The entire process begins when two haploid germ cells called gametes come together during conception. Once the zygote it formed, it undergoes cell division. Thus, one cell divides into two; the daughter cells, called blastomeres, then cleave into four; these cleave into eight, and so on. When the embryo consists of a hundred or more cells it may form a solid mass, called a morula from its resemblance to a mulberry. In most species the mass then resolves itself into a single layer of cells forming a hollow sphere, the blastula. The next step is the formation of a double-walled sac or cup, the gastrula. The outer wall is called the ectoderm, and the inner wall is the endoderm. The endoderm surrounds a new cavity known as the primitive gut. In some cases these two layers are formed by delamination, or splitting, of a mass of cells, but more commonly they are formed by invagination, that is, the pushing in of a portion of the wall of the blastula. In all animals except the simplest, a third layer, the mesoderm, develops between the other two layers.
Thus, one cell divides into two; the daughter cells, called blastomeres, then cleave into four; these cleave into eight, and so on. When the embryo consists of a hundred or more cells it may form a solid mass, called a morula from its resemblance to a mulberry. In most species the mass then resolves itself into a single layer of cells forming a hollow sphere, the blastula. The next step is the formation of a double-walled sac or cup, the gastrula. The outer wall is called the ectoderm, and the inner wall is the endoderm. The endoderm surrounds a new cavity known as the primitive gut. In some cases these two layers are formed by delamination, or splitting, of a mass of cells, but more commonly they are formed by invagination, that is, the pushing in of a portion of the wall of the blastula. In all animals except the simplest, a third layer, the mesoderm, develops between the other two layers.
As you can see, all of this would have to occur from a single stem cell, which is strictly not possible. Embryonic stem cells, which are the one's you're describing, have the potential to develop into any organ or type of tissue in the body. This property is called pluripotency. Embryonic stems cells exist in fully developing embryos for a limited period of time (about three weeks), but do not have the capability to undergo what happens at the early stages of conception.
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