Most of the cells, most of the organisms actually on earth today, are single cells. Most of them are pretty small, they're bacteria like the ones that would form the very early sorts of life and are still all around us today. It's an interesting thought that there are far more bacterial cells in and around your body than there are cells of your own; ever thought of that?

Others are very big - this ostrich egg, for example, is a single cell before it starts to develop. But in general, to make larger organisms, you have to use lots of little cells, that's the way they work. This picture here shows us the beginning of a human being, the fertilisation of an egg by a sperm. That little cell, the fertilised egg just a pin prick, is going to grow and divide and grow and divide and we'll see a film of that in a moment, with another organism which develops into not so many cells as us - the nematode worm. The fertilisation happens and you see the two pro-nuclei coming together: that's the male and the female nuclei, you can see the separation - and you see the first cell division. It's going to go on and on dividing and we're going to watch that happen. Now as it does it, the same DNA gets packaged into all the cells, so how come it's able to put together an organism? How come we are able to be an organism when we're made of so many cells, all of which contain the same DNA? It's important that somehow the cells know what to do.

The way to get at these questions is not to study people in the first instance because we're too big, we're too complex and also rather precious - you don't go cutting out bits of people and messing about with them in the lab, but you can do it with simpler animals, like this nematode - which is why we see its development here; and like the fruit fly, which has given us a tremendous opportunity to discover these control genes.

Now under here, we have a fly which is antennapedia. Can you see what's wrong with this fly? It's an awkward angle. You can see that this is a leg growing out of its head, which is why it's called antennapedia. There was another fly which was discovered nearly a hundred years ago, which is this one. It has four wings instead of only two and this was what gave the clue to the idea that there would be control genes that somehow specify the different parts of the body, which of course is exactly what you want if you're going to make this great menagerie of cells into a defined differentiated structure. And to see a bit about how these genes work, we've got a model here. If I could get somebody to come and help me with this? Would you like to come? OK, come and help me put this fly together.

You can see there's a fly there and you can see we've got coloured bits of its body. The puzzle is to put the bits together. Can you have a go? See what you're doing, yes, there's the head. And you can see that the various parts are coloured. Now there's a good reason for that. It's because these parts actually come from strips, from segments in the early fly. There's a big one that goes here I think somewhere. Yes, there we are - you see the legs are appearing. In the early fly, these segments are laid out and then they get specified to have different fates. The genes that specify them are called the genes in the hox cluster, which define the fates from head to tail in these segments.

I can't do this one at all … perhaps we could put that one in, there we are, we're getting there now. You're nearly there, good, thank you, that's a great help. Well done.

Now the genes that label these segments are laid out in order here according to the colours, and that's not an accident because that's just the way they look along the fly's DNA. Quite remarkably, they're laid out in order along the fly's genome in the same order that they're laid out in on the fly's body, head to tail. And the same genes - or at least relatives of them - are found quite widely throughout the animal kingdom.

But first I want to tell you a little bit about how we can see how the fly begins. You can see the stripes being laid out. Here are the genes that control the early development of the fly being made to produce an enzyme which allows us to stain in stripes, the areas where they're being expressed and these are the genes that lay out the structure.

In fact a way of describing it, of thinking about it, is building a house. If you want to build a house, what happens is that you make a scaffold, a sort of area to work in. And that's a bit like the mother's body which is incubating this growing organism. And then you start to build the bricks and then you start to make floors and you start to divide into rooms - and then you specify the rooms for particular purposes. So you see, making an organ or an organism is a little bit like doing that: you get the overall structure first, as these genes are doing in the early part of the fly, and then you specify, as we saw in the model, the regions to do particular things.

And what happened with the four-winged fly is that a segment which was supposed to produce just little balancing pieces produced whole wings instead. And in the one under the microscope that we had there before, the area of the head that was supposed to produce an antenna, produced a little sort of stunted leg and the reason for that is that the regional specifiers were wrong.

Now what the regional specifiers go on to do is to control lots of other genes and it's quite hard to figure out what's going on. But we can often do well to link the genes to fluorescent molecules. This is one that comes from the jellyfish, the green fluorescent protein. And what we're seeing here are now nematode worms which Duncan is organising under the microscope. If we pop the light up, so that we can see the worms as well, there you can see what we're actually seeing: these are worms whose head, whose mouth part - that's the mouth where it pumps food in - is stained, because there's a particular sort of muscle there which has been linked to the fluorescent protein and when it's expressed, you can see it.

You can go on doing this throughout the body of a developing animal and see where and when genes are expressed, by linking them to molecules that you can see. It's a way of labelling things and showing where they act.

Now what do these genes go on to do, all these control genes?

Well, obviously sometimes they specify making keratin again. If it's an appropriate part of the body to make hair, then they'll specify keratin - so they'll specify actual parts of the body. But it's much more complex than that. Lots of them will specify signals that have to go from cell to cell so the cells can communicate with each other as the system grows. That's what makes it stable, that's what allows the whole thing to regulate itself so you don't just depend on accident; it constantly brings itself to the right form by very, very complex systems of communication.

Sometimes odd things happen. For example, while you develop, a lot of your cells die. That seems a bit daft in a way that you would do that, but it's actually the most economical way and when you look at this embryo in the uterus, you see how it's got a little sort of webbed foot like this. These are really the fingers that have not been separated yet and so, during early development, cells have to die in between your fingers so that you don't have webbed feet.

So all of this is programmed. All this elaborate process is played out and the tool kit is preserved, those hox genes. That cluster of control genes that we saw laid out on the fly are reused, not in precisely the same way but in quite analogous ways in laying out our body. They become more complicated. It went through a process where we get multiple copies of those so it's harder to work out. Again, this is why the fly and the worm are good for working things out; they have smaller genomes, fewer genes, they're simpler to deal with. But by looking at what's going on in those, we can then come and look in our own bodies and find what's going on there too.

But there's another sort of thing that happens in developing your bodies. You don't just make the thing sort of growing out as a mass and sort of roughly the right shape. Some cells have to grow out over others.

For example, wiggle your toes. Everybody wiggle their toes. What's going on when you wiggle your toes? You're sending signals from your brain, down your back and there are nerves that start in the back here from the spine and run all the way down your legs. Those cells don't get there in the first place. As the body is growing, they actually grow out along the other cells - or over the other cells. And here we see this happening in a dish: we see a group of nerve cells which are still growing. They're from a growing embryo and they are sending out these processes just as yours will have done as you grew as a baby, growing nerves out down your legs and wherever you needed them. And if you touch your toes, you can feel the sensation of that and then of course the signal is going back the other way into your back and again you have these long processes of nerve cells that are carrying that information.

So you see, coming back to our house and thinking about that, when your house is largely built, electricians come in and lay out the wiring and it's like your nerve cells; you also have plumbers come in and lay out the plumbing, that's like your blood vessels. And they grow out over the other tissues as well. And so you find that the whole thing is put together and how's Max doing?

Can Max come on camera for a little while again? That'd be great.

Good, Max. So you see, Max has been assembled in this way. Nobody told him what to do, there weren't electricians or plumbers, there were just control genes that told the DNA when and where to make materials. But the materials put themselves together as we've seen, the proteins folding right, sticking themselves together right, making cells, the cells sticking together and developing. And so Max assembled himself and he did it with DNA that's 99.8 percent the same as the DNA of anybody else in this room. We all share nearly all of our genome. We have it in common from the long process of evolution that's produced us and we use it to make ourselves. The variations are important and we'll be talking about those next time, but for the moment, Max assembled himself and a new human being was born.