 a kind of embryology, the gastrulation, first the contents, definition of gastrulation, cell invagination, blastopore, and an arcantiron formation. Let me start with gastrulation, definition of gastrulation, gastrulation is a process by which blastula is converted into gastrula, so it's blastula embryonic stages make a stage, first it's morula, then it's blastula, and after that it's gastrula stage. So when blastula is converted into gastrula, this process is called gastrulation. Or we have a very simple definition of gastrulation, that gastrulation actually becomes embryonic layers, the endoderm and mesoderm. So we can say that the process by which embryonic and germ layers are formed. The process by which embryonic layers are formed, that is actually the gastrulation process. To start the process of gastrulation, the morula, the ball of cells, later the gastrulation which is converted into the blastula, the blastula is basically a single layer of cells. So what it will do from one side, it will start invaginate them. Invagination will usually be from the vegetal pole. The upper cell is called the animal pole and the lower cell is called the vegetal pole. So the lower cells will gradually invaginate and start going inside. The result will be that the blasto-seal will start squeezing slowly and the new cavity of the blasto-seal will start becoming, that is called the orcantiron. So this way the blasto-seal will gradually replace the orcantiron. Now from where the cells are invaginating, we call that position or that opening the blastopore. The cavity inside the orcantiron is called the orcantiron. Blastopore and orcantiron both are very important because the blastopore has to make the anus. It has to make anus in the deuterostomes. If it was a deuterostome, it would have been a mouth. So the orcantiron will ultimately form the digestive system. So the orcantiron will form the gutter digestive system. In the diagram you can see clearly we have an early gastroesthetic stage. In the gastroesthetic stage you can see in the beginning that you can see black space. This is blasto-seal. The cells on the vegetal pole, they will start invagination. They will invaginate. When they invaginate, the blasto-seal will start squeezing. It will start working. And instead of that, they will start taking another cavity. And this new cavity is called the orcantiron. Now as they spread, the blasto-seal will start diminishing. It will start working when the size of the orcantiron increases. The opening of the blasto-seal will make the mouth, while the orcantiron will form the digestive system.