Final answer:
An epiphyseal line is visible as cartilage is replaced by bone during growth and disappears upon the completion of bone growth. This line indicates where the epiphyseal plate, a layer of hyaline cartilage responsible for the longitudinal growth of bones, once existed.
Step-by-step explanation:
As the cartilage between the bones is replaced, an epiphyseal line is visible and disappears once bone growth is complete. During the endochondral ossification process, cartilage is replaced by bone, and this transformation is marked by an epiphyseal plate in growing bones, which is evident in long bones as they elongate. This epiphyseal plate is a sheet of hyaline cartilage found in the metaphysis of immature bones. As the individual matures, this cartilage is progressively ossified until growth ceases, leaving the epiphyseal line as a marker of where the growth occurred.
Throughout fetal development and early human growth, the skeleton initially consists mainly of cartilage. This cartilage provides a template for bone growth, which proceeds from a primary ossification center. Later, a secondary ossification process occurs in the epiphyseal regions, eventually leading to the disappearance of the epiphyseal plate and the fusion of the bony components into a single adult bone, signifying the end of bone lengthening.