 Shall we remind ourselves of what is a zygote? What is this zygote you speak of? Remember, when a sperm combines with an egg, the result is a single cell called a zygote. This single cell, that's you. That's your baby-est baby picture. That's the first cell that was you. The zygote goes through mitosis. Not mitosis, like mad, and divides to become a grown-up you. And because I love you, I'm going to give you a balloon. That balloon has a long string, but look, I'm going to give you a glove, too, to hold that balloon. Don't say I never did anything for you. I'm going to look at how that zygote specifically divided to form your central nervous system. And the crazy thing is that this all started at about four weeks in utero. This is four weeks in your mama's belly. You were a zygote for four weeks. You're not even out yet. You're not even close to being out yet. You already are forming some brain structures. However, you probably are thinking about the brain structures that you're forming and going, dude, really? That does not look like my brain. It's called a neural tube, and you form it at four weeks. And that structure, okay, so you accept the fact that a single cell goes through mitosis and then cells differentiate, and they're going to focus on forming different structures. Like some of those cells are going to become your heart. Some of those cells are going to become skeletal muscles. Some of those cells are going to become your skin. And some of those cells are going to become your nervous system. Those cells that are going to become your nervous system first form a neural tube. The neural tube slowly, I mean, it actually happens pretty quick, change how they are going through mitosis. And so that one end of this neural tube starts going through mitosis faster than the rest of it, and it starts to kind of swell. Can you visualize that? So there are multiple cells in here, and they just start going through mitosis more than the cells down here. If you had to label one end of my tube as cranial, what do you think is the head end? The end that's swelling. This is the cranial end. And we could go with, okay, this is the caudal end. This is the tail end of my neural tube. So the cranial end starts to swell and get kind of a bulbousy thing. And again, why is it swelling? It just is going, those cells are just going through mitosis more. Not only do they start swelling in general, they start swelling kind of in a specific orientation or shape, and they form structures called primary brain vesicles. Okay, so I'm going to label that down here. No, I'm going to label, I'm going to label that up here. Primary brain vesicles. And how many primary brain vesicles do you see in my drawing? One, two, three. There are three primary brain vesicles. And watch, I'm going to tell you what their names are. The first primary brain vesicle, I'm going to give you its name down here, is called the pro-prose encephalon. The nice thing is that all of my brain vesicles, all of them end in encephalon. Ceph means head. So encephalon means toward the head, and cephalic means head end of things. So my pro-encephalon is this first most cranial bulb, primary brain vesicle. And the second one is called the mesencephalon. The mesencephalon means encephalon. The nice thing, so we have the pros encephalon, the mesencephalon, and then we have one more. And this guy is called the rhombencephalon. Oops, it's not number four yet. It's number three. It's the rhombencephalon. Primary brain vesicles. I don't like your brain yet, hopefully not. The primary brain vesicles are going to give rise. We're going to get a couple more differential growth patterns. And so watch this. We're going to have my primary brain vesicles give rise to who do you think? Secondary brain vesicles, right? And here's the deal. My pros encephalon actually kind of blobs into two of them. I'm expanding this so that you can see it better. My mesencephalon does not divide. It stays. The mesencephalon primary brain vesicle stays and doesn't become another secondary brain vesicle. It stays as the mesencephalon. And then my rhombencephalon becomes two secondary brain vesicles so that what I have here, and again, we'll just go all the way down just to keep it consistent. What I have are two secondary brain vesicles that came from the rhombencephalon. One secondary brain vesicle that came from the mesencephalon. And guess what its name is? So nice. It's the mesencephalon. So the primary brain vesicle mesencephalon gives rise to the secondary brain vesicle mesencephalon. And then the prosencephalon gives rise to two secondary brain vesicles. Let's write down their names. What I have, the most cranial, the most superior in my neural tube is called the telencephalon. What did I say about encephalon? All of them have encephalon after their names. But the prosencephalon doesn't just become telencephalon, it also becomes diencephalon. Tel, di, mes, oh, and then my rhombencephalon, I was about to finish my little chant. My rhombencephalon becomes tel, di, mes, metencephalon, metencephalon, and myelencephalon. Tel, di, mes, met, myel. You could do a little chant, a little song. You could do a parade, marching around, singing the tel, di, mes, met, myel song. And then you'll never forget your secondary brain vesicles. Is that the end of the story? Not in a million. In fact, my secondary brain vesicles give rise to mature brain structures that we all have, most of us have, all of them. And we're going to look at which adult brain structures come from each of these secondary brain vesicles. Very good. Just in case you are having a hard time visualizing the wind-ified version of baby neural tube growth, check this, it's a baby picture of you, your cute little self right here. We all look like this. What brain vesicles do you think we're dealing with here? What do you think these are? Dog pounds of a feather. This is your primary brain vesicles, prosencephalon, mesencephalon, rhombencephalon. And you can see that we don't stay that way. We actually end up with five secondary brain vesicles. And look, this is helpful to me because you can actually see how the tube fits inside a little embryo and how that tube, like we're just having growth happening here. And we'll see how basically your telencephalon, this is the one we're going to talk about first, your telencephalon like becomes gigantic in humans and kind of covers over everything else. And the brain that you actually look at, you're seeing a lot of telencephalon, very little if any diencephalon, unless you turn it over or cut it in half. Okay, let's get started looking at the brain as a whole and then we'll look at our specific structures and where they come from.