A 2-year-old boy is brought to the physician by his parents, who complain that their son continually loses his balance. They also report that his speech seems more slurred. Physical examination confirms the truncal ataxia and wide-based gait. The child appears lethargic, and there is bobbing of the head while he is sitting. Muscle tone is normal. A lumbar puncture is performed in the patient, and neoplastic cells are found. A suboccipital craniotomy is performed, and a tumor is resected. Microscopic examination of the surgical specimen would most likely reveal which of the following histologic patterns?