Throughout the germinal stage, the rapidly dividing mass of cells travels down the fallopian tube to the uterus, where it will become implanted and develop over the course of the pregnancy. The germinal stage ends about two weeks after conception, when the mass of rapidly dividing cells attaches or implants itself to the uterine wall. The embryonic stage lasts from about two to eight weeks after conception and is critical to development. During this stage, the cells begin to differentiate, or become specialized to develop into different organs. Cell division still occurs at a very rapid pace, but cells are no longer identical to one another.
Some cells begin to form the brain, others the bones, the digestive system, and so on.
At this time, all of the organs in the body begin to form and take shape. One particularly important development during the embryonic stage is the formation of the neural tube at about three weeks after conception. The neural tube will become the brain and spinal cord, but at first the neural tube is hollow and open at both ends. During the fourth week of prenatal development, the neural tube begins to close. This is an especially critical time for development because the failure of the neural tube to close at either end causes neural tube defects like spina bifida and anencephaly.