It is surprisingly hard to find a good analogy for the development of living tissue, but you can find partial similarities to particular aspects of the process. A recipe captures something of the truth, and it is an analogy that I sometimes use, to explain why 'blueprint' is not appropriate. Unlike a blueprint, a recipe is irreversible. If you follow a cake recipe step by step, you'll end up with a cake. But you can't take a cake and reconstruct the recipe - certainly not the exact words of the recipe - whereas, as we have seen, you could take a house and reconstruct something close to the original blueprint. This is because of the one-to-one mapping between bits of house and bits of blueprint. With conspicuous exceptions such as the cherry on top, there is no one-to-one mapping between bits of cake and the words, say, or sentences of its recipe.
What other analogies to human manufacturing might there be? Sculpture is mostly way off the mark. A sculptor starts with a chunk of stone or wood and fashions it by subtraction, chipping away until the desired shape is all that remains. There is, admittedly, a somewhat sharp resemblance to one particular process in embryology called apoptosis. Apoptosis is programmed cell death, and it is involved, for example, in the development of fingers and toes. In the human embryo, the fingers and toes are all joined. In the womb, you and I had webbed feet and hands. The webbing disappeared (in most people: there are occasional exceptions) through programmed cell death. That is a bit reminiscent of the way a sculptor carves out a shape, but it is not common enough or important enough to capture how embryology normally works. Embryologists may briefly think 'sculptor's chisel', but they don't let the thought linger for long.
Some sculptors work not by subtractive carving but by taking a lump of clay, or soft wax, and kneading it into shape (which may subsequently be cast, in bronze for example). That again is not a good analogy for embryology. Nor is the craft of tailoring or dressmaking. Pre-existing cloth is cut, to shapes set out in a pre-planned pattern, then sewn together with other cut-out shapes. They are often then turned inside out to disguise the seams - and that bit, at least, is a good analogy to certain parts of embryology. But in general, embryology is no more like tailoring than it is like sculpture. Knitting might be better, in that the whole shape of a sweater, say, is built up from numerous individual stitches, like individual cells. But there are better analogies, as we shall see.
How about the assembly of a car, or other complicated machine, on a factory assembly line: is that a good analogy? Like sculpture and tailoring, assembly of pre-fabricated parts is an efficient way to make something. In a car factory, parts are pre-made, often by casting in moulds in a foundry (and there is, I think, nothing remotely like casting in embryology). Then the pre-made parts are brought together on an assembly line and screwed, riveted, welded or glued together, step by step according to a precisely drawn plan. Once again, embryology has nothing resembling a previously drawn plan. But there are resemblances to the ordered sticking together of pre-assembled parts, as when, in a car assembly plant, previously manufactured carburettors and distributor heads and fan belts and cylinder heads are brought together and joined in correct apposition.
Below are three kinds of virus. On the left is the tobacco mosaic virus (TMV), which parasitizes tobacco plants and other members of the family Solanaceae, such as tomatoes. In the middle is an adenovirus, which infects the respiratory system in many animals, including us. On the right is the T4 bacteriophage, which parasitizes bacteria. It looks like a lunar lander, and it behaves rather like one, 'landing' on the surface of a bacterium (which is very much larger) then lowering itself on its spidery 'legs', then thrusting a probe down the middle, through the bacterium's cell wall, and injecting its DNA inside. The viral DNA then hijacks the protein-making machinery of the bacterium, which is subverted into making new viruses. The other two viruses in the picture do something similar, although they don't look or behave like lunar landers. In all cases their genetic material hijacks the protein-making apparatus of the host cell and diverts its molecular production line to churning out viruses instead of its normal products.
Most of what you see in the pictures is the protein container for the genetic material, and in ('lunar lander') T4's case the machinery for infecting the host. What is interesting is the way in which this protein apparatus is put together. It really is self-assembled. Each virus is assembled from several previously made protein molecules. Each protein molecule, in a way that we shall see, has previously self-assembled into a characteristic 'tertiary structure' under the laws of chemistry given its particular sequence of amino acids. And then, in the virus, the protein molecules join up with each other to form a so-called 'quaternary structure', again by following local rules. There is no global plan, no blueprint.
The protein sub-units, which link up like Lego bricks to form the quaternary structure, are called capsomeres. Notice how geometrically perfect these little constructions are. The adenovirus in the middle has exactly 252 capsomeres, drawn here as little balls, arranged in an icosahedron. The icosahedron is that Platonic perfect solid that has 20 triangular faces. The capsomeres are arranged into an icosahedron not by any kind of master plan or blueprint but simply by each one of them obeying the laws of chemical attraction locally when it bumps into others like itself. This is how crystals are formed, and, indeed, the adenovirus could be described as a very small hollow crystal. The 'crystallization' of viruses is an especially beautiful example of the 'self-assembly' that I am touting as a major principle by which living creatures are put together. The T4 'lunar lander' phage also has an icosahedron for its main DNA receptacle, but its self-assembled quaternary structure is more complex, incorporating additional protein units, assembled according to different local rules, in the injection apparatus and the 'legs' that are attached to the icosahedron.
Returning from viruses to the embryology of larger creatures, I come to my favourite analogy among human construction techniques: origami. Origami is the art of constructive paper-folding, developed to its most advanced level in Japan. The only origami creation I know how to make is the 'Chinese Junk'. I was taught it by my father, who learned it in a craze that swept through his boarding school during the 1920s.* One biologically realistic feature is that the 'embryology' of the Chinese junk passes through several intermediate 'larval' stages, which are in themselves pleasing creations, just as a caterpillar is a beautiful, working intermediate on the way to a butterfly, which it scarcely resembles at all. Starting with a simple square piece of paper, and simply folding it - never cutting it, never glueing it and never importing any other pieces - the procedure takes us through three recognizable 'larval stages': a 'catamaran', a 'box with two lids' and a 'picture in a frame', before culminating in the 'adult' Chinese junk itself. In favour of the origami analogy, when you first are taught how to make a Chinese junk, not only the junk itself but each of the three 'larval' stages - catamaran, cupboard, picture frame - comes as a surprise. Your hands may do the folding, but you are emphatically not following a blueprint for a Chinese junk, or for any of the larval stages. You are following a set of folding rules that seem to have no connection with the end product, until it finally emerges like a butterfly from its chrysalis. So the origami analogy captures something of the importance of 'local rules' as opposed to a global plan.
Also in favour of the origami analogy, folding, invagination and turning inside out are some of the favourite tricks used by embryonic tissues when making a body. The analogy works especially well for the early embryonic stages. But it has its shortcomings, and here are two obvious ones. First, human hands are needed to do the folding. Second, the developing paper 'embryo' doesn't grow larger. It ends up weighing exactly as much as when it started. To acknowledge the difference, I shall sometimes refer to biological embryology as 'inflating origami', rather than just 'origami'.
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