Your Body. How It Works. Human Development PDF Free Download
Development is the process by which a single cell becomes a multicellular organism. In humans, this process takes approximately 264 days, or 9 months. During that time, a single cell divides many times to produce many cells. These cells undergo a limitless number of events at the cellular, molecular, and genetic levels to shape this collection of cells into the form of a human. Development begins with fertilization, the fusion of a sperm cell with an egg cell. Fertilization produces the first cell that, in turn, will ultimately give rise to every cell in the body. This first cell and its progeny will go on to make important decisions at the molecular level as they divide and take on specific fates. Some cells will take on a neural fate, some cells will become muscle, and some cells will become skin. This collection of cells, the embryo, will take on form, and cells will begin to position themselves to reflect the eventual role they will take as development proceeds. Cells destined to become muscle will position themselves inside the embryo while cells destined to become skin will position themselves on the outside of the embryo. Axes will form that will define the front and back, left and right, and top and bottom of the developing embryo. The nervous system will form as will organs, and throughout this entire process the embryo and then fetus will continue to grow.