The nerve cell factory
The nerve cell factory in the developing embryo is the lining of the neural tube, which forms both the brain and spinal cord. Each new cell moves through to the outside of the growing brain. The earlier a cell is made, the deeper in the brain it will end up, like the rings in a tree trunk. The outer layer, the cortex, is built up between the 13th and 28th weeks of the embryo's life. But nerve connections carry on forming up to the age of two.