Final answer:
The fluid-filled cavity present within the metaphyseal portion of a growing long bone is called the epiphyseal plate. It is responsible for the lengthwise growth of long bones.
Step-by-step explanation:
The fluid-filled cavity present within the metaphyseal portion of a growing long bone is called the epiphyseal plate. It is a layer of hyaline cartilage that is responsible for the lengthwise growth of long bones.
As the bone grows in length, the cartilage in the epiphyseal plate is replaced by osseous tissue.