In this video, we're going to be talking about gastrulation or germ layer formation and this is happening approximately days 12 through 16 post conception. So a germ layer is just a primary or a very initial layer of cells that forms that will give rise to a specific group of tissues and organs. As we will see in some future videos, each of these layers is going to be highly specialized to go on to form very certain tissues and organs in the human body and the term gastrulation just refers to the actual conversion of our bilaminar or our 2-layered embryonic disc into a trilaminar or a 3-layered disc. So gastrulation forms the three primary germ layers, and those are the endoderm, which translates to inner skin, the mesoderm, which translates to middle skin—I always think of those 'm's going together, 'm' and the 'm' and mesoderm and the 'm' in middle—and then the ectoderm which is the outer skin. Now, the process of gastrulation is going to happen because of the formation of something called the primitive streak. So the primitive streak is a groove on the caudal, or the tail end, of the bilaminar embryonic disc and like I said that will facilitate gastrulation. And I will show you this in our image in just one second, but basically what happens is that cells will begin to migrate toward that primitive streak, and that migration essentially forms the third layer. So if you look at our image here, what we are looking at here is the bilaminar embryonic disc. We are looking straight down so we can only see the top layer which we have drawn here in blue and as you can see on the caudal end of it, we have this little groove that is our primitive streak. So if we were to take a little slice of that bilaminar embryonic disc and put it on its side, it would look something like this. You can see we have that blue layer right here and this yellow layer right there and you can see we have cells migrating toward that primitive streak and that migration is kind of bulking up and building up a middle layer and so over time this will turn into this and now we have a trilaminar embryonic disc or a 3-layered disc. So now you can see we have the blue ectoderm. We have this red mesoderm in the middle, and then we have that yellow endoderm along the bottom. And this can be kind of a hard thing to picture in your head and mentally rotate, so to kind of help you out we added one more image just showing what this looks like from the side over here. So, in the side view, you can see nicely how we have these three layers of the disc. So we have that blue ectoderm, that red mesoderm in the middle, and then that yellow endoderm along the bottom. And what this image shows really nicely is that the yolk sac is actually attached to the endoderm. So we can't see that over here but do be aware that the yolk sac is attached to the endoderm and you're going to see some images in future lessons with the side view and you will see the yolk sac and how that kind of evolves a little bit throughout development, in those videos as well. If you're wondering how this disc goes on to form anything that looks remotely like a human, what happens around week 4 is that this disc is going to begin to undergo folding and that folding is going to make it a bit more cylindrical and then from there it will go on to form a much more recognizable human shape. Alright. So that is gastrulation and I will see you guys in our next video. Bye-bye.