Complex sentence: The diencephalon is part of the prosencephalon (forebrain), which develops from the foremost primary cerebral vesicle and differentiates into a caudal diencephalon and rostral telencephalon.
Split into simpler sentences: The diencephalon is part of the prosencephalon (forebrain), which develops from the foremost primary cerebral vesicle. The prosencephalon differentiates into a caudal diencephalon and rostral telencephalon.