11) When do the cells organize themselves into an embryo? 12) The brain and spinal cord are created from what layers of cells? 13) How is it that cells do "different jobs"? 14) How is a gene "turned on"? 15) How is sex determined? 16) What is the embryo called 2 months after ferritization? 17) How many times larger does it grow? 18) When does the fetus respond to light? 19) When does hard bone form? 20) What are some important develops in the last 2 months