Moving away from invertebrates, let's take a look at the vertebrates, or animals that have vertebrae and a cranium. First, let's talk about the vertebrae. Remember we said that in many cases these are going to form from the notochord, and the vertebrae are a column of segmented bone. Right? So when you look at our spine, the actual spinal cord, like the nervous tissue, is contained within the vertebrae, and you have these segmented bone structures that wrap around the spinal cord. So here in this image, we're actually looking at 3 vertebrae, 3 individual vertebrae. And looking at this snake, you can see tons of individual vertebrae in this image. I mean each single one of these projections that you see coming out of either side is attached to a vertebra or vertebrae. So there you go.
And the cranium, the other feature that defines vertebrates, is the case that encloses the brain. Right? Our noodles, noggins, whatever you want to call it. These can be made of bone or in some cases cartilage, as we'll see. Now it's worth noting that the brains of most vertebrates tend to be divided between three regions, which we call the forebrain, midbrain, and hindbrain. Now, in the human brain, the forebrain is going to make up the most of what you see when you see a picture of a human brain, whereas these midbrain and hindbrain regions are going to be deeper structures and also structures on the brainstem, so not the prominent features of the brain. However, even basic vertebrates can have their brains divided between these regions.
Now, it's also worth noting that the jaws of the cranium are actually derived from a different place than the rest, I'm sorry, the jaws of the skull are derived from a different place than the cranium. They come from these embryonic cells called neural crest cells that actually contribute to many different structures. And the reason I'm bringing up jaws, is because jaws are actually an important milestone in the evolutionary history of vertebrates. Now, early lineages of vertebrates actually had bony exoskeletons, believe it or not. They had a lot of them. These bony exoskeleton skull kind of things. Now, we don't see that in later lineages, you know, we don't see vertebrates today, for example, that have these bony exoskeletons. So those are very early evolutionary lineages.
A big milestone, again, is going to be the evolution of jaws and the organisms that evolved jaws, we call gnathostomes and today, we see living examples of these organisms in things like sharks or the sort of fancy term for that, chondrichthyans. But you know, these guys, just sharks basically. Now, bony endoskeleton is the next big milestone in the evolutionary history of vertebrates, and the living relatives of these organisms that first made the leap are going to be what we call ray-fin and lobe-fin fish. And we'll see examples of all of these organisms as we move through this section.
The next big milestone is limbs for moving on land. Right? The proverbial fish crawling out of the oceans as people often like to talk about it. It's a little different though. And these organisms were called tetrapods. Right? Four feet basically. And their living relatives are going to be organisms like amphibians. Now, the last major evolutionary step that we're going to talk about for vertebrates is the amniotic egg. And we see the amniotic egg today in organisms like reptiles, and we call this class of organisms amniotes.
So with that, let's flip the page and talk about gnathostomes.
