Medulloblastoma, light micrograph. Embryonal tumours comprise about 20% of pediatric brain tumours. Medulloblastoma is the most commonly seen member of this group. Others include atypical teratoid/rhabdoid tumour and the so-called primitive neuroectodermal tumour. Histologically, they all share the presence of primitive appearing small round blue cells with high N:C ratio, hyperchromatic nuclei and brisk mitotic activity. Medulloblastomas occur in the cerebellum and usually arise from the vermis. The image shows a distinct variant of medulloblastoma called medulloblastoma with extensive nodularity. It is composed of pale islands of uniform tumour cells in a fine fibrillary background. The tumour cells are often arranged in streaming parallel rows.