A 12 year old girl complains of pain persisting in his leg for several weeks with a low grade fever. A radiograph reveals a mass in the diaphyseal region of the left femur with overlying cortical erosion and soft tissue extension. A biopsy of the lesion shows numerous small round cells, rich in PAS positive diastase sensitive granules. The most likely histological diagnosis is :
Osteogenic sarcoma. Osteoblastoma. Ewing's sarcoma. Chondroblastoma