay was devoted to conclusions on embryogenesis that Theodor Boveri drew from genetic mosaics, mainly the famous gynanders of Mr. Eugster’s honeybee hive. Here we shall return to the question of how gynandromorphism might come to be in the first place.

“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.”

Gradients, for decades a “dirty word” in developmental biology (Lawrence 1992), have become respectable of late and are likely to stay with us after their role in pattern formation has been demonstrated at the level of gene regulation (Driever and Nusslein-Volhard 1988). The present essay is to illuminate the often misunderstood “relativistic” gradient concepts of Theodor Boveri (who actually introduced the term Gefalle = gradient), and their influences on his students. Boveri also paved the way, by his studies on chromosomes and development (see Essays 15 and 16), for a chromosomal location of the Mendelian hereditary factors (since 1909 called genes). His most influential students reacted quite differently to his insights on both gradients and genes. Hans Spemann (1869–1941) was remarkably reluctant in linking either con-. cept with his organizing center (see Sander 1986; Wolpert 1986). Waldemar Schleip (1879–1948), however, extended Boveri’s idea of interplay between cytoplasmic factors and genes in development, yet maintained a specific “Boverian” definition of gradient. Leopold von Ubisch (1885–1965) finally achieved a synthesis linking the emerging modern concept of morphogenetic gradients with differential gene activity in order to generate spatial patterns.

Im Laufe der Jahre zeigte uns die Praxis, dass die Krankheit „Sucht“ zunehmend auch in unseren Facheinrichtungen fur Gerontopsychiatrie ein ernstzunehmendes Thema wurde. Sie wurde sowohl als Primar- wie auch als Sekundardiagnose gestellt und hat alle Beteiligten vor neue Herausforderungen gestellt. Durch die raumliche Nahe zu einer renommierten Suchtklinik gab es schon langer die Moglichkeit, Bewohner aus unserer Einrichtung in Behandlung zu begleiten.

To obtain a clearer understanding of the evolutionary transition between short- and long-germ modes of embryogenesis in insects, we studied the expression of two gap genes hunchback (hb) and Krüppel (Kr) as well as the pair-rule gene even-skipped (eve) in the dipteran Clogmia albipunctata (Nematocera, Psychodidae). This species has features of both short- and long-germ mode of embryogenesis. In Clogmia hb expression deviates from that known in Drosophila in two main respects: (1) it shows an extended dorsal domain that is linked to the large serosa anlage, and (2) it shows a terminal expression in the proctodeal region. These expression patterns are reminiscent of the hb expression pattern in the beetle Tribolium, which has a short germ mode of embryogenesis. Krüppel expression, on the other hand, was found to be rather similar to the Drosophila expression, both at early and late stages. eve expression starts with six stripes formed at blastoderm stage, while the seventh is only formed after the onset of gastrulation and germband extension. Surprisingly, no segmental secondary Eve stripes could be observed in Clogmia although such segmental stripes are known from higher dipterans, beetles and hymenopterans. We therefore also studied another nematoceran, Coboldia, to address this question and found that some segmental stripes form by intercalation as in Drosophila, although belatedly. Our results suggest that Clogmia embryogenesis, both with respect to morphological and molecular characteristics represents an intermediate between the long-germ mode known from higher dipterans such as Drosophila, and the short-germ mode found in more ancestral insects.

We have studied the binding pattern of antibody mAB 2B8 directed against even-skipped orthologous proteins (EVE) in honeybee embryos. Primary and secondary EVE stripes form in roughly anterior-to-posterior succession; there are 8 primary and 16 secondary stripes. The most posterior primary stripes appear only after the onset of gastrulation. The secondary stripes form by a splitting of primary stripes; they demarcate the parasegmental pattern. While these findings resemble EVE expression in long-germ beetles, the honeybee differs from both beetles and dipterans by two transient pair-rule traits in the parasegmental EVE pattern: the secondary stripes in head and thorax alternate in strength, yet out of register with the Drosophila pattern, and over the whole pattern the odd-numbered stripes vanish earlier than their even-numbered counterparts. As in Drosophila, however, the strong EVE stripes coincide with the weak engrailed (EN) stripes. These findings are taken to indicate that (1) honeybee and beetles share a conserved mode of EVE stripe formation whilst Drosophila has diverged in this respect, (2) honeybee and Drosophila have diverged from the beetles in specific pair-rule traits during the parasegmental expression of both EVE and EN, and (3) some of these traits differ in the register of segment pairing and thus may reflect regulatory divergences at the pair-rule level between dipterans and the honeybee.

This award lecture reviews concepts of embryonic pattern formation developed since the time when, early in this century, insect embryogenesis first became a topic of causal research. These concepts, considered here in their historical sequence, are: (1) preformed patterns of determinants in “mosaic eggs”; (2) “regulative” pattern formation directed by a differentiation centre; (3) pattern specification by antagonistic polar gradients; and (4) the complex interactions of developmental genes recently revealed by genetical and molecular methods. These methods have triggered an epoch-making breakthrough in developmental biology, but they also can reveal surprising evolutionary aspects.

The organizer effect was the only achievement that the Nobel committee mentioned when awarding the 1935 prize for physiology or medicine to Hans Spemann. The paper by Spemann and Hilde Mangold describing this effect — published in this journal in 19241 — had a great impact for some time but then gradually lost in attraction. However, its citation rate rose again — quite dramatically indeed — when some interactions relevant to “Spemann’s organizer” became amenable to molecular analysis (Fig. 1).

Blocking epiboly during gastrulation in teleosts and amphibians leads to so-called ring-embryos that have a uniform head and tail end while the trunk consists of two separate lateral halves forming a ring around the persisting yolk plug. Comparable malformations probably due to delayed gastrulation can be traced throughout the chordates from amphioxus to man, revealing a fundamental unity in morphogenesis. The present contribution describes the discovery and early interpretation of ring-embryos in the Anamnia and recalls a mistaken fate map derived from them that must have substantially delayed the discovery of the amphibian organizer effect.

The engrailed expression in embryos of a beetle, four midges and a fly has been analysed with special reference to the terminal regions. In all six species the segmental expression pattern is very similar but variability occurs in the clypeolabrum, foregut and hindgut. In some cases, segmental engrailed expression seems to be extended into the hind- and/or foregut. The engrailed expression of these species is compared with published data from other insects.

A century ago, Horatio S. Greenough, an American living in Paris, persuaded the firm of Carl Zeiss to construct the first low-power stereomicroscope. Fitted by Zeiss with Porro prisms for image erection, this instrument became the ancestor of all stereoscopic dissection microscopes ("binoculars") now in use. On the basis of original documents and earlier publications, the present Centennial Essay traces both the history of the Greenough stereomicroscope and the scantily documented life of its inventor, including his probable descent from a family of famous New England sculptors. The aim is to provide some historical background for a truly timeless instrument that was fundamental to developmental biology, beginning almost from the days of its proclamation by Wilhelm Roux.

This report surveys data and interpretations presented by speakers in the Arthropod Session of the 1994 BSDB Spring Symposium. After a short review of phylogenetical aspects in premolecular insect embryology, the following topics are discussed: the ancestral germ type of pterygote insects, correlations between oogenesis and embryonic pattern formation, the universality or otherwise of bicoid as the anterior morphogen, novel functions in the insect Hox complex, the formal asymmetry between evolution and decay of complex gene networks, novel regulatory interactions as the main cause of evolutive changes, the repeated activity of conserved gene networks in successive steps of ontogenesis and strategies for future research. Interspersed are some unpublished data on oogenesis and pattern formation in lower dipterans, and their possible evolutionary implications.

We discovered extremely small genomes (1C similar to 100 Mb) in the dipteran insects Coboldia fuscipes (Scatopsidae) and Psychoda cinerea (Psychodidae). The small genomes of these species cannot be explained by a fast developmental rate, which has been shown to correlate with small genome sizes in animals and plants but might accommodate the combined effects of other developmental traits, including small egg size, thin blastoderm layer, and long-germ development.

Specification of developmental pathways by specific determining substances prelocalized in the egg cytoplasm is duscussed using the so-called germ cell determinants as an example. Some theoretical considerations speak against the assumption that in insects the various elements of the basic body plan are specified by a prelocalized mosaic of specific determinants. Experimental evidence also points towards a largely epigenetic mode of pattern specification. The process of axial pattern specification can be altered drastically by experiment: in some insects, tail ends may be formed in place of head parts and identical wequences of bocy segments may be specified two or even three times along the longitudinal egg axis. The experimental results indicate that polarity and regional character of pattern elements formed are specified by one and the same influence, and that this influence can be shifted to or simulated in various other egg regions by transposition or elimination of egg components, or by UV irradiation. Evidence obtained from several types of experiment in the chironomid midge Smittia points towards a key role for local metabolsim or energy charge in determination of cell polarity and in pattern specification. A model for embryonic pattern specification involving differential reaction of cells to a system of longitudinal gradients, which was proposed in 1960, can in principle formally account for all results described. Some striking coincidences of model and experimental results with Wolpert's concept of positional information are noted. Finally it is pointed out that universality of mechansims for pattern specification is much more likely with respect to formal principles than at the level of their physiological realization.

We discuss the interplay between evolution and development as reflected in data and concepts since about 1800. Darwin and his “continental apostle” Haeckel put the striking similarity between early vertebrate embryos in an evolutionary context. Haeckel's partly illicit generalizations discredited evolutionary thinking among early experimental embryologists who moreover noted riddles incompatible with contemporary concepts of homology and evolution. Relevant solutions were suggested by the more recent concept of ontogenetic networks that embody complex regulatory properties and genes with partly overlapping functions. Molecular data on development increasingly reveal evolutionary opportunism, for instance when a widespread signaling chain involved in primitive immune defense was apparently recruited later on for dorso-ventral axis determination in some evolutionarily advanced insect groups. Recently, Rickettsia-related bacteria colonizing many arthropod species were found to “manipulate” the first steps of host development to the advantage of their own propagation, but by ways that could also promote host speciation. Molecular genetics can now document evolutionary steps in ontogenetic networks. In the fruit fly for instance, the novel bicoid gene has superseded a crucial patterning function within a pre-existing network—a case of “molecular caenogenesis.” The expression patterns of conserved genes that antagonistically determine dorso-ventral polarity support a literal revolution envisioned almost 200 years ago. This is the dorso-ventral inversion of the body plan in some metazoans—ascribed then to the Articulata, now to the Chordata. The final section posits that the opportunistic character of evolutionary innovations is detrimental to parsimony in development. J. Exp. Zool. (Mol. Dev. Evol.) 302B:69–91, 2004. © 2004 Wiley-Liss, Inc.

Ernst Haeckel (1834-1919) must count among the most widely known biologists of his time. His monographs on radiolarian skeletons, sponges and medusae immediately became standard works, owing partly to lavish illustrations that later on culminated in his "Art Forms in Nature", which markedly influenced the "art nouveau" of the early 20th century. Haeckel's main impact, however, came from his numerous popular books that were crucial in transferring Darwin's ideas to continental Europe. Haeckel's main addition was his claim that ontogeny recapitulates phylogeny. It was partly founded on pre-Darwinian observations by J.F. Meckel and K.E. v. Baer who noticed that vertebrate embryos of different species resemble each other more strongly during early ontogenesis than later on. Wishing to illustrate this clearly, Haeckel clandestinely generalized some figures showing early embryos of animals and Man. This "fraud" provided ammunition for his many adversaries, most of whom felt provoked by his antireligious campaigns. The resulting controversies continued well into the 20th century but then subsided. Quite recently, however, they have flared up again, perhaps in connection with progress in molecular embryology that revealed an amazing evolutionary conservation of genes and their cooperation in signal transduction chains. The scientific publications that triggered this flare, and a selection of "Letters to the Editor" in both international science magazines and the German popular press, serve here to show that Haeckel's idea of recapitulation, while having proven its heuristic value, is still causing considerable irritation. This results from the widespread intuition that the marvels of ontogenesis and other biological phenomena must reflect some "intelligent design" rather than Darwinian evolution.

As described in the previous Essay, Theodor Boveri, by exploiting the “natural experiment” of sea urchin dispermy, demonstrated that embryogenesis requires “a certain combination of chromosomes”, possibly the full chromosome complement. The main topic of this Essay is his reflections on the methods he employed, and on their value for gaining previously inaccessible insights.

Theodor Boveri, in the words of Edmund B. Wilson (1918), “throughout his life held with extraordinary tenacity to the investigation of a group of problems that were already clearly in his view during his first researches. What is the egg, and what does it contain? How are hereditary traits represented in the egg and determined during its development? What is the background from which they emerge into view, one after another, as development goes forward? By what mechanism (if mechanism it be) do the unseen potentialities hidden within the germ become realities that appear before our eyes in the embryo and adult?”

Had Hans Driesch died while his Analytische Theorie der organischen Entwicklung (see the previous Essay) was in the press, he might have become the Franz Schubert of developmental biology: a genius and fruhvollendet, with unheard-of treasures sprung from his creative imagination waiting to be acknowledged and admired by posterity. Instead, he lived on happily for nearly half a century. Yet the fates asked a price for this: he was to ruin his reputation — that is, among those professing the mechanistical creed — by preaching vitalism.

To realize his plans to eliminate individual blastomeres of the ascidian embryo (see Fischer 1992a, b), around 1885 Laurent Chabry devised a sophisticated set of micro-tools for use under the microscope. It is the aim of this essay to describe these tools, to explain his methods of constructing and employing them and to follow their possible influences on subsequent research and on tools developed by later experimental biologists.

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.

In the very year, 1883, when Wilhelm Roux was pondering on the meaning of mitosis (see Sander 1991), he started experimenting on amphibian eggs. The first problem he set out to solve — or rather, the first question he discussed in writing — was the orientation of the prospective embryo in the egg. In amphibians this is a formidable problem, partly owing to the highly complex gastrulation movements, which in some respects have remained enigmatic to this day. Indeed, on trying to ascertain dorsoventral and antero-posterior polarity in the frog’s egg, Roux was confused by aberrant gastrulation movements in quite a spectacular way. Irrespective of this, he gained remarkable and still valid insights concerning the prospective median plane in frog eggs and the epigenetic factors that bring about its orientation — the latter study (Roux 1887) being adorned with further speculations on equal and unequal mitoses during early cleavage.

Roux’s Archives of Developmental Biology, founded nearly 100 years ago as Archiv fur Entwickelungsmechanik der Organismen, was the first journal devoted exclusively to the causal analysis of ontogenetic development. After a varied history (see back of volume title page), it is now entering Volume 200. On this occasion, the editor and the publisher have decided to initiate a series of essays highlighting problems and achievements in developmental biology in the discipline’s early years. We are encouraged in this venture by the increasing interest taken by practising developmental biologists in various countries in the history of their discipline and in the views of its pioneers — views that in our day may be appreciated in a more detached and (hopefully) more objective vein than by earlier generations of developmental biologists, who had to struggle with the pioneers’ inevitable errors while taking their achievements for granted.

A number of genes of the developmental gene hierarchy in Drosophila encode transcription factors containing Cys2His2 zinc finger domains as DNA-binding motifs. To learn more about the evolution of these genes, it is necessary to clone the homologs, or more correctly the orthologs, from different species. Using PCR, we were able to obtain apparently orthologous fragments of hunchback (hb), Krüppel (Kr), and snail (sna) from a variety of arthropods and partly also from other animal phyla. Sequence alignments of these fragments show that the amino acid differences can normally not be correlated with the evolutionary distances of the respective species. This is due to an apparent saturation of potential replacements within the finger domains, which is also evident from the frequent occurrence of convergent replacements. Another recurrent feature of these alignments is that those amino acids that are directly involved in determining the DNA-binding specificity of the fingers are most conserved. Using in vitro bandshift experiments we can indeed show that the binding specificity of a hunchback finger fragment from different species is not changed. This implies that there is a high selective pressure to maintain the regulatory target elements of these genes during evolution.

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.