 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 outermost is the syncytiotrophoblast, which secretes the beta-HCG we detect in pregnancy testing. Next is the cytotrophoblast, followed by extraembryonic mesoderm. Inside of which is a space known as the caryonic cavity. Sitting in this cavity and also surrounded by extraembryonic mesoderm is the developing embryo, the parts of which we'll discuss next. The epiblast sits atop the hypoblast, and together these are known as the bilaminar disk. Cells of the hypoblast extend out to form this space called the yolk sac. By this stage, the epiblast has developed inside it the amniotic cavity. We'll remove the outer elements in order to take a closer look at the bilaminar disk. At the cranial end of the hypoblast lies the procordal plate, an area in which columnar cells are present. It serves as a marker of the embryo's cranial aspect. The end point of gastrolation is the formation of a tri-laminar disk made up of ectoderm, mesoderm, and endoderm. The cells that make up these layers were all once of the epiblast. This transition is achieved as follows. Cells of the epiblast migrate inward, downward, and then differentiate. From either side, this continuous passage begins like symmetrical waterfalls. To be clear, it is the moving cells which produce this form as shown here. Macroscopically, this makes a visible groove in the surface of the epiblast known as the primitive streak. Its presence marks the beginning of gastrolation. 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 mesoderm, force their way 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 procordal plate and is known as the notocordal process. We take a coronal 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 notocordal process grows longer, the embryo continues to grow in all dimensions. Around day 18, the notocordal process fuses with the endoderm, from this to this. It's now known as the notocordal 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 neuranteric canal, and the space we can see into is the yolk sac. It's theorised 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 notocordal plate come toward each other and eventually fuse. They become a solid rod, the notocord. Noto is Greek for back, and the notocord act as the structural backbone of the developing embryo. By this stage, intra-embryonic mesoderm fills almost all the space between the ecto and endoderm. Apart from the space the notocord occupies, it is absent also at the oropharyngeal membrane, located where the procordal 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 gastrolation, from bilaminar to trilaminar embryonic disc. Hit subscribe for more videos simplifying this complex topic. Thanks for watching, and we'll see you next time.