Question: when do primary ossification centers appear in an embryo
[{'answer': 'prenatal development', 'provenance': [{'wikipedia_id': '6175955', 'title': 'Ossification center', 'start_paragraph_id': 8, 'start_character': 104, 'end_paragraph_id': 8, 'end_character': 124, 'bleu_score': 1.0, 'section': 'Section::::Types of ossification centers.\n'}]}, {'provenance': [{'wikipedia_id': '6175955', 'title': 'Ossification center', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 0, 'bleu_score': 1.0, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14637775166913162648}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '6175955', 'title': 'Ossification center', 'start_paragraph_id': 8, 'start_character': 0, 'end_paragraph_id': 8, 'end_character': 487, 'bleu_score': 0.9567517979328871, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14637775166913162648}, 'section': 'Section::::Types of ossification centers.\n'}]}, {'provenance': [{'wikipedia_id': '6175955', 'title': 'Ossification center', 'section': 'Section::::Types of ossification centers.\n', 'start_paragraph_id': 8, 'end_paragraph_id': 8, 'meta': {'evidence_span': ['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.', '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 except clavicle) but some irregular bones such as the os coxa (hip) and vertebrae have multiple primary centers.', '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.']}}]}, {'provenance': [{'wikipedia_id': '11849683', 'title': 'Zona hatching', 'section': 'Section::::Abstract.', 'start_paragraph_id': 1, 'end_paragraph_id': 1, 'meta': {'evidence_span': ['Zona hatching is a phenomenon occurring during prenatal development.', 'Zona hatching is a phenomenon occurring during prenatal development. Prior to this event, the predecessor of the embryo, in the form of a blastocyst, is surrounded by a glycoprotein sphere called the zona pellucida. To be able to perform implantation on the uterine wall, the blastocyst first needs to get rid of the zona pellucida. This lysis of the zona pellucida is called the zona hatching. In humans, this happens at the end of day four. In some situations, the term "hatching" is used only for artificial']}}]}]