The field of discourse, then, is embryology. There has traditionally been a deep divide between two different attitudes to the way single cells turn into adult creatures. The official names for them are preformationism and epigenesis, but in their modern forms I shall call them the blueprint theory and the recipe theory. The early preformationists believed that the adult body was preformed in the single cell from which it was to develop. One of them imagined that he could see in his microscope a little miniature human - a 'homunculus' - curled up inside a sperm (not egg!). Embryonic development, for him, was simply a process of growth. All the bits of the adult body were already there, preformed. Presumably each male homunculus had his own ultraminiature sperms in which his own children were coiled up, and each of them contained his coiled up grandchildren... Quite apart from this problem of infinite regress, naive preformationism neglects the fact, which was hardly less obvious in the seventeenth century than now, that children inherit attributes from the mother as well as the father. To be fair, there were other preformationists called ovists, rather more numerous than the 'spermists', who believed that the adult was preformed in the egg rather than the sperm. But ovism suffers from the same two problems as spermism.
Modern preformationism does not suffer from either of these problems, but it is still wrong. Modern preformationism - the blueprint theory- holds that the DNA in a fertilised egg is equivalent to a blueprint of the adult body. A blueprint is a scaled-down miniature of the real thing. The real thing - house, car, or whatever it is - is a three-dimensional object, while a blueprint is two-dimensional. You can represent a three-dimensional object such as a building by means of a set of two-dimensional slices: a ground plan of every floor, various elevation views, and so on. This reduction in dimensions is a matter of convenience. Architects could provide builders with matchstick and balsa-wood scale models of buildings in three dimensions, but a set of two-dimensional models on flat paper - blueprints - is easier to carry around in a briefcase, easier to amend, and easier to work from.
A further reduction to one dimension is necessary if blueprints are to be stored in a computer's pulse code and, for example, transmitted by telephone line to another part of the country. This is easily done by recoding each two dimensional blueprint as a one-dimensional 'scan'. Television pictures are coded in this way for transmission over the airwaves. Again, the dimensional compression is an essentially trivial coding device. The important point is that there is still one-to-one correspondence between blueprint and building. Each bit of the blueprint corresponds to a matching bit of the building. There is a sense in which the blueprint is a miniaturised 'preformed' building, albeit the miniature may be recoded into fewer dimensions than the building has.
The reason for mentioning the reduction of blueprints to one dimension is, of course, that DNA is a one-dimensional code. Just as it is theoretically possible to transmit a scale model of a building via a one-dimensional telephone line - a digitised set of blueprints - so it is theoretically possible to transmit a scaled-down body via the one-dimensional digital DNA code. This doesn't happen but, if it did, it would be fair to say that modern molecular biology had vindicated the ancient theory of preformationism. Now to consider the other great theory of embryology, epigenesis, the recipe or 'cookery book' theory.
