1. The Developing Brain: Cerebral Cortex
Gray matter consists of neurons and their axons and dendrites which have not been myelinated, as well as glial cells and capillaries.
The layer below the gray matter is formed predominantly of axons which have been myelinated; these connect neurons in different regions of the cerebral cortex with each other and with other neurons in the nervous system.
Myelinated axons are white in appearance, hence the term "white matter."

2. The Developing Brain: White Matter
Unlike gray matter, the brain's white matter—wire-like fibers that establish neurons' long-distance connections between brain regions—thickens progressively from birth in humans.
A layer of insulation called myelin progressively envelops these nerve fibers, making them more efficient.

3. The Developing Brain: A Closer Look
Gray matter is from 1/12 to 1/6 inch thick, with 6 layers of neurons.

5. The Developing Brain: Gender Differences (Age)

6. The Developing Brain: Gender Differences (Brain Activity)
Top row –men Bottom row – women
Left column – rest Center column - active
Blue – low blood flow and low brain activity
Yellow and red – higher blood flow and higher brain activity
“The upper row are the men; the lower row are the women. And you just compare the baseline difference; the resting brains of women look as hot as the activated brains of men. You can see the amount of activity in resting women is about as high as the amount of activity in men who are working on problems.”
-Ruben Gur, Professor of Psychiatry, University of Pennsylvania