An egg and sperm fuse, bringing two genetic legacies together to make a complete new set.
Week 1
The fertilized egg cleaves again and again to form a globe of cells. By day 4, it gains the most basic of structures: an outside and an inside. Soon after, it implants into the wall of the womb.
Week 2
The inner cells split into two layers--one that develops into the fetus, and one that later becomes part of the external life-support tissue, such as the amniotic sac and umbilical cord.
Week 3
Embryonic cells converge along an axis, forming the midline of the growing embryo. Cells at one end of the axis form an elongated node, which will later become a brain and its web of nerves.
Week 4
The embryonic disc rolls into a tube, giving rise to a third dimension. Cells diverge into different layers, destined to become various organs of the body. By the end of the first month, the embryo is fully implanted in the womb and begins to draw sustenance from its mother.