The early history of embryology was riven between two opposing doctrines called preformationism and epigenesis. The distinction between them is not always clearly understood, so I shall spend a little time explaining these two terms. The preformationists believed that the egg (or sperm, for the preformationists were subdivided into 'ovists' versus 'spermists') contained a tiny miniature baby or 'homunculus'. All the parts of the baby were intricately in place, correctly disposed to each other, waiting only to be inflated like a compartmentalized balloon. This raises obvious problems. First, at least in its early naive form, it requires what everybody knows to be false: that we inherit only from one parent - the mother for the ovists, the father for the spermists. Second, preformationists of this kind had to face a Russian-doll-style infinite regress of homunculi within homunculi - or if not infinite, at least long enough to take us back to Eve (Adam for the spermists). The only escape from the regress would be to construct the homunculus afresh in every generation by elaborately scanning the adult body of the previous generation. This 'inheritance of acquired characteristics' doesn't happen - otherwise Jewish boys would be born without foreskins, and gym-frequenting body-builders (but not their couch-potato twins) would conceive babies with rippling six-packs, pecs and glutes.
To be fair to the preformationists, they did face up, fairly and squarely, to the logical necessity of the regress, however absurd it seemed. At least some of them really did believe that the first woman (or man) contained miniaturized embryos of all her descendants, nested inside each other like Russian dolls. And there is a sense in which they had to believe that: a sense that is worth mentioning because it prefigures the nub of this chapter. If you believe Adam was 'made' rather than being born, you imply that Adam didn't have genes - or at least didn't need them in order to develop. Adam had no embryology but just sprang into existence. A related inference led the Victorian writer Philip Gosse (the father in Edmund Gosse's Father and Son) to write a book called Omphalos (Greek for 'navel') arguing that Adam must have had a navel, even though he was never born. A more sophisticated consequence of omphalogical reasoning would be that stars whose distance from us is more than a few thousand light years must have been created with ready-made light beams stretching almost all the way to us - otherwise we wouldn't be able to see them until the distant future! Making fun of omphalogy sounds frivolous, but there is a serious point here about embryology, which is the subject of this chapter. It is quite a difficult point to grasp - indeed, I am only in the process of grasping it myself - and I am approaching it from various directions.
For the reasons given, preformationism, at least in its original 'Russian doll' version, was always a nonstarter. Is there a version of preformationism that could be sensibly revived in the DNA age? Well, perhaps, but I doubt it. Textbooks of biology repeat time and again that DNA is a 'blueprint' for building a body. It isn't. A true blueprint of, say, a car or a house embodies a one-to-one mapping from paper to finished product. It follows from this that a blueprint is reversible. It is as easy to go from house to blueprint as the other way around, precisely because it is a one-to-one mapping. Actually, it's easier, because you have to build the house, but you only have to take some measurements and then draw the blueprint. If you take an animal's body, no matter how many detailed measurements you take, you can't reconstruct its DNA. That's what makes it false to say that DNA is a blueprint.
It is theoretically possible to imagine - maybe that's the way things work on some alien planet - that DNA might have been a coded description of a body: a kind of three-dimensional map rendered into the linear code of DNA 'letters'. That really would be reversible. Scanning the body to make a genetic blueprint is not a totally ridiculous idea. If that is how DNA worked, we could represent it as a kind of neo-preformationism. It wouldn't raise the spectre of the Russian dolls. It isn't clear to me whether it would raise the spectre of inheritance from only one parent. DNA has a breathtakingly precise way of intersplicing half the paternal information with exactly half the maternal information, but how would it go about intersplicing half a scan of the mother's body with half a scan of the father's body? Let it pass: this is all so far from reality.
DNA, then, is emphatically not a blueprint. Unlike Adam, who was fashioned directly into his adult form, all real bodies develop and grow from a single cell through the intermediate stages of embryo, foetus, baby, child and adolescent. Maybe in some alien world living creatures assemble themselves from tip to toe as an ordered set of three-dimensional bio-pixels read out from a coded scan line. But that is not the way things work on our planet, and actually I think there are reasons - which I have dealt with elsewhere and so won't go into here - why it could never be so on any planet.*
The historical alternative to preformationism is epigenesis. If preformationism is all about blueprints, epigenesis is about something more like a recipe or a computer program. The Shorter Oxford English Dictionary's definition is rather modern, and I'm not sure that Aristotle, who coined the word, would recognize it:
epigenesis: A theory of the development of an organism by progressive differentiation of an initially undifferentiated whole.*
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