Question: when do primary ossification centers appear in an embryo?
Answer: prenatal development

A primary ossification center is the first area of a bone to start ossifying. It usually appears during prenatal development in the central part of each developing bone.  In long bones the primary centers occur in the diaphysis/shaft and in irregular bones the primary centers occur usually in the body of the bone. Most bones have only one primary center (e.g. all long bones) but some irregular bones such as the os coxa (hip) and vertebrae have multiple primary centers.