The cortex has six layers of neurons, one on top of the other, each with different types of neurons. In this six-layered sheet, the neurons are organized in different ways in different places. The density of cells of different types varies as well. Furthermore, in distinct parts of the cortex the pattern of arrangement of cells is different. This is because the different parts of the brain serve distinct functions.
Neurons develop substantially after birth.
It turns out that neural projections, e.g., the dendrites of neurons, are enriched through experience. Another rat, rat B, is put in a nice cage with toys. After six months, the animals are sacrificed, and their brains are studied. This experiment demonstrates how experience and interaction with the environment enhance neural development. Thus, it is desirable to live in an enriched environment because it helps our brains to mature.