As the worm turns

Stephen Jay Gould

How can you evolve a vertebrate from an invertebrate? Invert it.

When Hamlet, in the most celebrated soliloquy of English literature, weighs the relative values of life and death, he describes suicide ("not to be") as an escape from active insults, including "the oppressor's wrong, the proud man's contumely." But writers and intellectuals worry far more about an opposite fate on life's potential "sea of troubles"--erasure and oblivion, the pain of being simply ignored. Samuel Johnson, as recorded by Boswell, expressed this silent arrow of outrageous fortune in a famous aphorism: "I would rather be attacked than unnoticed. For the worst thing you can do to an author is to be silent as to his works."

I therefore felt a special poignancy when I recently read an anecdote about the last years of a great English physiologist, Walter H. Gaskell (1847-1914). After a distinguished career of solid experimental work on the function of the heart and nervous system, Gaskell switched gears and devoted the entire second half of his professional life (from 1888 on) to promoting and defending his idiosyncratic theory for the origin of vertebrates. The last paragraph of Gerald L. Geison's long article on Gaskell in the Dictionary of Scientific Biography reads:

His final years were clouded . . . by a

feeling that his deeply loved theory of the

origin of vertebrates was not receiving a

fair hearing. Even at Cambridge, where

Gaskell lectured on the topic until his

death, his audience decreased over the

years until, near the end, the poignant

scene is drawn of Gaskell closing his

course by shaking hands with a lone

remaining auditor.

We may grieve for Gaskell's personal fate as an intellectual pariah, but, truth to tell, he had been pushing a pretty nutty theory for the origin of vertebrates. Gaskell believed with all his soul, and with a striking absence of critical questioning, that the evolution of animal life must follow a single pathway of progressive advance mediated by an increasing elaboration of the brain and nervous system. Gaskell wrote in his major work of 1908, The Origin of Vertebrates (the source of all quotes from Gaskell in this essay):

We can trace without a break, always

following out the same law, the evolution

of man from the mammal, the mammal

from the reptile, the reptile from the

amphibian, the amphibian from the fish,

the fish from the arthropod [insects and

their allies], the arthropod from the

annelid [segmented worms], and we may

be hopeful that the same law will enable

us to arrange in orderly sequence all the

groups in the animal kingdom.

Gaskell identified this controlling principle of linear advance as the "law of the paramount importance of the development of the central nervous system for all upward progress." In a rhetorical flourish, he then inverted the preacher's famous argument (Ecclesiastes 9:11) for randomness and aimless change without direction: "The law of progress is this--The race is not to the swift, nor to the strong, but to the wise."

Advocates for a single line of progress encounter their greatest stumbling block when they try to find a smooth link between the apparently disparate designs of invertebrates and vertebrates. In addressing this old problem, Gaskell adopted the standard strategy of linear progress theorists from time immemorial: identify the most complex invertebrate and attempt to forge a link with the simplest vertebrate. Gaskell, again following tradition, selected arthropods as the invertebrate pillar for his bridge, and then tried to build the span under his law of neurological complexification:

This consideration points directly to the

origin of vertebrates from the most highly

organized invertebrate group--the

Arthropoda--for among all the groups of

animals living on the earth in the present

day they alone possess a central nervous

system closely comparable in design with

that of vertebrates.

So far, so conventional. Gaskell's theory becomes idiosyncratic and a bit bizarre in his chosen mode for forging the improbable link of arthropod to vertebrate. Among the plethora of prominent differences between these phyla, one central contrast has always served as a focus for discussion and a chief impediment to any linear scheme. Arthropods and vertebrates share some broad features of general organization--elongated, bilaterally symmetrical bodies with sensory organs up front, excretory structures in the back, and some form of segmentation along the major axis. But the geometry of major internal organs could hardly be more different, thus posing the classical problem that has motivated several hundred years of dispute and despair among zoologists.

Arthropods concentrate their nervous system on their ventral (belly) side as two major cords running along the bottom surface of the animal. The mouth also opens on the ventral side, with the esophagus passing between the two nerve cords and the stomach and remainder of the digestive tube running along the body above the nerve cords. In vertebrates, and with maximal contrast, the central nervous system runs along the dorsal (top) surface as a single tube culminating in a bulbous brain at the front end. The entire digestive system then runs along the body axis below the nerve cord. But could evolution (or a sensible, divine Creator, for that matter) turn an arthropod with belly above nerve cords into a superior vertebrate with brains on top and a gut below?

Gaskell proposed a pretty wild scheme for such a transformation, and his loss of respect (and students) followed his inability to construct a cogent defense. Gaskell argued that the dorsal gut of arthropods evolved into the vertebrate brain and spinal cord as a proliferation of nervous tissue fueled the upward march of progress. This new nervous tissue began to surround the old gut, eventually choking off all digestive function like a strangler fig around a host tree or an anaconda squeezing the lifeblood from a pig. The modern vertebrate brain surrounds the old arthropod stomach, thus explaining the ventricles--the interior spaces between the folds of the brain--as remnants of an ancestral digestive space. Similarly, the central canal of the spinal cord represents the old arthropod intestine, now surrounded by nervous tissue.

But this putative solution only engendered an even more troubling problem: if the arthropod gut became the vertebrate nervous system, then what organ can serve as a precursor for the vertebrate gut? When this problem stymied Gaskell, he opted for a deus ex machina that eventually satisfied no one but himself (and perhaps his one last auditor): the vertebrate digestive tube simply arose de novo, to suit an obvious need. (Ya gotta eat, after all.) He concluded:

Vertebrates arose from ancient forms of

arthropods by the formation of a new

alimentary canal, and the enclosure

of the old canal by the growing central

nervous system.

Can we extract any message from Gaskell's failed theory beyond a stodgy, if appropriate, warning about the virtues of caution and sobriety? I certainly think so, for I have long held, and expressed as a mainstay of these monthly essays, that when fine scientists strongly espouse theories later judged as nutty or crazy, interesting and instructive reasons always underlie the paradoxical advocacy. This principle certainly applies in Gaskell's case because we can identify both a generally constraining bias and a personally compelling reason that drove Gaskell to the odd idea of stomachs turning into brains and new guts arising from nothing but inchoate potentiality.

Gaskell's dubious but unquestioned conviction about linear progress served as the general bias that led him to propose an almost alchemical scheme of transmutation from arthropod to vertebrate. But an understanding of the history of this subject also reveals a particular reason that interacted with his general conviction about progress to lead him down a path of increasing irrelevance and loneliness. In short, Gaskell proposed his own nutty theory because he couldn't abide the older and standard account, also judged by history as a prime case of nuttiness, for linking arthropods and vertebrates.

Think about the basic contrast and the most obvious way to produce alignment. Arthropods have ventral nerve cords with the gut above; vertebrates develop with a darsal nerve tube and a gut below. Presto turno--and one becomes the other. Why not just invert a segmented worm or an insect to produce the vertebrate pattern? Turn a bug on its back (as Kafka did, come to think of it, when he changed his protagonist into a roach in The Metamorphosis, and the internal geometry of a vertebrate emerges--nerves above guts).

I don't mean to be frivolous or cavalier about complex and serious matters. I and all participants in the history of this debate know perfectly well that an inverted worm or insect doesn't become a vertebrate, tout simple and all nice and clean. More than a few knotty problems and inconsistencies remain. To cite the dilemma most widely discussed in the literature, the esophagus of an inverted bug runs upward through the nervous system (right in the area that will become the vertebrate brain) to emerge at a mouth on top of the head. Clearly this will not do (and has not done in any real vertebrate). So the inversion theory for deriving vertebrates from arthropods must argue that the old brain-piercing mouth atrophied and closed up, while a new ventral mouth developed at the front end of the vertebrate gut. Forming a new mouth at the end of an old tube may not be quite so bold or improbable as making an entirely new gut from no preexisting structure (as Gaskell's theory required), but no evidence for such a scenario exists either, and the whole tale smacks of fatuous special pleading to save an otherwise intriguing idea.

In any case, I am not spinning an abstract fairy tale as a hypothetical alternative to Gaskell's solution. The inversion theory has a long and fascinating history in the discussion of vertebrate origins. The founding version dates to the early nineteenth century and became the centerpiece of a movement that may be called transcendental biology, or the attempt to reduce organic diversity to one or a very few archetypal building blocks that could then generate all actual anatomies as products of rational laws of transformation. Some of Europe's greatest thinkers participated in this grand, if flawed, enterprise. Goethe, Germany's preeminent poet-scientist, tried to explain the varied parts of plants as different manifestations of an archetypal leaf. In France, Etienne Geoffroy Saint-Hilaire attempted to portray the skeleton of vertebrates as a set of modifications upon an archetypal vertebra.

In the 1820s, Geoffroy extended his ambitious program to include annelids and arthropods under the same rubric. With boldness verging on a mania too sweeping to be entirely right but also too ingenious to be completely wrong, he argued that arthropods also built their bodies on a vertebral plan, but with one central difference. Vertebrates support their soft parts with an internal skeleton, but insects, with their external skeletons, must live within their own vertebrae (a reality, not a metaphor, for Geoffroy). This comparison led to other strange consequences, including the claim that a vertebrate rib must represent the same organ as an arthropod leg--and that insects must therefore walk on their own ribs!

Geoffroy also recognized that the opposite orientation of gut and nervous system posed a problem for his claim that insects and vertebrates represent different versions of the same archetypal animal--and he proposed the first account of the inversion theory to resolve this threat to unification. Geoffroy's initial version of 1822 makes much more sense than the later evolutionary scenarios of linear transformation that so enraged Gaskell. Geoffroy was an early evolutionist in these decades before Darwin, but he did not devise the inversion theory as a genealogical proposition--that is, he did not argue that an arthropod ancestor evolved directly into a primitive vertebrate by turning over. Geoffroy pursued the quite different aim of establishing a "unity of type" that could generate both arthropods and vertebrates from the same basic blueprint.

He then argued, quite cogently within his own framework, that this grand Platonic blueprint paid scant attention to such "insignificant" questions of nitty-gritty daily reality as which side of a universal design happened to point toward the sun. The one grand design has a gut in the middle and the main nerve cords at a periphery. Arthropods orient this periphery down and away from the sun--so we call their nerve cords ventral. But vertebrates orient their spinal cord up and toward the sun--so we call the same structure dorsal in our own kin. One common design in two orientations, insignificantly inverted with respect to an external axis of sunlight and gravity.

But later evolutionary theorists of linear progress had to advance the overtly physical and historical claim that an ancestral lineage of arthropods actually turned over to become the first vertebrates (for the classic statement of the inversion theory in this genealogical form, see William Patten, The Grand Strategy of Evolution, 1920). Gaskell could not abide this indecorous version of his beloved linear progress theory. He could not bear to imagine that the grand march from jellyfish to human, orchestrated by an ever increasing mass of nervous tissue yearning for consciousness, once paused in a stately and orderly progression toward a human pinnacle in order to execute a fancy little flip, a clever jig of inversion, just at the sublime and definitive moment of entrance into the vertebral home stretch.

Gaskell therefore had to keep his stately soldiers upright and uniformly oriented throughout their journey toward the human pinnacle--and he fulfilled this need by crafting the vertebral brain and spinal cord from an arthropod digestive tube, while forming a completely new gut below. By this device, he could keep tops on top and bottoms at the bottom throughout the linear history of animal life, while placing nerves below gut in arthropods, but above guts in vertebrates. Gaskell thought that his move would rescue the theory of linear progress, with its necessary transition of arthropod into vertebrate, from the absurdities of the old inversion theory. "How is it then," he wrote, "that this theory has been discredited and lost ground? Simply, I imagine, because it was thought to necessitate the turning over of the animal." Gaskell therefore invented his peculiar alternative as a refutation of the venerable inversion theory. He wrote of the first vertebrate: "If the animal is regarded as not having been turned over. . . then the ventricles of the vertebrate brain represent the original stomach, and the central canal of the spinal cord the straight intestine of the arthropod ancestor."

How ironic. In order to avoid the "nutty" theory of inversion, Gaskell invented the even nuttier notion of stomachs turning to brains with new guts forming below. No wonder then that subsequent theory cast a plague on both speculative houses and opted instead for the obvious alternative: arthropods and vertebrates do not share the same anatomical plan at all, but rather represent two separate evolutionary developments of similar complexity from a much simpler common ancestor that grew neither a discrete gut nor a central nerve cord. After all, we now know that arthropods and vertebrates have been separated for more than 500 million years, and that "simpler" arthropods did not turn into "complex" vertebrates at some halfway point on a march to a single evolutionary apex.

Furthermore, this sensible idea of independent derivation meshed beautifully with the triumph, from the 1930s on, of a strict version of Darwinism based on the near ubiquity of adaptive design built by natural selection, with little constraint imposed by strictures of a common anatomic ground plan like Goethe's leaf or Geoffroy's vertebra. If adaptation and natural selection wield such unimpeded power over the fate of each evolutionary sequence, why should we search for deeper commonalities in lineages long separate? Arthropods and vertebrates do share several features of functional design. But these similarities only reflect the power of natural selection to craft optimal structures independently in a world of limited biomechanical solutions to common functional problems--an evolutionary phenomenon called convergence.

After all, if you want to fly, you have to develop wings of some sort because nothing else can work. Bats, birds, and pterosaurs (flying reptiles of dinosaur times) all evolved wings independently because natural selection knows no other solution and has the capacity to build such intricate convergences as independent illustrations of its predominant power. Therefore, if both arthropods and vertebrates evolved guts and nerves in reversed orientations, why worry about different expressions of a common constraint? The two phyla have been separate for half a billion years and undoubtedly evolved their digestive and neurological organs along separate pathways of adaptation.

This new consensus seemed so compelling that Ernst Mayr, the dean of modern Darwinians, opened the ashcan of history for a deposit of Geoffroy's ideas about anatomical unity. We now appreciate the immense power of natural selection to build and rebuild every feature; to change, and then to alter again, nearly every nucleotide of every gene in the interest of better adaptation. Lineages that have been separate for 500 million years cannot possibly retain enough genetic identity to encode any important common constraint of design. In his epochal book of 1963, Animal Species and Evolution, Mayr wrote:

In the early days of Mendelism there was

much search for homologous genes that

would account for such similarities. Much

that has been learned about gene

physiology makes it evident that the

search for homologous genes is quite futile

except in very close relatives.

The verdict of history had descended. Gaskell had proposed a weirder theory to reject Geoffroy's union of arthropods and vertebrates by inversion. But Geoffroy's theory turned out to be quite weird enough all by itself. Evolutionary studies would finally abandon such romantic nonsense and move into the light of unimpeded natural selection.

Except for one small matter. Darwin himself told us in his last book, The Formation of Vegetable Mould Through the Action of Worms, that we should never underestimate the collective power of worms on the move. Our general culture also recognizes two primary metaphors, one inorganic and one organic, for the reversal of received opinion. Well may traditionalists fear the turning of these two objects: tables and worms. The inversion of the humble worm, especially when disturbed, may bring down empires. Shakespeare told us that "the smallest worm will turn being trodden on." And Cervantes wrote in his author's preface to Don Quixote that "even a worm when trod upon, will turn again."

How wonderfully symbolic and real in the double meaning. Geoffroy proposed a theory to unite the architecture of complex animals by comparing vertebrates with segmented worms and arthropods turned over. This theory for the archetype of complex animals became, instead, the archetype of nutty ideas in biology--so nutty that Gaskell felt driven to invent an even crazier theory for the origin of vertebrates, explicitly to avoid the bizarre concept of his own kin as inverted worms. But turning worms also serve as our cultural metaphor for upheaval of accepted ways and thoughts.

I have always loved the boldness of Geoffroy's theory, but I never dreamed that he might be right--even though I have long embraced, as a centerpiece of my own career, his larger view about the importance of inherited architectural pathways as constraints upon the optimizing power of natural selection. Well, the worm has turned twice during the past year--in both actual and symbolic styles. Geoffroy, it seems, was correct after all--not in every detail, of course, but surely in basic vision and theoretical meaning. And the triumph of this surprise, the inversion of nuttiness to apparent truth, stands as a premier example of the most exciting general development in evolutionary theory during our times.

I published my first technical book, Ontogeny and Phylogeny, in 1977. I took pride in this long work on the relationship between embryology and evolution, but I also became quite frustrated because we then knew so preciously little about the potential key to a resolution: the genetic basis of development. How does the genetic code help to orchestrate this greatest miracle of everyday biology--the regular and usually unerring production of adult complexity from the apparent formlessness of the tiny fertilized egg? We knew practically nothing, but we assumed (as documented above) that the major animal phyla, all evolutionarily separate for at least 500 million years, could share no constraining common plan or genetic architecture. Pure Darwinism reigned triumphant and natural selection had built each basic anatomy for its own adaptive utility.

But we can now determine, easily and relatively cheaply, the detailed chemical architecture of genes; and we can trace the products of these genes (enzymes and proteins) as they influence the course of embryology. In so doing, we have made the astounding discovery that all complex animal phyla--arthropods and vertebrates in particular--have retained, despite their half billion years of evolutionary independence, an extensive set of common genetic blueprints for the building of bodies.

Many similarities of basic design among animal phyla, once so confidently attributed to convergence and viewed as a testimony to the power of natural selection to craft exquisite adaptation, demand the opposite interpretation that Mayr labeled as inconceivable: the similar features are homologies, or products of the same genes, inherited from a common ancestor and never altered enough by subsequent evolution to erase their comparable structure and function. The similarities record the constraining power of history, not the building skills of natural selection independently pursuing an optimal design in separate lineages. Vertebrates are, in a sense, true brothers (or homologs)--and not mere analogs--of worms and insects.

Examples of this primary reversal of standard theory have been accumulating for the past fifteen years. I have written three previous essays on earlier cases. In 1995, for only the second time in history, the Nobel Prize honored an evolutionary study--as my colleagues Edward Lewis, Christiane Nusslein-Volhard, and Eric Wieschaus won the award for their work on unraveling the developmental genetics of the fruit fly Drosophila and discovering homologs of the same genes in vertebrates.

In the first, path breaking case, the homeotic genes of insects, responsible for specifying the separate identities of segments along the main body axis (by orchestrating the growth of antennae, mouthparts, legs, and so on in their proper places), were also discovered, in minimally altered form, in vertebrates. (The homeotic genes were first recognized in oddball mutants with body parts in the wrong places--legs growing out of the head where antennae should be, for example. In Drosophila, the homeotic genes occur in two arrays on a single chromosome. Interestingly, in vertebrates, these same arrays exist in multiple copies as four sequences on four separate chromosomes.) These vertebrate homologs do not control the basic segmentation of the vertebral column (so insect segments are not simple homologs of vertebrae, as Geoffroy had originally proposed). But the homeotic genes of vertebrae do regulate the embryonic segmentation of the mid- and hind-brain, and they do strongly influence other important repetitive structures, including the positioning of cranial nerves along the body axis.

A second case then seriously compromised the classic textbook example of convergence--the paired eyes of three great phyla: vertebrates, arthropods (with the multiple-faceted eye of flies as a primary example), and mollusks (particularly the complex lens eye of squids, so similar in function to our own, but built of different tissues). We had always assumed that eyes in the three phyla must have completely independent evolutionary origins because they differ substantially in basic anatomy. And we viewed this supposed convergence as a premier example of natural selection's power to produce organs of similar and optimal function, but built from different materials and evolved from entirely separate starting points.

But we now know that eyes in all three phyla share an inherited embryological pathway largely orchestrated by a gene (called Pax-6 in its vertebrate form) retained in all these phyla from a common ancestor--and remaining similar enough to work interchangeably (for the fly version will induce the formation of eyes in vertebrates, and vice versa). The end results vary substantially (the multifaceted fly eye is not homologous with our single-lens eye), but the embryological blueprints share a common ancestry, and the eyes of different phyla can no longer be viewed as a case of pure convergence.

The reversal of opinion during the past decade has been astonishing. Mayr argued that we shouldn't even bother to look for genetic homology and shared embryological pathways between distinct phyla. We have now moved to the opposite pole of being surprised when we identify a basic gene of developmental architecture in Drosophila and then do not find a homolog in vertebrates. Charles B. Kimmel began a recent paper on this subject (Trends in Genetics, September 1996) by writing: "We have come to find it more remarkable to learn that a homolog of our favorite regulatory gene in a mouse is not, in fact, present in Drosophila than if it is, given the large degree of evolutionary conservation in developmentally acting genes."

Still, I guess I haven't fully adjusted to the change--even though it so suits my hopes and fuels my theoretical prejudices--for I never dreamed that my all-time favorite theory of interphyletic union, Geoffroy's hypothesis of inversion, could possibly be right as well. The basic structuring from front end to back? Fine. Eyes? Why not. But the arthropod belly as the vertebrate back? Kind of silly, however intriguing.

Except that Geoffroy's inversion theory, appropriately reexpressed in the language of modern genetics and developmental biology, turns out to be true. In several papers published during the past two years--and based on work done primarily in the laboratory of Eddy M. De Robertis at UCLA--all essentials of Geoffroy's theory have been strikingly affirmed in contemporary terms (see especially S. A. Holley, P. D. Jackson, Y. Sasai, B. Lu, E. M. De Robertis, F. M. Hoffmann, and E. L. Ferguson, "A Conserved System for Dorsal-Ventral Patterning in Insects and Vertebrates Involving Sog and Chordin," Nature, vol. 376, 1995; and E. M. De Robertis, and Y. Sasai, "A Common Plan for Dorsoventral Patterning in Bilateria," Nature, vol. 380, 1996).

Geoffroy's vindication began with the sequencing of a vertebrate gene called chordin. In the toad Xenopus (but working, so far as we know, in a similar manner in all vertebrates), the chordin gene codes for a protein that patterns the dorsal (top) side of the developing embryo and plays an important role in formation of the dorsal nerve cord. When these scientists searched for a corresponding gene in Drosophila, they discovered, to their surprise, that chordin shares sufficient similarity with sog to make a confident claim for common ancestry and genetic homology. But sog is expressed on the ventral (bottom) side of Drosophila larvae, where it acts to induce the formation of ventral nerve cords. Thus, the same gene by evolutionary ancestry builds both the dorsal nerve tube in vertebrates and the ventral nerve cords of Drosophila--in conformity with Geoffroy's old claim that vertebrate backs are arthropods' bellies, and that the two phyla can be brought into structural correspondence by inversion.

This intriguing fact cannot affirm Geoffroy's inversion theory by itself, but De Robertis and colleagues then sealed the case with two additional discoveries. First, they found that a major gene responsible for specifying the dorsal side of flies (and called decapentaplegic, or dpp) has a vertebrate homolog (called Bmp-4) that patterns the ventral side of Xenopus--another reversal consistent with Geoffroy's hypothesis. Moreover, the entire system seems to work in the same way--but inverted--in the two phyla. That is, dpp, diffusing from the top to the bottom, can antagonize sog and suppress the formation of the ventral nerve cords in Drosophila--while Bmp-4 (the homolog of dpp), diffusing from the bottom to the top, can antagonize chordin (the homolog of soy) and suppress the formation of the dorsal nerve cord in vertebrates.

Second, these scientists also found that the fly gene can work in humans, and vice versa. Vertebrate chordin can induce the formation of ventral nerve tissue in flies, while fly sog can induce dorsal nerve tissue in vertebrates. I regard these three discoveries as forming a tight and well-documented case for Geoffroy's old theory of inversion.

Moreover, current results vindicate Geoffroy's version, not the later scenarios of linear evolution. These data do not support the silly notion that, at a defining moment in the march of evolutionary progress, an arthropod literally flipped over to become the first vertebrate. Rather, as Geoffroy argued nearly two centuries ago, the two phyla share a common architecture, but in reversed arrangement. In evolving separately from common ancestry, vertebrates oriented the shared design in one manner, annelids and arthropods in the opposite direction.

Evolution displays enormous ingenuity and versatility in iterating a set of common genes and developmental pathways along so many various routes of ecology and modes of life. But our brotherhood and sharing, like the still waters of legend, run far deeper than we had dared to imagine. A substantial blast from the past underlies the signs of new designs.

To end on an admittedly fatuous note, devotees of B-movies will remember one of the all-time classics--the original version of The Fly (not that dreadful remake with Jeff Goldblum as our hybrid hero). Focus on the unforgettable last scene: the fly with the man's head lies ensnared in a spider's web, as ugly Ms. Eight Legs moves in for the gruesome kill. In a shrill voice of fear, the fly keeps shouting, "Please help me." Finally, and mercifully (for the fly's head on the man's body has died, so the two creatures cannot be unmixed and properly reconstituted), another character throws a rock at the web, putting fly-man out of his misery. Perhaps at this crucial moment in the next remake, the rock-wielding mercy killer can offer some zoological advice instead: "Turn over and be a man."

Stephen Jay Gould teaches biology, geology, and the history of science at Harvard University. He is also Frederick P. Rose Honorary Curator in Invertebrates at the American Museum of Natural History.

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