once the blast assistance formed, it has to implant in the uterus. This is called implantation, getting really creative with the names here. Now HCG is a chemical secreted by the implanted blastocyst, and it kind of lets the body know that pregnancy is underway. In fact, most early pregnancy tests actually test for the presence of HCG. Now, HCG is not the end all be all hormone involved in this and eventually it will sort of be replaced by secretion of estrogen progesterone. But all of this is a little beyond the scope of what you need to know for by a one. So what you should know is that those trophoblastic cells will develop into the placenta and they will actually take over the estrogen and progesterone production, and they will continue to produce it throughout pregnancy. Those hormones air very important thio pregnancy and to supporting the developing organism. Now, once we have our blast assist and planted, it can start to undergo what's called gassed relation, which is the formation of these three germ layers which forms a gas strolla. So as you can see here we have our blast assist. Right? Um has cells around the outside blast, Ecole, that fluid filled cavity inside. And what happens is it starts to kind of poke inward like we see happening right here. So it pokes inward. This is gassed relation. And the result is what we see here. The gas Strolla. And let me just highlight in red. This is an what I'm highlighting in red is like the mass of cells. There's space inside. However, there's hollow cavity. Now let's go over a little terminology real quick. We have our blast. Oh, poor. That's the opening to this space. Uh, we have also our three germ layers, right? So those three germ layers three germ layers are the acted ERM which are the cells that give rise thio nerves. The adrenal medulla, which is a gland in the body, Uh, skin, brain eyes, inner ear or your vestibular system, if you will. You also have the miso durm on the meso Derm. You can see there's actually, uh, one little patch right here. This is also me, Zod. ERM over here, you take myself out of shots. So it's easier to see all of this. So we have miso Durham on both sides, and these are internal cells that will give rise to organs the adrenal cortex, which is also part of the adrenal gland. Different part, though, um, the blood, bone, gonads and soft tissues. And then, lastly, we have the endo derm. In the end, O derm is the layer of those innermost cells that you can see kind of in this mustard color. I'm going to circle it in blue right now. So this is Art Endo Durham. Those innermost cells form the epithelial linings of the digestive tract of liver, pancreas and lungs. So in a large part they formed. The inner cells of the what's called the Elementary Canal are basically the cavity that runs through our bodies. All right, let's flip the page.