This stage the baby is called an embryo, this is when the baby forms its major organs and body systems. It starts at about two weeks lasting to about 8 weeks. The embryo has three layers the ectoderm, endoderm, and mesoderm. Ectoderm forms the teeth, hair, skin, sense organs, brain, and spinal cord. The endoderm forms the respiratory system, digestive system, liver, and pancreas. The mesoderm forms the muscles, blood, bones, and circulatory system.