 Well, in this lecture, we will be studying amphibian embryology. Amphibian embryology means that is the early development in frog, salamander, and so on. So, the cleavage in amphibians is radially spritrical and holoblastic. In fact, these eggs, these animals lay their eggs in shallow water. Or, at a time, they are laid down by the female. And these eggs, they come out by the pressing of the belly by the males. And at the same time, the sperm, they are sprayed on it, and the fertilization occurs. And then, in that type of water, the cleavage occurs. The cleavage starts, and in this, there is an animal pole, then the vegetal pole. On the animal pole, there is a nucleosis present, whereas in the vegetal pole, there is a yolk. And this yolk prevents the cell devion. In the animal pole, the cell devion is very quick. And then, like a vegetal pole, the cell devion is rather slow. And this, first of all, there is a cleavage, which takes its start from the animal pole and then spreads down the vegetal pole. And it becomes a two-cell stage, or two-celled embryo. After that, the second cleavage, that is at right angles to the first one, and in this way, four-celled stage is formed. Then the third cleavage, that will occur, that is, at right angles to the first two, and in this way, as we cut the orange from one side, then horizontally, so in this way, four-cells, which are relatively smaller, why are they smaller? Because the third cleavage, that is towards the animal pole, and in this way, there are eight-celled stages formed. After that, the cell devion is quite fast, and consequently, the upper part has more number of cells and are smaller. Whereas in the lower half, the cell size is larger, and in this, there is a vegetal pole. It is towards the vegetal pole. There are two hemispheres formed. There is the animal pole and the vegetal pole. So these are all things that have been explained in this text. So as cleavage progresses, the animal region, the animal pole, that becomes packed with numerous small cells, while the vegetal region, the vegetal pole, that contains only a relatively small number of large, yolk-laden macromeres. In the upper cells, the animal pole, the cells, we call them as the micromeres, or its opposite, they are said to be the megameres or macromeres. So as amphibian embryo containing, that is about 16 to 64 cells, comes to the stage when we call it Morula. Okay. After this, the next stage, that will be the blastula. So at the 128-cell stage, the blasto-seal that becomes apparent, this animal pole develops a cavity, and we call that cavity a blasto-seal. And this embryo, that will be called as blastula. So as a gastula or blastula, it cannot differentiate from the outside, but when we will cut it, and if we look at the cavity, then we can say that this is a blastula, a blastula stage. Then this is the benefit of this blasto-seal. It permits cell migration, that is during gestulation, and secondly, it prevents the cell beneath it from interacting prematurely with the cell above it. So this is a kind of barrier, which does not allow interaction, but it is very important, that is the cavity. Okay. Next process is the gestulation. The gestulation, it is the phase, which is early and embryonic development of most animals, and in all these types of coordinates, this stage comes out. And what is this stage? This is a rearrangement of cells, so as to cells literally move from one place to the other, and so as to develop a three-layered embryo, and that embryo will be called as gastula. And this occurs by the process, which we call the gestulation process. And then it is multi-layered, and in this layer, they can be differentiated, there is the ectoderm, the mesoderm, and endoderm. So in this diagram, we can see that this is an animal pole. It has developed to a certain extent. This is a section, a sedative section, this diagram is a diagrammatic representation. So all of these are said to be the micro-mears, and these below are called mega-mears. And then the point where the micro-mears and mega-mears are differentiated from each other, is that then there appears a cavity, and it appears a dent. A dent appears, and this dent then progresses within the body, within the embryo, and here, as we talked about, there is a blastoseal, it is a cavity, and it can only be seen if we cut it from the inside. And especially these cells, which are said to be the bottle cells, these bottle cells are the dorsal lip of the blastopore, that is a kind of boundary. So they are the cells which move inward first, and then it leads to a cavity and this cavity then later on it develops into the, there is archentron, and this archentron as it develops, this is the gut cavity. So in this way, this is the dorsal lip of the blastopore, and then later on it develops into the ventral lip of the blastopore, and these micro-mears, these upper ones, like the micro-mears on the animal pole, they start extending over the mega-mears, and as they extend, and in this process, we give the name of api-boli, and as they spread, then another structure develops, which we give the name of yolk plug, and the cells that appear or appear from that yolk plug, they are the endoderms. So endodermal cells, and those cells which remain outside, which do not migrate, neither dorsal lip of the blastopore, nor ventral lip of the blastopore, those cells which remain outside, they don't migrate inward, so they are said to be the actoderm, and this is the actoderm, which later on then develops into the skin, nervous system, and so on. This is the embryology, these are very important points, and we can call it gastrointestinal. So this gastrointestinal, as soon as the archentron develops, this blastocele gets its pressure, and it gets obliterated, and when the archentron gets bigger, ultimately, these three layered embryos very clearly, which we can differentiate. So the gastrointestinal begins at the future dorsal side, future dorsal side, from the dorsal side, this gastrointestinal starts from there. Then below the equator, the equator is where the actoderm, sorry, micrometers or megamiers differentiate on that phase, but it is towards the, that is on the megamiers side. So when there is a prospective endodermal cell, which is said to be the bottle cell, that is invaginate, and form it, there is a slit, which is called blastopore. So the bottle cells, that is the line, the early archentron, A, that is the starting point, bottle cells, they are the cells, which enter first, or A, that is the sign of the endodermal cells. Okay. Now after the bottle cells, the next cells that enter the embryo, that form the pre-cordal plate, and the mesoderm of the notochord, and these then become notochordal cells. And on this stage, notochord appear, because notochord, that is the sign of its being, that is vertebrate, or it is skeletal rod, in fact, which later on, that develops into the vertebral column. So there is simultaneously the animal cells undergo epiboli, the spreading of the atoderm on to the, that is, the spreading of the micromeres, Radha. The micromeres over the megaberes. This movement, this specific movement, is called epiboli. So they completely surrounding the surface of the embryo, until a ventral lip of the blastopore forms. So this is a dorsal lip of the blastopore, or ventral lip of the blastopore. Ventral lip of the blastopore, the mesodermal cells, which are outside, they migrate inwards. And in this way, there is a dorsal lip, there is a ventral lip, on the lateral lip. And this is a rounded structure, and in this rounded structure, we can see the yolk, which is called the yolk plug. So the last endodermal cells on the surface appear as the yolk plug. The yolk plug is eventually covered by the actoderm. So these are the stages, these are the steps of early development of the amphibian embryos. So it takes its start from the egg, and this egg, which is not like a shell egg, or a hen's egg, or a bird's egg, or a ptyl's egg, it is not a shell egg. So this is covered by a gelatinous substance. And as soon as it comes into the water, in fact, it swells up. There is an imbibition, so it swells up. And after that, this jelly-like substance, this covering, in fact, it protects the embryo. And this is all about the amphibian embryology. So this is a representation, and there is an amphibian scheme. So this is the, you can say, the main steps of the Gestula Formation. So this is all about the amphibian embryology. So this is OK.