Electron micrograph of a portion of a dissociated nucleolar core isolated from an oocyte of Notophthalmos viridescens. During early growth of amphibian oocytes, a thousand or more extrachromosomal nucleoli are formed in each nucleus. When these are isolated and dissociated on water, the fibrillar core of the nucleolus is unraveled. The bottle-brush or Christmas tree-like segments each appear to represent one gene coding for the precursor of ribosomal RNA. It shows very clearly that many molecules are simultaneously synthesized on each gene. This remarkable micrograph was the first visual demonstration of an individual gene and its associated transcription product. Enhancement of 9C3001.