Simplify the following text for a Grade 12 native English-speaking person.
Original: The prosencephalon , the mesencephalon ( midbrain ) , and rhombencephalon ( hindbrain ) are the three primary portions of the brain during early development of the central nervous system .
Simplified: The prosencephalon ( or forebrain ) , the mesencephalon ( midbrain ) , and rhombencephalon ( hindbrain ) are the three primary portions of the brain of vertebrates . They develop very early during development of the central nervous system .