Final answer:
A diaphysis is the central shaft of a long bone, and an epiphysis is the wider section at each end. The fusion of diaphysis and epiphysis in a skeleton implies that the individual has reached maturity, typically occurring in early adulthood between 18-21 years.
Step-by-step explanation:
The diaphysis is the tubular shaft that runs between the proximal and distal ends of a long bone. The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow marrow and surrounded by compact bone. On the other hand, the epiphysis refers to the wider section at each end of a long bone that is filled with spongy bone and red marrow.
When observing a skeleton, if the diaphysis and epiphysis are fused, it indicates that the individual has reached maturity and the growth in bone length has ceased. This fusion typically occurs in early adulthood (around 18-21 years). The growth plate, known as the epiphyseal plate, has converted into an epiphyseal line due to the replacement of cartilage by osseous tissue, signaling an end to the bone's growth phase.