Between 4 and 5 weeks,
the brain continues
its rapid growth
and divides
into 5 distinct sections.
The head comprises about 1/3
of the embryo's total size.
The cerebral
hemispheres appear,
gradually becoming
the largest parts of the brain.
Functions eventually controlled
by the cerebral hemispheres
include thought, learning,
memory, speech, vision,
hearing, voluntary movement,
and problem-solving.