By day 13, the blast assist is firmly implanted in the endometrium. We'll now make sagittal cuts through each constituent part one after another. The ademosst is the syncediotrofoblast, which secreats the beta-HCG, we detect in pregnancy testing. Next is the site of troughoblast. Followed by extra embryonic mezzarderm. Inside of which is a space known as the charyonic cavity. Sitting in this cavity and also surrounded by extra embryonic mezzarderm is the developing embryo. The parts of which we'll discuss next. The epi-blast sits atop the hypoblast. And together, these are known as the bilaminate disc. Cells of the hypoblast extend out to form this space called the oaksac. By this stage, the epi-blast has developed inside it the amniotic cavity. We'll remove the outer elements in order to take a closer look at the bilaminate disc. At the cranial end of the hypoblast lies the pro-cordal plate, an area in which columnar cells are present. It serves as a marker of the embryo's cranial aspect. The end point of gastrelation is the formation of a trilaminate disc made up of ectoderm, mezzoderm and endoderm. The cells that make up these layers were all once of the epi-blast. This transition is achieved as follows. The epi-blast migrate inward, downward and then differentiate. For me, the side, this continuous passage begins like symmetrical waterfalls. To be clear, it is the moving cells which produce this form as shown here. Microscopically, this makes a visible groove in the surface of the epi-blast known as the primitive streak. Its presence marks the beginning of gastrelation. At the cranial end of the streak, cells are migrating thickly, forming this circular depression known as the primitive node. As the embryo grows, more cells migrate and the primitive streak elongates. Cells which differentiate into endoderm, displace cells of the hyperblast and eventually replace it entirely. Cells which remain in the upper layer differentiate into ectoderm, and cells which differentiate into mezzoderm or stay away between these two layers. Around day 16, cells migrating through the area known as the primitive node begin to form a hollow rod which curves toward the cranial end of the embryo. It extends toward the pro-cordal plate and is known as the notecordal process. We take a car and all cut now, we see the hollow rod, and if we move through it now, we should also see its connection to the primitive node. As the notecordal process grows longer, the embryo continues to grow in all dimensions. Around day 18, the notecordal process fuses with the endoderm, from this to this. It's now known as the notecordal plate. Being continuous with the primitive node, this now allows free passage between the amniotic cavity and the yolk sac. Let's take a look through to solidify that, starting at the primitive node. We're looking down through what is known as the Neuronteric Canal, and the space we can see into is the yolk sac. It's theorized that this brief communication between amniotic cavity and yolk sac allows pressure equilibration between the two cavities of the early embryo. Once that has been achieved, the two edges of the notecordal plate come toward each other and eventually fuse. They become a solid rod, the notecord. Notecord is Greek for back in the notecord act as the structural backbone of the developing embryo. By this stage, intraembryonic mesoderm fills almost all the space between the ecto and endoderm. Apart from the space the notecord occupies, it is absent also at the Orofaryngeal membrane, located where the pro-cordal plate once was, and the cloacal membrane. These are structural elements of the embryo that currently exist as ecto and endoderm tightly adhered. That concludes the process of gastroation, from biolaminar to triolaminar embryonic disc. Hit subscribe for more videos simplifying this complex topic. Thanks for watching, and we'll see you next time.