Final answer:
The middle division of the three primary divisions of the vertebrate brain is the midbrain, which is small relative to the forebrain and hindbrain. It forms part of the brainstem, and though established early in the embryonic stage, remains distinct throughout development.
Step-by-step explanation:
The middle of the three primary divisions of the developing vertebrate brain is known as the midbrain, or mesencephalon. This region develops early in the embryonic stage and is sandwiched between the forebrain and the hindbrain. It remains a distinct structure throughout development and into adulthood, though it is relatively small in comparison to the other brain regions. The brainstem includes the midbrain, along with the pons and medulla, which align with the mesencephalon, metencephalon, and myelencephalon, respectively. In the mature brain, the cerebellum develops from the metencephalon and is considered a separate area, while the forebrain differentiates to form the cererebrum and diencephalon.