Flexible tough material that makes up vertebrate skeletons or parts of
the system

Cartilage is the flexible, tough material that makes up vertebrate skeletons or parts of the skeletal system.

Explanation:

Cartilage is the part of skeletal system that acts as a connective tissue too. It is comprised of chondrocytes cell types (formed by chondrification) and produce collagen and proteoglycan. It provides elasticity and found in the rib cage, nose, ear, and in the Intervertebral discs. It is abundantly present in infants but at later growth stages, bone maturation process takes place which replaces/hardens the cartilage with hard bones.