Final answer:
The border between the epidermis and the dermis undulates due to the structure of the dermis layer.
Step-by-step explanation:
The border between the epidermis and the dermis undulates due to the structure of the dermis layer. The dermis layer is made up of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers. These fibers give the dermis its strength and elasticity, causing it to fold and form undulations.