Three to four weeks after conception, one of the two cell layers of the gelatin-like human embryo,now about one-tenth of an inch long, starts to thicken and build up along the middle. As this flat neural plate grows, parallel ridges, similar to the creases in a paper airplane, rise across its surface.Within a few days, the ridges fold in toward each other and fuse to form the hollow neural tube. The top of the tube thickens into three bulges that form the hindbrain, midbrain and forebrain. The first signs of the eyes and then the hemispheres of the brain appear later. How does all this happen? Although many of the mechanisms of human brain development remain secrets, neuroscientists are beginning to uncover some of these complex steps through studies of the roundworm, fruit fly, frog, zebrafish, mouse, rat, chicken, cat and monkey. Many initial steps in brain development are similar across species, while later steps are different. By studying these similarities and differences, scientists can learn how the human brain develops and how brain abnormalities, such as mental retardation and other brain disorders, can be prevented or treated. Neurons are initially produced along the central canal in the neural tube. These neurons then migrate from their birthplace to a final destination in the brain. They collect together to form each of the various brain structures and acquire specific ways of transmitting nerve messages. Their processes, or axons, grow long distances to find and connect with appropriate partners, forming elaborate and specific circuits. Finally, sculpting action eliminates redundant or improper connections, honing the specificity of the circuits that remain. The result is the creation of a precisely elaborated adult network of 100 billion neurons capable of a body movement, a perception, an emotion or a thought. Knowing how the brain is put together is essential for understanding its ability to reorganize in response to external influences or to injury. These studies also shed light on brain functions, such as learning and memory. Brain diseases, such as schizophrenia and mental retardation, are thought to result from a failure to construct proper connections during development. Neuroscientists are beginning to discover some general principles to understand the processes of development, many of which overlap in time.