Refers to the area (s) in in the midportion of the shafts of long bones where bone cells are formed so that the cartilage-model bones of the fetal skeleton begin ossifying, from the center outward, to form bone shafts.

The area wherein bone cells (of long bones) are formed from cartilage of the fetal skeleton is called the primary ossification center. The primary ossification center appears primarily during embryonic growth and it occurs in the diaphysis or the central area of long bones.