If testosterone is present, the embryo will develop male characteristics, and if the hormone is absent, the embryo will develop female characteristics. By the end of this period, at the eighth week of gestation, the basic tissues exist and the bones begin to harden, which marks the beginning of the period of the fetus. The period of the fetus lasts from the ninth week of gestation until birth, at about thirty-eight weeks. This period is a time of remarkable development in which the fetus grows ten times larger. During this time the fetus becomes more active and begins to move its arms and legs, close its fingers, and flex its toes.
Sexual differentiation becomes complete.
The rest of the major organs develop during this time, and the fetus becomes capable of surviving outside the womb. Brain development begins in the first few days after conception, although the cerebral cortex does not begin to form until about six weeks into gestation. Brain development begins in the first few days after conception, although the cerebral cortex, which is the center for higher order thought, does not begin to form until about the sixth week of gestation. The reflexes that become apparent at the end of the embryonic period are controlled by the rudimentary spinal cord, and do not indicate thought. It is not until seven or eight months that the fetal brain shows measurable electrical activity.