Hello and welcome to the rest is science. I'm Michael Stevens.

And I'm Hannah Frye. And I've got a question for you, Michael.

Okay.

It's a bit of a, it's a bit of a weird one, right?

Good.

It's sort of a philosophical question in a way, but maybe also deeply scientific.

If you start out with an embryo and it's just this perfectly symmetrical sphere of cells, how does it ever

decide where the head goes, right? Why doesn't it just, how does it ever end up with any structure?

I mean, when there's already a little bit of structure there, like I get it. Maybe there's

hormones that come from the head cells that let the neck start forming.

But when you're a blastocyst, like just a ball, a symmetric ball of cells, how does it decide,

all right, guys, final positions, you're the butt, you guys are the toes, you're going to be the brain, get to work.

Ready, steady, go.

Which way's up and down? Does it have to do with like local gravity or the parent's body? I don't know. How does it seed those? How does it seed those exactly? But then also, I mean, I mean, you said if there's a little bit of structure there, maybe it makes sense. but at the same time, when you look at physics, if you take, I don't know, like a glass of water, for instance,

and you put a little drop of ink in it, the process that happens there is diffusion. And diffusion

is like the destroyer of patterns. You don't get structure from physical processes. So what is it about biology that means that you end up

with structure? It's a bit of a puzzle. It's a bit of a puzzle. Well, yeah. It's really blowing my

mind because I've seen a blastocyst, a human one through a microscope when it's like eight cells.

And you're like, wow, cool. But how in the world does it

start assigning roles to each cell? And how do you make sure that the two cells on opposite

ends don't both decide that they're going to start forming the brain tube? Right. Am I going to find out

today? I think you are. I think you are. There's going to be an

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