 mechanism of development, the nuclear equivalents, first the contents, rock swissman hypothesis, hands dries discovery, and hands payment discovery. So first the rock swissman hypothesis. Before the start of modern research or the modern technology, the scientist was of the view that the developmental process it take place as the embryo divides on RRG, zygote is being divided into different types of cells. But the question was that one zygote divided into multiple cells. This was a question in 19th century, 19th century the scientists have this major question, how the cells which are derived from single zygote, the cells which are derived from single zygote, how they are capable to form different body organs. How they make different body organs. Now for this, different scientists and different theories of the page have one important hypothesis, that was of the rock swissman. Rocks and swissman they were through German scientists basically, embryologists, they give the idea that as the embryo keep on dividing, different cells receive different part of genome. Means, as the cell divides, the genome of the cell is converted into fragments. One cell of the genome will receive the other cell. So the cell which will receive the genome will develop the same organ in the future. This was initially hypothesis that the genome will be split and divided. So in this way, the scientists had this concept that since the cells are all the same, but among them, the genome is split, the genome is in the form of fragments that some cells are getting one fragment, some are getting the second fragment, some are getting the third fragment. In this way, from the breakage of the genome, the part which will receive the genome will develop in the future. Then in 1892, near the end of the 19th century, a new discovery of Hans Dries. He noticed that if sea urchin zygote, when it is at two cell stage embryo, if the both cells, both blastomere, they are separated from each other, they are capable to grow into two normal individuals. So what was it? That Hans Dries had an experiment in 1892. Because when the sea urchin zygote, when it is divided into two parts, it will make two cells. If we separate these two cells, then they convert into two new sea urchins. Now this concept that as soon as the cell is divided, half of the genome will get one, half of the genome will get the other, then it will come here and it will be abolished. Why? Because if that concept was right, then both cells should have made half of the larvae. Half of the larvae should have been made, half of the larvae should have been made in the other larvae. But both larvae are in normal individuals, then the embryo will go ahead and convert into normal larvae. They will become two sea urchins. So in this way, this concept was that during development, different fragments of the genome, different cells are found here and it is finished. Which means that all the cells will get the same genome. This genome will not be divided. This was the conclusion of the Hans Dries experiment. Later on, we have a lot of experiments of the Hans Spearman, which have been done to understand the developmental mechanism. Hans Spearman's most striking experiment was that he took the salamander fertilizer, but he took the fertilizer of the salamander, that is, the zygote. After that, he pinched one of his ligatures in two parts. He tied the zygote with a string like this. Half of the zygote is on one side, half of the zygote is on the other. Now he put this drop on it and then put it back in the medium. Then he saw that the side on which there was a nucleus, that half used to start the cell devion. When there was no nucleus on that side, there is no cell devion. Ultimately, when that half, where the cell devion is, where the nucleus was, when it reached the 16th cell stage, when it reached the 16th cell stage, then one nucleus escaped from here and went that side where there was no nucleus. That was the nucleated side. As soon as the nucleus reaches the second side, the cell devion will start. This way, Hans Spearman proved that there is a nucleus which is involved in the cell devion process. What is involved in the development process? There is a nucleus involved. The more there is a nucleus, the more there will be a development. The more there is a nucleus, the more there will be a development. Hans Spearman's some experiment as well, that sometime, the nucleus is present on both sides, but one side is developing properly, and the other side is developing abnormally. When Hans Spearman noticed that the fertilized egg has a gray crescent, when he saw it in detail, he said, actually, this gray crescent is going to one side, and the other side is not going to the gray crescent. So, the direction in which the gray crescent is going, it is developing there. Whereas, the direction in which the gray crescent is not going, the cell is dividing there, but abnormally dividing there. So, he took out two conclusions from this. Number one, he concluded that for the developmental process, nucleus is essential. Nucleus is essential. And then, when the nucleus went to the other side on the 16th cell stage, the nucleus that was on the other side also developed in a new larvae. So, this means that on the 16th cell stage, if one nucleus goes to the other side on the 16th cell stage, the other side will also become a cambary, and the 15 cells that are left there will also become a cambary. So, it means that the nucleus carries all the genetic information. So, this says that number one, nucleus is essential for the developmental process. Number two, nucleus contains all the genetic information. Number three, gray crescent is also required for the developmental process. In the developmental process, the important role of gray crescent is that it will be developed without gray crescent.