The fertilized egg divides many times to form a cell mass, which gradually grows and begins to differentiate. One part becomes the fetus, and the other part becomes the accessory organ that supplies nutrition and protects the fetus. This is the first week of life. In the second week, the little life grows very fast and the spine is formed. Brain tissue, spinal cord, nervous system and eyes all have certain prototypes. At the other end of the spine is a small tail. Blood vessels are beginning to form at this point, and the heart is not yet formed, but where the heart is formed there is a heartbeat. The heart appears in the fourth week, and the limbs appear in the next week. eyes and ears follow