Fertilization occurs when an egg from a female and a sperm from a male join together. This new cell, called the zygote, receives half of its genes from its mother and half from its father. 24 hours after fertilization, the cell divides into 2 daughter cells. After another 24 hours, it divides into 4 new cells. Then it divides into 8, 16, 32, etc. This is often called the morula stage. As the cells begin to form an inner cavity, they enter the blastocyst stage. At one end of the blastocyst is a ball-like cluster of cells that grow faster than the other cells. This is the inner cell mass, which is the beginning of the embryo. The rest of the blastocyst forms the placenta, amniotic sac, and other parts of the embryo/fetus support system.

Every person is made up of three types of tissue. The ectoderm makes up "paclaging material" like skin, as well as the nervous system. The mesoderm makes up major structural parts of the body, such as muscle. Finally, the endoderm consists of cells that line organs and vessels.

The inner cell mass separates into a layer of ectoderm and a layer of endoderm. Between these two layers, some mesoderm cells appear and form a long rod-like structure. This is the notochord, the forerunner of the vertebral column (backbone).

Through chemical reactions, the ectoderm lying above the notochord is caused to divide more rapidly in order to form the neural plate. A crease called the neural groove soon appears and deepens as the embryo grows. The sides (neural folds) of the groove fold over and fuse together, thus forming the neural tube, soon to become the nervous system.

The neural tube grows in both directions and closes completely except for the two open ends (neuropores). The neuropores do end up closing. If they do not, serious defects could occur (see diseases/disorders section).

After the tube closes completely, the anterior (front) forms three vesicles which mark the spaces for the two cerebral hemispheres and the brain stem. The first and third of these divide into five more vesicles, later to become the cerebral hemispheres, diencephalon, midbrain, pons, and medulla oblongata.

The midline of the neural tube is where the sides of the tube fused together. At this line are packets of cells called neural crest cells. These move throughout the embryo and form the peripheral nerves, roots, and ganglion cells of the peripheral nervous system.

Inside the tube, the cells divide rapidly and cause the wall and the anterior end of the tube to thicken. Some of these cells become neurons while others become glial cells. The determination of what the cell will become is still being researched.

After this period of division, some of the primitive nerve cells (neuroblasts) move away from the inside of the tube to the wall of the tube. Through this migration, the wall thickens selectively and begins to resemble the mature nervous system. As this becomes more and more difficult, the cells form a "rope ladder," formed by radial glial guide cells. The neuroblasts then migrate along this "ladder." These cells move to the thickening neural tube vesicle, which is becoming the cerebral cortex. The cortex consists of six cell layers, so each group of migrating cells must pass through the layer of cells that came before, thus following an "inside-out" method of migration. The migration is controlled by chemicals.

Many fiber connections grow below the cerebral cortex (especially in the thalamus) in the areas that they will develop before the nerve migrate and receive the fibers. Without these target cells, the fibers would turn away or wither away, so decoy cells are sent to act as neurons until the neuroblast migration is complete several weeks later. After the migration, these decoys disappear. Since there is a possibility of error during this process, various cognitive and emotional disorders, including some forms of dyslexia, may occur later in life.

Once the neuroblasts have migrated to their final position, they begin to grow two main extensions. These are the dendrites and the axon. The dendrites are small, but the size of the axon varies. Some may grow up to five feet long. The corpus callosum is also developed during both the prenatal and postnatal period. This structure is a massive bundle of nerve fibers that connect the two cerebral hemispheres.

Myelinization is used as a measure of maturity of the nervous system. Fibers that are used for the senses or motor skills are myelinized right after birth. Those involved in associative and cognitive processes myelinate later. It is believed that fibers of the prefrontal lobes (executive functions, intentions, future planning, etc.) are the latest to myelinate. In fact, the operation may go on into young adulthood.

Synapse formation begins in the mid or late second trimester and continues throughout an individual's life. The formation of synapses is at its highest rate during the first 6-8 years of life and decreases at puberty. During the first 6-10 years of life, the individual has achieved the highest density of synapses per unit volume of neural tissue that he or she will ever have. This is important because it is during this time, too, that enormous amounts of information is absorbed into the brain.