Others thought this was ridiculous. To be sure, they argued, God was ultimately responsible for the order of nature, but the notion that He had deliberately engineered defective eggs or sperm as a sort of creative flourish was absurd. If bodies were clocks, then there seemed to be a lot of clocks around that were hardly to the Clockmaker’s credit. Monsters were not evidence of divine design: they were just accidents.
The conflict between these two radically different postitions, between deformity as divine design and deformity as accident, came to be known as la querelle des monstres – the quarrel of the monsters. It pitted French anatomists against one another for decades, the contenders trading blows in the Mémoires de l’Académie Royale des Sciences. More than theology was at stake. The quarrel was also a contest over two different views of how embryos are formed. Duverney and his followers were preformationists. They held that each egg (or, in some version of the theory, each sperm) contained the entire embryo writ small, complete with limbs, liver and lungs. Stranger yet, this tiny embryo (which some microscopists claimed they could see) also contained eggs or sperm, each of which, in turn contained an embryo… and so on, ad infinitum. Each of Eve’s ovaries, by this reasoning, contained all future humanity.
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.