4. How the body works:Implantation of the embryo

Within a few hours of fusion of the sperm and the ovum, cell division begins. The number of cells doubles with each successive division until a hollow ball of rapidly dividing cells is produced. Throughout the process the ovum is moving down the Fallopian tube toward the uterus. Five to six days after the fusion, the ovum reaches the uterus and implants itself in the soft wall. The hollow ball of cells now contains up to one hundred cells, arranged as an outer ring and an inner mass. The outer ring grows outward and penetrates the uterine tissue, while the inner mass divides into two separate layers, an inner endoderm and an outer ectoderm. These two layers become separated from the outer ring and by nine days form two cavities with the hollow ball. The ectoderm later may spread around the periphery of the smaller cavity, forming a sac, the amnion which will surround and protect the developing embryo. The endoderm similarly encloses the larger cavity to form a yolk sac, which will have a nutritive function.