Final answer:
The primary ossification center is where ossification begins and is located in the diaphysis, not the epiphysis, and is different from the epiphyseal line.
Step-by-step explanation:
The primary ossification center is where ossification begins during the development of most appendicular bones through a process known as endochondral ossification. It is not avascular, as blood vessels do play a role in the ossification process. It is not located in the epiphysis but rather in the diaphysis, which is the central shaft of long bones. The primary ossification center is different from the epiphyseal line, which develops after the epiphyseal plate has ceased bone lengthening and the bone has reached its adult size, signifying the previous location of the growth plate.