Preformationism was an ingenious theory and won prominent adherents. Yet many seventeenth- and eighteenth-century philosophers, among them freethinkers such as Buffon and Maupertuis, preferred some version of the older theory of ‘epigenesis’, the notion that embryonic order does not exist in the egg or the sperm per se, but rather emerges spontaneously after fertilisation. At the time of the querelle, many thought that the preformationists had the better side of the argument. Today, however, it is more difficult to judge a victor. Neither the preformationists nor the epigeneticists had a coherent theory of inheritance, so the terms of the debate between them do not correspond in any simple way to a modern understanding of the causes of deformity or development. Preformationism, with its infinite regress of embryos, seems the more outlandish of the two theories, though it captures nicely the notion that development errors are often (though not invariably) due to some mistake intrinsic to the germ cells – the cells that become eggs and sperm – or at least their DNA. But the epigeneticists speak more powerfully to the idea that embryos are engaged in an act of self-creation which can be derailed by external influences, chemicals and the like, or even chance events within their dividing cells.
HOW TO MAKE A CONJOINED TWIN
What makes twins conjoin? Aristotle, characteristically, covered the basic options. In one passage of The generation of animals he argues that conjoined twins come from two embryos that have fused. That, at least, is where he thought conjoined chickens (which have four wings and four legs) come from. But elsewhere he suggests that they come from one embryo that has split into two.
To modern ears his notion of how an embryo might split sounds odd, but it is a sophisticated account, all of a piece with his theory of how embryos develop. Having no microscope, Aristotle knows nothing of the existence of sperm and eggs. Instead he supposes that embryos coagulate out of a mixture of menstrual fluid and semen, the semen causing the menstrual fluid to thicken rather as – to use his homely metaphor – fig juice causes milk to curdle when one makes cheese. This is epigenesis avant la lettre. Indeed, preformationism was very much an attack on the Aristotelian theory of embryogenesis and, by extension, its account of the origins of deformity. Sometimes, says Aristotle, there is simply too much of the pre-embryonic mix. If there is only a little too much, you get infants with extra or unusually large parts, such as six fingers or an overdeveloped leg; more again, and you get conjoined twins; even more mix, separate twins. He uses a beautiful image to describe how the mix separates to make two individuals. They are, he says, the result of a force in the womb like falling water: ‘…as the water in rivers is carried along with a certain motion, if it dash against anything two systems come into being out of one, each retaining the same motion; the same thing happens with the embryo’.
For Aristotle, the two ways of making conjoined twins bear on their individuality. He rules that if conjoined twins have separate hearts, then they are the products of two embryos and are two individuals; if there is only one heart, then they are one. The question of conjoined twin individuality haunts their history. Thomas Aquinas thought that it depended on the number of hearts and heads (thereby ensuring perpetual confusion for priests who wanted to know how many baptisms conjoined infants required). When twins are united by only by a slender cartilaginous band – the case with the original Siamese twins, Eng and Chang (1811–74) – it is easy to grant each his own identity. More intimately joined twins have, however, always caused confusion. In accounts of Ritta and Christina Parodi, the girls often appear as the singular ‘Ritta-Christina’, or even ‘the girl with two heads’, rather than two girls with one body – which is what they were.
Until recently, the origin of conjoined twins has been debated in much the terms that Aristotle used: they are the result either of fusion or fission. Most medical textbooks plump for the latter. Monozygotic (identical) twins, the argument goes, are manifestly the products of one embryo that has accidentally split into two; and if an embryo can split completely, surely it can split partially as well. This argument has the attraction of simplicity. It is also true that conjoined twins are nearly always monozygotic – they originate from a single egg fertilised by a single sperm. Yet there are several hints that monozygotic twins who are born conjoined are the result of quite different events in the first few weeks after conception than are those who are born separate.
One difference between conjoined and separate twins is that conjoined twins share a single placenta and (as they must) a single amniotic sac. Separate twins also share a single placenta, but each usually has an amniotic sac of its own as well. Since the amniotic sac forms after the placenta, this suggests that the split – if split it is – happens later in conjoined twins than in separate twins.
Another suggestive difference comes from the strange statistics of twin gender. Fifty per cent of separate monozygotic twins born are female. This is a little higher than one would expect, since, in most populations at most times, slightly fewer girls than boys are born. But in conjoined twins the skew towards femininity is overwhelming: about 77 per cent are girls. No one knows why this is so, but it neatly explains why depictions of conjoined twins – from Neolithic shrines to the New York Post – are so often female.
Perhaps the best reason for thinking that conjoined twins are not the result of a partially split embryo is the geometry of the twins themselves. Conjoined twins may be joined at their heads, thoraxes, abdomens or hips; they may be oriented belly to belly, side to side, or back to back; and each of these connections may be so weak that they share hardly any organs or so intimate that they share them all. It is hard to see how all this astonishing array of bodily configurations could arise by simply splitting an embryo in two.
But where are the origins of conjoined twins to be found if not in partially split embryos? Sir Thomas Browne called the womb ‘the obscure world’, and so it is – never more so than when we try to explain the creation of conjoined twins. The latest ideas suggest, however, that Aristotle’s dichotomy – fission or fusion – is illusory. The making of conjoined twins is, first, a matter of making two embryos out of one, and then of gluing them together. Moreover, the way in which two embryos are made out of one is nothing so crude as some sort of mechanical splitting of the embryo. It is, instead, something more subtle and interesting. Indeed, although we perceive conjoined twins as the strangest of all forms that the human body can take (as recently as 1996 The Times referred to one pair of twins as ‘metaphysical insults’), they have shown us the devices by which our bodies are given order in the womb.
ORGANISE ME
On the seventh day after conception, a human embryo begins to dig. Though only a hollow ball made up of a hundred or so cells, it is able to embed itself in the uterine linings of its mother’s womb that are softened and swollen by the hormones of the menstrual cycle. Most of the cells in the hollow ball are occupied with the business of burrowing, but some are up to other things. They are beginning to organise themselves into a ball of their own, so that by day 9 the embryo is rather like one of those ingenious Chinese toys composed of carved ivory spheres within spheres within spheres. By day 13 it has disappeared within the uterine lining, and the wound it has caused has usually healed. The embryo is beginning to build itself.
Its first task is to make the raw materials of its organs. We are three-dimensional creatures: bags of skin that surround layers of bone and muscle that, in turn, support a maze of internal plumbing; and each of these layers is constructed from specialised tissues. But the embryo faces a problem. Of the elaborate structure that it has already built, only a minute fraction – a small clump of cells in the innermost sphere – is actually destined to produce the foetus; all the rest will just become its ancillary equipment: placenta, umbilical cord and the like. And to make foetus out of this clump of cells, the embryo has to reorganise itself.
The process by which it does this is called ‘gastrulation’. At about day 13 after conception, the clump of cells has become a disc with
a cavity above it (the future amniotic cavity) and a cavity below it (the future yolk sac). Halfway down the length of this disc, a groove appears, the so-called ‘primitive streak’. Cells migrate towards the streak and pour themselves into it. The first cells that go through layer themselves around the yolk cavity. More cells enter the streak and form another layer above the first. The result is an embryo organised into three layers where once there was one: a gastrula.
The three layers of the gastrula anticipate our organs. The top layer is the ectoderm – it will become the outer layers of the skin and most of the nervous system; beneath it is the mesoderm – future muscle and bone; and surrounding the yolk is the endoderm – ultimate source of the gut, pancreas, spleen and liver. (Ecto-, meso- and endo- come from the Greek for outer, middle and inner derm – skin – respectively.)
The division sounds clear-cut, but in fact many parts of our bodies – teeth, breasts, arms, legs, genitalia – are intricate combinations of ectoderm and mesoderm. More important than the material from which it builds its organs, the embryo has also now acquired the geometry that it will have for the rest of its life. Two weeks after egg met sperm, the embryo has a head and a tail, a front and a back, and a left and a right. The question is, how did it get them?
In the spring of 1920, Hilda Pröscholdt arrived in the German university town of Freiburg. She had come to work with Hans Spemann, one of the most important figures in the new, largely German, science of Entwicklungsmechanik, ‘developmental mechanics’. The glassy embryos of sea urchins were being bisected; green-tentacled Hydra lost their heads only to regrow them again; frogs and newts were made to yield up their eggs for intricate transplantation experiments. Spemann was a master of this science, and Pröscholdt was there to do a Ph.D. in his laboratory. At first she floundered; the experiments that Spemann asked her to do seemed technically impossible and, in retrospect, they were. But she was bright, tenacious and competent, and in the spring of 1921 Spemann suggested another line of work. Its results would provide the first glimpse into how the embryo gets its order.
Then as now, the implicit goal of most developmental biologists was to understand how human embryos construct themselves, or failing that, how the embryos of other species of mammal do. But mammal embryos are difficult to work with. They’re hard to find and difficult to keep alive outside the womb. Not so newt embryos. Newts lay an abundance of tiny eggs that can, with practice, be surgically manipulated. It was even possible to transplant pieces of tissue between newt embryos and have them graft and grow.
The experiment which Spemann now suggested to Hilda Pröscholdt entailed excising a piece of tissue from the far edge of one embryo’s blastopore – the newt equivalent of the human primitive streak – and transplanting it onto another embryo. Observing that the embryo’s tissue layers and geometry arose from cells that had passed through the blastopore, Spemann reasoned that the tissues at the blastopore’s lip had some special power to instruct the cells that were travelling past it. If so, then embryos that had extra bits of blastopore lip grafted onto them might have – what? Surplus quantities of mesoderm and endoderm? A fatally scrambled geometry? Completely normal development? Earlier experiments that Spemann himself had carried out had yielded intriguing but ambiguous results. Now Hilda Pröscholdt was going to do the thing properly.
Between 1921 and 1923 she carried out 259 transplantation experiments. Most of her embryos did not survive the surgery. But six embryos that did make it are among the most famous in developmental biology, for each contained the makings of not one newt but two. Each had the beginnings of two heads, two tails, two neural tubes, two sets of muscles, two notochords, and two guts. She had made conjoined-twin newts, oriented belly to belly.
This was remarkable, but the real beauty of the experiment lay in Pröscholdt’s use of two different species of newts as donor and host. The common newt, the donor species, has darkly pigmented cells where the great-crested newt, the host species, does not. The extra organs, it was clear, belonged to the host embryo rather than the donor. This implied that the transplanted piece of blastopore lip had not become an extra newt, but rather had induced one out of undifferentiated host cells. This tiny piece of tissue seemed to have the power to instruct a whole new creature, complete in nearly all its parts. Spemann, with no sense of hyperbole, called the far lip of the newt’s blastopore ‘the organiser’, the name by which it is still known.
For seventy years, developmental biologists searched in vain for the source of the organiser’s power. They knew roughly what they were looking for: a molecule secreted by one cell that would tell another cell what to do, what to become, and where to go.
Very quickly it became apparent that the potency of the organiser lay in a small part of mesoderm just underneath the lip of the blastopore. The idea was simple: the cells that had migrated through the blastopore into the interior of the embryo were naive, uninformed, but their potential was unlimited. Spemann aphorised this idea when he said ‘We are standing and walking with parts of our body that could have been used for thinking had they developed in another part of the embryo.’ The mesodermal cells of the blastopore edge were the source of a signal that filtered into the embryo, or to use the term that was soon invented, a morphogen. This signal was strong near its source but gradually became fainter and fainter as it dissipated away. There was, in short, a three-dimensional gradient in the concentration of morphogen. Cells perceived this gradient and knew accordingly where and what they were. If the signal was strong, then ectodermal cells formed into the spinal cord that runs the length of our back; if it was faint, then they became the skin that covers our body. The same logic applied to the other germ layers. If the organiser signal was strong, mesoderm would become muscle; fainter, kidneys; fainter yet, connective tissue and blood cells. What the organiser did was pattern the cells beneath it.
It would be tedious to recount the many false starts, the years wasted on the search for the organiser morphogen, the hecatombs of frog and newt embryos ground up in the search for the elusive substance, and then, in the 1960s, the growing belief that the problem was intractable and should simply be abandoned. ‘Science,’ Peter Medawar once said, ‘is the Art of the Soluble.’ But the soluble was precisely what the art of the day could not find.
In the early 1990s recombinant DNA technology was applied to the problem. By 1993 a protein was identified that, when injected into the embryos of African clawed toads, gave conjoined-twin tadpoles. At last it was possible to obtain – without crude surgery – the results that Hilda Pröscholdt had found so many years before. The protein was especially good at turning naive ectoderm into spinal cord and brain. With a whimsy that is pervasive in this area of biology, it was named ‘noggin’. By this time techniques had been developed that made it possible to see where in an embryo genes were being switched on and off. The noggin gene was turned on at the far end of the blastopore’s lip, just where the gene encoding an organising morphogen should be.
Noggin is a signalling molecule – that is, a molecule by which one cell communicates with another. Animals have an inordinate number of them. Of the thirty thousand genes in the human genome, at least twelve hundred are thought to encode proteins involved in communication between cells. They come in great families of related molecules: the transforming growth factor-betas (TGF-?), the hedgehogs and the fibroblast growth factors (FGFs) to name but a few, and some families contain more than a dozen members. The way they work varies in detail, but the theme is the same. Secreted by one cell, they attach to receptors on the surfaces of other cells and in doing so begin a sequence of molecular events that reaches into the recipient cell. The chain of information finally reaches the nucleus, where batteries of other genes are either activated or repressed, and the cell adopts a fate, an identity.
When noggin was first discovered, it was supposed that its uncanny powers lay in an ability to define the back of the embryo from the front – more precisely, to instruct naive ectodermal cells to become spin
al column rather than skin. This was the simplest interpretation of the data. Noggin, the thinking went, spurred ectodermal cells on to higher things; without it, they would languish as humble skin.
The truth is a bit more subtle. The probability that a cell becomes spinal column rather than skin is not just a function of the quantity of noggin that finds its way to its receptors, but is rather the outcome of molecular conflict over its fate. I said that our genomes encode an inordinate number of signalling molecules. This implies that the cells in our bodies must be continually bathed in many signals emanating from many sources. Some of these signals speak with one voice, but others offer conflicting advice. Noggin from the organiser may urge ectoderm to become neurons, but as it does so, from the opposite side of the embryo another molecule, bone morphogenetic protein 4 (BMP4) instructs those same cells to become skin.
The manner in which the embryo resolves the conflict between these two signals is ingenious. Each signal has its own receptor to which it will attach, but noggin, with cunning versatility, can also attach to free BMP4 molecules as they filter through the intercellular spaces, and disable them. Cells close to the organiser are not only induced to become neurons, but are also inhibited from becoming skin; far from the organiser the opposite obtains. The fate of a given cell depends on the balance of the concentration between the two competing molecules. It is an ingenious device, only one of many like it that work throughout the development of vertebrate bodies, at scales large and small, to a variety of ends; but here the end is a toad or a child that has a front and a back. In a way, the embryo is just a microcosm of the cognitive world that we inhabit, the world of signals that insistently urge us to travel to one destination rather than another, eschew some goals in favour of others, hold some things to be true and others false; in short, that moulds us into what we are.
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