All of us are developed from a single cell, zygote. The information carried by the zygote acts like a commander, he commands his general (daughter cells), and each general commands their own subordinates (daughter cells). As the result, 0.2 million billion of cells (approximate number of cells in an adult man), are generate from (or controlled by) the only one zygote; no doubt, zygote is a perfect commander that he command a huge number of cells and seldom make mistakes, because we all have right number of eyes, ears, limbs, brain, heart and so on, and they are in right position. Some people will say it feels like a delicate machine that does a lot of work but no mistake. I would like to say: no, our body is much better than a machine. “One of the critical differences between you and a machine is that a machine is never required to function until after it is built. Every animal has to function even as it builds itself.”(From our text book) That is one reason that developmental biology attracts me.
When I am reading the developmental history of the leopard frog, the most amazing thing for me is the formation of the ectoderm, mesoderm and endoderm. If the embryo chooses another way to develop, that differentiates by organ at the beginning. There may be one cell; its mission is to from the head, and other single cells they have to become hand, heart, liver all by themselves. For the one “head cell”, it have to develop into variety of nerve cells to build the brain, it also should generate skull and skin to protect the brain. Considering hair, eyes, teeth and so on, that single cell acts similar as zygote. It may be too much work for that cell and may be easier to make mistake. Another problem is, if all parts of head is formed by one cell, it is hard for it to connected to the body as many vessels, nerves, muscles need to be recognized and joined together smoothly. So, forming three layers of cell is a smart way of building an animal, although it seems strange at the beginning that the shape of the embryo doesn’t like an animal at all.
Another amazing thing is the appearance of mesoderm. It is not only the precursor of the important organs (heart, blood, skeleton, gonads and kidneys), but also provide a wonderful space for the organs to stay. Food is taken in from mouth and goes through the digestive tract then move out from anus. So all the space surrounded by digestive tract (formed by endoderm) is used for digestion. Of course organs cannot be putted there as they may be digested as well. Respiratory tract (formed by endoderm) is also not suitable for organs as it is direct connected to the air, they may suffer the risks from outside. So, endoderm cannot provide a space for organs. Obviously, the space outside the ectoderm is not suitable; it is silly to have heart above the skin for the vulnerability. For the similar reason, the coelenterate cannot have complex organs as they do not have mesoderm. So the smart choice is the space between endoderm and ectoderm, and mesoderm serves for that.
When I say it is smart, I am not saying the animal itself knows these, even the smartest species human being cannot fully understand these till now. I would like to believe that, the nature is the smartest one in the world, even she cannot think at all.