In order to study possible mechanisms of fibrogenesis within living cells, experiments have been carried out using solutions of nucleo-proteins to determine the conditions which lead to the building up of spontaneously ordered structures. Nucleo-protein solutions which separate into a two-phase system produce orientated fibres by a process of aggregation. These fibres are birefringent and show visual dichroism after staining with vital stains. A method of measuring concentrations by interference microscopy in the living cell has shown that the concentration of high molecular-weight material within the nucleus would be expected to maintain ordered structures of the type observed in the solutions of isolated nucleo-protein. It is shown that the formation of fibres takes place by a similar process in the cytoplasm during spermatogenesis. These fibres also undergo a process of spiralization which is analogous to the behaviour of chromosomes.