WHEN SPERM meets egg, the merger of a father’s and mother’s DNA triggers the start of a new life. Encoded in the tens of thousands of genes that make up a human being are a substantial fraction that will create the brain and central nervous system. You won’t find the child’s personality, emotions, and ideas buried in the code; they arise instead as the brain develops and interacts with its environment after birth. Nevertheless, the explosion of cell development that begins with conception is the first step toward forming the brain and all of the hopes and dreams it will one day contain.

As an embryo develops into a fetus, the brain grows and differentiates rapidly.
As an embryo develops into a fetus, the brain grows and differentiates rapidly.


In its first phases of development, the fertilized egg, or zygote, undergoes a rapid series of divisions. One cell becomes two, two become four, four become eight, and so on until the exponential divisions Create a tiny, hollow ball of hundreds of cells nearly uniform in design. Two weeks after conception, the sphere of cells, still dividing, takes the first step in the series of physical changes to construct a differentiated body and begin the process of becoming human.

First, a dent appears in the sphere. Cells move into the indentation, which folds under the surface of the sphere. The folding creates three layers of cells: an outer layer called the ectoderm, an inner layer called the endoderm, and a middle layer called the mesoderm. In the following weeks, these three layers grow into the tissues that give rise to the body’s major systems: Endoderm becomes digestive tract; mesoderm creates muscles, skeleton, heart, and genitalia; and ectoderm forms brain, spine, nerves, and skin.

Lots of gentle handling produced increased serotonin, a neurotransmitter that dampens aggression, in baby rats. Grown into adults, the rats lived longer and handled stress better.


The nascent brain makes its first appearance at about four weeks after conception, when a thin, spoon-shaped layer of cells called a neural plate emerges at the head end of the embryo. Major characteristics of the future brain already are in place just one month into fetal development. Hemispheres later will develop on either side of a groove down the center of the neural plate, creating the bilateral symmetry of the human brain.

As the fetus grows, the bowl of the spoon will become the brain itself, while its handle grows into the spinal cord. And as the neural plate folds to form a tube, swellings in the original spoon shape become the forebrain, midbrain, and hind brain. As they develop, they work together to form the major sections of the brain, from the cerebrum at the top of the head to the thalamus, hypothalamus, cerebellum, and spinal cord at the back and lower end.

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