In mammals, the considerable expansion of the cerebrum and cerebellum also led to an expansion of the fiber tracts which travel from the body to the cerebellum, from the cerebellum to the cerebrum, and from the cerebrum to the cerebellum.  At the base of the cerebellum in mammals, there is an expansion of the myelincephalon where all of these fiber tracts are organized.  This expansion is known as the pons, which only mammals possess (Romer p. 577).

The expansion of the cerebrum and cerebellum in higher primates and humans corresponded to expansions in the pons as well.

OPOSSUM

CAT

SHEEP

PIG

HUMAN