After fertilization, the cells in the zygote divide rapidly to form a morula, which becomes the embryo after approximately five days. The fetus is present about eight weeks after the fertilization takes place.
After fertilization, the zygote begins to divide rapidly. Twenty-four hours after fertilization, the zygote contains two cells, and three days after fertilization the zygote has 16 cells and is called a morula. The cells continue to divide and form a hollow bundle called a blastocyst. The blastocyst implants in the endometrium, and the cells begin to form the embryo.
The embryo is present at five to six days after the zygote is fertilized. The embryo is a clump of cells inside the formerly hollow blastocyst, and is surrounded by the trophoblast. The trophoblast contains cells that direct hormone production and implantation.
After the embryo forms, the cells form the embryonic disk. The embryonic disk begins to fold, allowing the cells within the disk to touch other cells. The cells then start forming each of the bodily systems. For example, the organs and nervous system begin to develop at this stage. After approximately three weeks, each of the cells in the developing embryo are part of one of the developing systems in the body. After eight weeks, the embryo becomes a fetus.