 mammalian embryology, cleavage and blastolation, first the contents, cleavage, compaction, modulation, totipotent and polyurepotent cells, blastola and the diagrammatic representation of the blastolation process. So first the cleavage. Students, let me just start from the fertilized egg of mammals. If we talk about the fertilized egg of mammals, the mammals we are specifically discussing here are the placental mammals. The egg of the placental mammal is the smallest one. The smallest egg of the animal is the placental mammal. The yolk is present in it as it is not present. So because of which all the cell divisions will be holobelastic. That is, it will completely divide the cells in two. Because of the yolk, there will be no half or restriction because of the yolk. So all the cell division in mammals, they are the holobelastic cleavage. Or holobelastic cleavage is present in it. So initially, the pattern of cell division, if we look at it, it is very, very slow. When the embryos of mammals start dividing, the cell division of mammals is very slow. Number one. And the second important thing is that the pattern of cell division is not the one that we see in the rest of the embryos. In this, that is, we will find a special kind of unique feature. First of all, that the cleavage started, the cell division started, it is very slow. Number two. The pattern of the cleavage is different from the rest of the animals. How? The first division will be longitudinal or meridinial, which is said to be divided in one cell, two cells from the top to the bottom. Two bolastromers will be formed. Now the second cleavage will be divided in two cells differently. One cell will be divided vertically again and one will be divided horizontally. So in this way, the division pattern of the two cells will be different. So that's why we call it rotational cleavage. This type of cleavage is called the rotational cleavage. Second one. The pattern of cleavage is different from the rotational cleavage. Along with that, one more important thing is that the cells are not dividing synchronously. Synchronous means they are not dividing together. Some cells are first dividing, some are dividing later. That is why we do not get two cells, four cells, eight cells, sixteen cells. In fact, at some time we can get a stage with the order number. It is possible that instead of four or eight cells, we can get five cells for some time. Or we can get a stage with four or seven cells. They are divided in three cells, one cell is dividing and the other is slowly dividing. So in this way, since the cell division is not regular or synchronous, some cells are first dividing and some will divide later. That is why we do not get the number of two, four, eight, sixteen, thirty-two. The third important thing is that when the cell division is in the rest of the animals, the messenger RNA comes from the mother for protein synthesis at that time. That is, the egg comes from the first messenger RNA. And during the cell division, the messenger RNA makes proteins. This is not the case in mammals. The egg of the mammal does not come from the mother to the messenger RNA. In fact, when it is necessary, it makes its messenger RNA and starts the cell division. That is why in the case of mammals, the maternal effect will be less. That is, there is no maternal dominance, but the cell will utilize its machinery as its own and start the cell division. Here if we look diagramtically, as I said, after two cell stages, one cell will divide in one way. If one cell is dividing horizontally, then the other cell will be dividing vertically. As you can see here, the division of this cell is like this. Whereas the other cell is dividing vertically. Or the opposite can be that the one cell is dividing vertically and the lower cell is dividing horizontally. In both cases, what we will say is that this is rotational cleavage. After that, if we look further, then again, the pattern of the cell division can be any way. In any way, the cells can divide themselves and form any form. This is why this pattern is called the rotational cleavage. This rotational cleavage is typically found in mammals. Cleavage, as soon as these cells are dividing, then when the cells will be up to eight cells, that is, two cells, four cells, and then eight cells, the cells in the eight cell stage are not in a closely compact form, but they are loosely arranged. As soon as the eight cell stage comes, the cells will release specific proteins. And the specific protein will become tightly connected to each other. This is called the compaction, which we will call compaction. Then, after the eight cell stage, the next stage is the sixteen cell stage. We will divide the eight cell stage and make sixteen cells. The sixteen cell stage is a solid cell. The cell is in the form of a solid wall. We will name this wall of cells as Morula. We will get two types of cells in Morula. Some of those cells are called inner cell mass. And the cells on the sides are called the trophoblast. The embryonic cells that are actually going to be animals are going to be made out of the inner cell mass. The trophoblast will be made out of the layer of the membrane. The actual thing is the inner cell mass. When the embryo was on the eight cell stage, it was made out of the eight to sixteen cell stage. Some of the sixteen cells are made out of the inner cell mass and some are made out of the trophoblast. So, the cells on the eight cell stage can be made out of the ICM and the trophoblast. So, the cells on the eight cell stage, the blastomeres, they can be made out of either of the two, that's why they are called the totipotent cells. Whereas, the cells on the inner cell mass, the cells on the inner cell mass can be made out of different embryonic layers. So, the cells on the inner cell mass are called pluripotent. The cells on the eight cell stage are called totipotent. Whereas, the cells on the inner cell mass are actually pluripotent cells. So, the cells on the sixteen cell stage, the cells on the inner cell mass and the trophoblast, they are compactly arranged. Now, the trophoblast cells will secrete some material. They will release the material because of which a fluid filled cavity is created in between. And that cavity is called the blasto-seal. So, the blasto-seal will be produced and all the cells will line the blasto-seal. So, the trophoblast cells are in the inner cell mass and the trophoblast cells are in the trophoblast cells. Here, you can see the overall diagram. This is the four cell stage. After the four cell stage, we have eight cell stages. The cells are compact. The cells on the four cell stage are not compact. What is this here? It is a compaction. Now, if we look at the further diagram, then the marula is also a compact form. After that, if we look at the gourd, the cells in between are in the inner cell mass. This is in the inner cell mass and the green cells in the side are basically the trophoblast cells. The trophoderms are called trophoblast cells. In the further step, the trophoblast cells will release some kind of material which will make the blasto-seal. And the cells will line all the other cells. The blasto-seal will be placed on the embryo stage.