Embryonal brain tumour. Light micrograph of a section through the cerebellum in the brain, showing an embryonal tumour (dark patch, centre to lower left). Embryonal tumours are masses of rapidly growing cells believed to be derived from embryonic (foetal) tissue. They can be benign (non-cancerous) but many are highly malignant (cancerous). This tumour is thought to be derived from primitive neural cells. It is known as a medulloblastoma.