What is the bulk of the human brain formed primarily by during development?

The human brain develops primarily from densely packed neurons and myelinated nerve fibers, which together form the structural and functional foundation of the nervous system. During brain development, neurons proliferate, migrate, and differentiate to form the complex networks that are crucial for processing information.

As these neurons develop and communicate with one another, many become myelinated—a process in which the axons of neurons are coated with a fatty substance called myelin. This myelination is essential for increasing the speed and efficiency of electrical signal conduction between neurons. Therefore, the bulk of the human brain is characterized by both a high density of neurons involved in information processing and a network of myelinated fibers that facilitate rapid communication across different brain regions.

This combination is critical for the functioning of various brain areas, including those involved in cognition, emotion, and motor control, making the correct answer a blend of both components. The other options, while relevant to brain structure, do not encompass the complete picture of the developmental foundation of the brain.